[alsa-devel] pcm_rate API question

Rob Sykes aquegg at yahoo.co.uk
Sat Sep 28 11:58:40 CEST 2013

> From: Rob Sykes <aquegg at yahoo.co.uk>
>To: "alsa-devel at alsa-project.org" <alsa-devel at alsa-project.org> 
>Sent: Friday, 27 September 2013, 16:28
>Subject: Re: [alsa-devel] pcm_rate API  question
>> From: Clemens Ladisch <clemens at ladisch.de>
>> To: Rob Sykes <aquegg at yahoo.co.uk>; "alsa-devel at alsa-project.org" <alsa-devel at alsa-project.org>
>> Cc: 
>> Sent: Friday, 27 September 2013, 13:38
>> Subject: Re: [alsa-devel] pcm_rate API  question
>> Rob Sykes wrote:
>>>          void
(*convert_s16)(void *obj, int16_t *dst, unsigned int 
>> dst_frames,
>>>                              const int16_t *src, unsigned int src_frames);
>>>  is the plugin obliged always to consume src_frames and generate dst_frames 
>> samples?
>> Yes.  src_frames and dst_frames are the period sizes of the respective
>> devices; their values will never change, and they define the actual
>> conversion ratio.
>Thanks, Clemens. Typically there is some buffering/delay within a resampler -- how does the pcm_rate API allow for this?  E.g. from the libsamplerate FAQ:
>Q6 : I'm use the SRC_SINC_* converters and up-sampling by a ratio of 2. I reset the converter and put in 1000 samples and I expect to get 2000 samples out, but I'm getting less than that. Why?
>The short answer is that there is a transport delay inside the converter itself. Long answer follows.  By way of example, the first time you call src_process() you might only get 1900 samples out.
> However, after that first call all subsequent calls will probably get you about 2000 samples out for every 1000 samples you put in.

Also, just noticed that when performing a rate conversion from 44100 -> 48000, the buffer sizes used by alsa are 5512 and 6000, however the actual ratio for the required conversion is 5512.5 to 6000.

If, as you say above, the buffer sizes define the conversion ratio, then this seems to imply that alsa is unable to perform accurate rate conversion for one of the most common uses! — am I missing something here? A pointer to the API documentation, or perhaps some relevant threads, would be very useful. 


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