[mythtv-users] *huge* difference in picture quality..

Rudy Zijlstra mythtv at edsons.demon.nl
Wed May 26 16:24:28 EDT 2004


Craig Tinson wrote:

>
>
> I'm in the UK and I have a cable box in the bedroom.. I was running a 
> 10-12 meter coax from its secondary output to the tv card (aver media 
> tvstudio) on the backend server (hidden behind my tv in the front room)
>
> Out of curiosity I moved the backend server to the bedroom.. and 
> exchanged the coax to a 1 meter composite cable I had laying around.. 
> this plugged into the cable box via a scart connector (with 
> passthrough back to the portable tv in the bedroom)
>
> *wow*
>
First a question: What do  you mean by a cable box? Beside a description 
I would like the make and model.

The secondary output. A lot depends on what this is.

Option a) its a remodulated signal
Option b) its RF loop through.

Lets analyze option a)

This is usually in the neighbourhood of 300MHz carrier freqeuncy. By the 
way, Analog Cable signals range from 112MHz to 865MHz. Satellite is in 
GHz range.

See comments from maarten which i will quote:

maarten> The second and even more 
maarten> important reason is that the videosignal has to be modulated onto that HF 
maarten> signal (which is called the carrier).  And most all domestic modulators and 
maarten> de-modulators are of bad to mediocre quality.  So you you lose three times:
maarten> First you modulate a video signal, then you send it over coax, and you 
maarten> demodulate it again to get that "same" video signal you started out with.

Although I agree with the quality statement of the re-modulators (term usually used on the modulator in a home appliance like a VCR), I strongly disagree with the statement on the de-modulaters. If the de-modulators were of low quality you would *always* have lousy image quality.

Now for option b)

I suspect this to be your case. You are talking about an seconday box in the bedroom. This probably means you already have a pretty long cable length to that box. Which means the signal strenght at entrance to the box is already low. The loop through typically has a 4dB signal loss. Than another 12m cable with its assosiated signal loss and you were likely seeing low quality signal because of a bad signal (or rather: a bad Signal/Noise Ratio). 
When you decreased cable length and removed the cable box, you strongly improved the SNR (stronger signal), thus your suddenly way better quality image.

Cheers,

Rudy




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