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Inside the Apple-1's unusual MOS clock driver chip

Apple's first product was the Apple-1 computer, introduced in 1976. This early microcomputer used an unusual type of storage for its display: shift register memory. Instead of storing data in RAM (random-access memory), it was stored in a 1024-position shift register. You put a bit into the shift register and 1024 clock cycles later, the bit pops out the other end. Since a shift-register memory didn't require addressing circuitry, it could be manufactured more cheaply than a random-access memory chip.1 The downside, of course, is that you had to use bits as they became available, rather than access arbitrary memory locations. The behavior of shift-register memory was a good match for video circuitry, though, since characters are displayed on the screen in a fixed, repeating order (left to right and top to bottom).2

The Apple-1 was sold as a bare board, so users needed to make a case for it, or mount it in a briefcase as shown here. Note the cassette drive used for mass storage. Photo cropped from Binarysequence, CC BY-SA 4.0.

Shift-register memory chips required clock pulses with high current and unusual voltages: from +5 volts to -11 volts. These pulses were provided by a special chip, the DS0025 Two-Phase MOS Clock Driver. This chip, introduced in 1969, was the first monolithic (i.e. integrated circuit) clock driver. In this blog post, I look inside the chip and explain how it was implemented.

Die of the DS0025 clock driver. Click this image (or any other) for a larger version.

The photo above shows the silicon die under the microscope. This chip is very simple, containing four large NPN transistors, four diodes, and four resistors. The silicon appears blue-gray in this image, while the metal layer on top appears speckled white. Around the outside of the die, are six dark rectangles, the pads where golden bond wires connected the die to the chip's external pins.

The die was encased in an epoxy package. To expose the die, Eric (@TubeTimeUS) tediously sanded through the plastic package until the die was visible. Some bits of epoxy remained, caught in the bond wires, so I cleaned up the die with a few drops of boiling sulfuric acid.

The Apple-1's display

The Apple-1 displayed 24 lines of forty characters on a television monitor. Like most computers at the time, the Apple-1 stored characters rather than pixels to reduce memory requirements. A character-generation ROM converted each character into a 5×7 matrix of pixels as it was displayed. To reduce memory even more, the display didn't store full bytes, but 6-bit characters, supporting upper-case letters, numbers, and some symbols.

The six-bit display characters were held in six 1024-bit shift registers. A seventh shift register tracked the cursor position.3 The diagram below shows the shift registers and the clock driver on the Apple-1 circuit board. These chips are in 8-pin packages, so two chips fit into the space of a regular TTL chip.

Apple-1 circuit board, showing the 1024-bit shift register chips and the clock driver chip. Original image from Achim Baqué, CC BY-SA 4.0.


Next, I'll discuss the components of the chip. Because the chip generates high-current pulses, it uses large NPN transistors, with a different construction from most integrated circuit transistors. Each transistor consists of 24 emitters, paralleled in two groups. (You can consider it one large transistor, 2 transistors, or 24 small transistors.) The transistor is structured vertically with the collector (made of N-doped silicon) underneath, a thin P-type base in between, and the N-type emitters embedded in the top, forming the N-P-N layers of the transistor. The doped silicon regions are faintly visible with black lines around their boundaries.

Half of a transistor, with 12 emitters.

In the photo above, you can see the metal wiring for the transistor's collector, base, and emitter. The collector wiring is on the outside, with base wiring in between. The collector and emitter wiring is tapered: at one end, the wiring needs to support the full current load, while at the other end it only handles 1/12 of the current. The tapered approach saves space, since it is thicker only where it needs to be thick.


The resistors are formed from silicon doped to have higher resistance. The doped silicon rectangle is faintly visible in the die photo. At each end of the resistor, a contact connects the silicon to the metal layer on top. The 1000Ω resistor on the left is longer than the 250Ω resistor on the right, giving it more resistance.

Two resistors as they appear on the die.


The chip has a single layer of metal wiring, which poses a problem if two signals need to cross. The solution is to put one signal in the silicon layer so it can pass under the metal layer. In essence, a low-valued resistor is used to pass under the metal layer. The image below shows how a tunnel appears on the die.

The conductive silicon strip at the top connects the metal regions on either side. The conductive strip at the bottom doesn't fulfill a wiring need, but ensures that both paths encounter the same resistance.

One problem is that the silicon has relatively high resistance compared to metal, so the tunnel adds resistance. The chip is carefully designed so both "sub-transistors" encounter the same resistance, to avoid one transistor turning on before the other. You can see that the input path in the upper left has a tunnel to pass under the metal wiring, while the path in the lower right has a tunnel of identical dimensions that doesn't go under any metal. While the second tunnel appears pointless, it assures that both paths have the same resistance.

The chip's circuit

The shift register requires a two-phase clock, that is two clock signals in alternation that step the bits through the circuit. To support this, the clock driver chip has two identical driver circuits. The schematic below shows one of the circuits. When the input goes high, it turns on transistor Q1, pulling its collector low. This pulls the output low through diode CR2. When the input drops, Q1 turns off. This lets R2 provide a current to the base of Q2, turning it on, and pulling the output high. Thus, the circuit is essentially an inverter, but one that can provide up to 1.5 amps of output.4

Schematic of the DS0025, from the application note.

The image below shows the various components of the schematic as they appear on the die. Most of the chip is occupied by the large power transistors. Although the chip is mounted in an 8-pin package, only six pins are used; the corresponding pads are labeled below. The chip consists of two identical mirror-image drivers; one is labeled. There are a few blackened regions in the transistors; we suspect this is where the chip failed.

Die with the components labeled. Note: diodes are labeled CR (crystal rectifier) in the schematic but D here.


This chip provides an interesting view of computer technology in the 1970s. The Apple-1 used shift-register memory, a technology that rapidly became obsolete as RAM prices dropped. Shift-register memory required a specialized clock driver integrated circuit, a chip that contained just four large transistors. With billions of transistors in modern integrated circuits, it's hard to imagine that it was once worthwhile to build a chip that was this simple. The Apple II, introduced just a year later in 1977, used RAM chips for all its storage, making shift-register memory a thing of the past.

I announce my latest blog posts on Twitter, so follow me @kenshirriff. I also have an RSS feed. Thanks to @TubeTimeUS for supplying the chip. I've written about the Intel 1405 shift register memory if you want to know more about this type of storage.

Notes and references

  1. Looking in an old Byte magazine from 1976, a 1-kilobit shift register chip cost $9 ($34 in current dollars), while a 4-kilobit DRAM chip cost $20 ($75 in current dollars). Thus, it appears that even by the time the Apple-1 was released, DRAMs had become cheaper than shift registers. (This also illustrates the amazing drop in memory prices since the 1970s, as described by Moore's Law.)

