On 3/28/16 12:38 PM, Georg Chini wrote:
On 28.03.2016 17:18, Georg Chini wrote:
On 28.03.2016 16:16, Pierre-Louis Bossart wrote:
On 3/22/16 4:11 AM, Georg Chini wrote:
Hi,
Sorry I missed this thread last week.
At the risk of being pedantic, maybe you should consider two
different concepts.
- cold latency: the time it takes for the audio device to render the
first sample when first opened
- continuous latency: the time it takes to hear a sample after it's
written to the ring buffer.
Yes, but it looks like the alsa USB driver assumes that a sample can
be heard immediately
after it left the buffer, which is not true. So there is some
additional continuous latency
that is not reported. This is visible as a "startup delay" - the time
between the moment
the first sample is written to the buffer and the moment when it is
reported that the
first chunk of audio has been played.
More correctly the time between snd_pcm_start() and the moment when the
first chunk
has been played minus the chunk time, since the buffer is filled before
snd_pcm_start()
is called.
The cold latency is mostly what happens in the .prepare step at the
driver level. It's very hard to estimate by software since you can't
observe the analog output. It can also vary depending on platform
states.
Well, I can observe the analog output. In my setup I have an
oscilloscope connected
to input and output of the loopback, that is why I detected the
difference between
configured and real latency at all.
Ups, looks like I misread your sentence above. You are right, in
software you can't
see the output. But what you can see is the time between dispatching the
audio
to the USB bus and the time when the bus reports back that the audio was
played.
I am not sure if I read the code right, I don't know anything about USB,
but if the
URB's you are submitting to the bus in the prepare step are handled in
chronological
order, it means that
a) you have to wait for the next URB to retire before you can send any
real audio
b) after you submitted the first audio, it is at the end of the queue
and all the other
URB's containing silence will be processed first.
The USB driver will submit N silence URBs on startup in the prepare and
you will have to wait for those URBs to retire before the samples are
queued. There is very little 'USB processing'. If you want to reduce
this delay you have to use smaller periods, it'll decrease the size of
the URBs. I guess it could be possible to change the URB size after the
start but that's not implemented atm.
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