[LAU] Use of 96 kHz sample rate to lower latency

[Bill Gribble]

Wait a minute.  This discussion is making my head spin!  How is there any way at all that increasing the sampling rate, and changing nothing else, will improve the lowest reliable latency of a system?

Lowest latency in my experience is a function of the longest time (in microseconds or milliseconds) that you can expect your system to be unavailable for audio processing due to other kernel work such as disk IO, plus whatever per-cycle setup overhead you have in your application, plus data processing time.  If that is, say, 5ms at 48 kHz, the kernel latency part (which generally dominates) will be 5ms at 96 kHz (or 32 kHz, or...) unless you reconfigure your system to improve kernel behavior, or improve your app's performance.   

Or are you just talking about the boundary case where kernel-dependent latency is very low and you are limited by the smallest buffer size that the ALSA driver supports?  In that case I guess this would work.  But you are more than doubling the amount of work your system is doing, and if you are at the buffer size lower limit already, is it really an audible improvement?

Am I missing something?

Thanks,
Bill Gribble

> On Jan 2, 2014, at 0:25, Joel Roth <joelz at pobox.com> wrote:
> 
> Harry van Haaren wrote:
>>> On Wed, Jan 1, 2014 at 1:21 PM, Joel Roth <joelz at pobox.com> wrote:
>>> 
>>> I was curious, if doubling the sample rate is a
>>> practical way to reduce latency for live effects
>>> processing. I would think it would reduce latency by half.
>> 
>> It would: you mention "practical", i'm not sure I'd call it that.
>> 
>> 
>>> If one wanted to avoid the tradeoff of handling twice the
>>> usual amount of audio data,
>> 
>> CPU load will go up, since there is 2x more  of data to process,
>> which also means every plugin / host has  2x more work to do.
>> Adds up quickly if you're doing things like convolution reverbs
>> or other CPU intense processing..
>> 
>> I was curious if ALSA sample
>>> rate conversion, or some other clever hack could be used to
>>> get low latency advantage of the high sample rate, while
>>> actually dealing with 48k streams through JACK.
>> Theoretically possible I suppose, it seems like an awful lot of
>> effort to get a few less ms latency..
>> 
>> Latency below ~3ms isn't percievable at all IMO: most will agree.
>> Why not run jack at 64 frames, 2 buffers? That'll achieve approx
>> 3ms (on 44.1kHz and 48kHz).. which is fine for the purpose?
> 
>> Perhaps I'm missing something, are you doing mulitple passes
>> trough the sound-card that you're adding its latency two or more times?
> 
> For a live submix, the routing I want to use is exemplified by:
> 
> system:capture_5 --> Nama:sax_in_1 --( ecasound )--> Nama:sax_out_1
> Nama:sax_out_1 --( ecasound )--> system:playback_11
> 
> If I understand correctly, the latency not due to Ecasound
> in this graph is the soundcard roundtrip plus the number of
> hops which contribute (frames*buffers/sample-rate) latency
> per.
> 
> I'm adding one more hop, so that would be 3ms 
> at 64/2, 4ms at 64/3, and 5.3ms at 128/2.
> 
> Regards,
> 
> Joel
> 
> 
>> Cheers, -Harry
> 
> -- 
> Joel Roth
> 
> 
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