Re: [alsa-devel] what is the difference between SNDRV_PCM_FMTBIT_S24_LE and SNDRV_PCM_FMTBIT_S24_3LE
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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.
Most devices that support 24 bits use SNDRV_PCM_FORMAT_S32_LE, which stores the sample's bits in the upper bits of a 32-bit word.
But how to distinguish the SNDRV_PCM_FORMAT_S24_LE and the REAL SNDRV_PCM_FORMAT_S32_LE?
If a 24-bit sample with the value 0x123456 is stored in memory, it looks like this: SNDRV_PCM_FORMAT_S24_LE: 56 34 12 00 SNDRV_PCM_FORMAT_S32_LE: 00 56 34 12
Regards, Clemens
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
On Thu, Apr 11, 2013 at 08:27:21AM -0700, Gabriel M. Beddingfield wrote:
On 04/11/2013 02:35 AM, Clemens Ladisch wrote:
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 CPU DAI is responsible for converting the SNDRV_PCM_* format into the serial format. Therefore, the conversions that it supports is totally hardware-dependent.
The drivers should in general be defaulting to using exactly the right number of bits on the wire, aside from the marginal efficiency gains if hardware has limitations they tend to be an inability to cope with extra BCLKs so this improves robustness and compatibility.
participants (3)
-
Clemens Ladisch -
Gabriel M. Beddingfield -
Mark Brown