[mythtv-users] coax vs. composite

[Brian Wood]

On May 15, 2006, at 12:03 PM, John Brooks wrote:

> Hey, all. I have a Hauppauge PVR-250, and I was looking into the  
> possibility of increasing my video quality; from what I've learned  
> so far, the ladder appears to be as follows: Coax < Composite < S- 
> Video < Component video.
>
> Now, my STB doesn't possess an S-Video output (sadly), so I'm down  
> to composite and coax. I'm currently using coax, and that's served  
> me all right, but I know that there must be some signal degradation  
> since all the audio and video data is squeezed down that one pipe.  
> Now, composite video, on the other hand, at least separate the  
> video out from the audio channels. It would seem to me that this  
> would be a better way to go. This leads me to the following  
> question: does anyone know the respective qualities I'm likely to  
> get out of coax and composite? In other words, what are the upper  
> bounds of quality I'm likely to see for each- MPEG-2 PS? DVD- 
> Special 2?
>
> I've looked around on ivtv for this sort of information, but maybe  
> my search fu is not up to snuff.

When you say "coax" I assume you are referring to an RF signal with a  
TV signal modulated onto it. I mention this because "composite" video  
is normally carried on a co-axial cable.

With absolutely top-notch modulation and demodulation it is possible  
to get very good quality from an RF modulated signal, unfortunately  
equipment of that grade costs in the tens of thousands of US dollars,  
and is not normally found in consumer equipment (for obvious reasons).

"Composite" refers to a "baseband" or "video" signal of 1 volt peak- 
to-peak (nominally) running on a coax cable. With NTSC the color is  
carried on a subcarrier of 3.579545 Mhz. and is "interleaved" into  
the horizontal sync frequency. In a signal which has been transmitted  
through an analog TV system (VSB or vestigial sideband) lumunance  
components are limited to about 4.2 Mhz., because the sound is  
carried 4.5 Mhz. above the visual carrier, and the filters used to  
keep video out of the sound channel start to cut out at about 4.2  
Mhz. With a composite video signal that has *not* been through a TV  
transmitter luminance components can go as high as 6 Mhz.

There are other distortions induced by the RF modulation process, the  
major ones being luminance to chrominance delay inequality ("group  
delay") and phasing problems.

Suffice it to say that the process of putting a TV signal through an  
analog RF modulation/demodulation process really screws up the signal.

S-Video is a system which carries the luminance and chrominance  
signals separately, avoiding the distortions possible with  
interleaving etc.

Normally "composite" video is of a better quality than that derived  
from an RF signal, but if the composite signal has been derived from  
demodulating an RF signal (normal for a cable signal from a TV tuner  
card or an STB) then it just depends on which demodulator is more  
crappy, the one in the STB or the one in your capture card. Since the  
signal has been through an analog TV system in any case, even the S- 
Video signal, normally considered "better", will only be so if the  
circuit which separates the luminance from the chrominance is better  
than the one in your TV set, because it has to get separated  
*someplace*.

So, while you will hear it said that S-Video is better than  
Composite, which is better than RF, if you are talking about a signal  
derived from a cable system this is not necessarily true. If your TV  
set has a better demodulator than your STB, for example, you will  
actually get a better picture by feeding the RF signal to the TV than  
you will by taking the composite signal from the STB, it all depends  
on which gear is the more crappy (because it is *all* crappy or we  
couldn't afford it).

Often there is a convenience factor as well, it is far easier to  
change the channel on the RF tuner of a capture card than to use an  
IR blaster to change the channel on an STB, as an example, and some  
folks will put up with an inferior picture in order to make things  
more convenient.

There are so many variables that it is impossible to state with  
certainty which setup will be "better", and the only real solution is  
to try it different ways and see what make you the happiest.

Even this summary is extremely brief, and leaves out most of the  
subject matter. You probably don't really want to know all the  
details, just use whatever looks good to you :-)