[mythtv-users] noobie sound questions

Jamie Stotz wstotz at toad.net
Sun Nov 21 01:11:34 UTC 2004

I've been digging through the list for a few months in preparation for 
building my first Myth (and Linux) box. While trying to decide which 
motherboard to buy I realized I don't understand enough about how sound 
is handled to know which features I need.

I plan on using an nForce2 mATX motherbord and a Hauppauge PVR250 tuner 
card. My main use will be to time shift standard def TV programing and 
feed it through my AV receiver to a standard def TV. Since I already 
have a stand alone DVD player, I probably won't be using it to watch 
commercial movies. However, I might want to remove commercials and 
record TV to DVD's if my hard disk fills up. I won't be using MythMusic 
or MythGame.

Here is my current understanding of how the sound works. Please add or 
correct anything.:

1. SDTV signals ( over-the-air or through analog cable TV ) can contain 
mono or 2 channel stereo audio signals, depending on the source 

2. Additionally, the 2 channel stereo signal can have a Dolby Prologic 
surround sound signal encoded in it.

3. This analog stereo signal (with or without the embedded surround 
signal ) is captured and digitized by the PVR 250 tuner card and stored 
un-decoded in the MPEG2 file that also contains the video.

4. Upon playback, this stereo data is separated out by the MPEG2 
playback software and sent to:

5. Some collection of sound related software and hardware on the 
motherboard or sound card. ( I'm kind of fuzzy here )

6. The black box in #5 can:

6a: Convert this digital stereo data into an analog waveform and send 
it out the analog out port ( 1/8" stereo jack ) to my AV receiver where 
the surround signal ( if present ) is extracted and sent to the 
appropriate speakers.


6b: Send the digital stereo data out the S/PDIF optical/coax port to my 
AV receiver where the surround signal is extracted and sent to the 
appropriate speakers?


6c: Extract the surround data from the digital stereo data and send the 
digital surround data out the S/PD/IF optical/coax port to my AV 
receiver where the surround signal is extracted and sent to the 
appropriate speakers?

So, I'm wondering:

A. If 6b and 6c are possible, is there any noticeable advantage in 
quality over 6a? So do I really need a S/PDIF port for TV or only if I 
want to play commercial DVD's?

B. Some motherboards use the Cmedia CMI9739 sound chip, others use the 
Realtek ALC650. What function(s) does this chip actually perform? I 
assume is is involved with #5 and 6 above. Should I care which chip is 

C. I imagine that if I was going to play commercial DVD's the S/PIF 
port would be important. But since I have an AV receiver with 5.1 
support I don't need the motherboard to decode 5.1 surround.

D. Does the nVidia MCP-T chip offer anything useful for MythTV/Linux 
that the regular MCP doesn't?

Thanks for your help. I'll be posting some video related question soon.


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