[sdiy] Envelope attack smoothing for drum question

Mattias Rickardsson mr at analogue.org
Fri Aug 21 23:21:16 CEST 2020

Hi Florian,

On Fri, 21 Aug 2020 at 17:50, Florian Anwander <fanwander at mnet-online.de>

> Hi Mattias, Didier
> To be honest: one of the interesting parts of the 808 *is*, that there
> is no oscillator to sync, but that the trigger initializes the resonance
> of a T-filter. This means also: if the circuit is still resonating when
> a trigger comes in it depends heavily on in which "direction" the
> T-filter is swinging at that moment. That is how a real drum head is
> behaving: if the stick hits the drumhead, while it is moving up, then
> the resulting sound will be different from the sound of a drumhead
> moving down, while hit by the drumstick.
> If you do the drum synthesis with an oscillator+VCA concept then you
> should think about provide a choice whether to sync the oscillator or not.

True, it's a valid point in a way. But it's not like using an unsynced
oscillator, since the new trig pulse in the 808 BD always comes in the same
direction and in the same amount. Also it's normally much stronger than the
residual oscillation from the previous trig.
The click is always strong, but the boomy tail has an amplitude that
depends on the mix with the residual.
Making a sharp drum sound from an unsynced oscillator, on the other hand,
gives different click strengths at different oscillator phases.

And another note about syncing. I remember a drumsynth (don't know which
> one - was it the 909?), which did not reset the oscillator by the
> trigger, but the trigger switched the frequency of the oscillator or
> T-filter to a higher frequency for a very short moment. This has the
> same effect as a sync, but sounds more "natural".

Sounds 808-ish, but perhaps something else? Please tell us if you wake up
realizing which machine it was. :-)


Am 20.08.20 um 00:01 schrieb Mattias Rickardsson:
> > 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 <mailto: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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