    The Apple-1 used 4-kilobit RAM for data and program storage. It's possible, though, to build a computer that uses shift-register storage for its main memory. The Datapoint 2200 is one example. If memory is accessed sequentially, shift-register storage is efficient since the bits are provided sequentially. However, if you access memory out of sequence, the processor has to wait while the memory cycles around, until the desired bits become available. In a way, shift-register memory is a throwback to very early computers such as EDSAC(1949), which used mercury delay lines for main storage. 

  2. The IBM 2260 video display terminal (1965) used a technique similar to shift registers: it stored data in a sonic delay line, sending torsional pulses through a 50-foot nickel wire. But unlike the Apple-1, this delay line stored pixels, not characters. For more about this system, see my blog post

  3. The display circuitry has some additional complexity. Characters can't be taken directly from the display shift register: since each character is made up of eight scan lines; a line of character must be processed eight times. To handle this, a second shift register (six 40-bit registers) buffers a line of characters and feeds each character into a display ROM. Another 1024-bit shift register keeps track of the cursor position. For more details, see this post. The Apple-1 schematic is in the Operation Manual

  4. The large current is required because of the design of the shift-register memory. The clock line snakes through the chip, providing a clock signal to each stage of the shift register. As a result, the clock line has a fairly high capacitance, about 150 picofarads. This clock line must be switched between +5 volts and -11 volts at a 1 megahertz rate. The combination of large capacitance and large voltage swing with the fast rate requires a high current. 

Reverse-engineering the waveform generator in a 1969 breadboard

How hard could it be to fix a vintage solderless breadboard that doesn't quite work? The "elite 2 circuit design test system" below combined a solderless breadboard with some supporting circuitry: power supplies, a waveform generator, a pulse generator, switches, and lights. CuriousMarc found one of these breadboards on eBay, but the function generator didn't work, so we set out to repair it.

The E&L Instruments elite 2 solderless breadboard has a variety of supporting circuitry.

I figured that the waveform and pulse generators would be simple circuits, but they turn out to be implemented with a board crammed full of components, including over 40 transistors. I reverse-engineered the circuitry and found some interesting circuits inside, including op-amps implemented from discrete transistors. This complexity probably explains the shockingly high price of this breadboard: $1300 in 1969 (equivalent to $10,000 in current dollars).1

The circuit board for the function/pulse generator is crammed full of components. The upper part holds the waveform circuitry while the lower part holds the pulse generator.

The waveform generator

The breadboard has a waveform generator that produces triangle, square, and sine waves over a wide frequency range, up to 1 megahertz. These waveforms are generated through a complex circuit that charges and discharges an integrator to produce the triangle wave. A comparator turns the triangle wave into a square wave. Finally, a sine-wave shaping network produces a sine wave from the triangle wave.

Triangle-wave generator

The oscillator's frequency is selected by resistors and capacitors. The faster the capacitor charges through the resistor, the higher the frequency. Thus, increasing the capacitance and resistance slows the oscillation. The frequency range knob turns a vintage wafer rotary switch to select one of seven different resistors and capacitors, allowing frequencies over a wide range from 1 Hertz to 1 megahertz. A potentiometer adjusts the frequency within the range to provide the exact desired frequency.

The triangle wave is generated from an op-amp integrator circuit, using the approach below. This circuit uses the capacitor to integrate the input voltage, producing the output voltage. The result is that the square wave input increases or decreases the output linearly, yielding a triangle wave. (The op-amp keeps that right side of R1 at ground, so the current that charges the capacitor is proportional to the input voltage. In contrast, in a simple R-C charging circuit, the capacitor charges exponentially; the charging current drops as the capacitor's voltage increases.)

A simplified op-amp integrator, based on image by Rutujadeshpande, CC BY-SA 3.0.

To make an oscillator, the other piece is a comparator to provide the square wave input to the integrator. The comparator reverses direction at the top and bottom of the triangle wave, as shown below. When the input to the integrator is positive, the integrator output climbs linearly. This output is fed into the comparator, along with a limit level (red dots). When the output exceeds the upper limit at "A", the comparator output becomes negative. Since the comparator output is used as the integrator input, the integrator now discharges and the signal drops. When the signal drops below the limit at B, the comparator switches on and the process repeats. A hysteresis circuit changes the comparator level (dotted red line) at A and B, setting the upper and lower limits of the triangle wave.Athleta skirt skort

The triangle-wave generation process.

The comparator is a 710HC, a simple differential comparator integrated circuit introduced by Fairchild around 1965. This is one of just two integrated circuits on the board.3 The IC is packaged in an 8-pin circular metal can. This package was common at the time for analog integrated circuits, as the metal can provided shielding.

The 710HC comparator is packaged in a round metal case.


The op-amp is a key component of the integrator. Although integrated-circuit op-amps date back to 1963, this board builds op-amps out of discrete components. The integrator op-amp consists of seven transistors, along with a bunch of resistors and capacitors, as shown below. The heart of the op-amp is the differential pair (Q30 and Q32), a standard analog circuit. A fixed current is fed into the differential pair transistors. If one transistor has a slightly higher input than the other, that transistor turns on and most of the current will go through that transistor. Thus, the differential pair amplifies the difference between the inputs, the key function of an op-amp. Additional amplification is provided by Q33, while Q34 and Q35 buffer the dual outputs.

Implementation of an op-amp in the waveform generator.

The output amplifier for the waveform circuit uses another discrete op-amp. This one has two stages of differential pairs for additional amplification, followed by power transistors to produce a high-current output. Another op-amp circuit is used in the sine-wave shaper, discussed below.

Sine-wave shaper

The board uses a surprising technique to generate sine waves: it synthesizes a sine wave from the triangle wave. Specifically, a resistor-diode network shapes the sine wave using piecewise-linear segments. The idea is to use diodes as switches that turn on as the signal level crosses various points. This adds resistance into the circuit, changing the slope. The result is a sine wave with less than 1% distortion.

This diagram explains the sine-wave shaping network. From HP Journal, Nov 1965.

The sine-wave shaper appears to be inspired by the similar circuit in the HP 3300A Function Generator, introduced in 1965. The schematic below shows the HP 3300A's sine-wave shaper; the breadboard's network is similar. The resistances are carefully chosen to achieve the sine wave. Similar resistor-diode networks were also used in analog computers to implement arbitrary functions, sometimes with user-adjustable resistances to change the function.

The sine-shaping circuit from the HP 3300A is very similar to the circuit in the breadboard. The resistor-diode network is highlighted. The surrounding circuitry biases the network and amplifies the output. From the Hasbro FurReal Check Up Zandi Interactive Monkey Talking Sound E Fig 6-2.

Pulse generator

The pulse generator produces pulses from 100 ns to 100 ms wide. These pulses can be triggered by the waveform generator, an external trigger input, or a "one-shot" pushbutton. The pulse width is controlled by a switch-selectable resistor-capacitor network.

