[sdiy] Frequency shifted from BBD?

[Dave Leith]

Analog pitch shifter
https://www.chasebliss.com/thermae


On Sun, Oct 6, 2024 at 3:18 PM Tom Wiltshire <tom at electricdruid.net> wrote:

> I'm not claiming that digital control solves any of the inherent problems,
> but I think it moves solving them to a much more manageable domain! If we
> have the BBDs clock rate and volume under voltage control, and we move the
> CV generation to a uP, then we have full liberty to do whatever we like
> *within the limitations we have*. It doesn't remove any of the limitations,
> but it makes them a whole lot easier to deal with. If that's just making it
> "smaller and cheaper" I'll take that as a win, since this theoretical
> pitch-shifting unit is highly likely to be large and expensive anyway!
>
> But as I said, I think this needs more thinking about. The complex way
> BBDs shift the pitch of signals passing through them is often
> underestimated, and in this instance, the details totally matter, so I
> think Brian and I are really on the same wavelength here.
>
> Tom
>
> > On 6 Oct 2024, at 22:32, brianw <brianw at audiobanshee.com> wrote:
> >
> > Digital control of a BBD does not solve any of the major problems
> inherent in using a BBD. The BBD is not a CV-controlled technology, and
> digital generation of CV signals would not solve anything that's directly
> related to the BBD. Thus, a modern implementation of a BBD pitch shifter
> would have precisely the same major limitations as the decades old
> solutions.
> >
> > The BBD itself is partly analog. Although it is a discrete time sampling
> technology, the sample values themselves are continuous.
> >
> > The number one challenge with a BBD is that all samples move through the
> buckets based on a single clock. Technically, there are two clock inputs,
> but they have a precise relationship and might as well be thought of as
> one, since they cannot vary in period. Thus, adding a new sample requires
> all prior samples to be moved to the next bucket, and the oldest sample is
> lost as soon as it appears on the output.
> >
> > By the time a modern solution is built around a BBD, with VCA and EG
> units, it really doesn't matter whether the clocks and CVs are generated
> from analog or digital sources. It's actually fairly easy to synthesize a
> ramp wave that is phase locked with another ramp wave that's offset 50% of
> the period, and generate the trapezoidal envelopes - perhaps simply by
> clipping a triangle wave. Granted, these analog synthesis signals can be
> generated easily digitally, but that is a very minor challenge. Basically,
> digital control would merely make the circuit smaller and cheaper. The
> tough problems remain tough.
> >
> > No doubt, most BBD designs from back in the day used digital logic chips
> to generate the required clocks (which cannot overlap without shorting all
> of the buckets together and destroying the stored voltages).
> >
> > Brian
> >
> >
> > On Oct 6, 2024, at 11:44 AM, Tom Wiltshire <tom at electricdruid.net>
> wrote:
> >> At the time that this was a tough problem, people were trying to solve
> "the pitchshifting problem" using entirely analogue means. The problem
> didn't really get solved until Eventide did it digitally with the
> Harmonizer.
> >>
> >> But we don't have the same limitations today. It seems to me that a
> BBD-based pitch shifter would be possible if you had a uP to produce the
> required control voltages. Of course, such a thing is still *ludicrous* by
> most measures, but it would be *possible*. You need at least two and
> possibly several BBD delay lines with voltage-controlled clocks and VCAs on
> their outputs. Each delay line then has a CV for the clock rate, and a CV
> for the volume. A decent uP+DAC and a decent programmer can then generate a
> pile of control voltages to give you up/downshifted delayed signals, and
> all the necessary crossfading to avoid the glitches when the ramps jump
> back to zero.
> >>
> >> I'll have to think harder about this to work out for how long a given
> delay line length can give you a specific pitch shift. That'll help
> determine how many delaylines would be required. More shift is steeper
> ramps (e.g. higher modulation frequency) and probably more BBDs required?
> >
>
>
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