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@xxxxxxxxxxxxx/ 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
