The Making of an Electronic Music Synthesizer

A 40 year Bet

1  “Bet is still open”
3  {
4    "started": "1975",
5    "continued-to": "2017",
6    "Polysynthy": {   
7      "status": "OPERATIONAL",
8      "front-panels": "mounted",
9      "bugs": "fixed"
10   }
11   "Turan's-school": {
12     "status": "LAST-SEMESTER in Law School", 
13     "history": "having transferred from 
14                    Engineering decades ago"
15   }
16  }

Started as a hobby Project

Grew into a BET – TURAN finishes school first or ALI finishes the ‘organ’ first?

Hakkı, the “bet originator and referee” named the Project as

Polysynthy: 61-key fully polyphonic 12-harmonic additive synthesizer!
Latest Information

Bet is likely to finalize this year (2017) !!!

A 200 year-old French piano (with its ivory keyboard) is used for its furniture

NOTE: Bet does not allow new technology.


I express my appreciation for the great work conducted by the engineers behind the book: “Integrierte Schaltungen für Elektronische Musikinstrumente” that provided the initial design for Polysynthy. Also thanks to Hakkı Göçeoğlu for finding and gifting me this book. The authors were from ITT Intermetall, Germany, and are listed below:

Wilfried Gehric

Joachim Hollmann

Marian Lorkovic

Horst Mielke

Günter Peltz

K. –E. Reinarz

W. Stern

Inside the machine



Range (vibrato intensity): adjustable from none to +/- half an octave frequency shifts (minimum), Depending on the Glissando adjustment, higher or lower parts of the frequency altering may be clipped – there are upper and lower limits for the frequency.

Frequency (rate of tone shifts – frequency modulating frequency): 0.5 Hertz – 1 KHz.

Waveforms: sine , square, triangle , sawtooth, reverse sawtooth (however, only sinus is found to be sufficient for practical musical use !)


(Overall tone frequency adjustment)

+/- half an octave.

Can be used for tuning the instrument for example to match an accompanying violin. One control knob. Also octave shift to a higher or lower octave (8‘) MAY be possible through one switch (not implemented).

Waveform Synthesis

Linear volume control over the first 12 harmonics for any combination of played keys: through 12 control knobs, for sinusoidal sources or square. A balance control is provided to shift linearly from all sine to all square that allows the mixture of two source waveforms. Control over 5 decades of volume – no absolute zero! The lowest value on any harmonic is a low volume that is negligible. A true zero MAY be possible by turning off the harmonic if lowest adjustment is made (not implemented).


Range (tremolo intensity): Range (tremolo intensity): adjustable from zero (leaving the volume at full) to full (sound can be modulated to change continuously from 0 to its full volume). Tremolo is adjusted to allow full volume when off – subtracts from the full volume as a function of the tremolo waveform, as the tremolo intensity increases.

Frequency (rate of volume shifts – volume modulating frequency): 0.5 Hertz – 1 KHz.

Waveforms: sine , square, triangle , sawtooth, reverse sawtooth (however, only sinus is found to be sufficient for practical musical use !). Also, instead of separate sawtooth and reverse sawtooth options, a symmetry control is being provided, for any shape.


The rising and falling of the sound amplitude as a key is pressed and released: various combinations of attack, decay, decay tremolo, sustain, and percussion effects are adjusted through 3 knobs and 3 switches. Also velocity sensitivity is available for some combinations of those functions.


61 Keys are used on the keyboard, leaving the right-most 27 keys for future use. Each key commands an envelope circuit. Envelope circuits produce analog gate control signals for the 61*12 gates. The gate block is organized as one row for each 12 of the harmonics: 12 rows. Each row contains 61 gates that are organized in groups of 12 (per octave corresponding to the keyboard). Highest octave has 13 keys and hence, gates block also have 13 gates for the highest octaves on each harmonic. Gates block is the biggest board, as the central and backbone unit. It has key inputs on it, fed from the bottom of this vertically positioned board – also allocating the 61 envelope circuits on it.

