[ardour-users] Exporting and latency

Sampo Savolainen v2 at iki.fi
Fri Feb 24 08:25:41 PST 2006

On Fri, 2006-02-24 at 13:57 +0100, Alex Polite wrote:
> On 2/24/06, Sampo Savolainen <v2 at iki.fi> wrote:
> >
> > xruns don't have anything to do with latency. Do you mean this in the sense
> > that the larger buffers jackd uses, the more latency you get and thus less
> > xruns?
> OK, what are xruns then?

xruns are buffer underruns. The buffers in question are the signal
buffers for the soundcard DA unit. Consider a buffer size of 256 frames.
If the soundcard is running at 44.1kHz, 256 frames means:

 256 frames * 1 sec / 44.1 samples per second = 5.8 msec

This means that every buffer equals about 5.8 milliseconds of sound.
Also, this means that the computer has only 5.8 milliseconds to process
each buffer. An xrun is when the computer misses this deadline.

If you connect your soundcard inputs straight to the soundcard outputs,
the latency you get is _not_ 5.8 msec, though. When calculating total
latency through the AD unit -> jack -> DA unit you must take the number
of "periods" into account. 

If you have 2 periods, your latency will be 11.6 msecs. These "periods"
are extra buffers the drivers use to make processing more efficient and
less xrun prone.

PS. It seems you didn't "reply-to-all" when you answered me. It's
required for your replies to get posted on the original mailing list.

Sampo Savolainen <v2 at iki.fi>

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