linuxaudio.org

On Mon, Apr 18, 2011 at 05:21:46PM -0400, Paul Davis wrote:

quoted text
> On Mon, Apr 18, 2011 at 4:52 PM, S. Massy  wrote:

> > Thanks,

> > Builds and runs fine on Debian Wheezy AMD64. What latency would be

> > considered average? What numbers should one expect?

>

> the "excess" value is generally on the order of several tens of

> samples in each direction. it can reach as high as the low hundreds.

> with USB audio (and potentially some firewire devices, the excess can

> even get up into the thousands, but its dependent on a variety of

> factors. this assumes you are doing the obvious thing and measuring a

> loopback via a D/A and A/D converter pair.

>

> for reference, my RME HDSP system, connected to a Frontier Designs

> Tango24 converter, meaures 68 samples of total excess latency, so

> about 34 in each direction.

That's a very typical value.
If you have an HDSP or any other card with ADAT (or MADI) interfaces

it can be interesting to measure the excess latency with a digital

loopback as well.
If this is much less than the value obtained when going through

the DA/AD converters, then the difference is an indication of the

type and lenght of the anti-aliasing filters used in the converters.
If these filters are linear-phase (i.e. a symmetric FIR), then

each of them will add a delay equal to half the length of the

filter, so the excess delay is a measure of the filter length.
Values I've seen so far appear to be much less than e.g. the filter

lengths used by libsrc or zita-resampler at high quality settings. 
Ciao,
--

FA

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