[mythtv] Digital audio passthrough

[mythtv at meric.id.au]

> I am an ALSA developer.
> From your description of how myth works, I am amazed that AC3
> passthrough ever worked.
> The "device" open will override any mixer controls, so the iecset will
> be forgotten once you open the device.
> 
> The correct way to do AC3 passthrough is by opening the device with
> the device name:
> iec958:AES0=0x6,AES1=0x82,AES2=0x0,AES3=0x2
> and not using any "iecset" utility.
> 
> This is how xine does it, and it has always worked. I wrote the source
> code in xine to do it.
> When switching from non-audio to PCM, one has to close and open the
> device again.

Hmmm.. well, in my case it didn't work.

The funny thing is that the device open doesn't actually clear the
non-audio flag.  It isn't until you set the hw_params (and prepare the
device), that it gets wiped out.

Jean-Yves, if you happen to be making changes at some point in the
future, I think that the snd_pcm_prepare() call in SetParameters() can
be removed.  My understanding is that the snd_pcm_sw_params() call does
this implicitly.

Just out of curiosity - I understand what the AES0 flags do, but what do
the rest actually mean?  My setup seems to work without them.

Richard.