[alsa-devel] Fast Track Ultra latency and new streaming logic.

Felix Homann linuxaudio at showlabor.de
Thu May 3 10:39:29 CEST 2012


2012/5/1 Grant Diffey <gdiffey at gmail.com>:
> I can't seem to get below 512frames in flight (2 256sample frames or 4 128
> or 8 64's) without consistant overruns.

at least at 44.1 and 48 kHz I can get it to run without consistent
xruns with 3 periods/buffer and 128 frames/period.
For 88.2 and 96 kHz I need to go to 4 periods/buffer. Less than 128
frames/period lead to consistent xruns no matter how many
periods/buffer I use.

I've been using a kernel freshly cloned from Tiwai's git with the
latest rt patch on a rather slow laptop. I've switched to Fedora on
that machine a couple of days ago so I don't know yet if my system
settings are properly adjusted for Jack, yet.

> This leads to rather high latencies (22ms+) or is this conclusion bogus?

Yes, I guess that this conclusion is bogus.

First, even at 44.1 kHz I think that you rather get 11.6 ms latency
(512/44.1). At 96 kHz this reduces to 5.3 ms.

I've even done some measurements using jack_iodelay. At 96 kHz I get a
total round trip latency of 17.5 ms. According to the jack_iodelay
manpage this corresponds (if I get it right) to an hardware
input/output latency of the device of ~584 frames (6.1 ms).

This would lead to the (maybe bogus) conclusion that we have a total
output latency of 11.4 ms.

By the way, if you're using jack2 you would probably want to start it
in synchronous mode (with the -S option ). Otherwise you get an
implicit extra period/buffer. (I guess you already know that.)

@Daniel: I could not find any difference between my latest
"old-streaming-logic" kernel and my latest "new-streaming-logic"



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