[sdiy] Tap tempo question

Jacob Watters jacobwatters at gmail.com
Tue Feb 2 15:45:37 CET 2021


Thanks for the info. So a microprocessor is always used? I was thinking
that maybe a shift register and PLL loop would get the job done, but with
the cost of processors these days, I guess it is easier to just use one and
not try to do it all with logic chips.

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


On Tue, Feb 2, 2021 at 9:39 AM Tom Wiltshire <tom at electricdruid.net> wrote:

> With a BBD you’ve got a known relationship between clock rate and delay,
> so you can simply calculate the required clock rate and generate that
> directly. Either by dividing down a high frequency clock, or by using a NCO
> to do a similar job. Note that I’m assuming that we’re using a
> microprocessor to measure the tapped tempo (which is virtually always the
> case) and that therefore we have a modern uP’s resources at our disposal -
> timers and NCOs.
>
> With the PT2399 you’ve also got a known relationship between clock rate
> and delay, but you can’t provide a clock directly. Instead, you’re trying
> to control a not-very-accurate VCO, so the only way to do better than
> roughly-good-enough is to measure the clock rate on pin 5 of the chip and
> then adjust your input until you get the clock rate you require. The actual
> current-control of the VCO can be done various ways; transistor current
> mirrors, vactrols, digipots, etcetc.
>
> Tom
>
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
>
>
>
> On 2 Feb 2021, at 14:25, Jacob Watters <jacobwatters at gmail.com> wrote:
>
> 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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