[mythtv-users] Bug with 32K AC3 audio from FireWire over optical

[Yeechang Lee]

I'd appreciate suggestions on an issue that has bedeviled me and
others for some time regarding digital-audio output from my Turtle
Beach Riviera card, Onkyo 5.1 receiver, and the SP/DIF connection
between the two.

As Rob Baumstark and others report
(<URL:http://www.gossamer-threads.com/lists/mythtv/users/207600#207600>),
certain FireWire recordings come with audio tracks that MythTV
interprets as AC3 in 32khz, which either give my receiver fits (even
to the point of shutting itself down to, I presume, avoid physical
damage to the speakers) and/or result in faint, "chipmunk"-like audio
and sped-up video to match.

Here's what I know about it:
* The affected recordings are only from basic (i.e., analog cable
  channels under 100) channels over FireWire.
* It's not consistent. One recording from TCM or Cartoon Network, for
  example, is fine while the very next, also from the same channel, is
  affected. Whether a recording is affected appears purely arbitrary,
  but whichever the case the state remains the same from the beginning
  to the end.
* This has nothing to do with the audio tracks for a particular
  program; even if the aforementioned recordings both begin in the
  middle of commercial or other promotional spots before the actual
  program being recorded, this problem will appear in one but not in
  the other.
* As far as I can tell this has nothing to do with ALSA or
  .asoundrc. I use Fedora Core 6 and neither have nor need an
  .asoundrc. (The .asoundrc on the Wiki didn't make a difference,
  anyway.)  AC3 and DTS passthrough work fine for me everywhere else,
  whether from MythTV or mplayer. (mplayer has the same problem
  mythfrontend does with the MythTV recordings that are affected,
  though.)
* Affected recordings played without problems when, before the digital
  receiver, I used two-channel analog output from the Riviera.
* I can work around the issue by timestretching. Shifting to another
  speed (1.05X, say) forces mythfrontend to remix the AC3 audio into
  two-channel stereo, which the receiver plays without problems. Also
  fixes the video speed, too.

Based on the above I presume that something occurs during the
FireWire-stream recording process that affects certain recordings'
audio tracks, but I'll be darned if I know how or what.

If developers or others have thoughts on this issue, please check
<URL:http://svn.mythtv.org/trac/ticket/2813>.

-- 
Yeechang Lee <ylee at pobox.com> | +1 650 776 7763 | San Francisco CA US