[LAU] mystic 1ms border in sound cards or ALSA?

[Florian Schmidt]

On Monday 12 January 2009, Giso Grimm wrote:
> Hi all,
>
> in several audio devices I observe a minimal block size setting of 1 ms:
>
> - In the Echo Layla 3G, it is possible to configure smaller block sizes,
> but the driver or device seems to use 3 fragments instead of 2.
>
> - With the RME HDSP9652, I hear artifacts when I configure a fragment
> size which is smaller than 1 ms.
>
> - With the RME HDSP9632 I am not able to configure a smaller fragment
> size at all.
>
> Does anyone know about the source of this mystic border? If it is in the
> ALSA abstraction or driver layer, what is the reason for this? Is it
> simply the assumption that nobody needs shorter fragment sizes, and
> shorter fragment sizes are likely to produce xruns? Or is this just
> caused by the hardware?

Either the hardware or the driver. I have used sub-millisecond latencies 
(roundtrip) on a delta66 card and a properly setup -rt kernel without XRUNS. 
E.g. 8 frames per period, 2 periods at 48khz (2*8/48000 is ca 0.3ms latency). 
ALSA itself (the framework, not each and every device-specific driver) and 
jackd can handle these latencies just fine. 

Just as a sidenote: There is additional latency caused by the DA and AD 
converters themselfes which varies from device to device. I think Fons once 
released a program to measure these additional latencies.. I don't know the 
figures for the delta 66.

Regards,
Flo

-- 
Palimm Palimm!
http://tapas.affenbande.org

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