[PD] jack periods 32 = no pd

[Peter P.]

* Peter P. <p8rpp at aol.com> [2014-01-28 00:35]:
> * Miller Puckette <msp at ucsd.edu> [2014-01-28 00:02]:
> > Probably that's so - but Pd should still be able to deal with the mismatch.
> > OTOH, since Pd is built-in 64 blocksize, you wouldn't be able to reduce Pd's
> > latency by reducing jack's blocksize below 64.
> Allright, then I can live without going lower.
>  
> > I'm not sure about this, but I believe that Pd running standalone (using ALSA
> > driver) can achieve as low or perhaps slightly lower latency than Pd running
> > under jack.  This is only of interest if Pd's the only audio application
> > you're running on that particular audio interface, however.
> I can currently start pd using
> -alsa -rt -audiobuf 2
> and get click-free performance, as long as I don't the classic "Select
> all.." and move things around in Tcl/Tk, which of course causes
> drop-outs. Haven't measured latency with that setup yet, but it sure
> sounds promising. Using the oss emulation of alsa works too, but
> slightly more clicks when moving objects on the canvas.

Made a latency test with pd's own latency testing patch 
/usr/local/lib/pd/doc/7.stuff/tools/latency.pd

-alsa -rt -audiobuf 2 -channels 18 on an HDSPe card with Multiface and
a loopback analogue cable gives 6.5ms, which I consider great!

Interestingly Pd gives this latency value regardless of setting
-audiobuf to values 1-6, which I suppose is because of rounding this
value to something usable by the dsp part. Pd also reports on startup
"...buffer_time 2902 us opened"
"using mmap audio interface"

Now what is weird, is that I can't get clean audio under alsa with any
values larger than -audiobuf 6. The sound is very distorted.

Using the -oss emulation and -rt, clean audio is available for values larger
than -audiobuf 2 such as:

-audiobuf 3     with measured latency of 7.9ms (Pd choses blocksze 56)
-audiobuf 5     with measured latency of 9.4ms (Pd choses blocksze 56)
-audiobuf 6     with measured latency of 10.9ms (Pd choses blocksze 56)
and so on.

Oh this is on a Debian system running kernel 3.2.0-4-rt-amd64

Nice that Pd can do so low latencies, but that's a strange thing about alsa though...
> 
> best, P
> 
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