[mythtv-users] Dolby Digital, DTS, HDA, Xbox, and Z-5500

Yeechang Lee ylee at pobox.com
Fri Sep 29 18:44:41 UTC 2006

I haven't ever paid much attention to the many threads on AC3
passthrough with MythTV because a) I don't have a surround-sound
speaker set or even a receiver--just a set of Klipsch 2.1 speakers
that have performend well for years--and b) I'd always vaguely assumed
that were I to upgrade to 5.1 speakers I'd have to get a new,
"higher-quality" soundcard.

However, now that I think about it, the whole point of MythTV passing
recordings' Dolby Digital (DD) or DTS signals through to a receiver or
other device with a DD/DTS decoder is to have the latter do
all the hard work in turning the bits and bytes into surround sound.

So, then, if I understand correctly, assuming I buy something like the
Logitech Z-5500 5.1 speaker system ($279.99 with no shipping or tax at
Amazon.com!), which does contain a Dolby Digital/DTS decoder, then
connect one of the three 1/8" jacks on the back of my Intel
HDA-equipped (ALC880) MythTV box to it:

* If I'm only interested in using my MythTV box to play MythTV
  recordings (Do any TV shows broadcast with anything other than
  stereo?) or DVDs or MP3s with surround sound, after fiddling with
  the proper ALSA and mythfrontend settings, mythfrontend will pass on
  through the recordings' DD/DTS signals to the Z-5500 and I'll have
  glorious 5.1 sound. The "quality" of the Intel HDA chipset for
  *generating* audio is irrelevant because all the Intel HDA is acting
  as in this context is as a conduit for digital data from A to B;
  i.e., a built-into-the-motherboard "free" audio chipset like HDA
  should be exactly equivlaent to a high-end audio card.

* Were I also interested in using the MythTV box to play games with
  surround-sound support, then I might or might not want a different
  sound card depending on how I satisfied I am with the quality of the
  audio it generates.

Things I don't as yet understand:

* What role, if any, does the Intel HDA have in producing sound from
  recordings, DVDs, and MP3s without DD/DTS audio tracks? Will the
  DD/DTS passthrough also pass on non-DD/DTS audio tracks directly to
  the Z-5500, which then handles the task of turning this into audio?
  Or is it that the passthrough only handles sending on the extra
  tracks needed for 5.1 sound, while the HDA handles the generating of
  audio from the mono and stereo tracks?

* Right now I'm using my LCD flat panel as a poor man's receiver of
  sorts. I have audio out from the panel plugged into the Line In jack
  on my MythTV box, and have enabled the jack with alsamixer, thus
  letting me control the audio from it with the method I use for
  normally controlling MythTV audio volume. If I tune the panel to
  component/audio-input #1, I get video and audio from a cable box
  (bypassing MythTV completely). If I tune it to component/audio-input
  #2, I get video and audio from my Xbox.

  If I get the Z-5500, I'd of course plug the audio from the Xbox (Or,
  more specifically, the TOSLINK output from the Microsoft
  High-Definition AV Pack. Is there an alternative? Apparently this is
  nowadays a rare beast.) into it, so the Xbox's DD/DTS signals can go
  directly to the Z-5500's decoder, which means that in order to use
  the Xbox I'd now have to a) switch to input #2 on the panel *and* b)
  switch to the Z-5500 input I have the Xbox's audio plugged in
  to. Would a "real" A/V receiver handle these tasks together?  Or is
  this somehing I should just plan on handling by telling my MX-500
  remote to perform both switching actions with the same button?

* While I want good-quality 5.1 sound I neither have nor want a
  full-fledged stereo system with a receiver. If I plan on using my
  MythTV box as my only source of audio-visual entertainment, is
  something like the Z-5500 sufficient for my needs given the above

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

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