The most unusual part of the pulse generator is how the output circuit adjusts the pulse amplitude. Instead of simply adjusting the amplification, the circuit changes the voltage that powers the output amplifier. This variable voltage is produced by an LM305 voltage regulator IC, adjusted by the amplitude knob on the breadboard. A four-transistor circuit produces the matching negative voltage.4 These voltages power a fairly complex output stage with two circuits. One circuit produces positive pulses, while the other produces negative pulses.

The LM305A integrated circuit is in an old-fashioned metal can.


A prototyping breadboard may seem like a simple product, but everything becomes more complicated when built with 1969 technology. This breadboard includes a precision waveform generator and power supplies, designed for high accuracy, so it was almost like having test equipment included. But these features came at a steep price, equivalent to $10,000 today.

After I reverse-engineered the board,5 CuriousMarc used the schematic to fix the problems. The breadboard turned out to be in bad shape with a broken wire, a bunch of bad transistors, and a failed bridge rectifier in the power supply.6 It's unclear why the board had so many problems, more than you'd expect from age alone. Maybe the power supply over-voltaged the components at some point? My full schematic for the board is here.7

Some of the bad transistors that needed to be replaced.

CuriousMarc now has a video about the breadboard, so check it out:

Follow me on Twitter @kenshirriff for more posts. I also have an RSS feed.

Notes and references

  1. The breadboard is described in a 1971 brochure

  2. The hysteresis circuit is critical to the stability of the triangle wave. If the comparator input doesn't immediately switch to the lower level at "A", the comparator will switch again as soon as the integrator output drops slightly. Then the integrator will start rising, causing the comparator to switch again. The result is undesired high-speed oscillations (around a megahertz) with the triangle wave remaining stuck. This happened to us while attempting to repair the circuit when we replaced the hysteresis transistor with one that was a bit too slow. 

  3. One unusual feature of the comparator chip is its asymmetrical power supplies. The positive supply (Vcc+) can go up to 14 volts, while the negative supply (Vcc-) is limited to -7 volts. This is inconvenient for the breadboard, which uses ±12 volt supplies internally. The solution is that the breadboard uses a 6.2-volt Zener diode to reduce the negative supply to the chip. R-C filters in the supply lines to the chip reduce noise. 

  4. The negative-voltage circuit produces a negative voltage to match the user-selected positive voltage. In essence, it uses feedback from a resistor voltage divider between the positive and negative rails. When the two voltages are equal (and opposite), the voltage divider will yield 0 volts. If the voltages don't match, the signal from the voltage divider provides feedback to increase or decrease the negative rail as necessary. 

  5. To reverse-engineer the board, I used a process that I've developed recently that works well. I took photos of both sides of the board and used the GIMP software to mirror one image and then align the images using the perspective tool. Next, I created a schematic in EAGLE, putting the symbols in their approximate locations. I wired up the schematic by drawing connections in EAGLE and marking the traces in GIMP as I handled them. The result of this was a "physical" schematic, approximately matching the board's layout.

    The next step was to rearrange the components in EAGLE to create a more logical "functional" schematic. (I find that EAGLE works better than KiCad for this.) This schematic helped me understand how the circuitry was implemented, and we used it to trace through the circuitry and diagnose its problems. 

  6. The breadboard has five different power supplies: three user-adjustable power supplies, and two supplies for internal use, providing unregulated ±32 volts and regulated ±12 volts. Modern systems typically use compact switching power supplies, but the breadboard uses two large and heavy power transformers. Five large power transistors provide regulation, along with massive capacitors. Overall, the power supply illustrates how much power supplies have improved since the 1960s. 

  7. Disclaimer: the schematic isn't completely accurate. In particular, I didn't look up component numbers, so I'm kind of guessing on NPN vs PNP transistors and there are definitely errors. I also didn't record resistor and capacitor values. (The purpose of the schematic was to guide the repairs, not to completely document the device.) 

A look inside the chips that powered the landmark Polaroid SX-70 instant camera

The revolutionary Polaroid SX-701 camera (1972) was a marvel of engineering: the world's first instant SLR camera. This iconic camera was the brainchild of Dr. Edwin Land, a genius who co-founded Polaroid, invented polarized sunglasses, helped design the optics for the U-2 spy plane, and created a theory of color vision. The camera used self-developing film2 with square photos that came into view over a few minutes.3 The film was a complex sandwich of 11 layers of chemicals to develop a negative image and then form the visible color image. But the film was just one of the camera's innovations.

Edwin Land and the Polaroid SX-70 camera were featured on the cover of Life magazine, October 27, 1972.

The camera required complex new optics to support the intricate light path shown below. The components included a flat Fresnel mirror, aspherical lenses, and a moving mirror. These optics could focus from infinity down to a closeup of 10 inches. The optics are even more amazing when you consider that the camera folded flat, 3 cm thick and able to fit in a jacket pocket.

Diagram from Life magazine, Oct 1972, showing the light path through the camera.

But I'm going to focus on the camera's electronics, powered by a custom flat battery pack. When the shutter button is pressed, the camera carried out several tasks with precision timing. First, a solenoid closes the shutter, blocking the entry of light into the camera. Next, the motor is turned on, causing the camera's internal mirror to flip up to uncover the film. The solenoid is then de-energized, causing the shutter to open. The film exposure time depends on the light level; at the appropriate time, the solenoid closes the shutter again to stop exposure. Finally, the motor runs again to eject the film and reset the mirror.

I'm helping the openSX70 project by reverse-engineering the chips on the exposure control board. This board contains three surface-mount ICs: a chip to read the light intensity from a photodiode, a timer chip to control how long the shutter blades are open, and a power control IC to drive the motor and solenoids.4

The exposure control circuit board, manufactured by Texas Instruments. Photo from openSX70.

The development of this board was contentious, with Fairchild and Texas Instruments battling to supply the electronics for millions of cameras.5 The exposure control board went through three designs as Fairchild and Texas Instruments struggled to meet Polaroid's price target of just $5.75. First, Texas Instruments built a control board from a ceramic substrate with laser-trimmed resistors. The expensive components put the board way over budget at $100 so Polaroid used these boards only in prototype cameras. Fairchild's design was accepted by Polaroid even though its cost of $20 still exceeded the target. Fairchild's board was used from 1972 to 1973, but Texas Instruments fought back with an all-new design that cost only $4.10. This TI board was the long-term winner, and is the one I am examining in this post.

The optical chip

The exposure control board automatically adjusts the exposure time based on the amount of ambient light. Ambient light is measured by the optical chip, a package that combines a photodiode and a silicon die in one small package. The silicon die is protected by epoxy, but the larger photodiode is exposed so external light can fall on it.

The optical chip contains a photodiode and a silicon die in one package.

