Re: [Ekiga-list] A comparison ALSA-PULSE

[Alec Leamas <leamas alec gmail com>]

Andrea wrote:
> Alec Leamas wrote:
>   
> > > It is at the beginning of the 2 files (the same):
> > >
> > > Its setup is:
> > >   stream       : PLAYBACK
> > >   access       : RW_INTERLEAVED
> > >   format       : S16_LE
> > >   subformat    : STD
> > >   channels     : 2
> > >   rate         : 44100
> > >   exact rate   : 44100 (44100/1)
> > >   msbits       : 16
> > >   buffer_size  : 1764
> > >   period_size  : 441
> > >   period_time  : 10000
> > >   tstamp_mode  : NONE
> > >   period_step  : 1
> > >   avail_min    : 441
> > >   period_event : 0
> > >   start_threshold  : 1
> > >   stop_threshold   : 1764
> > >   silence_threshold: 0
> > >   silence_size : 0
> > >   boundary     : 1849688064
> > >
> > >   
> > >       
> > Actually, I thnk I'm wrong. If  the buffer is 1764 frames:. Time for 1
> > frame = 1/44100 sec = 1000/44100 ms. Time for buffer 1764 * 1000/44100=
> > 40 ms. So, it really not that small, and increasing  it is a problem.
> >     
>
> Looking at the code in PSoundChannelALSA::Write(), where len=1764
>
>     /* the number of frames to read is the buffer length
>        divided by the size of one frame */
>     r = snd_pcm_writei (os_handle, (char *) &buf2 [pos], len / frameBytes);
>
> I would say 1764 is a number of bytes.
>
> The header file confirms this
>
>     virtual PBoolean Write(
>       const void * buf, ///< Pointer to a block of memory to write.
>       PINDEX len        ///< Number of bytes to write.
>     );
>   
There are really two things here: the total size of the hw buffer, and 
the size of a period. Data is transferred in chunks of one period, 
that's what's called  buffer size in this code as I see it. The buffer 
is actually four periods, period time is 10000 ns = 10ms. So everything 
seems to adjust to this model. To summarize, a period is 1764 bytes == 
buffer above, the total buffer is 1764 frames i. e., 7056 bytes. If I'm 
right, that is...

After some serious googling I found a document I've used before. Maybe 
it could be of interest:
http://www.stderr.org/doc/libasound2-doc/html/pcm.html . Note the 
hostname, it looks bad, but I actually trust these docs.