PCM format support (24 bit data stored in 32-bit word) in ALSA?

PCM format support (24 bit data stored in 32-bit word) in ALSA?

[X Jin]

Hi,

I have a capture card using a TI PCM1803a chip, which is a 24-bit AD
converter. However a 24-bit digital information is stored in a 32-bit
word. I searched the ALSA PCM code and found that the format
SNDRV_PCM_FORMAT_S24_LE (and others) defines the physical bit width as
32. I guess these definitions are designed to handle exactly the
situation I'm facing now. In other words, ALSA can handle the 24-bit
format in a 32-bit physical data.

Is my understanding correct?

TIA
-XJ

Re: PCM format support (24 bit data stored in 32-bit word) in ALSA?

[Takashi Iwai]

At Mon, 23 Jun 2008 23:46:11 -0700,
X Jin wrote:
> 
> Hi,
> 
> I have a capture card using a TI PCM1803a chip, which is a 24-bit AD
> converter. However a 24-bit digital information is stored in a 32-bit
> word. I searched the ALSA PCM code and found that the format
> SNDRV_PCM_FORMAT_S24_LE (and others) defines the physical bit width as
> 32. I guess these definitions are designed to handle exactly the
> situation I'm facing now. In other words, ALSA can handle the 24-bit
> format in a 32-bit physical data.
> 
> Is my understanding correct?

Yes.

- The data is stored in lower 24bits of 4 bytes ==>
   SNDRV_PCM_FORMAT_S24_LE,BE
- The data is stored in upper 24bits of 4 bytes ==>
   SNDRV_PCM_FORMAT_S32_LE,BE  (and set msbits hw_params if needed)
- The data is stored in 3 bytes ==>
   SNDRV_PCM_FORMAT_S24_3LE,3BE


Takashi