Crumpet & Variphon Modules

Crumpet is based on the Trumpet module of the Variophon breath-controlled systhesizers. The board is designed to also allow some of the other Variophon modules.

Controls

Pulse In: audio input, e.g. from Blip!
Brass Out: audio out
Breath In: control voltage, e.g. from breath controller or envelope generator
Tone knob: Brightness
Level knob: Breathe modulation of BPF level
Shape knob: Breathe modulation of VCAs
Response knob: BPF level

Background

The Variophon was a 1970s wind synth formerly shrouded in mystery: the method of synthesis came from academic work not well-known outside Germany/Austria, the user selected various modules like presets but these were potted and inaccessible, the chief engineer left the company, design notes could not be found, and very few were made to produce sound samples.

But in the last 20 years, the situation has improved. The work of academics Reuters and Oehler has provided information in English on the method and simulated circuits (for Trumpet and Bassoon modules), some modules (Oboe and Tuba) have been unpotted with acid and reverse engineered, contact with the engineering team was re-established, some circuit diagrams were discovered, and there are more audio samples on the WWW which allow guess at the configuration (I have done this for Sax.)

Design

It seems that the Variophon modules are all:

Based on the pulse forming method. This is what the fricko Blip! module provides. The modules seem to use the triangular shape, which can be obtained from the Sharkstooth output with the shape knob at about 8 oclock. For some modules, low notes will have slightly larger pulse width.
Use the TCA740 chip, which provides a dynamic band-pass filter followed by two VCAs in series. These all respond to the breath: so when breath is 50% VCA1 will produce 50% volume but VCA2 will produce %50x%50 = %25% volume.
Have two or three fixed formant filters in parallel with the direct output. These are either fed by VCA 1 or VCA 2, so that blowing louder changes the relative levels of some formants. “Treble emphasis function”.
Have quite noisy breath transducers, which provides some instability to the amplitude, formants etc.

Module Specifics

Parameter	Trumpet	Tuba	Oboe	Bassoon	Sax
Pulse shape	Rect 50ns edge	Tri (Flat tri?)	Tri	Rect? 50ns edge
Pulse width	pp=500us ff=400us		1ms	Low: pp=2.5ms, ff=1.2ms High: pp=1ms, ff=.76ms
Pre LPF	4.5kHz, Q=0.5		NA?
TCA 740 HPF	? (600 Hz?)		1Hz, Q1.7		300?
TCA 740 HPF	? (2.5k->10k?)		44Hz		500?
VCA1 to	Formant A, B?	Formant C, Direct	Formant C, Direct		Formant C, Direct
VCA2 to	Direct ??	Formant A, B	Formant A		Formant A, B (D, E?)
Formant A	975Hz, Q=1		NA		1.2k (+1.8?)
Formant B (to external pot as well?)	2kHz, Q=4.5		4Hz +7dB, bw 2.8kHz		4.8k (+5.3)
Formant C (with dry)			3.2kHz +22dB, bw 1.6kHz		12k
Direct LPF	115Hz		2k
Noise					30 – 50 Hz?

WWW

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