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

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).

-- 
mail: adi@email-addr-hidden  	http://adi.thur.de	PGP/GPG: key via keyserver
_______________________________________________
Linux-audio-dev mailing list
Linux-audio-dev@email-addr-hidden
http://lists.linuxaudio.org/listinfo/linux-audio-dev