[sdiy] Tap tempo question

ColinMuirDorward colindorward at gmail.com
Tue Feb 2 16:00:59 CET 2021


My dream tempo-sync delay controls would be a tap button for coarse adjust
plus a +/- narrow-range knob (maybe 1/4 tap length?) to fine tune it on or
off the beat.
Food for thought :)
Colin

On Tue, Feb 2, 2021, 7:48 AM 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>
> 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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