To measure the ambient light, the chip implements the integrator circuit below. The photodiode generates a small current that depends on the light level. This current is integrated over time using a capacitor until a threshold is reached. By opening the shutter during this interval, the film is exposed for the desired amount of time. (The film's exposure depends on the total amount of light received, which is the same value that the integration calculates.) The op-amp die outputs the voltage across the capacitor without draining the capacitor in the process.

The optical chip is essentially an integrator.

The photo below shows the silicon die under a microscope. The chip, made by Texas Instruments, is dominated by the zig-zags forming two interlocking JFET transistors. A JFET is a special type of transistor, used before MOSFETs became popular. These transistors have very low input currents, so they won't drain the capacitor as it charges. The interlocking layout ensures that both transistors are at the same temperature, so the circuit will stay accurate even if the chip heats up unevenly. The chip also contains NPN and PNP transistors, resistors, and a capacitor (the large pink square labeled 28710).

The optical chip. Photo from siliconPr0n, (CC BY 4.0).

By reverse-engineering the die, I created the schematic below. It is an op-amp, measuring the difference between two inputs (one tied to ground). The two JFET (Q12/Q13) transistors are configured as a standard differential pair circuit. A fixed current (from the current mirror Q8/Q6) is fed into the transistors, and whichever transistor has the higher input will pass almost all the current. The result is amplified by Q5 for the output. In addition to the op-amp circuitry, the chip contains a reset circuit to discharge the capacitor before use (Q1/Q2). (The internal capacitor C1 stabilizes the op-amp; the integration capacitor is external.)

Schematic of the optical chip. Click for a larger version.

The power driver chip

Next, the power chip drives solenoids and the motor to activate the camera's mechanisms. The high-current power transistors (the golden triangular shapes) take up most of this chip. Smaller transistors below form the control circuitry.

The power driver chip. Photo from siliconPr0n, (CC BY 4.0).

My reverse-engineered schematic shows that the chip has three parts. First, a simple inverter. This probably interfaces the logic chip to the motor control board.

Schematic of the inverter in the power driver chip. Click for a larger version.

Second, a high-current driver. This uses the large power transistor at the left of the die. This probably drives a solenoid.

Schematic of the driver in the power driver chip.

Finally, a high-current driver with a separate circuit for the solenoid hold current. (That is, the solenoid is pulled into position with a high current, and then held in that position with a lower current.) This uses the large power transistors at the right of the die. There's a single large transistor underneath the main "triangle" of transistors; that transistor is for the hold current. This circuit uses separate power and ground pads from the rest of the chip.

Schematic of the driver/hold circuit in the power driver chip.

The driver circuits are more complex than I'd expect, using current sources and current mirrors. Maybe this design minimizes standby current use.

The logic chip

The camera is controlled by a complex logic chip that controlled the timing of the various mechanisms, running the motor and solenoids. It had to handle four different use cases: ejecting the protective cover sheet when a film package was inserted, taking a photo, taking a photo with the flash, and ejecting an empty film package.

This chip was constructed from Integrated Injection Logic (I2L), an obscure 1970s logic family featuring high density and low power. Because the camera ran off a battery in the film pack, minimizing power consumption was a critical factor. At the time, I2L was a good choice for dense, low-power circuitry, although it was soon overtaken by CMOS. Texas Instruments did a lot of development with I2L, including digital watch chips and the 76477 sound chip so it's not surprising that they chose I2L for the camera chip.

The logic chip. Photo from siliconPr0n, (CC BY 4.0).

I2L gates can be packed together at high density, as shown below. Each vertical gray rectangle is two transistors (one above the horizontal centerline and one below), corresponding to two gates. The chip has very little wasted space, especially compared to TTL logic, which was commonly used at the time but required multiple transistors and bulky resistors for each gate.

Closeup of the logic chip.

I2L is a bit tricky to understand since an I2L gate has one input and multiple outputs. How can that work? The schematic below shows an I2L gate, with one input and three outputs. Normally the current from the injector (ICC) turns on the output transistor, pulling the output low. But if the input is low, the output transistor turns off and the output will be high. Thus, the gate inverts the input. (You can think of the injector as a pull-up resistor on the input.)

Implementation of an I2L gate. Note that it has a single input and multiple outputs. Icc is the injected current. From "Integrated Injection Logic: A Bipolar LSI Technique".

Since the circuit above has a single input, it may seem to be just an inverter. But by wiring several signals together at the input, you get an AND gate "for free": if any signal is low, it will pull the wire low, and otherwise the signal is high. This is called "wired-AND". The wired-AND input to the I2L inverter results in a NAND gate.

One problem arises with wired-AND: if you connect an output to more than one wired-AND, everything gets shorted together. The solution is to have multiple outputs from the inverter. Thus, each I2L NAND gate has a single input and multiple identical outputs. In the diagram below, the outputs from various gates (A and B below) are connected together and fed to the input of an I2L gate, creating a NAND gate.

Diagram of a NAND gate implemented in Integrated Injection Logic (I2L). From "Integrated Injection Logic: A Bipolar LSI Technique".

The transistors in I2L have multiple collectors, which may seem strange, but the diagram below shows how they are constructed. Each collector has an N region (purple) with a P region (tan) below for the base, and another N region (green) at the bottom, forming an NPN transistor. The multiple collectors are built by creating multiple N regions. Physically, the injector PNP transistor is just a P region for the emitter, reusing the emitter and base's N and P regions; this makes the injector more compact than a "full" transistor.

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Die photo and cross-section diagram of an I2L gate. The transistor base, collectors, and emitters are labeled along with the current injection.

I haven't reverse-engineered this chip yet. I believe that it contains an oscillator and a chain of flip flops for timing, as well as a comparator for the light level and some miscellaneous control logic.


While the electronics of the SX-70 camera aren't impressive by modern standards, they were cutting edge at the time. They made the SX-70 easy to operate by handling the exposure and timing automatically. Texas Instruments split the electronics across three chips: a precision JFET op-amp with a photodiode, a high-current power driver chip, and a complex logic chip using dense, low-power I2L logic.

Unfortunately, innovative technology wasn't enough for Polaroid. The company declined after competition from Kodak, the expensive failure of the Polavision instant home movie system, and the rise of digital cameras. Polaroid declared bankruptcy in 2001 and the company was broken up. The SX-70 has seen a resurgence in popularity, with film and cameras sold by, which acquired the Polaroid name in 2017.

Follow me on Twitter @kenshirriff for more posts. I also have an RSS feed. Thanks to Joaquín De Prada and Peter Kooiman of openSX70 for providing the chips and John McMaster for decapping them.

The openSX70 project is building extensions to the SX-70 camera.

For more about the SX-70, see the interesting and quirky 10-minute movie below, which markets the SX-70, explains how to use it, and discusses the internal operation. This movie was made in 1972 by the famous designers Ray and Charles Eames.

