[alsa-devel] what is the difference between SNDRV_PCM_FMTBIT_S24_LE and SNDRV_PCM_FMTBIT_S24_3LE
Gabriel M. Beddingfield
gabrbedd at gmail.com
Thu Apr 11 17:27:21 CEST 2013
On 04/11/2013 02:35 AM, Clemens Ladisch wrote:
> (please do not drop the mailing list from replies.)
>
> Zhang wei wrote:
>> I do not really understand. For example,an codec is wm8985 ...
>
> The SNDRV_PCM_* symbols specify the format in which samples are
> stored in memory.
>
> I do not know how ASoC maps these symbols to on-the-wire formats.
The SNDRV_PCM_* symbol specifies the format of the audio in RAM, and
transfers that audio to the digital audio interface (DAI) on the CPU
side. (E.g. the OMAP McBSP.)
The on-the-wire format is set up by the ASoC machine driver (i.e. "sound
card.") It will configure the CPU DAI (e.g. the OMAP McBSP) and the
codec (e.g. wm8985) to expect the same "on-the-wire" format. For
example, in sound/soc/omap/omap-twl4030.c (beagleboard's machine driver)
the on-the-wire format is done in omap_twl4030_hw_params() like this
(edited for brevity):
fmt = SND_SOC_DAIFMT_I2S |
SND_SOC_DAIFMT_NB_NF |
SND_SOC_DAIFMT_CBM_CFM;
/* Set codec DAI configuration */
ret = snd_soc_dai_set_fmt(codec_dai, fmt);
/* Set cpu DAI configuration */
ret = snd_soc_dai_set_fmt(cpu_dai, fmt);
The CPU DAI is responsible for converting the SNDRV_PCM_* format into
the serial format. Therefore, the conversions that it supports is
totally hardware-dependent.
-gabriel
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