Frame-based access (via mmap) and cross-device synchronisation

Frame-based access (via mmap) and cross-device synchronisation

[Metathronius Galabant]

Hi all,

I could not find any examples, plus the alsa documentation and the
pcm.c example on mmap-style access is rather slim.
Consider the following setting:

I have 2 seperate HW devices that are NOT clock sync'ed; their period
is set to 10ms, and the total SW buffer is set to 20ms (= classic
double buffering) or 40ms.
Now I need to read from the 1st device and write to the 2nd device -
and read from the 2nd device and write to the 1st device.

Now, to synchronise - because their clocks do drift - I'd like to have
the following semantics:

At any given time x0, x1, ... xn I'd like to take some kind of
"snapshot" of both devices, ask them "how many frame do you have?".
This MUST NOT be restricted to the period, but it should be like
device1 has 80 frames, device2 only has 78 frames. In short I would
need pure frame-based access at any given time.
I will then manually discard or duplicate frames to keep both streams happy.

How do I achieve this kind of access method via alsa?

Any help is welcome. Thanks!
M.

Re: Frame-based access (via mmap) and cross-device synchronisation

[Clemens Ladisch]

Metathronius Galabant wrote:
> At any given time x0, x1, ... xn I'd like to take some kind of
> "snapshot" of both devices, ask them "how many frame do you have?".
> This MUST NOT be restricted to the period, but it should be like
> device1 has 80 frames, device2 only has 78 frames.

Call snd_pcm_avail().  The granlarity of its return value is device
specific; e.g., chips like Emu10k1 or DS-1 move data in blocks of 256
or 512 samples, and USB uses 1-ms frames.  You are not guaranteed to
get a result that is more recent than the last period boundary,
although most devices can do better.


Best regards,
Clemens