Paul Braman wrote: > I'm writing a program to open an ALSA device (USB S/PDIF) and capture > the bitstream. Aside from having to tackle the bits about deciding > whether the stream is PCM versus something else, PCM streams are marked with the "valid" bit. > So, assuming I can find the card and open it using snd_pcm_open I need > to then configure the hardware parameters. > > snd_pcm_hw_params_set_access(..., ..., SND_PCM_ACCESS_RW_INTERLEAVED) > snd_pcm_hw_params_set_format(..., ..., SND_PCM_FORMAT_S16) > snd_pcm_hw_params_set_channels(..., ..., 2) > snd_pcm_hw_params_set_rate_near(..., ..., 48000, 0) This is correct. (Although "_near" does not make sense with S/PDIF). > However, I'm not entirely clear what I'll get from this device. You'll get PCM samples. (Or in the case of non-PCM streams, compressed data that pretends to be PCM samples.) > The S/PDIF standard talks about 32-bit words that can be parsed to get > the headers and payload. This parsing is done by the device. The information in the headers is made available with a special mixer control, but not for USB devices. > Also, what if the device to which I am connected is a CD player > outputting data at 44.1 KHz? If you had an analog-to-digital converter, configuring it for a specific sample rate would control how often it generates a new sample. In the case of an S/PDIF receiver, you _cannot_ control this; how often you get a new sample is determined by the sender. The sample rate you set is either ignored or used to drop data that arrives with the wrong rate. (And because the USB Audio spec doesn't have a mechanism to access the header bits, you have to guess.) Regards, Clemens ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ Alsa-user mailing list Alsa-user@xxxxxxxxxxxxxxxxxxxxx https://lists.sourceforge.net/lists/listinfo/alsa-user
