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DSS, Comb Filters, and Ultimate TV (1 Viewer)

Dustin B

Senior HTF Member
Mar 10, 2001
I recently did a little reading on comb filters which raised a question that the article I was reading didn't have the answer to. DVD stores the C and Y signals seperately, so if you use the svideo or component inputs a combfilter isn't used. On a laser disc however the C and Y signals are not stored seperately and you need to decide if the combfilter in your laser disc player or tv is better. So my question is with DSS (most specifically ExpressVu and StarChoice, that is if it differs at all between providers) are the Y and C signals stored seperately or are they compined. So in other words with the Svideo by pass the use of a compfilter or should I be using the composite connection so the 3D Y/C Digital Filter of my 43" Sony can do the seperation instead the likely inferior one in the DSS receiver.
For my second question, why can't a PVR record more than one show at one time with only one line hooked up from the satellite? ExpressVu recently anounced a PVR but it can't record a show while watching another show. This lead my to look up Ultimate TV where I found out it can only record two shows at a time and requires two lines to be run to it. From the commercials I thought it could record more than 2 shows at a time. To my mind it only has to convert one show to analog to be viewed (unless PIP functions are available or desired). The other shows just require the portion of the digital bit stream coming down to be stored on the drive. So why can't the machine simply store the digital info for multiple channels while decoding and displaying the one being watched? Or do the digital signals for each channel come on multiple over lapping carrier waves that require an expensive peice of hardware in the box to issolate one at a time. And is this why the second line is required, since if two overlapping signals are wanted at the same time and extracting one destroys the other you need the second line to get it.
I don't feel like rereading this again right now, so I hope it makes sense, if not I'll fix it later tonight.


Senior HTF Member
Aug 22, 2000
Real Name
A PVR has two parts: a tuner and a decoder.
The tuner part works like the tuner in your TV or on your radio. Each satellite channel is on a different frequency. During recording, the tuner is set to a frequency corresponding to the channel you want to record.
The decoder take the MPEG bitstream and converts it to an analog signal with audio and video. With a PVR, you are never really watching something live because the signal is buffered through the hard drive.
With this setup, you can watch and record a show at the same time or record a show and watch a previously recorded show.
Two tuner models need two connections to the sat because you can't split the signal like regular cable TV. To squeeze more channels on the satellite, odd channels have a left hand polarity and even channels have a right hand polarity. For example channel 100 may be at 12.1 Mhz right hand and channel 101 will be at 12.1 Mhz left hand. The doubles the number of channels that can be carried. Because of this, if you split the coax cable using a standard cable splitter between two tuners, one tuner gets odd channels and one tuner gets even channels.
Disclaimer: I may have over simplified this process especially with the polarity/Mhz stuff but I tried to explain the concept to the best of my knowledge.

Bob McElfresh

Senior HTF Member
May 22, 1999
I'll try the first question:
The MPEG2 encoding stores the Y & C seperated so using a SVideo or Component connection CAN give you a superior picture from your DSS box. (Your typical DSS box does NOT have a comb filter).
When I first got my DSS system, I hooked both SVideo and Composite up and compared the picture. To my shock, they were nearly identical. (This is NOT the case with my DVD player).
Here is the trick: your signal will only be as good as the source created it. In my case, I was comparing a local station via Cable, DSS with Composite, and DSS with SVideo. The local station (TV Movie) was not a high-quality source so I did not see much improvement.
I was told that using a premium movie channel like HBO/Showtime may have shown a difference.

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