Digital cable: better picture with s-video or coax? (1 Viewer)

Fred G · Oct 26, 2003

I recently got digital cable and upon receiving the box I noticed it had a digital audio out, as well as an S-Video out.

Now the analogue digital audio out has obvious benefits, as the cable box outputs a signal that is split into 5 channels.

However I don't see how the S-Video can give you any benefit when the original source (the cable from the cable company) is a coax cable. Wouldn't a coax-in (into the cable box from the wall) and a coax-out (from the cable box to the TV) be the "cleanest" connection? I would think so, since as with anything, you loose quality the more you "process" the signal.

It's kind of like saving a .jpg on your computer. Every time you save that picture, it gets compressed and the picture degrades a bit.

However after calling the cable company, one of the tech representatives said that the S-Video will give me a better picture because of the way the signal is processed. But this just doesn't make any sense to me. "You can’t get a better picture than the original source," I thought. Wouldn't that be violating some law of of physics?

After doing some unscientific testing, I've found that my untrained eye can't tell the difference between the coax signal and the s-video signal. However in the back of my mind I'd always like to think I'm not "loosing" picture quality by using one over the other.

So does anyone know the truth behind these digital cable boxes? Can S-Video give you a better picture even if the original source itself is coax?

John Garcia · Oct 27, 2003

It doesn't matter how the signal gets to the box, since what you see on the TV is being decoded at the box, so it is possible to get a better picture with S-video. Without changing settings on the TV, you will not see a difference between the two. Using a calibration disc such as Digital Video Essentials or Avia will likely help.

ChrisLazarko · Oct 27, 2003

Well I don't own a digital cable box but... with my DVD play I used to use the composite out (yellow line) and when I upgraded to Component (Red,Green,Blue) I noticed a big difference on my TV. The colors didn't bleed anymore, the signals were a bit sharper as well.

I did have S-Video for a while and I noticed a little imrpovement as colors wouldn't bleed with black like on a normal coax cable.. so I would say go for it, you can find S-Video cables for a good price usually.

Lew Crippen · Oct 27, 2003

However I don't see how the S-Video can give you any benefit when the original source (the cable from the cable company) is a coax cable. Wouldn't a coax-in (into the cable box from the wall) and a coax-out (from the cable box to the TV) be the "cleanest" connection? I would think so, since as with anything, you loose quality the more you "process" the signal.

If the same signal were being sent out the S-Video output and the coax output, and your TV handled the video from both signals in the same way, there probably would be no discernable difference. But this is almost for sure, not the case.

I do not know your cable box, nor how it decodes and passes on the signals, but I suspect that what is happening is very different.

To begin, S-Video separates the video signal into two components: chrominance and luminance (color and light). This means that your TV will need to have electronics that can understand how to processes the S-Video signal and turn it into a picture. Not all TV displays will do this. This means that if S-Video is sent out a coax output, some TVs would not have the electronics to be able to process the information and would not therefore be able to display a picture.

It follows then, that all TV coax expect video information to be input in the same way. And this most basic method is composite (which is also the one used by the composite (yellow RCA jack) input. This then means that the coax output must be this basic composite.

Just as you intuitively feel that the audio will be better if taken from the digital output instead of the analogue or coax output, because the signal is broken into six channels (at least for DD shows), the video with S-Video is superior as it is broken into two components.

You ought to expect something like a 20% improvement. Your ability to see this improvement depends on the actual display, your set’s electronics and your set’s calibration.

Click to expand...

Fred G · Oct 27, 2003

Thanks for the responses.

John, I don't know why I didn't think of that before! It's a digital signal so it shouldn't even matter how the original source was delivered.

Somehow this slipped my mind.

I guess I'm surprised at how good the coax signal is. I can definitely tell the difference between coax and composite. Composite is horrible in regards to lack of sharpness and increased color bleeding. However coax seems pretty much on par with S-video, which I found somewhat strange as I always thought coax was the lowest quality of analogue signals (a frame of mind stemming from using "rf adapters" for various types of video sources). I will continue to use S-Video since it provides convenient audio/video switching via my receiver.

Allan Jayne · Oct 29, 2003

I cannot explain why the channel 3/4 RF coax feed and the S-video feed from the cable box are both better than the composite video feed. You might try a different composite cable into the TV.

For digital cable, the coax feed from the wall is digital and not the same as the coax feed from the box to the TV which is analog.

For the coax feed to the TV, the video comes into the cable box, the proper channel is selected, if digital it is converted from component video into S-video into composite video and is remodulated onto channel 3 or 4 then sent to the TV where it goes through the TV tuner, comb filter and color decoder. If the channel is analog the box remodulates it onto channel 3 or 4 and then sends it to the TV where it goes through the TV tuner first.

For the composite feed to the TV, the video comes into the cable box, the proper channel is selected, if digital it converted from component video into S-video into composite and sent to the TV where it goes through the TV comb filter and color decoder. If the channel is analog it goes directly to the TV where it goes through the comb filter first.

For the S-video feed to the TV, the video comes into the cable box, the proper channel is selected, if digital it is converted from component video into S-video and sent to the TV where it skips the comb filter and goes to the color decoder. If analog the cable box comb filters it (usually mediocre-ly) and sends the resulting S-video to the TV and the color decoder gets it next.

I can't find any part of the video signal path used by composite and not by the other two, where the degradation might occur.

Because of the signal paths as described, sometimes analog channels will be better via composite as opposed to component or S-video.

Video hints:
http://members.aol.com/ajaynejr/video.htm

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Digital cable: better picture with s-video or coax? (1 Viewer)

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Fred G

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John Garcia

ChrisLazarko

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Lew Crippen

Fred G

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Allan Jayne

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