Notes and references

  1. The name SX-70 comes from its inventor Dr. Edwin Land. He numbered all his "special experiments" in a notebook and his instant picture experiment was number 70. Although the camera was 30 years after special experiment 70, he felt that it embodied the system he had envisioned in the mid-1940s. 

  2. Land introduced instant photography in 1947, and then color instant film in 1963, based on a peel-apart technology. The SX-70 eliminated the problems of the peel-apart instant photos. As Polaroid said, “No pulling the picture packet out of the camera, no timing the development process, no peeling apart of the negative and positive results, no waste material to dispose of, no coating of the print, no print mount to attach, no chance for double exposure, no chance to forget to remove the film cover sheet and spoil a picture, no exposure settings to make, no flash settings to remember, no batteries to replace.”  

  3. Although Polaroid photos develop in a minute or so without any user intervention, shaking the photo was a common custom. "Shaking the Polaroid" was the theme of a 1998 Polaroid ad. Outkast's 2003 song Hey Ya! featured the refrain "Shake it like a Polaroid picture". 

  4. I haven't investigated all the chips in the SX-70. The motor control module contains a linear control IC and power transistors. The camera also takes a flashbar with five flashbulbs. The flash circuit has another chip that checks the bulbs to find an unused bulb. 

  5. "The Battle for the SX-70 Camera", IEEE Spectrum, May 1989 discusses in detail the battle between Fairchild and Texas Instruments to win the Polaroid contract.

    The internal circuitry of the camera is described in "Behind the Lens of the SX-70", IEEE Spectrum, Dec 1973. This circuitry, however, is for the earlier Fairchild version and the implementation is considerably different from the Texas Instruments circuitry that I examined. The Fairchild implementation is also described in "Camera Electronics, A New Approach", WESCON 1973. 

Yamaha DX7 chip reverse-engineering, part 6: the control registers

The Yamaha DX7 digital synthesizer (1983) was the classic synthesizer in 1980s pop music. It uses a technique called FM synthesis to produce complex, harmonically-rich sounds. In this blog post, I look inside its custom "OPS" sound chip and explain the control registers for this chip. By reverse-engineering the circuitry, I found a few undocumented test functions. (This post covers some fairly obscure details of the DX7; you might prefer my previous DX7 posts1 starting with "DX7 reverse-engineering".)

Die photo of the YM21280 chip with the main functional blocks labeled. Click this photo (or any other) for a larger version.

The die photo above shows the DX7's OPS sound synthesis chip under the microscope, showing its complex silicon circuitry. Unlike modern chips, this chip has just one layer of metal, visible as the whitish lines on top. Around the edges, you can see the 64 bond wires attached to pads; these connect the silicon die to the chip's 64 pins. In this blog post, I'm focusing on the control registers, highlighted in red. I'll outline the other functional blocks briefly. Each of the 96 oscillators has a phase accumulator used to generate the frequency. The sine and exponential functions are implemented with lookup tables in ROMs. Other functional blocks apply the envelope, hold configuration data, and buffer the output values.

The DX7 synthesizer. Photo by rockheim (CC BY-NC-SA 2.0).

The DX7 generates sounds digitally using a technique called FM synthesis. Each note has six oscillators (called "operators") that can be combined in different ways (called "algorithms"). An algorithm is represented by a diagram (below), where an oscillator modulates the oscillator below, as shown by the lines. For instance, in algorithm 1 below, oscillator 6 modulates oscillator 5 which modulates 4 which modulates 3. Oscillator 2 modulates oscillator 1. The output is taken from the bottom oscillators (1 and 3). Meanwhile, oscillator 6 modulates itself, controlled by a user-selectable feedback level. With 32 different algorithms, the DX7 can generate a wide variety of sounds. In the DX7 synthesizer, all 16 notes must use the same algorithm. But from my reverse-engineering, it appears that the chip supports different algorithms for each note, even though the synthesizer doesn't make use of this.

Four of the 32 "algorithms" that can be selected on the DX7.

To the programmer of the DX7 firmware, the sound chip appears to have two write-only registers that control the chip. The diagram below shows the layout of the chip's s two registers, as described by Anthony Richardson. The desired algorithm and feedback are written to address 1.2 Address 0 has bits to turn the "key sync" feature3 on and off. As for the Mute and Test Register Select bits, my investigation provides some explanation.

Address | Bit 7 | Bit 6          | Bit 5        | Bit 4 | Bit 3 | Bit 2 | Bit 1 | Bit 0 |
0       | Mute  | Clear Key Sync | Set Key Sync | Test Register Select                  |
1       | Algorithm Select (0..31)                              | Feedback Level (0..7) |

(The functionality above is pretty limited, so you might wonder how the synthesizer controls which notes are played. Most of the synthesizer functions are controlled through a second custom chip, the envelope generator chip (EGS). Note and envelope data is written to registers in the EGS chip, which then sends frequency and amplitude data to the sound chip over a special bus.)

The diagram below shows the main components of the register circuitry. The large block at the bottom is the A-register, which holds the algorithm/feedback entries, as 16 8-bit values.4 The most puzzling feature of the A-register is its size; it holds 16 entries, one for each note, but the DX7 uses the same algorithm/feedback setting for all 16 notes. The second puzzling feature is that although the chip appears to have two 8-bit registers, the implementation is one 9-bit register, and one 5-bit register. Moreover, the 5-bit register can only be modified by writing through the 9-bit register.


Main functional blocks of the register circuitry.

The chip has one address pin (called "DS") which selects between the two registers. When a byte is written to the chip, to either address, the 8 bits along with DS (the address bit) are stored in the 9-bit latches. If DS is 0 (i.e. write to the control address 0), the bits are decoded to perform any special functions, and the lower 5 bits are loaded into the 5-bit register on the right. If DS is 1 (i.e. write to the algorithm/feedback address 1), the 8-bit algorithm/feedback value is stored into the A register, in a location controlled by the 5-bit register.

Updating the algorithm/feedback

The algorithm/feedback A-register register holds data for 16 notes. It can be updated in two ways. The first way, used by the DX7, updates all entries with the same value. The second way updates a single entry, allowing different notes to have different algorithms. Both cases involve a write to address 0, followed by writing the algorithm/feedback byte to address 1.

To update all entries, address 0 must be written with a value with the bit pattern 0??1?0?? (where ? indicates a "don't care" bit that can be 0 or 1).5 This pattern triggers a circuit that constantly loads the value in the data latch into the storage register.

The DX7's CPU controls the OPS chip in this way. Specifically, an update of the algorithm and feedback is performed by writing either 0x30 (if sync is on) or 0x50 (if sync is off) to address 0, and then writing the algorithm/feedback byte to address 1.2

The second update path will change the algorithm and feedback for a single note. (The DX7 does not use this feature.) is triggered by writing the bit pattern 0??0nnnn, where nnnn specifies one of the 16 notes.

