iec958 and 24-bit audio

iec958 and 24-bit audio

[pl bossart]

When I try playing 24-bit audio files on my SPDIF output, I get this

[alsa-lib]$ aplay -Diec958 file24bits.wav
aplay: set_params:1059: Sample format non available
Available formats:
- S16_LE
- S32_LE

Is there an built-in assumption that the iec958 device can only take
as input what the slave hardware device supports? The IEC subframes
can convey 24 bit audio, and it's possible to insert the 24 bits
starting from bit 4 when adding the preambles and status bits. What am
I missing?
Thanks,
-Poerre

Re: iec958 and 24-bit audio

[Jaroslav Kysela]

On Mon, 1 Nov 2010, pl bossart wrote:

> When I try playing 24-bit audio files on my SPDIF output, I get this
>
> [alsa-lib]$ aplay -Diec958 file24bits.wav
> aplay: set_params:1059: Sample format non available
> Available formats:
> - S16_LE
> - S32_LE
>
> Is there an built-in assumption that the iec958 device can only take
> as input what the slave hardware device supports?

Use 'plug:iec958' device if you want to do the format conversion (from 
24-bit to 32-bit format).

 					Jaroslav

-----
Jaroslav Kysela <perex@perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.

Re: iec958 and 24-bit audio

[pl bossart]

>> [alsa-lib]$ aplay -Diec958 file24bits.wav
>> aplay: set_params:1059: Sample format non available
>> Available formats:
>> - S16_LE
>> - S32_LE
>>
>> Is there an built-in assumption that the iec958 device can only take
>> as input what the slave hardware device supports?
>
> Use 'plug:iec958' device if you want to do the format conversion (from
> 24-bit to 32-bit format).

That seems to work, but what I was asking is if there's a direct path
without conversion? With plug:iec958 there's a conversion to 32-bits
and then you drop the lsb.
Also PulseAudio seems to rely on iec958 and when it doesn't work it
tries with plug:iec958. What's the recommended way of using this
device? Should we always open plug:iec958?
Thanks
-Pierre

Re: iec958 and 24-bit audio

[Jaroslav Kysela]

On Mon, 1 Nov 2010, pl bossart wrote:

>>> [alsa-lib]$ aplay -Diec958 file24bits.wav
>>> aplay: set_params:1059: Sample format non available
>>> Available formats:
>>> - S16_LE
>>> - S32_LE
>>>
>>> Is there an built-in assumption that the iec958 device can only take
>>> as input what the slave hardware device supports?
>>
>> Use 'plug:iec958' device if you want to do the format conversion (from
>> 24-bit to 32-bit format).
>
> That seems to work, but what I was asking is if there's a direct path
> without conversion? With plug:iec958 there's a conversion to 32-bits
> and then you drop the lsb.

Almost all sound bridge chips works with 16-bit or 32-bit samples because 
it's more compatible with analog converters and the handling is more 
simple than for 3-byte formats. The conversion from the 32-bit format to 
the 32-bit S/PDIF frame is done inside hw (dropping the LSB, add S/PDIF 
frame status bits). Some hardware also supports raw S/PDIF frames 
directly.

> Also PulseAudio seems to rely on iec958 and when it doesn't work it
> tries with plug:iec958. What's the recommended way of using this
> device? Should we always open plug:iec958?

You can detect which formats are supported (like aplay does) and use the 
appropriate format to avoid using the plug layer in alsa-lib.

 						Jaroslav

-----
Jaroslav Kysela <perex@perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.