[sdiy] Envelope attack smoothing for drum question

Mattias Rickardsson mr at analogue.org
Fri Aug 21 22:58:57 CEST 2020

On Fri, 21 Aug 2020 at 14:59, Didier Leplae <didierleplae at yahoo.com> wrote:

> Very interesting! So, are you essentially feeding the oscillator signal
> thru a comparator and the incoming trigger is determining when that is fed
> to the frequency CV?

Yes, a couple of comparators can implement something like

at sync trig start:
   if VCO signal is too high
      start giving it the strongest possible control current in the
downward direction
   else, i.e., it's too low
      start giving it the strongest possible control current in the upward

when VCO signal is close enough to the desired start point:
   stop that extra control current
   set the VCO to the desired start direction (if it's a triangle core)

There are some aspects that might become tricky depending on the rest of
the design and the use case, like reaching a large enough control current
(quickly discharging a cap old-sync-style corresponds to a very large
current), dealing with the little delay created, not affecting the
precision of the normal VCO operation, etc.


> On Aug 21, 2020, at 2:04 AM, Mattias Rickardsson <mr at analogue.org> wrote:
> Discharging the cap to any phase could be tricky, yes. Maybe not worth it.
> Instead of discharging the oscillator cap in ordinary fashion,
> synchronization can be implemented by speeding up the oscillator for a
> short while until it reaches the desired start phase. It kind of
> fast-forwards or fast-reverses the waveform to the starting point.
> This can be done by feeding it with a high CV spike generated by a
> comparator-based circuit that uses the oscillator state and the incoming
> sync pulse to start the CV spike, end it at the right moment, and also
> handle the direction control of the oscillator during and after the spike.
> This might seem very elaborate, but the added circuitry is not that
> different from the standard mechanisms already available in a triangle
> oscillator.
> Can't give any pointers to schematics since I don't remember ever seeing
> this in any other construction, but it ended up quite minimal needing very
> few extra components and was well worth a couple of days of thrilling
> research. :-)
> /mr
> Den tors 20 aug. 2020 23:27Didier Leplae <didierleplae at yahoo.com> skrev:
>> We are experimenting with using a jfet, in the same way the sync on a VCO
>> would work, but have not gotten it to work yet.
>> How would you be able to control the different start phases?
>> On Aug 20, 2020, at 1:16 PM, Mattias Rickardsson <mr at analogue.org> wrote:
>> On Thu, 20 Aug 2020 at 18:10, Didier Leplae <didierleplae at yahoo.com>
>> wrote:
>>> Yes! This makes so much more sense than what we were thinking.
>>> Thank you Matthias!
>> No problem!
>> Btw, when thinking about this it might be worthwhile checking out
>> different start phases for the synced oscillator waveform, if possible.
>> Perhaps starting in the middle isn't punchy enough (cf. the TR-808 bass
>> drum initial click, for instance). :-)
>> /mr
>>> On Aug 19, 2020, at 5:02 PM, Mattias Rickardsson <mr at analogue.org>
>>> wrote:
>>> Hi there!
>>> A smoothing of the envelope will likely not be slow enough to avoid your
>>> problem for low frequencies, and will affect the sound too much. And the
>>> drum hits will still be inconsistent when the oscillator is at random
>>> phases.
>>> Instead, not immediately simple but anyway:
>>> Implement an oscillator sync mechanism that resets the triangle wave to
>>> its center value and starts in a certain direction, so that it always
>>> starts in the same way when the envelope is triggered.
>>> It will likely not be obvious how to do this, but I've done it and I'm
>>> sure it will be possible in your design too with quite few components. In a
>>> drum synth it will be worth it! Unsynced oscillators are for string synths.
>>> ;-)
>>> /mr
>>> Den ons 19 aug. 2020 23:41Didier Leplae via Synth-diy <
>>> synth-diy at synth-diy.org> skrev:
>>>> We are working on an analog drum module that uses a simple envelope
>>>> created from a trigger to control the amplitude of a triangle oscillator
>>>> with a basic OTA based VCA.
>>>> We are having a problem with a slight clicking sound at the beginning
>>>> of many of the drum hits. We think this is because the attack of our
>>>> envelope is so sharp that the beginning of our drum hit looks like a
>>>> straight jump from 0V to wherever the triangle wave happens to fall.
>>>> Therefore the click is somewhat random in that it doesn't occur when the
>>>> triangle happens to be low at the time of attack.
>>>> Can anyone suggest a simple way to deal with this, like adding a slight
>>>> bit of attack time to the envelope? How could this be done without adding
>>>> too many parts?
>>>> Thanks,
>>>> Didier
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