Suzuki Omnichord

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hello everyone,
I'm really curious to understand how the Omnichord actually works.
This is the schematic for the System 2 model (OM-84):

I'm not new to electronic, but this one looks hard to understand for me. I can only understand that the sound comes from the circuit between the touch sensors and the amplifier IC (13x the same circuit, so I guess 13 note polyphony), but what about all that logic circuits?
First, have you held on in your hands and played it? That might actually answer some questions for you. Personally I love Omnichords.

Not sure what you want to know. A keyboard is a keyboard, whether it is on your computer or on a synthesizer. All of them take a switch closure of some sort, and tell the CPU it happened. The CPU looks up in ROM what it is supposed to do about it. Your keyboard array selects the base chord/key it is playing in.

The CPU then produces the notes of the chord selected. There is an array of 20 dividers upper right area, that produces individual notes. In between those dividers you can see the numbered feeds down to the touch sensor circuits.

The touch sensors then trigger the notes to come through the audio. Each sensor allows a cap to charge holding the base of each of the 13 transistors to an on condition, which gates the sound through. All 13 transistors are not draw, only the first and last. As the cap discharges, the gates comes closed. The sustain control is a adjustable voltage that can retard the discharge, thus lengthening the sound of the note.

Center right is a noise circuit - it generates white noise. Or at least something similar to white noise. Note upper right side of the CPU has many ports, notably ports P30-34. Note further they are also labelled BD SD CL CY HH, for bass drum, snare drum, claves, cymbal, and hi hat. The circuits aroound the noise generator gate the white noise to emulate percussion.

Look at the two noise generators, then to their right is a third transistor with an inductor in its collector lead and a 100k trimer in its emitter lead. That is the output from the noise generator. Note now to the left of the generator the HH SD and CY control signals all come in to gate that same third transistor. They use the same sound for each, the tone being a matter of the gate timing. Different size caps gate different length of time.

Don't be confused by the drawing. Up on the CPU the port numbers are on the IC and the function - HH CY etc - is in a circle with an arrow. But down at the noise circuit, the port number is in the circle and the function is just nearby letters. These are the same things.

Below the generator is a row of three pulse generators for percussive hits. Each is triggered. The left is BD, the middle CL, and the right is SD. They are the same except for the capacitors and a resistor in each that determines the tone. Note too that the snare drum triggers not only some white noise but also has its percussive hit. That makes a more realistic snare drum hit.

Does that help?
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many thanks Enzo, that helped a lot!
so I think I have clear in mind the CPU parts with all the switches to select the chords (on the upper left) and the connected circuit for timing.
The noise generator / drum simulation is clear too thanks to you.

btw I have some things to understand:
1) when pressing the touch sensor (or better, the 13 touch sensors) we are simply making a short circuit?
2) what's the "keyboard volume" (or better, the keyboard function) ? The manual doesn't explain that, and OM-27 doesn't have this function (only "chord").
3) what's that part on the upper-right (above the noise generator)? Seems like it's feeding the 13 transistors (on the collectors) of the sound generator..but what does that part feed? some sort of voltage? if yes, of what kind? variable or fixed?
4) please check the transistor on the center-bottom of the schematic (the one that have an inductance connected in series with a capacitor on the collector), is that little circuit responsible of the vibrato? There's a point called "E" that is connected to the 4040 circuit I was talking about, again, what sort of signal passes there? a voltage?

many thanks and sorry for all those questions :)
1. Sorta. You are not so much "pressing" it as you are just touching it. No pressure involved, the touch of your skin is enough to complete the circuit. You could lay a piece of metal there and do the same thing. Note that each sensor merely spikes a charge through a cap. So it strikes the note, but doesn;t sustain it. If you put your finger on it and leave it there, the note still will fade out as normal. There is that sustain control I mentioned that will hold the note, but that is not the same as holding you finger on it.

2. DO you have a working instrument there? I think playing with it a few moments would answer a lot. I have not seen one of these in quite a long time, but if I recall well, you have the accompaniment in both rhythm and chords, plus the option of steady held chords. I think the two controls Chord and Keyboard refer to those two different functions. To play, you can just push a chord select button and the system will remember it and continue to play that chord in the accompaniment. The keyboard mode allows yoou to push and hold the buttons and play them like an accordion. I think...

3. I think those gate a bass tone from P04 on the CPU to sound under the chord. I am not sure offhand.

4. The E signal is of course some kind of voltage waveform. Vibrato? I suppose it could be, not sure. Looking at it, that looks like a pretty good guess.
thanks again
but still I can't understand how can the pitch be set for each of the 13 generators?
I guess it's all about the voltage on the collectors of the transistor of each generator, which is connected to the logic circuit..but that's a digital counter and I don't think it can output a variable voltage
As regards the touch-sensing, I don't know, and although I looked at the circuit, I can't tell how the touch sensing works in this case, but there are numerous ways of implementing touch-sensing. Some require special resistive matrices, either multi-layer or cross stranded, and some work purely on capacitance.

It's fairly obvious how the touch of a finger can cause a change in resistance, either by pushing conductors closer together, or by bridging across them with a (slightly) conductive finger.

