[PD] JACK and blocksize

[Christof Ressi]

Hi Roman,

> I wonder if the blocksize of 64 is deeply baked into Pd or if it could
> theoretically be made configurable.
Pd's scheduler block size is a constant value (64 samples), defined as 
DEFDACBLKSIZE in "s_stuff.h".

In theory, you can change the #define in "s_stuff.h" and recompile Pd 
(and all externals, just in case some of them rely on it), but that's 
obviously not a good solution.

Now, the "callbackprocess" function in "s_audio_jack.c" checks if the 
Jack block size is multiple of DEFDACBLKSIZE. As you noticed, this 
breaks for Jack buffer sizes smaller than DEFDACBLKSIZE.

Actually, Pd's Jack callback implementation could use a small ring 
buffer for such situations. For example, if the Jack blocksize was 16 
samples, Pd would call m_audio_callbackfn() every 4 blocks to fill the 
buffer. Note that you would still have 48 samples of extra latency, but 
at least sound would be working :-)

Generally, there is no way to get lower I/O latency than 64 samples in 
Pd without changing DEFDACBLKSIZE and recompiling.

> I found the '-blocksize' flag, but it seems it only sets the blocksize
> for audio I/O and doesn't affect Pd's internal blocksize

Yes. "block size" is really a misnomer. At least it should be called 
"hardware blocksize" or even "hardware buffer size". It does *not* set 
Pd's scheduling block size, which is really fixed at 64 samples.

> (and doesn't
> have any effect at all when running with -jack).
In the case of Jack, the hardware blocksize can't be set dynamically in 
the client. Instead, Pd obtains the value from the Jack server. The same 
is true for the sample rate, btw.