Re: hardware channel mixing

[Lee Revell <rlrevell@joe-job.com>]

On Fri, 2004-09-03 at 08:59, Clemens Ladisch wrote:
> Patrick Dumais wrote:
> > When I read a description of what the soundblaster Live features, I notice
> > they say "131 hardware channels". Now is this what I think it is? does it
> > mean that I can actualy write multiple (upto 131) sounds and let the card
> > mix it by itself?
> 
> Essentially, yes, but the real number is 64 voices, and each stereo
> stream needs two of them, so you can play 32 streams simultaneously.
> 

Not quite, it's 32 mono substreams, 21 stereo.  Each mono substream
requires 2 voices, each stereo substream, three.  There is an extra
voice allocated per playback substream that is silent, and is just used
to generate the period interrupts.

I have pored over the code, and I don't understand exactly why the extra
voice is needed, Jaroslav said something like "if you don't use the
extra voice the interrupts are going faster than the voice position".  I
think it might be needed so that multiple can playback, each with
different period sizes.

There is currently no way to open an 8 or 16 channel playback
substream.  There are the various plughw surround plugins, but these
seem to work by allocating several mono/stereo streams.  This is
certainly wasteful - 5.1 surround wastes 2 voices.
 
I have been planning to add another playback device to the emu10k1
driver with one 16-63 channel substream, to correspond to the hw:0,2
capture device which can record up to 64 channels.

> > If so, I haven't seen any documentation about this.
> 
> On the SB Live, the PCM device has 32 subdevices.  The subdevice
> number can be specified as the third number in the "hw:x,y,z" device
> name when opening the device.  If you don't specify a subdevice
> number, the default is -1 which means "pick the first free one".

There is some documentation, but nothing that gives a high level
overview of the emu10k1's design and how the driver works, maybe I will
write one at some point.  For now you have to use the source.

Lee



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