Tone inputs are fed from the left and the top of the board. 8 octaves of sounds are used, that makes 96 tones. These tones are organized as 61 notes for the first harmonic whereas the second harmonic uses some of these same notes (a 3-octave range) and also uses a higher-octave range (as the second harmonic for the keys of the 5th octave). The lowest (1st) octave of the lowest keys have their fundamental (1st harmonic) sounds that are not used by any other key. Altogether, the gates are organized to use some of the sound inputs to be used more than once, for different harmonics of the different keys.

Additive Synthesis

The outputs are collected on the right, to be directed to Voltage Controlled Amplifiers (VCA) and filter circuits. One filter per sounds that are in the same one-octave frequency range is used. For example, second octave for the first harmonic is at the same frequency range with the first octave of the second harmonic. Although those two different output ranges (Octave 2 for Harmonic 1 and Octave 1 for Harmonic 2) are in the same frequency range, they may have different volumes. This is due to the additive (Fourier) synthesis requirement where every harmonic needs a separate volume control. Also, so far the outputs are based on square wave shaped input signals. We need filters (Low Pass Filters: LPF) to convert square waves to sine. Luckily, a mixed output of signals in about an octave range, can be converted to sine using only one LPF.

As a result, either the output of every octave range (there are 60 of them: 5 octaves per harmonic, 12 harmonics) needs to be volume controlled through a VCA, or a different filter should be supplied for each 12-tone output where filter outputs should be volume-controlled. Summary: first volume control then sine conversion or vice-versa. Since there will be one volume control per harmonic, any octave on that harmonic should have the same volume. All octave ranges (5 of them) in one harmonic needs to be adjusted by one level control, before reaching the corresponding filter. Octave ranges as output from different octaves in different harmonics, that correspond to the same frequency range, get fed to the same filter after volume adjusted. Current architecture uses VCAs. An alternative architecture (Architecture 2) under experimentation is not using VCAs: 60 different octave outputs have dedicated (60) filters. All sounds in one harmonic are added and sent to final mixer through one volume control (potentiometer).

In both architectures, keeping square wave signals in addition to the sine waves requires extra circuitry: In Architecture 1 (current) inputs of our 14 filters are added to produce the square output – they are already synthesized in the VCA’s for different volumes per harmonic. Outputs, similarly, are added to produce the sine wave synthesized output. A final 2-channel mixer adjusts square/sine ratio. In Architecture 2, 2-gang potentiometers are used to adjust the square and sine wave signals simultaneously: These signals come from the added square/sine outputs from each harmonic. So there are again 12 potentiometers for the synthesis. The 12 square and 12 sine signals are fed to a mixer after such a volume adjustment.

Frequency control for glissando and vibrato effects are through voltages that control the master Voltage Controlled Oscillator (VCO) that spans 0.5 –2 MHz range. Its output drives a 12-tone synthesizer that produces the highest 12 notes (for octave 8). Binary dividers divide those notes 7 times each to provide the notes for the lower 7 octaves. So, 96 notes are produced and fed to the main board (gates). On the right, after the filters, sounds are added in mixers. To modify the amplitude for the tremolo effect, currently the Control Voltages for the VCA’s are being produced from the output of the tremolo Low Frequency Oscillators (LFO). The LFO output is sent to blocks of VCAs after being divided by a potentiometer for the corresponding harmonic’s level. 12 potentiometers handle the additive synthesis, by controlling levels for each harmonic. LFO output, when lowest, provides full DC voltage to 12 pots so they send highest voltage to the VCAs. As tremolo intensity is increased, the alternating part of the control voltage increases, subtracting form the full voltage. An alternative way to yield tremolo is modulating the overall output through one VCA, that is being considered also.


About final assembly

left:VCAs(5 per board), right: filters (2 per board)