On Wed, 24 Dec 2003, Peter Hunnisett wrote:
> >What's looping sound? You can initiate endless ring buffer looping when
> >you set stop_threshold to boundary (sw_params). I think that this
> >major drawback does not exist.
> >
> >
>
> Thanks for pointing out that sound looping is possible in a reasonable
> and straightforward way. We had actually realized that we could have
> worked around this even if ALSA didn't provide support using a PCM
> callback and some double buffering but it wasn't obvious which is why
> the link you quoted above indicates the lack of looping support. The
> online documentation that we looked at certainly wasn't clear or
> particularly informative in showing this reasonably common usage so may
> I suggest improving it.
OK, I've added a few words to snd_pcm_sw_params_set_stop_threshold()
and snd_pcm_sw_params_get_stop_threshold() functions.
> We have been unable to find a way to tell ALSA, through
> snd_pcm_mmap_begin(), where we want to write data when using the mmap
> api. OSS doesn't have this restriction and this is the only documented
> way we can see to support sound engines which mix in new sounds using
> looping buffers and "randomly" writing ahead of the playing position. We
> have seen that there are undocumented ways to support this functionality
> and we are leery of using undocumented functionality since it could
> disappear or change. Can you suggest a documented way to achieve the
> desired effect or would you be in favour of modifications to the API?
Use snd_pcm_forward() and snd_pcm_rewind() functions to move the
application pointer in the ring buffer before snd_pcm_mmap_begin().
Note: The pointer for these functions is the stream pointer
[0 ... (boundary - 1)]. The offset to the ring buffer is:
stream_pointer % ring_buffer_size
Jaroslav
-----
Jaroslav Kysela <[EMAIL PROTECTED]>
Linux Kernel Sound Maintainer
ALSA Project, SuSE Labs
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