[mythtv-users] AC3 and/or S/PDIF (again)

Michael T. Dean mtdean at thirdcontact.com
Tue Jan 24 00:42:55 UTC 2006

On 01/23/2006 02:00 PM, Ben Suffolk wrote:
>> As a matter of fact, I don't expect to use digital connections for 
>> any of my audio. I can't live without timestretch, anymore (thanks, 
>> Mark)--on TV and for DVD's--and decoding and re-encoding an AC-3 
>> (*lossy* compression algorithm) stream to allow timestretch seems 
>> like it negates the benefit of a digital audio connection. After all, 
>> the audio is running over short connections and has to be analog 
>> between the receiver and the speakers, anyway, so proper wiring of 
>> analog connections will probably give at least as good--if not 
>> better--quality as resampled AC-3.
> A quick question if I may. Am I right in thinking that if I buy a new 
> sound-card with 6 analogue channels on the output it will drive my 5.1 
> amp (it has 6 channel input) for surround sound.
Yes (assuming you mean by 6-channel input that you have at least 6 RCA 
connectors or 3 headphone connectors for input).

Note, though, that many cards use line in for surround output in 
surround modes, so if you've got a software encoder that requires a 
sound card for audio, you may have to keep your on-board card enabled as 
well for sound capture.
> i.e. I guess I am asking does myth decode the 5.1 happily from DVDs 
> and from DVB-T broadcasts?
Yep.  It's been doing this for a long time--just disable AC-3 pass 
through.  Mark Spieth recently created a patch to support timestretch 
with multi-channel streams.  He did the analog patch in no time, but 
it's the re-encoding to AC-3 that's taking some time.  I don't think the 
patch has been put in SVN, yet, though, so it may not make it into 0.19.
> Does it also work happily with the internal volume controls as well 
> (which after all is what I am trying to achieve)
That's a question of your ALSA drivers.  For some, the Master controls 
only 2-channel audio and has no effect on surround channels (i.e. your 
front left/right will get quieter and your center/rear channels will 
stay at full volume).  Since Myth's internal volume controls can only 
use either PCM (which is always 2-channel audio) or Master (which is 
sometimes 2-channel audio), you may be forced to use an external script 
to change volume (thereby losing the OSD volume indicator).  However, 
using the mythtvosd program, you can make a "poor-man's" volume OSD.

For example, the script volume.sh (attached) will allow you to 
increase/decrease or mute/unmute all channels of your surround, even if 
ALSA's master control won't work.  To use it, disable internal volume 
controls in Myth and use something like the following in your LIRC 
config (~/.lircrc) (note that the mute button is being used to toggle 
the mute/unmute):
  prog = irexec
  button = volume-up
  config = /path/to/volume.sh up &
  repeat = 3
  prog = irexec
  button = volume-down
  config = /path/to/volume.sh down &
  repeat = 3
  prog = irexec
  button = mute
  config = /path/to/volume.sh mute &
  config = /path/to/volume.sh unmute &
  repeat = 0

That being said, I still like the IR transmitter to my receiver 
approach.  ;)



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