S24LE and S32LE format equivalency

[Takashi Sakamoto]

Hi,

On Sun, Dec 12, 2021 at 08:16:57PM +0900, Marios Sioutis wrote:
> Hi all,
> 
> Quick question, I was troubleshooting a 24bit USB device and got down the
> rabbit hole of sample formats. The device advertises itself as S24LE (4byte
> payload) but it appears as S32LE under alsa, something that bothered me a
> lot initially. However, If the conversion between the two formats is to just
> << 8 i.e. put a zero byte as LSB, then the two formats are essentially
> equivalent.
> 
> Is this actually the case? If yes, who performs the conversion? Is it on the
> alsa side, or the device already adds a zero byte LSB?
> 
> Thank you in advance,
> 
> Marios
 
I think the format of sample is 'left-justified' or 'right-padding' 24
bit in 32 bit frame. For the case, in ALSA PCM interface, [S|U]32 sample format is
used with 'msbits' hardware parameter. The snd-ua101 USB driver might be
an good example for the case.

 * https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/sound/usb/misc/ua101.c#n615

In detail, please refer to my previous patch:

 * https://lore.kernel.org/alsa-devel/20210529033353.21641-1-o-takashi@sakamocchi.jp/

Unfortunately, as long as I know, no userspace application evaluates the
msbits parameter with sample format, however in theory the conversion
should be done in userspace in advance of transmission of audio data
frame.


Regards

Takashi Sakamoto