[mythtv-users] OT RANT on DirecTV DD5.1 offerings!

Michael T. Dean mtdean at thirdcontact.com
Thu Mar 9 07:24:40 UTC 2006


On 03/08/06 23:25, Steven Adeff wrote:
> On 3/8/06, Michael T. Dean <mtdean at thirdcontact.com> wrote:
>   
>> On 03/08/06 21:47, Jim Minihane wrote:
>>> Oh well. I guess I'll have to settle for ProLogic until I play a DVD
>>> or ripped DVD. OK… I guess I'll just buy a few PVR-500s and stuff them
>>> into a case for capture. I suppose this simplifies things.
>> Should I mention that the audio chips on the PVR-x50's/500's only decode
>> the stereo portion of a ProLogic signal (they don't use the matrixed
>> data for the other channels), so the audio that's encoded into the
>> recording has only 2 channels? Probably not... :)
> they don't decode anything,

Actually, they do...  TV audio in the US (and several other countries) 
is broadcast with 2-channel stereo provided in multichannel television 
sound ( http://en.wikipedia.org/wiki/Multichannel_television_sound ) 
encoding (an analog--not digital--encoding) that typically further uses 
dbx encoding (another analog encoding) to increase the signal-to-noise 
ratio/provide noise reduction.  In addition, the stereo signal--once 
decoded from MTS--may be encoded in Dolby Surround, which would allow a 
ProLogic decoder to retrieve additional channel information from the 
matrix encoding.

The PVR-x50's/500's output either MP1 or MP2 encoded audio within an 
MPEG-2 container.  Since the audio isn't left in the original 
format--MTS--the card must decode the MTS to a standard stereo signal to 
be able to record the audio.

>  they just record. and since the signal is
> analog when you play it back, your receiver will be able to extract
> the prologic matrix from the analog signal.
>   
OK, let me put it another way.  After the card decodes the MTS signal 
into a stereo signal,  the stereo signal is never decoded using 
ProLogic.  If it were, it would be possible for the card to encode that 
surround data into an AC-3 stream--a format designed specifically for 
multi-channel surround sound representation.

Instead, the stereo signal is encoded into a 2-channel MP{1,2} format 
for storage in the MPEG-2 container.  I had always heard that the lossy 
encoding of MP{1,2} was not conducive to preservation of Dolby Surround 
information and that some audio parameters further degraded the surround 
matrix encoding--parameters such as stereo format (stereo, dual stereo, 
or joint stereo), bitrate (any encoding using less than 256kbps), and 
the sampling frequency (the higher the better).

However, after your post, I did some research and--although there are a 
lot of people claiming one way or the other--there was little definitive 
information on how the MP{1,2}-encoding affects Dolby Surround 
encoding.  This post, however, 
http://www.videohelp.com/forum/userguides/132976.php seems to be the 
most authoritative source I found--and shows that whether using joint 
stereo (which I had heard was the worst) or stereo, as long as you use 
192kbps or greater you get a good reproduction of surround information 
up to 16kHz and at 256kbps or 384kbps, you lose little information.

So, I stand corrected.  Although ProLogic is not used onboard the PVR to 
decode the surround information for encoding to a multi-channel audio 
format, the Dolby Surround information survives relatively well at 
reasonable bitrates.

Mike



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