[alsa-devel] Discovery of device characteristics

[Tim Hewett]

I'm writing software looking to discover the available sample rates, channel formats etc. for a device. I have been taking the approach of testing a sample rate from a list of potential rates, then setting the device to that rate to then test the channel formats etc..

The structure is initialised using snd_pcm_hw_params_any() then snd_pcm_hw_params_test_rate() is called for a set of rates followed by snd_pcm_hw_params_set_rate() if the previous call is successful, then snd_pcm_hw_params_test_format() for a set of formats followed by snd_pcm_hw_params_set_format() if that is successful, then snd_pcm_hw_params_test_channels(), the point being to narrow down the characteristics available for given sample rates and channel formats.

For example I have a device which supports up to 8 channels at sample rates of 44.1kHz and 48kHz but only 2 channels at 96kHz.

It works ok usually but keeps failing to detect that the device supports 24-bit channel formats, both for SND_PCM_FORMAT_S24_LE/BE and SND_PCM_FORMAT_S24_3LE/3BE. It detects the 16-bit formats without any problem.

Online source taken from http://download.atmark-techno.com/misc/demos/usb-audio-a500-howto/hw_params.c does detect the 24-bit format but doesn't set the sample rate first - each characteristic is tested in isolation - snd_pcm_hw_params_any() is called before each one.

I am wondering if my way is not the correct way to determine device characteristics.