The implementation of this is a bit tricky because the chip uses shift registers for storage, not RAM. The A-register consists of 8 shift registers (one for each bit), each with 16 stages (one for each note). An entry can only be updated when it is shifted out the end of the shift register, and a new value can be inserted. (This is unlike RAM, where an arbitrary entry can be written.) To update an entry in the shift register, a 4-bit comparator circuit (below) compares the number of the current note with the number of the desired note in the control register. When there is a match, the new value is written to the shift register.

The 4-bit comparator determines when the shift register is at the desired note position. It is built from four exclusive-NOR gates.

Special command sequences

The logic circuitry recognizes several bit patterns when they are written to address 0, and causes special actions when they are detected. These are not used by the DX7; I think they were used for testing the chip during manufacturing to make tests more predictable and faster.

1???????: Setting the top bit triggers several special actions. Earlier analysis has labeled this bit as "Mute", but I suspect it is more of a "Test Reset" function, resetting the chip to a known state so tests will be predictable. This bit clears the phase accumulators. This bit disables the scale factors, so the output data is unshifted. It also bypasses the output latch, which may output digital note data at a higher rate.

1??????1: In addition to the previous action, this pattern resets the counters that count through the operators and notes, controlling the actions of the chip. This is probably used start testing the chip from a known state, so the outputs can be compared with expected values.

1?????1?: This causes the low-order bits of the phase register to generate the waveform, rather than the high-order bits. I think this is used for testing so the low-order bits can be examined more directly to find flaws. It also will increase the frequencies by a factor of 1024, which may help run through waveforms faster for testing.


By looking inside the chip and reverse-engineering the silicon circuits, I learned some details about the internal registers. One interesting discovery is that the chip appears to support separate algorithms for each voice, even though the synthesizer doesn't use this feature. I also uncovered some test functionality.

The Yamaha YM21280 OPS integrated circuit package with the metal lid removed, revealing the silicon die.

I plan to continue investigating the DX7's circuitry, so follow me on Twitter @kenshirriff for updates. I also have an RSS feed. Thanks to Jacques Mattheij and Anthony Richardson for providing the chip and discussion.6

Disclaimer: I figured out the behavior described in this post studying the die. It hasn't been tested on an actual DX7 so I don't guarantee that it is correct.

Notes and references

  1. My previous posts on the DX7: DX7 reverse-engineering, The exponential ROM, Handmade Knit Yellow Newborn Baby Booties, How algorithms are implemented, and The output circuitry

  2. Looking at the ROM shows how the synthesizer's CPU communicates with the OPS chip. Since the DX7 ROM code has been disassembled, you can view the code that writes to the sound chip here

  3. Oscillator Key Sync is a feature of the DX7. According to the manual, Operator Key Sync "enables you to set the operator so its 'oscillator' begins at the start of the sine wave cycle each time you play a note. When Oscillator Key Sync is off, the sine wave continues so that subtle differences will occur even when you play the note repeatedly." 

  4. The DX7/9 Service Manual shows the "A-register" holding the algorithm and feedback level, so I'll use that name. 

  5. The bit pattern 0??1?0?? looks a bit random. I don't know why this pattern was chosen. The first two bits can be explained, but I don't see a purpose for the last 0 bit. 

  6. For more information on the DX7 internals, see DX7 Technical Analysis, DX7 Hardware, OPLx decapsulated, and the video Emulating the DX7 the hard way

Yamaha DX7 chip reverse-engineering, part V: the output circuitry

The Yamaha DX7 digital synthesizer (1983) was the classic synthesizer in 1980s pop music. It uses a technique called FM synthesis to produce complex, harmonically-rich sounds. In this blog post, I look inside its custom sound chip and explain how the chip's output circuitry works. You might expect it's just a digital output fed into a digital-to-analog converter, but there's much more to it than just that.

Die photo of the YM21280 chip with the main functional blocks labeled. Click this photo (or any other) for a larger version.

The composite die photo above shows the DX7's OPS sound synthesis chip under the microscope, revealing its complex silicon circuitry. Unlike modern chips, this chip has just one layer of metal, visible as the whitish lines on top. Around the edges are the 64 bond wires attached to pads; these connect the silicon die to the chip's 64 pins. The three blocks in red are the focus of this post. The output buffers hold the 16-bit digital values for the 16 notes. The output is controlled by a counter and PLA (Programmable Logic Array). The synthesizer's digital-to-analog conversion uses a sample-and-hold circuit, controlled by the "S/H ctrl" block.

I've discussed the chip's other functional blocks in earlier posts1, so I'll just give a brief summary here. Each of the 96 oscillators has a phase accumulator used to generate the frequency. The oscillators are combined using the operator computation circuitry in the middle of the chip, under the control of the algorithm ROM. The signal synthesis uses sine and exponential functions, implemented with lookup tables in ROMs.

The Yamaha DX7 synthesizer with its 61-key keyboard and digital controls. Photo by Disney Pixar Toy Story 4 Figural Bag Clip Blind Bag - Forky (CC BY-NC-SA 2.0).

The synthesizer's output circuitry

Before I dive into the details of the chip, I'll explain the synthesizer's output circuit. The heart of the synthesizer is the OPS (Operator S) sound chip that digitally generates the notes. It provides digital values to the digital-to-analog converter (D/A). The resulting analog signal goes through a low-pass filter (LPF). The volume is controlled by a foot pedal and the synthesizer's volume control. Finally, the signal is amplified for the line and headphone outputs.

Block diagram of the output circuit. Based on the DX7/9 Service Manual.

The digital-to-analog conversion is more complex than you might expect. The process starts with the digital-to-analog converter (DAC) chip2 that takes a 12-bit digital value from the sound chip and converts it to an analog value in the range 0 to 15 volts.3 The multiplexer allows the overall synthesizer volume to be controlled by MIDI, but with just 8 levels.

Schematic of the volume control and DAC circuit. Based on the DX7 schematics.

The DAC provides 12 bits of resolution, but an additional circuit (below) provides approximately two more bits. This scaler circuit divides the analog signal by 1, 2, 4, or 8, using a resistor network and IC switch. The scaler is controlled by the sound chip through the scale factor signals SF0-SF3. The scaler adds more dynamic range to the digital value; the result is similar to a floating-point value with a sign bit, 11-bit mantissa, and two-bit exponent.4

The scaler divides the voltage by 1, 2, 4, or 8. Based on the DX7 schematics.

Next, the signal goes to a sample-and-hold circuit that samples the analog voltage at a point in time and holds it in a capacitor, kind of like an analog memory. An op-amp buffers the capacitor's voltage so it can be "read" without draining the capacitor. There are two hold circuits, used in alternation, so the last two samples are stored and summed to form the circuit's output.5 The SH1 and SH2 control signals load the analog value into a capacitor, using IC52 as a switch. Finally, the output from the sample-and-hold circuit is filtered,Lululemon Peloton Shirt the volume is adjusted,7 and the signal is amplified for the output (circuitry not shown).

