Re: [Christmas] Both ALSA and OSS sound devices

Hi Jan,

On Sun, Jan 04, 2009 at 06:33:57PM +0100, Jan Willies wrote:
>
> > I would not recommend using plughw since it does not unswap the left and
> > right channels, and also does not implement volume control.
>
> It was recommended on the alsa list for reasons I didn't understand
> anyway: http://thread.gmane.org/gmane.linux.alsa.devel/51290/focus=51291

The EM8300 chip has several problems as a soundcard (for the analog
ouput):
 - it only supports 2 channels big-endian 16-bit data;
 - it only supports a few frequencies (as far as I remember 32kHz,
   44.1kHz and 48kHz);
 - the left and right channels are swapped;
 - no volume control.

As for the digital output:
 - it only supports big-endian IEC958 subframes;
 - it only supports a few frequencies;
 - no volume control.

All these problems can be solved in software. And what is great with
ALSA is that alsa-lib provides all that is required to solve all these
probems in userspace. Hence the kernel-space driver simply reflects what
the hardware supports.

Most userspace applications require a specific format, number of
channels and frequency.

When you use hw devices (like hw:0,0), it directly uses the kernel-space
driver, which generaly does not work because of the limitations of the
hardware.

When you use plughw devices (like plughw:0,0), it uses alsa-lib to adapt
the sound format, number of channels (and perhaps frequency as well) to
what the kernel-space driver supports). It works better since it solves
the worst problem, but you still have no volume control, and the left
and right channels are still swapped. Note that it only works for anaog
output since plughw does not understand IEC958 subframes.

To go further, you need a specific configuration, and that's exacty what
/usr/share/alsa/cards/EM8300.conf provides. With this configuration
in place, you should be able to use analog output with the
"default:EM8300" or "default:0" (if your EM8300 card is your first ALSA
audio card) device. As for digital output, "iec958:EM8300" or "iec958:0"
should work, but they lack volume control (because volume control is
impossible with digital pass-through of AC3 or DTS). With a specific
configuration, it should be possible to have volume control with a
digital output for PCM audio.

As for your problem I wonder whether your ALSA installation is fine...
What is you distro? Is it up-to-date?
Would it be possible for you to test your ALSA installation with a
different ("real") soundcard?


Cheers,

Nicolas

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