[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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