The sample-and-hold circuit. IC52 looks complicated because it uses pairs of switches in parallel. Based on the DX7 schematics.

To summarize, the sound chip interacts with the output circuitry in three ways. The 12-bit digital value (DA1-DA12) is most important as it specifies the output value for each voice. The scale factor signals (SF0-SF3) are also a key contributor to the signal. The sound chip also provides the sample-and-hold control signals (SH1 and SH2).

Time-division multiplexing

The DX7 has 16 voices, so it can play 16 notes at once. Each note is produced by an "algorithm" that combines 6 oscillators in a particular way, so there are 96 oscillators in total. An oscillator can modulate the frequency of another oscillator to generate complex sounds with FM synthesis.

The chip performs all its processing sequentially, one oscillator at a time, rather than computing the notes in parallel. Internally, the chip has one "operator" calculation circuit to combine oscillators. As shown below, the chip starts by processing operator 6 for note 1, then operator 6 for note 2, and so forth through note 16. Then it processes operator 5 for notes 1 through 16. Finally it processes operator 1 for notes 1 through 16, generating the output sound values. It takes a bit over 20 µs to compute all 16 notes in a complete processing cycle.

Timing diagram of sound production. This time interval corresponds to 49.096 kHz. From the DX7/9 Service Manual.

You might expect the chip to combine the 16 notes into a single digital output. However, the sound chip outputs the 16 notes sequentially, using a technique called time-division multiplexing. Each time interval (~20µs) is divided into 16 intervals and one note is output from the chip per interval. (Note that these intervals don't line up with the intervals in the diagram above.) Thus, digital values are output at 786 kilohertz, 16 times the underlying frequency, and the DAC chip converts them to analog at this rate.

As an example, consider two notes that are sine waves with different frequencies. The digital output would look like the image below. You might think that this signal is unusable since it jumps around wildly from point to point.

Output data with two multiplexed sine waves. (Theoretical, not actual DX7 data.)

However, applying a low-pass filter smooths out the waveform (essentially summing nearby points). The result is the waveform below, which shows the sum of the two sine waves.8 The point is that time-division multiplexing data may look strange, but the analog circuitry's filtering creates a "normal" waveform.

Output data after filtering with a 16 kHz low-pass filter.

Output buffer

Inside the chip, the output buffer stores values for the 16 notes as they are generated, and outputs them in sequence. Rather than RAM, the chip uses shift registers for storage. The shift registers are arranged in a loop of 16 stages, one stage for each note. On each clock cycle, the values in the shift register move to the next stage. The output value is fed back into the shift register's input so the value is retained. Alternatively, a new value can be stored in the shift register. Shift registers provided an efficient way to store data, but they cannot be accessed arbitrarily; instead, data must be processed as it becomes available.

The schematic below shows how one stage of the shift register is implemented. The chip uses a two-phase clock. In the first phase, clock ϕ1 goes high, turning on the first transistor. The input signal goes through the inverter, through the transistor, and the voltage is stored in the capacitor (kind of like DRAM). In the second phase, clock ϕ2 goes high, turning on the second transistor. The value stored in the capacitor goes through the second inverter, through the second transistor, and to the output, where it enters the next shift register stage.

Schematic of one stage of the shift register.

The die photo below shows the output buffer, with the 16 shift-register loops arranged in columns. These hold the 16-bit sound values (four scale factor bits and 16 data bits.) Each shift register is 16 stages long to hold the 16 notes. In the next sections, I'll discuss the bit shifters, the logic, and the output latches.

Closeup of the die, showing the output buffer circuitry.

The scale factor: pseudo floating point

The DX7 uses a 12-bit digital-to-analog converter chip, but the scaling circuit (discussed earlier) will scale the voltage by 1, 2, 4, or 8, which adds more resolution. This isn't quite equivalent to 14-bit resolution; it's more like a floating-point number with a sign, 11-bit mantissa, and 2-bit exponent. This provides more resolution for low signals and reduces signal noise.

Inside the chip, scaling is implemented with a shifter that shifts the data bits by 0 to 3 bit positions. (This is unrelated to the shift registers that hold data.) The shifter (below) is implemented as eleven chevron-shaped logic gates; each gate selects one of four potential bits for each mantissa position.

A data sample is shifted 0 to 4 bits by this shifter circuit.

The operator circuitry generates data as 15 bits (2's complement, so one of the bits provides the sign). The output from the chip is 12 bits, so three bits must be discarded. Normally these are the low-order bits, but by using the shifter, high-order zero bits can be discarded instead, and the external scaler counteracts this. The result is more bits of precision in the output.

The shifter is controlled by the logic circuitry to the left of the buffer, which controls the amount of shift based on the number of leading zeros. (For a negative number, leading 1's.) With 5 leading zeros, the number is shifted left by 3 positions. With 4 leading zeros, the number is shifted by 2 positions. With 3 leading zeros, the number is shifted by 1 position. With 2 or fewer leading zeros, the number is unshifted.

Note that the circuit leaves two leading zeros when it shifts, so it's "wasting" two potential bits of precision. I assume this is because the scaler won't be perfectly linear (due to the resistor imperfections9), so you want to avoid switching scale levels for large signals (which don't really need the extra bits).10

The output latches

As mentioned earlier, the 16 notes are output individually, spaced across the interval. This timing doesn't line up with the timing of the output buffer, which shifts to a new note every clock cycle. To fix the timing, two 16-bit latches sit between the output buffer and the output pins. While one latch outputs the current note, the other latch grabs the next note as it is shifted out of the shift register. At the appropriate time, the latches swap roles; the second latch outputs the note while the first latch waits for the next note.

The timing for the latches is fairly tricky to make sure the note data is loaded into the right latch at the right time. These latches are controlled by the chip's master counter, which is the subject of the next section.

To summarize, the sound chip runs at 4.7 MHz. Data values are produced at this rate (but intermittently) and stored into the output buffer. The output latches provide data values to the DAC chip at 786 kHz for an overall audio rate of 49096 kHz.

Keeping track of 96 clock cycles: the chip's counter and timing PLA

One complete cycle of the sound chip takes 96 clock cycles: processing all 16 notes through the 6 operators that form an algorithm. Because data can only be accessed when it exits a shift register, everything must be timed so the right data is available at the right time. A critical part of the chip is the counter that keeps track of the current note number and operator number to keep everything synchronized.

On the right of the die photo below is the counter, consisting of seven toggle flip flops: four to count the note number (0-15) and three to count the algorithm number (0-5). On the left is the PLA that defines what happens for particular time slices. (A Programmable Logic Array (PLA) is similar to a ROM, but implements arbitrary logic.) The PLA has 39 columns, each one implementing an AND gate triggered by a particular counter output, corresponding to a particular operator and note. Below the PLA is some logic; mostly buffers with a few gates.

