[PD] Pd and RME HDSP alsa vs jack performance

Peter P. p8rpp at aol.com
Tue Apr 15 22:07:03 CEST 2014

Hi list,

following up onto some earlier postings I had made about the current
problems running Pd with any buffersize larger(!) than 5 using alsa,
the mmap code and an RME HDSP Multiface on Debian Linux, here are some
more results from tests I made recently:

Alsa only works with audiobuf smaller 6, otherwise causing totally
garbled sound. With that short buffer settings it is unusable for
everything more CPU intense than sine test tones though.

The only way to get Pd to run reliably under some load on my box using
the HDSP is to use jack with 256 frames at 2 periods/buffer.

Now what is weird is that the built in soundcard of my box is able to
do the same thing without problems with just 64 frames at 3
periods/buffer in jack.

Again with the HDSP, once I start adding Gem and let it render a
single geo, I do get many audio dropouts unless I increase jack's
buffers to a minimum 2048 frames at 2 frames/buffer.

I tested this using two different HDSP cards on two different Debian
boxes, using the -rt flag, realtime and non-realtime kernels,
optional callbacks and different sleegrain setting.

Somehow it seems very sad that one of the best pro multichannel sound
cards under Linux seems to be unusable with Pd under Alsa reliably.

I would like to contribute to any ways of improving this situation and
desperately hope that someone can comment on my findings.

thank you!
best, Peter

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