Synth Shop Home

Old Crow's Synth Shop: Remaking the Yamaha CS Voice

Note: Images are 1152x864, about 100K each.

Here is the output from pin 7 of the IG00153, with the scope set to 0.5mS/div and 0.5V/div:

Notice there is in fact a definite one-shot pulse that leads the ramp. This pulse is about 0.45V amplitude and 30 microseconds in width.

Another look at the waveform, at 1mS/div:

Here is the one-shot pulse viewed at 10uS/div, where it can be seen that the pulse is abvout 30uS wide. This matches up with the external RC time constant that is t=(R)(C) = (24.5K)(1200pF) = (24500)(1.2E-08) = 2.97E-05 or about 30uS:

Lastly, the sawtooth after being inverted in the IG00158 WSC chip:

So, it is apparent the one-shot pulse is a factor in the final waveform/sound of the CS VCO.

Update: CS VCO built as module from standard parts completed.

Here are a couple of images of the 1U module:

The 1U module has frequency, vibrato (FM) level and initial pulse width controls, plus a to-be-added fine tune control. CV inputs are 1V/Oct pitch, FM CV IN and PW CV IN. Outputs are sawtooth, pulse and sine.

While this version is scaled for 1V/Oct operation, it is a simple matter to make it do V/Hz (linear) response with a different front-end current control circuit. Well, I say simple but the method to still obtain exponential vibrato with V/Hz pitch tracking is a little tricky. Thanks as usual to JH for the solution to that one.

Scope images of my VCO:

The sawtooth at 6Vp-p:

The 30 microsecond reset pulse:

The not-quite calibrated sine:

Audio examples

Note: I did not use my usual setup to record these as it is packed up and sitting in storage pending my move to Chicago. Still, the samples serve their demo purpose well enough.

VCO octave test
Mid-range test of sawtooth
VCO raw sawtooth timbre comparison -- guess which is the CS VCO ;)

More later.

Scott Rider -- 0ldcr0w@0ldcr0ws.n3t