Subject: Re: [linux-audio-dev] Exact description of PCM system for ALSA
From: joy (joy_AT_pingfm.org)
Date: Wed Jul 17 2002 - 23:16:39 EEST
i finally figured out the magic of snd_pcm_mmap_begin/commit, and
what i did wrong all the time. its really simple, but to figure it out
was somehow a 'long way'.
the baisc steps how i did it now are:
frames_available = snd_pcm_avail_update(alsa_handler)
gets the available space in audio-buffer to write to.
frames_transmit = frames_available;
snd_pcm_mmap_begin(alsa_handler, &area, &offset, &frames_transmit);
prepare the areas and get back area, offset and frames_transmit.
frames_transmit gives back the REAL value of available frames which
could be written to in the mmaped-area. its sometimes for some unknown
reasons < frames_available. this is what i did wrong all the time. i
had the wrong idea that frames_transmit is always == frames_available.
outbuffer = ((char *) area->addr + (area->first + area->step * offset)
/ 8);
this mysterious formula setup and calculates the mmaped-buffer which could be
written to. it works only with '2 chn, Signed 16-bit'-streams for
me. should be modified for 'usigned 8-bit'-streams', but i currently
don't know how.
result = snd_pcm_mmap_commit(alsa_handler, offset, frames_transmit);
gives back the virtually written (writable) frames which should be
always == frames_transmit.
a simple
memcpy(outbuffer, data, result);
finally transfers the current data into the mmaped-buffer.
-- regards____- joy
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