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Discussion Starter · #1 ·
Hi all,


Got a problem here with my analog cable tv feed. A few months ago my picture suddenly became "grainy". That's the best way I can describe it. Prior to that the picture was near studio quality. In fact I've been burning all kinds of video discs (S)VCDs, hybride DVDs, DVDs and Mini-DVd, and all with outstanding quaility using my cable tv feed as the source. The change was sudden and severe.


I've replaced the entire feed, (frome the input to my house) and even set up a brand new distribution system. All new cable (RG-6 and good stuff) and all connections and amps, splitters and such are top grade. While the "grain" has reduced, it's still there.


I'd like to test the signal strength at various points .. The system is a "home run" with a multiple output amp feeding (at this point in time) a few devices directly. The grain is not objectionable for causual viewing, however it is pretty bad when I try to use this source for capturing. (I've been doing this for a few years with perfect results)


So my first question is, "Does anyone know where I can purchase or rent a signal strength meter that will show me the db at low middle and high end frequencies for the typicial cable TV feed. I'm not dealing with digital here, all analog and believe the high end would not be much over 900 mHz or so.


I had a cable guy come out to the house to just check the signal at my house's feed. He told me it was as good as it's going to get. And said he measured between 7 and 9 db from one end of the spectrum to the other.


Isn't that a little low? I designed my system thinking I was dealing with a signal strength of around 12 db. (the FCC wants no more than 15 at the TV.) It almost looks like my amplifer is actually amplifiing the noise derived from my low signal? to the house. The entire neighborhood is fibre optic and the main breakout point from fibre to coax just happens to be buried in my front yard. So there's only about 20 or so meters of actual coax to get to my house. Then about 10 meters more (all new) to my distribution system. The entire neighborhood has all utilities (including the cable TV) buried.


It's hard to troubleshoot this by taking a portable TV to different points and viewing the picture with that small screen portable. The problem when viewing that way is subtle. It really shows up on the larger screens and as I mentioned is very bad when used as source video for authoring. I was going to post a sample of the before and after here, but can't figure out how to imbed it in the text of this message.


So I'm looking to find a device that will measure the actual signal strength at various points in my system. All the ones I've seen are several hundred bucks. I thought of building a small tuned circuit for about 100 to 1000 mHz and then place a 75 ohm load at the test point and using an rf probe, measure the signal. Geeshhh, there's gotta be a better way ??


Ahah .. I just figured out how to imbed an example. The frame on the left was from a video made several months ago (a black screen frame). The frame on the right was the same movie (rerun) and same frame but captured recently. Actually those are highly magnified small cuts from the full screen. While you can barely see anything in the left frame, the right one shows a moire like effect. When viewed at normal size the moire looks more like a fine grain. This is driving all my encoders bananas and producing poor quality mpegs. The problem is easiest to see with a full black frame.

http://www.pcphotovideo.com/Downloads/example.jpg
 

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The easiest way to calculate it is to do the math. If you have 7-9DbMV, coming into the house then all you have to do is figure how many splitters you have and what their values are, they usually say on them, figure out the gain of any amps that you may have and what the signal is coming to them and what it is leaving the amp and subtract the signal from any splitters after that.

Keep in mind that signal is not necessarly an important factor, Carrier to noise is more important, if you have 7-9db of signal but a C/N of 20 dB than pictures are going to suck, and amplifiers are going to increase the signal but they will increase the noise more so you will then have a signal 0f say 25 db but a C/N of 15 dB. Also the Inverse is true, you can have a Signal of -5dB but a C/N of 40 dB and the picture will rock. Think of it this way: garbage in, garbage out. Your problem may not be yours, it may be your cable company and there is nothing you can do to improve it. Except nag them to fix it.

Natas
 

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Discussion Starter · #3 ·
Thanks Natas,


When I yanked out all the old wiring and set forth to design my own system, the first thing I did was to call the cable company and ask what the signal was at the line coming into the house. They told me between 12 and 14 dbmv. And that's what I designed the system for, figuring on losses for cable length, gains for amps, losses for filters splitters etc.


But still the problem existed. I think you may have hit upon something with the signal to noise ratio. When they actually came out and measured my system at the street to house point they said the signal was 7 to 9 dbmv. Which was several db below what I planned for.


I could be amplifing the noise.


I'll try dropping down the amp somewhat. But I'm afraid the problem is most likely some noise on the line to the house .. which means they will have to dig up my front yard and part of my driveway. I don't think they will do that without a fight. Sigh .. Thanks again. I'll work on the "noise" aspect of things and see what it looks like when I feed one TV directly from the street line with all amps and such removed. If the noise is still there, then you've hit upon the problem. The feed to me most likely has a "low" signal to noise ratio .. or too much noise to begin with.
 

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I'm at the beginning of this journey. My picture just recently went grainy. I have a ChannelPlus distribution system and I've pulled it out of the loop and tested with a direct run to my 60 inch projection set. No improvement.


I guess I'll make that call to the cable company...
 

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Rich,

THere should be no need to dig up the yard and the driveway if it is the cable company, Your line is going to introduce no to very little noise itself, what more than likely has happened is that somewhere before your house there is a signal problem that hasn't or can't be easily repaired so they band-aid it with an amplifier, then the C/N issue comes into effect but they hope no one will notice and not be doing any distribution like you are.

Amplifiers amplify signal but they also amplify and create noise. If there is a new neighborhood in your area, the cable company may have had to redesign that area to fit the new neghbohood into the plant, which may have meant another amp to get the signal to your house and it could be one amp too many.

Call the cable company up and ask them to come by and check the C/N at your house, I personaly like to see 42db of C/N at 10 D/B of signal, it's hard to do that but it can be done. this is on analog though. on Digital your Signal to Noise will be higher so if they tell you some outrageous number like 80 than you know they are talking about the digital signal and that's not going to help you at all. I don't have any expierence with digital so I can't help you there. Stay on them and they may remedy the situation or you may get a tech that wants to get it remedied.

One more thing, lowering the output of the amp may not help because you will basically be bringing the signal closer to the noise floor and reducing the C/N.

Natas
 
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