[pulseaudio-discuss] What is latency? And other related questions

[Lennart Poettering]

On Fri, 12.02.10 09:20, David Henningsson (launchpad.web at epost.diwic.se) wrote:

> > What happens here either there is a large period (~30 seconds to
> > 90 seconds) of silence before audio begins to play fine, or I get
> > bursts of audio (5-10 seconds) followed by very long bursts of
> > silence. I know this is related to prebuffering, but beyond that I
> > am lost.
> 
> First, make sure that your tlength is at least two-three time as large
> as mixlen. My guess is that you supply a tlength but for some reason
> fail to fill it properly, so you get underruns on the PA frontend, which
> makes PA increase tlength even more, leading to higher prebuffering (and
> latency).

Uh, 2-3x seems very arbitrary to me. I don't see where such a rule
would ever apply.

> > 2. How do the meaning of pa_stream_writable_size and pa_simple_get_latency differ in terms of the fill state of the buffer?
> 
> pa_stream_writable_size returns the number of bytes that may be written
> using pa_stream_write. 

Actually, to be fully correct you are welcome to write both less or
more than pa_stream_writable_size() returns. 

Generally however the rule is that you should pass at least what it
returns. A good reason to pass more is when your app generates blocks
of PCM and hence cannot generate the exact number of samples
_writable_size() indicated.

A reason to pass less is for example when you are reaching the end of
your stream, or otherwise want to (temporarily or not) stop passing
audio data.

Lennart

-- 
Lennart Poettering                        Red Hat, Inc.
lennart [at] poettering [dot] net
http://0pointer.net/lennart/           GnuPG 0x1A015CC4