[sdiy] Tap tempo question

Tom Wiltshire tom at electricdruid.net
Tue Feb 2 15:57:24 CET 2021


People *have* done tap tempo with discrete logic of one sort or another, but they’re much braver than I am! Let’s just say that you won’t find a single commercial pedal that does it that way - uPs every time.

==================
       Electric Druid
Synth & Stompbox DIY
==================



> On 2 Feb 2021, at 14:45, Jacob Watters <jacobwatters at gmail.com> wrote:
> 
> 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 <mailto: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 <mailto: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 <mailto: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 <mailto: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 <mailto: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/ <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 <mailto: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?
>> > _______________________________________________
>> > Synth-diy mailing list
>> > Synth-diy at synth-diy.org <mailto:Synth-diy at synth-diy.org>
>> > http://synth-diy.org/mailman/listinfo/synth-diy <http://synth-diy.org/mailman/listinfo/synth-diy>
>> > Selling or trading? Use marketplace at synth-diy.org <mailto:marketplace at synth-diy.org>
>> > _______________________________________________
>> > Synth-diy mailing list
>> > Synth-diy at synth-diy.org <mailto:Synth-diy at synth-diy.org>
>> > http://synth-diy.org/mailman/listinfo/synth-diy <http://synth-diy.org/mailman/listinfo/synth-diy>
>> > Selling or trading? Use marketplace at synth-diy.org <mailto:marketplace at synth-diy.org>
>> > 
>> > 
>> > 
>> > 
>> > 
>> > 
>> > 
>> > _______________________________________________
>> > Synth-diy mailing list
>> > Synth-diy at synth-diy.org <mailto:Synth-diy at synth-diy.org>
>> > http://synth-diy.org/mailman/listinfo/synth-diy <http://synth-diy.org/mailman/listinfo/synth-diy>
>> > Selling or trading? Use marketplace at synth-diy.org <mailto:marketplace at synth-diy.org> 
>> > 
>> > -- 
>> > This email has been checked for viruses by AVG.
>> > https://www.avg.com <https://www.avg.com/>
>> > 
>> 
>> 
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at synth-diy.org <mailto:Synth-diy at synth-diy.org>
>> http://synth-diy.org/mailman/listinfo/synth-diy <http://synth-diy.org/mailman/listinfo/synth-diy>
>> Selling or trading? Use marketplace at synth-diy.org <mailto:marketplace at synth-diy.org>
> 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20210202/05d9caef/attachment.htm>


More information about the Synth-diy mailing list