Re: [Jack-Devel] [jackosx] MIDI timing and Jack (not OS-X-specific)

On Sat, Apr 23, 2011 at 2:11 PM, Eric Kampman <[hidden]> wrote:
> I'm enough of a newbie at audio that I've not yet run into the circumstances that would cause me to need to compromise jitter for the sake of latency or vice versa. I will use the advice you are giving and when my experience catches up a bit I will understand better.

you will run into these very quickly the moment you do any kind of
synthesis based on incoming MIDI (or equivalent) data. if you try to
"make the sound as soon as possible after the incoming event arrives",
you will get bad jitter, up to 1 block's worth of audio. the latency -
the delay between the person doing something and the sound being
audible - will vary between "zero" and 1 block.

if the block size is small (in JACK terms, frames-per-period is low),
then some musicians/performers won't notice it too much, but most
will. most humans are sensitive to variable jitter much more than they
are to a fixed latency. they will play a key and the sound will come
out anywhere from "zero" frames to 1 block's worth of frames later. if
the block size is big, you will *really* notice the effect quite
dramatically, and it will be essentially unplayable because of the
jitter.

you might think that the latency would be the issue there, but think
again, and specifically, think about instruments like pipe organs.
they take a bit of practice to get used to, but the latency from such
instruments is enormous compared to most digital audio - it can be
tenths of a second (or more) between a key press and the sound
starting. but because the jitter is basically zero, humans can play it
- the brain just shifts into a mode where it compensates for the fact
that the sound is always X msecs behind the fingers.

> AFA a2jmidid I confess to hasty conclusions -- I was basing my assessment on a2jmidibridge.c comment and code around line 94 in jack_callback(). The comment says:

a2jmidi_bridge.c is not part of a2jmidid:

From the Makefile:

     SOURCES = a2jmidid.c.* jack.c.* port.c.* port_thread.c.* port_hash.c.*

in the checkout i have of a2jmidid, there is quite a bit of clutter in
there that can be misleading like this.

--p

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