Re: [LAD] [64studio-users] MIDI jitter

From: Ralf Mardorf <ralf.mardorf@email-addr-hidden-dsl.net>
Date: Thu Jul 01 2010 - 11:22:11 EEST

On Wed, 2010-06-30 at 14:27 +0200, Adrian Knoth wrote:
> On Tue, Jun 29, 2010 at 06:20:30PM -0700, Niels Mayer wrote:
>
> > What's interesting to note is that most of the USB MIDI interfaces do
> > not have consistent latency (Other than the Roland UM2's consistent
>
> Let me just show you the result from my Midisport USB 2x2 (standard
> edition when it was still produced by midiman, not the anniversary
> edition):
>
> set_realtime_priority(SCHED_FIFO, 20).. done.
> clock resolution: 0.000000001 s
>
> > latency distribution:
> ...
> 3.1 - 3.2 ms: 1 #
> ...
> 3.9 - 4.0 ms: 1 #
> 4.0 - 4.1 ms: 9903 ##################################################
> ...
> 5.0 - 5.1 ms: 95 #
>
> > SUCCESS
>
> best latency was 3.13 ms
> worst latency was 5.00 ms, which is great.
>
>
> This is a vanilla 2.6.34 kernel, no realtime patches, no nothing.
>
> adi@email-addr-hidden:~$ cat /proc/asound/timers
> G0: system timer : 4000.000us (10000000 ticks)
> P0-0-0: PCM playback 0-0-0 : SLAVE
> P0-0-1: PCM capture 0-0-1 : SLAVE
> P0-1-0: PCM playback 0-1-0 : SLAVE
> P0-1-1: PCM capture 0-1-1 : SLAVE
> P0-2-1: PCM capture 0-2-1 : SLAVE
>
>
> Not even HR timers. And yes, this is SMP, and yes, I have the "ondemand"
> cpu governor, that is, the evil frequency scaling and the lot. Put
> simply, it's a plain Debian unstable system, no latency tuning at all.
>
> Ok, 4ms are not strikingly fast, but it's acceptable and shouldn't
> matter in praxis. Especially I don't see any noteworthy jitter.
>
> I'm going to measure the mainline kernels from 2.6.26 to 2.6.34 within
> the next days to see if this stability can be accounted to the parts of
> the RT patch that have been integrated into mainline.
>
> If this hypothesis is correct, I should see a lot of jitter with older
> kernels.
>
> I'll also try to compare it against an onboard MPU-401, a firewire based
> device and one or two more USB solutions.
>
>
> Long story short: Seems things are not black (USB) and white (PCI).

I'll test PCI as soon as possible too.
Btw. while ports from other computers and my whole MIDI hardware is able
to use my 80ies self-made MIDI thru box, the level of the USB MIDI's
output is to weak to do it (I didn't re-adjust the thru box). Could it
be, that a weak level has impact to the interpretation of the slew rate,
regarding to high and low? Of course without using this box, because it
won't work, I'm thinking of connecting the IOs of the USB MIDI and
running the ALSA latency test.

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Received on Thu Jul 1 12:15:01 2010

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