[linux-audio-dev] LADSPA and TiMidity++

[David Olofson]

On Wednesday 18 September 2002 19.18, Peter L Jones wrote:
[...]
> Well, when I had "-B 8,11" (8 x 2^11 byte buffers), the latency
> was, um, rather amusing :-).  Dropping this to "-B 4,4" means I
> can't feel any latency between key press and sound.

That's really rather good for an off-line synth, I'd say! ;-)


>  (Dropping to
> -B 3,3 caused very strange behaviour (that lead to an uncontrolled
> reboot)

Oops... If that's three buffers of 8 bytes, I'm not surprized - but 
still, the worst accident that should be possible (with SCHED_FIFO 
and no watchdog) should be a user space freeze. Was that what 
happened?


> and -B 2,2 was too small - very strange noises produced.)

Well, you should never use less than 3 buffers for anything if you 
can avoid it (applies to video as well, BTW), and you should probably 
stay above the minimum PCI DMA burstsize limit of 64 bytes. :-)


> > (I'm talking about low latency mixing with live MIDI input,
> > rather than playing back MIDI files with a large audio buffer.
> > And no, I don't know how well timidity performs in this regard.
> > Just pointing out some potential issues to keep in mind with any
> > soft synth setup, basically.)
>
> Yup - my default set up is tuned for file playback, so it's tuned
> to even out any problems caused by system performance.  For manual
> play, it needs to be tuned for low latency, which -B 4,4 appears to
> achieve (at least, I've not yet come across further problems...).

Well, buffering is obvious, but there's a *lot* more than that to it, 
if you want reliable real time performance. It's all about the design 
of the synth engine; chosing determinism before faster average speed, 
avoiding (standard) dynamic memory allocation, avoiding certain ways 
of talking to devices and other processes etc...


//David

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