Re: Digital Upgrade
- From: Terry Casey <k.type@xxxxxxxxxxxxxxx>
- Date: Thu, 21 Aug 2008 15:00:26 +0100
Java Jive wrote:
Ah! Now that takes me back ...
Some years ago I was asked to investigate an intermittent 'Network' fault on a CATV system.
Now, I was always suspicious of alleged 'Network' faults that only affected one customer - usually with good reason! (Network responsibility ended at the roadside cabinet.)
This was also the case here but, having tracked down the problem, I could fully understand why the service techs had given up!
The complaint was of intermittent patterning and channel changing but, of course, when I checked on the customer just after lunch, everything was perfect.
A little bit of history will help understanding of the rest of this post:
This particular CATV network used the Switched Star sysyem which HMG wanted to be used by all cable operators in the UK; the period of early licences was about 50% longer for Switched Star than for Broadband systems. Unfortunately, the technology was new and troublesome, which did little to promote CATV in the early years.
The trunk network of a Switched Star system is identical to a Broadband system - the difference is how the subscriber is connected.
With a broadband network, the entire RF spectrum is fed to a Set Top Box in the subs premises. With a Switched star Network, all the intelligence is in the street and only the channel requested by the customer is fed to the home on a single frequency (two, if a dedicated VCR feed is required.) Depending on vendor, frequencies of 40 & 56MHz or 64 & 72MHz were used. One advantage is that drop cable lengths can be much greater than where signals up to 750MHz are fed to the home (Cable attenuation more than trebles between 64 & 750MHz). One beauty of the system is that no scrambling is required - if the customer doesn't subscribe to a channel it is blocked at the switching point, thus there is no possibility of signal theft using a hacked STB.
The STB is a simple device comprising a simple upconvertor to UHF and electronics to transmit channel selection data back to the roadside cabinet (at between 6 & 11MHz, typically.)
Now back to the story:
I parked next to the cabinet and set up a Honda genny under my van, with a VHF TV inside and inserted a 20dB directional coupler into the sub's drop cable to monitor the feed directly. The sub watched tennis and I read a book, while keeping an eye on the TV via the rear view mirror - positioning the TV in the back of the van facing forward served the dual pupose of keeping the sun off the screen in fine weather and the rain off me when wet!
Time passed and the picture was perfect. Then suddenly, the tennis was reolaced by TV5. A few seconds later it was back to the tennis, then TV5 again. The picture quality was perfect, however, on both channels, yet when I 'phoned the customer, she said it was awful at her end of the same cable! My monitoring activities were obviouly a waste of time (though I had absolved the network of responsibility) so I packed everything up and went back to the customer, on the 10th floor of a tower block.
The picture was indeed awful with severe RF patterning - yet I knew that the subs equipment had already been eliminated - I was puzzled! On a whim, I called Customer Services and asked how many customers we had in the block. Only two, it turned out, so I asked for details of the other one and went off down to 3rd floor where I found a french speaking arab women watching a cookery programme on TV5 - and the picture there wasn't up to much, either!
Intrigued, I phoned the lady upstairs and switched of the STB on the 3rd floor. Instantly, the picture upstairs improved - so what the hell was happening! (I'm sure some of you Sherlocks out there will have sussed it already!)
In fact, I found the cause by accident. When I got out of the lift on the ground floor, I turned the wrong way and ended up at the rear entrance. I did a U-turn and came face to face with the problem. Our duct emerged from the ground next to the doorway and the drop cables went through the wall into an internal riser. There should have been a cover between the duct and the entry point but, in this case, there wasn't.
I found myself looking at a chop bone wedged in the top of the duct and two drop cables stripped bare for about 15 - 20 cm. The only thing that had defeated the rats was the copper plated steel inner conductor (1mm dia. in RG6 cable). The 2 bare wires were only about 10mm apart and, as the cables carried different RF signals but both at 64MHz, that was the cause of the problem. There was even enough coupling of control signals back to the switch to randomly allow each STB to change channels on both subs line cards at the same time!
I eased a little slack out of the duct and bent both cables away from each other as far as possible - which produced a vast improvement - and arranged for an install crew to do a re-pull the following day and protect the new cables properly. (The adjacent bin chute obviously provided the rats with plenty of food.)
The UK CATV system uses Harmonically Related Channels - all analogue carriers are an exact multiple of 8MHz and locked to a comb generator. The means that all distortion products fall precisely on the carriers and alows amplifiers to run 3dB higher than would otherwise be possible using Incrementally Related Channels (like the broadcast channels) - a very useful benefit!
It always struck me as odd that the synthesiser in each of the sub's line cards (there could be up to 64 in each switch) all had their own crystal reference oscillator. One master oscillator would surely have cheaper? In this case, the LF beat between the two carriers completely obscured any clues that might otherwise have been present.
Terry
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- References:
- Digital Upgrade
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- Re: Digital Upgrade
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