[mythtv-users] AAC 5.1 decode and analog playback

Ryan Steffes rbsteffes at gmail.com
Wed Nov 21 20:25:14 UTC 2007

On Nov 21, 2007 9:09 AM, Mark Fink <marksfink at yahoo.com> 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'd start off by worrying less about what everyone agrees on, and more
about what compromises are important to you.  You might try encoding a
variety of ways with various options and just deciding which one
balances convenience with quality the best for you.  You may find the
convenience of ripping xvid w/ AC3 sound beats the small gains of
using H.264.   Your results, and preference, is going to vary based on
the TV or Monitor you're intending to use (IE HD, Widescreen, etc),
how much disk space you are going to take up, how much time you're
willing to spend encoding, how precise you want your rips to be, how
many tools you're willing to learn to use, etc.

I've got my own pretty massive (in my mind, and my friends seem
inclined to agree) DVD collection ripped entirely to disk.  I've found
that various videos have "earned" different rates of quality in my
mind.  The bulk of my recordings are in stereo, with a decent receiver
doing the mixing.  The simple fact is a huge portion of surround sound
is completely wasted.  Those movies that actually have done a
particularly good job earn AC3 audio.  Aside from that, movies that
quality is less important visually, ie comedies, drama, earn one
bitrate, while higher action earn another.  Particularly high action
movies also get the more advanced, and slower, encoding options.

Of course, I automated most of these options with a script I use with
my most common options controlled by flags.

As for your original question, with a minute or two of looking, I
found a couple guides with two different methods for it.  I'm trying a
third, less complicated method right now based on what I know about
xvid, but I'm not sure it'll translate properly so I'm going to hold
off until I see the result before posting it.

These are the two I found quickly:


Should get you started somewhat.

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