Re: ALSA supported PCM formats

["Clemens Ladisch" <cladisch@fastmail.net>]

Assaf Hoffman wrote:
> I'm new to ALSA and I'm trying to understand the PCM formats and the way
> ALSA stack place/pick them from the DMA buffer.
> 
> My understanding concerns PCM formats that their physical representation
> bit size is bigger than the actual sound sample bit size.
> For example, defined in pcm_misc.c file under pcm_formats[]:
> 	[SNDRV_PCM_FORMAT_x24_xE]  = {.width = 24, .phys = 32, ...}
> 	[SNDRV_PCM_FORMAT_x20_3xE] = {.width = 20, .phys = 24, ...}
> 	[SNDRV_PCM_FORMAT_x18_3xE] = {.width = 18, .phys = 24, ...}
> How the ALSA stack (kernel 2.6.12.6) knows the offset of the sound
> sample bits inside the format physical size? For example, when the sound
> sample size is 24 (.width = 24) and the format physical size is 32
> (.phys = 32) the sound sample offset inside the physical frame can
> anywhere beteen 0-7 bit. How the ALSA stack can know where to look for
> the sound sample?

There are three sizes that describe a sample:
1) physical size: how many bits are occupied in memory, i.e., how many
   data is read from/written to the DMA buffer for each sample;
2) sample width: the number of bits, starting at the _least_ significant
   bit inside (1), used for sample data;
3) significant bits: the number of bits, starting at the _most_
   significant bit inside (2), actually used for sample data.

Which bit is least/most significant depends on whether the sample format
is little or big endian.


HTH
Clemens

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