[mythtv-users] AAC 5.1 decode and analog playback

Nick Rout nick at rout.co.nz
Thu Nov 22 00:59:08 UTC 2007

On Thu, November 22, 2007 3:09 am, Mark Fink wrote:
> Planning a MythTV HTPC build with...
> Asus P5K-VM mobo that uses Intel ICH9 / Realtek ALC883 chipset supporting
> 8.1 audio. The board has analog outputs for multi-channel audio (front,
> center, rear, LFE). It also has a coax port for AC3 passthrough.  Plan on
> using Intel Core 2 Duo E6750 (for what its worth).
> To the point...I want to backup my DVDs (that I have paid for) to
> H.264/AAC 5.1 mp4 files. I want to playback these files and still enjoy
> discrete 5.1 surround sound with no downmixing to stereo or DPL-II, and I
> have read volumes about this in forums.  And I think I have my solution,
> but am looking for a sanity check.
> I have a Sony receiver that has RCA inputs for 5.1 audio that has already
> been decoded by another device. It also has a coax input for receiving an
> AC3 signal for the receiver to do the decoding. The receiver is connected
> to a 5.1 speaker arrangement.
> Without having purchased or tried anything, I believe I have 2 realistic
> (in terms of time and effort) options:
> 1 - rip/transcode audio to AAC 5.1, then on playback, decode the AAC audio
> with software on the HTPC (with MythVideo or mplayer or xine, no strong
> preference), output the decoded multichannel sound out the Asus board to
> the 5.1 RCA inputs on the receiver.
> 2 - route AAC 5.1 audio through ac3filter which transcodes to AC3 on the
> fly and spits it out the coax S/PDIF to the receiver which then does the
> decoding.
> Intuitively, to me at least, the first option would seem to be better
> because of fewer conversions.
> I'm not into other solutions that involve muxing H.264/AC3 together into
> matroska or MPEG-TS because this appears to me to be complex and
> time-consuming.  Then again, I could just compress video with Divx or Xvid
> and maintain original AC3 audio, but would prefer to use H.264 as everyone
> universally agrees to its superior quality.
> Am looking for educated opinions.  Thanks!

I tend to send everything digitally to my amplifier.

I have successfully muxed h264 and ac3 into an avi container. It was not
complicated - I just chose avi as the output format in avidemux. (Having
established that trying to mux ac3 and h264 into an mp4 file didn't seem
to work).

In terms of the number of conversins, this only results in one conversion,
and thats by my (THX certified) amplifier at playback. All the other steps
maintain the ac3 audio from the dvd through to the final avi file.

Nick Rout

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