[alsa-devel] Understanding _snd_pcm_channel_area Struct (PCM Interface)!

[Guilherme]

Hum...

so .. the code

if ((areas[chn].step % 16) !=0 )

is because a 16bits format's been used.

If I wanted a 32bits sample format I should change that for
if ((areas[chn].step % 32) !=0 ) ??

and the offset for the first doen's need any changes, is it?


Just a opinion about all this, I think that alsa developers should design graphs for these things, as has in gstreamer documentation. 
It is a lot easer to understand that way. 

It is because I dont understand very well yet, but as I am going to work with sound stuff especially in linux environment, Ill be very soon offering help for this!

;o) 



 

Tks!

-------------------

Guilherme Longo
Dept. Eng. da Computação
Unaerp

Linux User - #484927

*Before Asking
http://www.istf.com.br/?page=perguntas

!- I'd rather die on my feet than live on my knees -!



Takashi Iwai wrote:
> At Tue, 30 Jun 2009 11:40:20 -0300,
> Guilherme wrote:
>   
>> Tks takashi....
>>
>> The doubt comes from this block of code:
>>
>> for (chn = 0; chn < channels; chn++) {
>>             if ((areas[chn].first % 8) != 0) {
>>                     printf("areas[%i].first == %i, abortando...\n", chn, 
>> areas[chn].first);
>>                     exit(EXIT_FAILURE);
>>             }
>>             //pega endereço e offset da area
>>             samples[chn] = /*(signed short *)*/(((unsigned char 
>> *)areas[chn].addr) + (areas[chn].first / 8));
>>            
>>          if ((areas[chn].step % 16) != 0) {
>>                 printf("areas[%i].step == %i, aborting...\n", chn, 
>> areas[chn].step);
>>                 exit(EXIT_FAILURE);
>>          }
>>            
>>                 steps[chn] = areas[chn].step / 8;
>>                 samples[chn] += offset * steps[chn];
>>         }
>>
>> first:
>>
>> areas[chn].first % 8   ( I am using chn = 1). So for the only area that 
>> I have, if I get the value of the parameter 'first' isn't divisible by 
>> 8, the program exits.
>> The  same doubt happens with "areas[chn].step % 16"  and  
>> "areas[chn].step / 8".
>>
>> This is the parameter I am using:
>>
>> static snd_pcm_format_t format = SND_PCM_FORMAT_S16;   //sample format 
>> unsigned 16 bit endian
>>
>> Perhaps with this practical example I can understand better.
>>     
>
> Oops, I wrote wrongly.  The first and step fields are not in bytes but
> bits.  That's why 8 and 16 appear there, corresponding to 1 byte and 2
> bytes.
>
>
> Takashi
>
>