18 Sep
2002
18 Sep
'02
6:08 p.m.
On Wednesday 18 September 2002 19.18, Peter L Jones wrote:
[...]
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
.- M A I A -------------------------------------------------.
| Multimedia Application Integration Architecture |
| A Free/Open Source Plugin API for Professional Multimedia |
`---------------------------> http://www.linuxdj.com/maia/ -'
.- David Olofson -------------------------------------------.
| Audio Hacker - Open Source Advocate - Singer - Songwriter |
`-------------------------------------> http://olofson.net -'
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...).