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

Scott list-mythtv at bluecamel.eml.cc
Fri Sep 29 19:23:25 UTC 2006

On Sep 29, 2006, at 2:44 PM, Yeechang Lee wrote:

> 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.

This sounds correct to me.

> 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:

Not quite. Check the connections on your motherboard and see if you  
either have a Optical Digital or Coaxial Digital connection. Commonly  
these connections are referred to as spdif. The optical style  
connection looks squarish with a flip down door to keep dust from  
getting in. The coaxial type connection look like an RCA plug, not a  
1/8 head phone plug. These should be located near your sound card.

if you don't have a spdif connection then you'll have to use analog  
connections to the Z-5500. It looks like the Logitech Z-5500 can  
accommodate this but it also means you'll have to setup a proper  
asoundrc file for your driver.

When using the analog connections expect the HDA Intel sound driver  
to be doing all the "hard work" figuring out which signals go to  
which speaker over the proper jacks. When using an spdif connection  
then it's up to the receiver (Logitech Z-5500) to handle the decoding.

The above is most likely simplified a bit but over all should close  
to accurate. :)

> * 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.

What sound formats you receive will depend on your broadcast  
provider. You could also hook the cable box or statellite receiver up  
to the Logitech Z-5500 speakers directly using spdif and see what  
gets decoded. This might make for an interesting test.

When the HDA Intel audio chip is simply passing data out the spdif  
it's really not doing to much. A "high quality" sound card might be  
able to pass a cleaner digital signal but I think you would be hard  
pressed to tell the difference between HDA Intel spdif and the spdif  
on a high quality sound 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.

Again, depends on the sound source format. Beyond a PS2 I'm not a big  
game person. You'll have to research and find out what sound formats  
your games provide.

> * 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?

Don't confuse DD with Stereo or other formats. AC3 is just a  
container for DD. It can have any number of channels (DD 2.0 aka  
stereo, DD 5.1 aka surround sound). DTS is also not the same as AC3.  
And mp3 is again, not the same as AC3.

The main answer you're looking for is that it depends on your  
connection to the decoder (Logitech Z-5500) from your sound card. See  
my previous answer :)

Hope this helps a bit!


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