Re: capture with non-interleaved mode

[Takashi Iwai <tiwai@suse.de>]

At Tue, 14 Oct 2003 17:30:40 +0200,
Cyril.Coquilleau@UTBM.fr wrote:
> 
> > >Hi. I'm developping a signal processing program which needs to compute data
> > 
> > >from each channel of soundcards separately. 
> > >After searches I found that I must open the soundcard with the 
> > >SND_PCM_ACCESS_RW_NONINTERLEAVED flag but it doesn't works well : for each
> > 
> > >readn operation i get the -32 error code . Meanwhile, I tested capture in 
> > >interleaved mode and it works well.
> > >I had to use hw:x,y for the interleaved capture and plughw:x,y for the
> > non-
> > >interleaved capture, I don't know if non-interleaved capture works with
> > hw:x,y.
> > 
> > [ We need a FAQ ... ]
> > 
> > ALSA's "hw" devices can only be used with parameters that match
> > capabilities offered by the underlying hardware. There's not much
> > hardware out there that supports non-interleaved I/O, so as a result,
> > most "hw" devices can't do this. The Hammerfall and ice1712 series are
> > notable exceptions. But there is a corollary: if you were using a
> > hammerfall, the "hw" device cannot do interleaved I/O either.
> > 
> > as to your error, its probably a bug in your code. JACK uses
> > non-interleaved mode by default, and it works with both plughw and hw
> > models of many different kinds. if you posted the relevant parts of it
> > here, somebody can probably help you.
> > 
> > 
> 
> I join the relevant parts of the source (without error handlers) :
> 
> #define BUF_SIZE 128
> 
> struct snd_card
> {
>   snd_pcm_t *handle;
>   snd_pcm_hw_params_t *params;
> 
>   short inputBuffer[BUF_SIZE*2];  
>   void *buffer[2];
> };
> typedef struct snd_card snd_card_t;
> 
> int 
> main (....)
> {
>   char card_id[32];
>   int err;
>   itn card = 0;
>   snd_card_t *card1;
>   
>   card1 = calloc(1,sizeof(snd_card_t));
>   card1->buffer[0] = card1->inputBuffer;
>   card1->buffer[1] = &card1->inputBuffer[BUF_SIZE];
>   snprintf (card_id, 32, "plughw:%d", card);
>   
>   snd_pcm_open (&card1->handle, card_id, SND_PCM_STREAM_CAPTURE, 0);
>   snd_pcm_hw_params_malloc (&card1->params);
>   snd_pcm_hw_params_any (card1->handle, card1->params);
>   snd_pcm_hw_params_set_channels (card1->handle, card1->params, 2);
>   snd_pcm_hw_params_set_access (card1->handle, card1->params,
> SND_PCM_ACCESS_RW_NONINTERLEAVED);
>   snd_pcm_hw_params_set_format (card1->handle, card1->params,
> SND_PCM_FORMAT_S16_LE);
>   snd_pcm_hw_params_set_rate_near (card1->handle, card1->params, ECHANT, 0);
>   snd_pcm_hw_params (card1->handle, card1->params);
>   snd_pcm_hw_params_free (card1->params);
>   snd_pcm_prepare (card1->handle);
> 
>   while (...)
>   {
>     err = snd_pcm_readn (card1->handle, card1->buffer, BUF_SIZE);
>     printf("err=%d, buff=%d\n", err, card1->buffer[0]);
>     ...
>   }
>  ...
> }
> 
> 
> 
> And here is the result of printf 
> err=-32, buff=-1

-32 is EPIPE.  it's likely buffer under/overrun.


Takashi


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