[sdiy] Tap tempo question

Jacob Watters jacobwatters at gmail.com
Tue Feb 2 15:25:38 CET 2021


I have always wondered how it is done in a circuit that uses a BBD or
PT2399.

1. Is a PLL used to sync the delay clock somehow? I assume a
divider/multiply would be required to get the clock rate slow enough to
sync to the tap rate.
2. How are the pulses stored for syncing in the PLL? Is it a shift register
in a loop with a consistent clock rate, or something else?


*Jacob Watters*
JacobWatters.com <http://jacobwatters.com/>


On Tue, Feb 2, 2021 at 7:21 AM Tom Wiltshire <tom at electricdruid.net> wrote:

> Yeah, I wondered about doing a software PLL too. Having something that
> gradually caught up with incoming tempo changes seems sort of “natural”, so
> it’s an appealling idea.
>
> Tom
>
>
> > On 2 Feb 2021, at 12:03, Richie Burnett <rburnett at richieburnett.co.uk>
> wrote:
> >
> > I don't know what the algorithm is in my Roland TR-8 drum machine here,
> but it definitely does some sort of averaging.  If you change the speed of
> your taps it pushes or drags the tempo of the pattern that's playing.  It
> doesn't immediately sync each beat to the new tap instants (because that
> would cause a glitch in the drum pattern) but it does eventually sync up if
> you keep tapping at a fixed speed.  Someone has obviously given it some
> thought, because whatever it is doing inside the box feels very intuitive
> to the user.
> >
> > It feels like the tempo jumps to the new tap tempo after 3 taps.  And
> then the "phase" adjusts gradually after that until the pattern is
> eventually in sync with the taps.  Maybe it uses some sort of software
> phase-locked-loop.
> >
> > -Richie,
> >
> >
> >
> > -----Original Message----- From: Tom Wiltshire
> > Sent: Tuesday, February 2, 2021 10:59 AM
> > To: Markus
> > Cc: SYNTH DIY
> > Subject: Re: [sdiy] Tap tempo question
> >
> > I am on the list.
> >
> > I don’t think there’s a “more sensible” or “accepted” way to do it. It
> depends on the purpose.
> >
> > Averaging makes some sense where you’ve got a human tapping, and Brian
> is right that weighted averages are a good idea to give more recent times
> more importance. Rather than doing hard sums, on a PIC it makes sense to do
> this weighting by using bit shifts (so choose from weights of 1, 0.5, 0.25,
> or 0.125!!).
> >
> > However, if you’re likely to use the tap tempo input for syncing the
> delay to incoming tempos from a drum machine or sequencer or similar, then
> just measuring the last two pulses and using those only makes the thing
> much more responsive. You don’t need to worry for a delay, but for my tap
> tempo LFOs, I also needed to think about the phase, and it’s important to
> reset or adjust the phase so that the LFO waveform is on the beat, not just
> in time.
> >
> >
> > ==================
> >      Electric Druid
> > Synth & Stompbox DIY
> > ==================
> >
> >
> >
> >
> > On 2 Feb 2021, at 00:43, Markus via Synth-diy <synth-diy at synth-diy.org>
> wrote:
> >
> >
> >
> > sounds like Tom Wiltshire from Electric Druid would be the expert on
> this. is he on this list?
> >
> > https://electricdruid.net/electric-druid-chips-the-next-generation/
> >
> > On Monday, February 1, 2021, 04:40:12 PM PST, Didier Leplae via
> Synth-diy <synth-diy at synth-diy.org> wrote:
> >
> >
> > I am currently working on a delay module using a PIC uC to control the
> delay time so adding in a tap function.
> > Thus far, I have the PIC measuring the time between two consecutive taps
> to determine the delay time. But it occurred to me that sometimes when I
> use a tap tempo, I tap multiple times with the assumption that the multiple
> taps are averaged.
> > Does anyone have an opinion on which is the more sensible or accepted
> way to do this?
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> >
> >
> >
> >
> >
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