Re: Affordable well-working USB interface with low latency at 48kHz?

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On 23.11.20 17:17, David Kastrup wrote:
> Giso Grimm <gg3137@xxxxxxxxx> writes:
> 
>> Here is data for the Focusrite Scarlett solo for all period sizes P from
>> 16 to 240 frames in steps of 16 frames:
>>
>> P  frames  ms
>> 16 114.847 2.393
>> 32 176.847 3.684
>> 48 232.847 4.851
>> 64 294.847 6.143
>> 80 368.847 7.684
>> 96 424.847 8.851
>> 112 498.847 10.393
>> 128 566.847 11.809
>> 144 616.847 12.851
>> 160 684.847 14.268
>> 176 752.847 15.684
>> 192 802.847 16.726
>> 208 876.847 18.268
>> 240 1000.847 20.851
>>
>> This is almost perfectly 4*P+48, for all P. I can also start it with P =
>> 37 (an arbitrary prime number), so it looks that there is no limitation
>> on the period size. Also for such odd numbers it remains 4*P+48 (the
>> exact delay varies a bit between jack starts, which is normal for USB
>> devices).
>>
>> This is jackd2 - I don't know if jackd1 behaves differently (although
>> both would use the underlying ALSA layer).
>>
>> I started jack with
>>
>> jackd --sync -P 90 -d alsa -d hw:$CARD -r $RATE -p $PERIODSIZE
> 
> --sync is not documented in jackd2's man page.  What does it do?
> 

without --sync jackd2 uses multiple cores whenever possible, at the cost
of one extra buffer. With --sync jackd2 behaves single-threaded but has
one buffer less delay (i.e., two buffers like in jackd1). Using --sync
is thus mostly relevant when delay is the primary concern, and not
performance.


-- Giso
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