[sdiy] Frequency shifted from BBD?

[brianw]

On Oct 4, 2024, at 12:26 PM, Ingo Debus wrote:
> Am 04.10.2024 um 19:49 schrieb Gordon:
>>> I recently re-read the section in Barry Klein’s Electronic Music Circuits on bucket brigade devices and noticed a brief mention of using them as a frequency shifter by modulating with a ramp wave.
>>> He acknowledges that there will always be an audible glitch and some noise issues. 
>>> I experimented with this idea a little to minimal success, but curious if anyone else has tried this or seen it used in interesting ways.
>>> 
>> 
>> Didn't the Eventide harmoniser basically do this by ramping two delays, and crossfading from one to the other when one was about to "jump" and the other was mid-sweep?
> 
> Surely this would result in a pitch shift, not a frequency shift.
> 
> Ingo

Yes, this would be a pitch shift.

The BBD uses a single clock for input and output. There is no way to clock voltages in at a different rate than they're clocked out, nor can the sample "address" be random access.

The pitch shift works because the clock is constantly increasing (or decreasing) so that the output rate is effectively different from the input rate that was active when that particular sample was taken. But the caveat is that new samples are already coming in at the new rate.

As Gordon mentioned, having two parallel setups and crossfading between them helps. You'll need a triangle wave (for the crossfade) and two ramps offset from each other. At the top of the triangle, one ramp will be in the middle (no glitch), while the ramp that's completely faded out is recycling (glitch, but fully attenuated). Then, at the bottom of the triangle, the reverse is true. I can't remember seeing the oscillator design that would create these three synchronized signals. The two ramps would have to be at exactly the same slope, but their reset point would be offset so that they alternate in sync with the triangle.

Brian