The chip's main counter, along with the control PLA.

Of the PLA's 39 columns, the 32 columns on the left control the data output latches,11 two columns control loading data values into the output buffer, one generates the chip's sync output signal, three reset the operator count, and the last increments the operator count.12


The chip outputs two signals to control the sample-and-hold circuitry, SH1 and SH2. These signals are activated in alternation to take an analog sample of each digital output.

The sound chip on the DX7 schematic has three missing pins, indicated in red.

The sound chip has three unused pins next to the SH1 and SH2 pins; the DX7 schematic doesn't show pins 6-8. I traced the chip's internal circuitry and found that these pins, in conjunction with the sample-and-hold pins, count out the 16 samples. It appears that the chip is designed to sample-and-hold all 16 notes individually, so the synthesizer could have had separate outputs for all 16 notes.13

Moreover, the chip has data buffers to hold separate algorithm algorithms for the 16 notes. This would let the chip drive 16 independent voices, each with a separate algorithm. My conclusion is that the sound chip supports much more flexibility than is used in the DX7 synthesizer.


The DX7 generates sounds digitally and then converts the digital values to the analog output. This process turns out to be more complicated than one would expect, with circuitry inside the chip interacting with synthesizer circuitry to scale and adjust the signal. My hope is that my analysis of this process will help DX7 emulators to achieve more accuracy. Looking at the chip's internal circuitry reveals the floating-point format of the output data as well as the function of the three unused pins.

I plan to continue investigating the DX7's circuitry, so follow me on Twitter @kenshirriff for updates. I also have an RSS feed. Thanks to Jacques Mattheij and Anthony Richardson for providing the chip and discussion.14

Notes and references

  1. My previous posts on the DX7: DX7 reverse-engineering, The exponential ROM, H M jean shorts, and How algorithms are implemented

  2. The DAC chip is the BA9221, a 12-bit D/A converter that produces an output current based on a 2's-complement input value. A datasheet is here. The DAC receives an input voltage reference. This voltage reference can be one of 8 values selected by a multiplexer. This allows the overall volume to be set via MIDI, but only with 3-bit resolution (see the DX7 ROM code here). The volume is an exponential function (so linear in decibels) except that 0 is off. Also see this DAC discussion and this StackExchange discussion.) 

  3. The output from the DAC is centered around 7.5 volts. In other words. a digital value 0 corresponds to 7.5 volts, with positive digital value above 7.5 volts and negative digital values below 7.5 volts. I would have expected the signals to be centered around 0 volts, which is what the DAC datasheet shows. I think strictly positive voltages were used because they work better with the TC4066 and TC4066 integrated circuits switches (IC 41 for scaling and IC 52 for sample-and-hold respectively). The DX7 converts the signals to zero-centered voltages for the low-pass filter. 

  4. The amplitude scaler is built from an R-2R resistor ladder, similar to a DAC circuit. However, the attenuator only has 4 useful values, not the 16 levels you might expect with four control lines, because only one control line can be activated at a time. Combinations of control lines do not yield useful outputs. For example, if the top switch is on, you get the maximum output regardless of the other switches. Other switch combinations are non-monotonic. Thus, the scaler only provides two additional bits of resolution, not four. 

  5. The benefit of keeping two samples is not clear to me. One theory is that this reduces intermodulation distortion between the voices, the effect of one signal on another. With one "solid" sample and one changing sample, the effect of the changing sample will be reduced. Another, more speculative, possibility is that the circuitry was originally designed for stereo, holding one sample for each channel. 

  6. The filter is a sharp low-pass filter around 16 kHz using a Sallen-Key topology. 

  7. The volume is controlled by an external volume pedal and a volume control on the synthesizer. The signal also passes through a relay, which cuts the output when the synthesizer is being reset (presumably to avoid random noise).

    The external volume pedal has an interesting circuit. The pedal is essentially a variable resistor, so you might expect the output signal to pass through it. Instead, the pedal is connected to a photocoupler with an LED and a cadmium sulfide photocell inside. The output signal passes through the photocell and is attenuated as controlled by the LED. I think the motivation behind using a cadmium sulfide photocell instead of a phototransistor is that the photocell is completely resistive, so there is no nonlinear distortion of the signal. 

  8. Time-division multiplexing and filtering isn't perfect, and will contribute some artifacts to the output. In particular, there will be some aliasing, where high frequencies turn into lower frequencies. The low-pass filter will eliminate most of the high frequencies—I believe the DX7's filter is at 16 kHz—but it's not perfect and will add its own color to the sound. Two notes could also interact differently based on their relative positions in the time slice. These artifacts probably contribute to the DX7's characteristic sound. You could consider the artifacts desirable if you're trying to duplicate the DX7 sound. 

  9. The scale resistors are marked on the schematic with Ⓑ, which probably indicates they are higher-precision resistors. Assuming they are 1% resistors, a 1% error in a large signal would be much more error than the benefit of additional bits of precision. For smaller signals, the additional bits reduce the quantization noise, which is probably more important than the nonlinearity error from scaling. 

  10. There's an interesting timing issue for the scale calculation. The scaling logic requires about 3 clock cycles to determine the scale factor, so the straightforward implementation would shift a voice based on the amplitude of an earlier voice. The solution is that the 5 bits for scale calculation are pulled out of the operator shift register six stages (3 clock cycles) earlier. Thus, these shift registers have two output; the "early" output gives the scale factor circuitry time to work. 

  11. The output buffer has two latches, used by alternating notes. Each latch has one control line to latch a data value and one control line to output the latched value, so there are four control lines in total. Curiously, it appears that the notes aren't output sequentially; the order is 1, 13, 5, 11, 3, 15, 7, 10, 2, 14, 6, 12, 4, 16, 8, 9. I don't know if there's a motivation for this; it's also possible that I'm misinterpreting the circuit. 

  12. A few notes on the PLA outputs in case anyone looks at them more closely. Because signals get delayed through multiple shift registers and clock cycles, things don't happen on the cycle you'd expect. For instance, SYNC is generated 6 cycles before the end. Likewise, loading of the output buffer is triggered midway through operator 6, about 26 cycles later than operator 1 started generating outputs. Most PLA columns are triggered for a specific voice and operator value. The exception is the last column, which increments the operator regardless of the operator value. Curiously, there are three counter reset lines. One resets near the end of operator 1 (as you'd expect). The other two reset near the end of the two invalid operator values (there are 6 operators but 8 possible bit values). Presumably this keeps the synth from starting up in a bad state. Below the PLA are some gates. These are mostly buffering and clock synchronization. 

  13. Someone with a DX7 could probe the three unused pins and verify that they count out the notes. 

  14. For more information on the DX7 internals, see DX7 Technical Analysis, DX7 Hardware, OPLx decapsulated, and the video Emulating the DX7 the hard way