[sdiy] Pitch shift in modulated BBD delays

[Tom Wiltshire]

> On 16 Jun 2026, at 21:21, Gordonjcp <gordonjcp at gjcp.net> wrote:
> 
> On Tue, Jun 16, 2026 at 02:20:42PM +0100, Tom Wiltshire wrote:
>> I've spent a long time wondering about how the LFO waveshape relates to the final pitch shift you get when modulating a BBD clock, as you would in a Chorus or Flanger circuit. Or even a delay-based analogue vibrato, come to that.
>> 
>> I recently managed to get some interactive demonstrations done, which I think make the whole thing a lot easier to understand. Here's the finished article. Perhaps some of you will find it interesting.
>> 
>> https://electricdruid.net/weird-wiggles-bbd-clock-modulation/ <https://electricdruid.net/weird-wiggles-bbd-clock-modulation/>
>> 
>> Any thoughts, comments, or error-spotting appreciated.
>> 
>> Thanks,
>> Tom
> 
> So where does that actually happen? The chorus in the Juno 106 at least is modulated with a triangle wave at the same amplitude and one of two speeds, and gives a very distinct pitch shift - but that shift is dead steady, sharp/flat/sharp/flat deedaahdeedaah like a toy police car.

I'm not entirely sure what you're asking. "Where does that happen?"...Where does what happen?! It happens in the circuit, right?!

If I'm interpreting you correctly,  I think that if you play with the sim, setting the LFO waveform to triangle, and the delay stages to something short (is it 256 or 512 in the Juno? I don't remember without looking it up) and most importantly set the modulation type to "Linear Period", you'll see a "deedahdeedah" pitch shift. It seems a lot like realising modulation of the clock period rather than the frequency was what gave Roland the edge in the chorus world. Given that, increasing the clock frequency/shortening the delay reduces the pitch shift. Lowering the clock frequency/lengthening the delay increases the pitch shift. Changing the LFO Frequency or Depth also has an effect. The sim shows the differences.

https://electricdruid.net/modulated-bbd-delay-simulation/ <https://electricdruid.net/modulated-bbd-delay-simulation/>

T.
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