Capacitive sensing is a little less obvious. Usually a piece of foil (on a PCB) is concealed behind a sheet of dielectric material. This foil pad has a capacitance with respect to ground, and can be made part of the resonator which controls the frequency of an oscillator. When a finger is brought close to the pad, the overall capacitance to ground changes (the finger having its own capacitance, and the dielectric being permeable by an electrostatic field) and hence the oscillator frequency changes. This can be detected by counting using a microprocessor. The foil capacitor may alternatively be charged by closing a (transistor) switch. A threshold detector and timer detect changes in the time taken to charge the capacitor to a known level, the charging process being repeated so quickly that no delay is perceptible to the user.

As for the pitches generated, these are oscillators which depend on resonance just as a guitar string does, but the resonators are electrical, as opposed to mechanical. There are many different types of electrical oscillators.
DOn't forget, you press a button to tell the system to play a certain chord. SO only related notes ar available at any given time. The notes are coming from that custom CPU. That matrix of boxes upper right is just dividers. All the notes of the chord are related and can be derived that way. You cannot play notes outside of the selected chord. This isn't like playing a synth. The CPU sends appropriate notes through the dividers.

Or maybe it kinda is like a synth. Imagine you know how to play a C chord on a regular keyboard. You don't know any other chord, just C. (You also know major, minor, 7th, but ignore that) Now to play a simple song you need C but also F and G. So instead of playing different chords, you continue to play only C, but you move the transpose control over to F or G. The same keboard input now produces the F or G notes.

The 13 input circuits with the 13 transistors are only triggers and gates, they are not generating notes. They each act as a valve for a particular note of the chord selected. Note that the same sensor will produce a different note of the scale depending on which chord has been selected. it is just a resistance thing. SO trigger #1 might be a C in one chord, and a D flat in another. The 13 triggers have no inherent note of their own, they only have relative places in the chord and scale. So if a chord is a note and its 3rd and its 5th, and we cover three octaves, then we might find trigger 1 is always the tonic first octave, #2 is always the third first octave, and #3 is always the 5th first octave, then #4 is tonic second octave, #5 the 3rd second octave, and so on. They control the same position in every chord chord, just the underlying chord is changed by the selector circuit and the CPU/divider circuits.

The dividers up there just sit there continuously making all their notes for the selected chord. Touching a sensor triggers on its transistor which then gates that note on through to the audio mixer until the sustain times out.

SO: the generators are the CPU and the boxes upper right. The 13 transistors are just triggers.

COunter culture, there is nothing fancy about these boards. The sensor board is just a pattern of traces in a row. Wipe your finger the length and you run down the chord notes. As far as I can recall, it is a one sided board. There are of couirse sensing circuit such as you describe, but in this little unit, ther is no oscillation, the sensors do not connect to the processor.

There are no tuned circuits here, just plain old notes made up in the CPU. The circuits all around the CPU are just there to gate its sounds to make it musical.

Maybe I am being a little overly informal. The CPU is the larger IC, and it is there for control, it sends out controlling signals. I have been including the tone generator as part of the CPU, when it really is a separate IC. So if it helps, the actual notes come from the 8253 chip, not the processsor chip itself. I look at those two plus support parts around tham as "the processor." Sorry if I have made it more confusing for doing that. The 8253 feeds the divider group.
that is a huge explanation, many thanks again :)
so the tones come from that digital chip..
I have some samples of the Omnichord and I noticed that the waveform of the sound is not a classic waveform like triangle, sine, or's more complex, and furthermore it changes its shape when it change the frequency.. How can a digital chip (to be precise, one from the beginning of '80s) can do that? do you think it's a sort of wavetable synthesis? I thought that omnichord sound generation was analog (because I read that once on the web) but looks like it's wrong.
I have no idea just how they generated the notes, I include the data sheet for the IC itself for you below. The CPU tells it whatever it needs to make those notes. I'm not an engineer, I run a commercial repair shop for pro audio. SO all I need to know is the flow of things and how to determine if parts are not working. So if I can identify the IC as bad and determine its cost and availability, that is where I stop.


  • M5L8253P-5.pdf
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don't worry you helped a lot already!
perhaps do you know how the connection are made for the 4040 and 4520 chips (upper-right on the schematic) ?
it looks weird, every Q on the chip is a single pin, but on the schematic they look like they have 2 pin, indeed every Q is chained with another. How can this be accomplished if each Q has only 1 pin?
The 4520 boxes all connect inside, they are all in the one IC. Pin numbers are on the drawing, consult the data sheet on the 4520. it is a dual binary counter, four bits.

Looking at the 4040 string, the pin numbers are on the schematic. Pin 10 takes the timing signal or clock, and is a straight counter. Look up the data sheet on the 4040, it should have a truth table on the IC. That is all one 4040 IC, the connections between the 12 boxes are internal.

These are both common CMOS logic ICs, and the data sheets tell the story of their insides.

This schematic may LOOK complicated, but really this is not all that complex a thing. The upper left quarter of the thing makes some tones depending on the buttons you push. The boxes upper right split those tones into useful intervals for music. The rest of the thing pretty much just gates these sounds to make it sound notes.
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