[PD] latency, -frags, and -blocksize

Andrew (Andy) W. Schmeder andy at eh2o.ws
Fri Jan 11 21:44:06 CET 2002

blocksize is the dsp vector size, also controlls the temporal resolution
of messages... ie. your midi messages can only be processed on block
boundaries.  Midi having a clock with 1 ms resolution means you want a
small blocksize to match.  bs = 64 => ~1.4 ms resolution which is

frags is latency... i.e. the number of blocks of lag.  IMHO its better
to have more frags than a large blocksize.

> My laptop has a i8x0 sound card built in that I'm using. For MIDI I use 
> the serial port and a MIDIator 124W. I have lowlatency patches applied 
> to a 2.4.18 kernel, and am using ALSA 9b10. Thanks for any and all help.

I have an i8x0 sound card in a laptop as well (tosh portege 3440ct) and
with lowlat patch and alsa 0.9b.  Basically the card sucks.  In full
duplex its impossible to to run with less than 15 ms latency... i.e.
blocksize 64, 10 frags.  However if I disable the dac or the adc (-nodac
or -noadc) then I can get it down to 1.5-3 ms latency.... but usually
that's not desirable.  I'm certain its a fundamental limitation of the
card.  Someday we will get a decent USB or pcmcia sound solution for
linux and everyone with a laptop will rejoice.


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