[Qtractor-devel] Correlation of alsa -p value and hw MIDI jitter

[Ralf M. <ral...@al...>]

Hi :)

delayed by a thunder-storm I could do another test.

64 Studio 3.3 alpha (= Ubuntu Karmic) amd64

LXDE, poff dsl-provider, cpufreq-selector -g performance, chgrp
audio /dev/hpet, sysctl -w dev.hpet.max-user-freq=64, modprobe
snd-hrtimer

Qtractor + HR timer playing FluidSynth DSSI drums and a standalone MIDI
drum module in unison.

$ jackd -Rch -dalsa -dhw:0 -r96000 -p16 -n2

FluidSynth DSSI and the hardware MIDI drum module are playing in unison.

$ jackd -Rch -dalsa -dhw:0 -r96000 -p512 -n2

Borderline, not out of timing, but not fine and already critical.

$ jackd -Rch -dalsa -dhw:0 -r96000 -p256 -n2

Borderline, not out of timing, but not fine.

$ jackd -Rch -dalsa -dhw:0 -r96000 -p128 -n2

Borderline, not out of timing, but not fine.

$ jackd -Rch -dalsa -dhw:0 -r96000 -p64 -n2

Borderline, not out of timing, but not fine.

$ jackd -Rch -dalsa -dhw:0 -r96000 -p32 -n2

It here might become usable.

$ jackd -Rch -dalsa -dhw:0 -r96000 -p16 -n2

Yes, for this test the problem at -p32 and -p16 is, that because of
phasing the kick become randomly very loud. It seems not to depend on
the sounds, but jitter, anyway I can't say this for sure, some sounds
can't be played in unison, even if there won't be jitter.

Values >= -p64 result in ... I need to hear it again ...

$ jackd -Rch -dalsa -dhw:0 -r96000 -p64 -n2

... 'in unison' becomes a little bit of an early reflection like effect,
but a very, very short delayed early reflection, still more a
fluctuating phasing.

$ jackd -Rch -dalsa -dhw:0 -r96000 -p1024 -n2

Completely out of sync, bad timing. Unusable.

I guess even with -p16 one should record all drums and the bass at the
same time, but recording every instrument one after another. This could
be possible even for -p512, but MIDI is completely out of timing at
-p1024, note, at 96KHz. Short attacked percussive sounds shouldn't be
played in unison.

For some usage PCI MIDI seems to be okay on my machine, but it's not
comparable to an Atari ST and SMPTE sync to a 4-track cassette recorder
or even to a C64 and click sync to a 4-track cassette recorder. PCI MIDI
still has to much jitter for a straight timing, that enables to record
one instrument after the other in perfect sync.

I guess as long as I could keep -p256 and I won't be able to do a lot
without increasing this value, hw MIDI could be usable for very simple
music, when the whole MIDI backing is recorded at the same time, but one
after the other instrument.

- Ralf