24 bit sample embedded in 32 bits

24 bit sample embedded in 32 bits

[Vipin Kumar]

Hello,

I assume that the alsa framework expects the 24 bit sample data to be 
packed one after the other. for example while recording a 2 channel, 24 
bit sample data, typically the dma reads a fifo (which receives data 
from adc) and prepares a buffer which is then passed to the upper layers.

This buffer is expeted to be 24byte left channel and then 24 byte right 
channel. Am I right?

If the above assumption is correct, I need to find a way to tell the 
framework that the data is 24bit but is embedded in 32 bit. ie 32bit 
left and then 32 bit right channel data

I am tending to think that there has to be a way but I am unable to find 
it. Can you please point me to the right direction

Regards
Vipin

Re: 24 bit sample embedded in 32 bits

[Clemens Ladisch]

Vipin Kumar wrote:
> I assume that the alsa framework expects the 24 bit sample data to be
> packed one after the other.

ALSA expects the sample data to conform to whatever sample format has
been configured.

Most hardware uses 32-bit sample words in memory to make the
interface to the PCI bus easier.

> I need to find a way to tell the framework that the data is 24bit
> but is embedded in 32 bit.

(HH = highest byte, LL = lowest byte, MM = middle byte)

LL MM HH     SNDRV_PCM_FORMAT_S24_3LE
xx LL MM HH  SNDRV_PCM_FORMAT_S32_LE
LL MM HH xx  SNDRV_PCM_FORMAT_S24_LE
HH MM LL     SNDRV_PCM_FORMAT_S24_3BE
HH MM LL xx  SNDRV_PCM_FORMAT_S32_BE
xx HH MM LL  SNDRV_PCM_FORMAT_S24_BE


Regards,
Clemens

Re: 24 bit sample embedded in 32 bits

[Vipin Kumar]

On 7/2/2012 4:31 PM, Clemens Ladisch wrote:
> Vipin Kumar wrote:
>> I assume that the alsa framework expects the 24 bit sample data to be
>> packed one after the other.
>
> ALSA expects the sample data to conform to whatever sample format has
> been configured.
>
> Most hardware uses 32-bit sample words in memory to make the
> interface to the PCI bus easier.
>
>> I need to find a way to tell the framework that the data is 24bit
>> but is embedded in 32 bit.
>
> (HH = highest byte, LL = lowest byte, MM = middle byte)
>
> LL MM HH     SNDRV_PCM_FORMAT_S24_3LE
> xx LL MM HH  SNDRV_PCM_FORMAT_S32_LE
> LL MM HH xx  SNDRV_PCM_FORMAT_S24_LE
> HH MM LL     SNDRV_PCM_FORMAT_S24_3BE
> HH MM LL xx  SNDRV_PCM_FORMAT_S32_BE
> xx HH MM LL  SNDRV_PCM_FORMAT_S24_BE
>

Thanks Clemens, Jaroslav,

This was very helpful..I would try more cases and come back for more 
doubts if there are any :)

Regards
Vipin

>
> Regards,
> Clemens
> .
>

Re: 24 bit sample embedded in 32 bits

[Jaroslav Kysela]

Date 2.7.2012 12:11, Vipin Kumar wrote:
> Hello,
> 
> I assume that the alsa framework expects the 24 bit sample data to be 
> packed one after the other. for example while recording a 2 channel, 24 
> bit sample data, typically the dma reads a fifo (which receives data 
> from adc) and prepares a buffer which is then passed to the upper layers.
> 
> This buffer is expeted to be 24byte left channel and then 24 byte right 
> channel. Am I right?
> 
> If the above assumption is correct, I need to find a way to tell the 
> framework that the data is 24bit but is embedded in 32 bit. ie 32bit 
> left and then 32 bit right channel data
> 
> I am tending to think that there has to be a way but I am unable to find 
> it. Can you please point me to the right direction

Use S/U24 or S/U32 formats - look to adriver.h for SNDRV_PCM_FORMAT
constants. S/U24 expects 24-bit samples with zero MSB in 32-bit words.
S/U32 expects 32-bit samples (LSB is zero for 24-bit data).

					Jaroslav

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