Understanding _snd_pcm_channel_area Struct (PCM Interface)!

Understanding _snd_pcm_channel_area Struct (PCM Interface)!

[Guilherme]

Hi all.

I am a bit confused on what I get with this function.
I finished my first project but I did not understand what exactly the

_snd_pcm_channel_area at

 http://www.alsa-project.org/alsa-doc/alsa-lib/struct__snd__pcm__channel__area.html

does.

There are 3 fields in the documentation.

addr, first and step.

It seems that addr is the memory address of the channel samples... so 
far so good. The "step" is the distance between 2 sample (but I am not 
sure) and the least, "first", I could not figure out what does it means.

Could I get some help from here?
 
Thanks in advanced!

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 -!

Re: Understanding _snd_pcm_channel_area Struct (PCM Interface)!

[Takashi Iwai]

At Tue, 23 Jun 2009 02:40:35 -0300,
Guilherme wrote:
> 
> Hi all.
> 
> I am a bit confused on what I get with this function.
> I finished my first project but I did not understand what exactly the
> 
> _snd_pcm_channel_area at
> 
>  http://www.alsa-project.org/alsa-doc/alsa-lib/struct__snd__pcm__channel__area.html
> 
> does.
> 
> There are 3 fields in the documentation.
> 
> addr, first and step.
> 
> It seems that addr is the memory address of the channel samples... so 
> far so good. The "step" is the distance between 2 sample (but I am not 
> sure) and the least, "first", I could not figure out what does it means.

This information is needed to understand how the multi-channel samples
are assigned in a stream.  The first is the offset of the channel
position to the given addr.  The step is the bytes to the next sample
of that channel.

For example, suppose you have a 2-channel stereo interleaved stream
with 16bit samples.  Then you'll have an array of snd_pcm_channel_area
with two elements, for left and right channels, containing like

	area[0].addr = base_address;
	addr[0].first = 0;
	addr[0].step = 4;
	addr[1].addr = base_address;
	addr[1].first = 2;
	addr[1].step = 4;

Both channels share the same base address but have the different
"first" offset bytes.  The step size is 4 = #chanel * sample-size.

For a non-interleaved stereo stream, it'll look like

	addr[0].addr = base_addr_0;
	addr[0].first = 0;
	addr[0].step = 2;
	addr[1].addr = base_addr_1;
	addr[1].first = 0;
	addr[1].step = 2;

Thus it looks like two mono streams with 16bit samples.


HTH,

Takashi

Re: Understanding _snd_pcm_channel_area Struct (PCM Interface)!

[Guilherme]

Takashi...

The base_address and the step sounds pretty clear to me.... just the 
offset I could not understand.

What the difference in being  .first = 0 or .first = 2??? Could you 
provide please a more in depth explanation. I tried really hard find 
this but there is nothing related in the documentation.

area[0].addr = base_address;
	addr[0].first = 0;
	addr[0].step = 4;
	addr[1].addr = base_address;
	addr[1].first = 2;
	addr[1].step = 4;

P.S. being a stereo or a mono pipeline I understand... just the way the offset works that is not so clear to me.

Thanks in advanced!


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, 23 Jun 2009 02:40:35 -0300,
> Guilherme wrote:
>   
>> Hi all.
>>
>> I am a bit confused on what I get with this function.
>> I finished my first project but I did not understand what exactly the
>>
>> _snd_pcm_channel_area at
>>
>>  http://www.alsa-project.org/alsa-doc/alsa-lib/struct__snd__pcm__channel__area.html
>>
>> does.
>>
>> There are 3 fields in the documentation.
>>
>> addr, first and step.
>>
>> It seems that addr is the memory address of the channel samples... so 
>> far so good. The "step" is the distance between 2 sample (but I am not 
>> sure) and the least, "first", I could not figure out what does it means.
>>     
>
> This information is needed to understand how the multi-channel samples
> are assigned in a stream.  The first is the offset of the channel
> position to the given addr.  The step is the bytes to the next sample
> of that channel.
>
> For example, suppose you have a 2-channel stereo interleaved stream
> with 16bit samples.  Then you'll have an array of snd_pcm_channel_area
> with two elements, for left and right channels, containing like
>
> 	area[0].addr = base_address;
> 	addr[0].first = 0;
> 	addr[0].step = 4;
> 	addr[1].addr = base_address;
> 	addr[1].first = 2;
> 	addr[1].step = 4;
>
> Both channels share the same base address but have the different
> "first" offset bytes.  The step size is 4 = #chanel * sample-size.
>
> For a non-interleaved stereo stream, it'll look like
>
> 	addr[0].addr = base_addr_0;
> 	addr[0].first = 0;
> 	addr[0].step = 2;
> 	addr[1].addr = base_addr_1;
> 	addr[1].first = 0;
> 	addr[1].step = 2;
>
> Thus it looks like two mono streams with 16bit samples.
>
>
> HTH,
>
> Takashi
>
>   

Re: Understanding _snd_pcm_channel_area Struct (PCM Interface)!

[Takashi Iwai]

At Fri, 26 Jun 2009 23:56:43 -0300,
Guilherme wrote:
> 
> Takashi...
> 
> The base_address and the step sounds pretty clear to me.... just the 
> offset I could not understand.
> 
> What the difference in being  .first = 0 or .first = 2??? Could you 
> provide please a more in depth explanation. I tried really hard find 
> this but there is nothing related in the documentation.
> 
> area[0].addr = base_address;
> 	addr[0].first = 0;
> 	addr[0].step = 4;
> 	addr[1].addr = base_address;
> 	addr[1].first = 2;
> 	addr[1].step = 4;
> 
> P.S. being a stereo or a mono pipeline I understand... just the way the offset works that is not so clear to me.

Actually, the non-interleaved formats can be represented also with
a single base_address (and is so in the actual implementation).

	addr[0].addr = base_addr;
	addr[0].first = 0;
	addr[0].step = 2;
	addr[1].addr = base_addr;
	addr[1].first = mono_buffer_len;
	addr[1].step = 2;
	addr[2].addr = base_addr;
	addr[2].first = mono_buffer_len * 2;
	addr[2].step = 2;
	...

The "first" is the offset of the first sample.  The address of the
first sample of the channel is simply calculated as addr + first.
This is because just we want to keep the base address same to all
channels as much as possible especially in mmap mode.


Takashi

Re: Understanding _snd_pcm_channel_area Struct (PCM Interface)!

[Takashi Iwai]

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

Re: 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
>
>   

Re: Understanding _snd_pcm_channel_area Struct (PCM Interface)!

[Takashi Iwai]

At Tue, 30 Jun 2009 12:01:53 -0300,
Guilherme wrote:
> 
> 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 ) ??

Depends.

The step field doesn't specify the sample format.  It specifies
the bits to the next sample of the given channel.

For such a purpose, checking the sample format bits via
snd_pcm_format_width() or snd_pcm_format_physical_width() would be
more suitable.

> 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!

Well, usually you don't have to care about the channel_info.  It's
just for mmap, and unless you really do want it inevitably, the mmap
access isn't needed.


Takashi