[Qtractor-devel] Repeated drum hits via drumkv1 sometimes sound different

[Frank N. <bea...@we...>]

Hi Rui and list,

I am observing an oddity with drumkv1 in Qtractor (actually, I believe this
issue is independent of Qtractor, but since I am not aware of a drumkv1
specific mailing list, I take the liberty of reporting it here).

In Qtractor, I created a new MIDI track with a drumkv1 instance, and in it I
have created a tiny 2-bar sequence with a short drum sample played on every
beat. Same note length, same velocity for every hit.

In drumkv1, I have turned off DCF, DCA, LFO and the limiter - so, there should
be no (or not a lot) of operations going on in the signal path.

If I now play back this sample (it's attached, for reference) and repeat over
the 2-bar sequence, I notice that every now and then the snare sounds
"slightly differently". I had first noticed the effect in a multi-track
session where I was thinking one of the other tracks is clipping somehow, but
I could now nail this down to this drumkv1 instance.

I have recorded the Qtractor output via jack_capture, and analyzed the onset of
every hit. I am attaching a screenshot of two hits which show significant
differences in their onset.

One suspicion was that when samplerate of that sample and of my jackd do not
match (say, 44100 <-> 48000), the problem could be within libsamplerate, but
I converted the sample to match jackd's samplerate, and the issue is still
there.

Any idea what could be causing this? My expectation would have been that
repeated hits with the same input produce the same (or almost the same
output) every time. Is there some additional processing going on in drumkv1
I should be aware of?

Note, if I import that sample directly into an _audio_ track in Qtractor
and copy&paste it a couple of times and play it back, the issue does not appear.
So it must be something inside drumkv1. My version of drumkv1 is 0.9.21,
to be precise Git commit 69224d.

This might sound harmless, but in the mentioned multi-track arrangement it is
creating pretty obvious clicks that have become quite disturbing once I
realized they are there.

If I should instrument my drumkv1 plugin somehow to sput debug information
about played buffer position, I'll be glad to try out anything you suggest.

Thanks,
Frank