[LAD] ALSA MIDI latency test results are far away from reality

[Ralf Mardorf]

On Wed, 2010-07-14 at 17:30 +0200, Robin Gareus wrote:
> On 07/14/2010 04:22 PM, Ralf Mardorf wrote:
> > Hi Robin :)
> > 
> > On Wed, 2010-07-14 at 15:44 +0200, Robin Gareus wrote:
> >> On 07/14/2010 03:23 PM, Ralf Mardorf wrote:
> >>> [..]
> >>> AND IT'S AUDIBLE THAT THERE IS MUCH MORE JITTER BUT 1.1 ms.
> >>>
> >>> Any hints how to solve this are welcome.
> >>
> >> Did you try to start jackd with -p64 instead of -p1024
> > 
> > A good argument, because when I made tests in the past for the USB MIDI,
> > things become better at  >= -p256 (when I had this Windows test install
> > latency for the EWX 24/96 audio was less high than for Linux). The
> > problem here is, that I need at least -p512 and even than I'm not safe
> > regarding to issues for JACK audio, that's why I used -p1024 instead of
> > -p512. For a test -p64 should work, but when recording music I would
> > need to increase it step by step until a minimum of -p512.
> 
> I'm sorry; don't understand that. Are you getting [audio] x-runs or what
> is the problem using -p64 (or even -p32)?

Yes there are lapse, even if there should be no messages about xruns.

> I was hinting that the audible midi-jitter could be a result of
> midi-messages getting 'quantizied' to jack-periods.

It seems to be like that. Take a look at my email with the 
                           subject: 
Correlation of alsa -p value and hw
MIDI jitter

> A JACK-MIDI app which does not honor 'jack_midi_event_t->time' but
> simply processes all queued midi-events on each jack_process_callback()
> will result in the symptoms you describe (snare & kick on the same
> beat). One example of such an app is "a2j".

I did use Qtractor, but had the same issue when using Rosegarden a long
time ago. But we are talking about ALSA MIDI to the hardware
MIDInterface?!

> > IIRC when I did tests for the USB MIDI with -p64 or even -p125 (I'm not
> > sure) it theoretically did work, but this isn't a solution, because at
> > some point JACK audio will fail.
> 
> How does it fail? x-runs?

All kinds of dropouts, even the left and right channel seem to get out
of phase. Just running FluidSynth DSSI playing a kick and a rim-shot was
without audible failure at -p16, but I'm sure I won't be able to do
multi-trach recording with -p16.

> 
> JACKd works quite robust here with the UA25 and FA101 at 64fpp.
> 
> >> using JACK-midi, I've encountered a similar issue with fluidsynth always
> >> synchronizing note-start/ends to jack cycles.
> >>
> >> Simply lowering the frames-per-period got me playing again so I did not
> >> check if it's related to JACK-midi or FluidSynth 1.1.1 in general.
> > 
> > At least FluidSynth DSSI (host is Qtractor) is able to play in unison
> > with any DSSI or virtual ALSA MIDI and JACK MIDI (-Xseq) synth on my
> > machine. Just 'in unison' for virtual synth to hw synth there sometimes
> > is more delay, but just an early reflection like effect.
> >
> > Note! It was hard to groove when I connected the master keyboard to ALSA
> > hw MIDI in --> DIRECTLY TO --> ALSA hw MIDI out and this to a 100% ok
> > drum module. Directly connecting the master keyboard to the drum module
> > there were no issues.
> 
> Aha, by this we can infer that your problem is ALSA or kernel/timing
> related.
> 
> To verify this, take everything up from there (eg. qtractor, fluidsynth)
> out of the picture for now.
> 
> 1) Use 'amidiplay' to send a some midi-song directly to your
> drum-module. -> Is there still audible jitter?

On a todo list, but there seems to be a correlation to JACK audio.

> 
> 2) Do you have a Hardware MIDI Sequencer? Have it play a simple
> metronome beat and dump incoming MIDI-messages. See if the timestamps of
> each midi-note-on message are identically spaced.

C64 and Atari ST are stable, but there are some issues for e.g the
monitor. My VGA isn't slow enough for the Atari. I'll try to do it with
the broken SM124.

> 'aseqdump' (at least version 1.0.22 which I currently use) does not
> print timing-info, 'kmidimon' does.
> 
> > I need to do something else now, but I take time off. From Friday
> > (perhaps earlier) until next Sunday noon I could spend the whole days
> > for this MIDI issue only.
> > 
> > Resume:
> > 
> > I assume that -p64 would solve this 'looooooong early reflection like
> > effect/async', but then hard disk recording will become impossible.
> > 
> > The target is, that Linux at least replace the Atari ST as sequencer +
> > an analog 4-Track machine synced by SMPTE. With -p64 4-track recoding
> > would become impossible.
> 
> I'm pretty sure that you can get a stable 64fpp setup, but one thing at
> a time. let's keep this thread to MIDI just now.

Thank you for your effort :).

> 
> > 'Read you later' today or at the latest on Friday.
> 
> enjoy a good long week-end.

Have a nice weekend too.
I'll spend the weekend for Linux audio.

> 
> > Cheers!
> > 
> > Ralf
> 
> ciao,
> robin

Cheers!

Ralf