VCDO - asking again + new idea

Paul Schreiber synth1 at
Wed Apr 14 17:16:45 CEST 1999

Grant Richter has been doing this for years.

see: Waveform City

Paul Schreiber

-----Original Message-----
From: Roman Sowa <rsowa at>
To: synth-diy at <synth-diy at>
Date: Wednesday, April 14, 1999 9:03 AM
Subject: ODP: VCDO - asking again + new idea

That was my idea I posted few months ago!
Then I thought of such problem:
The amplitude of VCO (assume saw at first) must be stable
within whole frequency range, and be acurate within, say, 1LSB.
Otherwise distortion will come up. OTOH this might be wanted
effect - by modulating saw amplitude you get waveshapping.
Why not put RAM after DAC and download waveform via sysex?
Possibilities are huge, and maybe someday I'll do such thing.
What you suggest later - to use it approach for waveshaping
is also a good idea. So good I bet not only you and me came up with it.

Want to make terrible noise? replace bits after ADC - I mean
instead of connecting b6 from adc to dac's b6, b5 to b5 etc
replace just two of them, say b5 to b2 and b2 to b5.

As for ADC - AD775, 20 MSPS (may run at 35MSPS),
8-bits, 13$(soic) 20$(dip)


> -----Oryginalna wiadomość-----
> Od: jbv [SMTP:jbv.silences at]
> Wysłano: 14 kwietnia 1999 15:12
> Do: synth-diy at
> Temat: Re: VCDO - asking again + new idea
> Let's say we have an audio range VCO producing a positive sawtooth, with
> a freq range from 0 to
> 20 KHz. As it ramps up periodically, the sawtooth is sampled through a
> fast ADC, the 6 MSB
> from the ADC are used to adress a ROM where various waveforms are stored
> (64 bytes for each).
> Of course, the most significant adress bits of the ROM are used for
> waveform selection. Then the data
> output of the ROM are sent to a fast DAC.
> In order to still have 64 samples for the highest freq (20 KHz), the
> sawtooth must be sampled at
> 1.28 MHz min. In the Farnell catalog I found a 6 bits ADC (CA3306CE by
> Harris) with a typical
> conversion time of 0.1 us. Then we need a fast 8 bits DAC, and there are
> plenty of those.
> Yes, I know it's not the cheapest solution for a VCDO, but my estimation
> is that it should come for
> less than $30.00. The other interesting thing is that a regular VCO can
> be used.
> But now, let's try to think further : instead of a fixed periodic
> sawtooth, what it we feed that thing
> with another kind of signal (tri, sine, PWM square...) or even natural
> sounds (strings, guitar,
> woodwind or human voice) ? What will happen to the signal ?

> jbv

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