Gee Mike, there's a question that could have a hundred different explanations, and they would all be right. It is the same as cable TV and used to be used to discern between MATV (Master Antenna) or perhaps SATV (Satellite Antenna) When I first dealt with cable, I think USA only used mid-band channels 14-22 in between channel 6 & 7, where broadcast TV has the huge gap for FM broadcast and other uses. Then they added Super-Band and Hyper-Band channels above 13. Then HRC and IRC (Harmonically and Incrementally Related Carrier) tuning offsets. Scrambling used to just be adding a sine wave or they notched out stations you didn't pay for with in-line filters. Then fancier Oak Orion, SA BMac, or Zenith SSAVI (Suppressed Sync and Active Video Inversion) came along. Then 2 way and digital cable hit the scene. Now you don't even know if you're watching the channel you think you're watching. Just because it says channel 2, doesn't mean that it is getting to you using the spectrum between 54-60 MHz.
The wierdest thing I noticed with cable was the pre-ghosting. The multpath distortion is backwards from the norm. You get the direct faint ghost, then the received and amplified and delayed signal from the cable company.
From Hal's and Les's posts, I can see how much "cable" has advanced since my UHF broadcasting days. There was a time that cable companies would just amplify or translate a station's signal to a new channel, and push it down a cable. Then they added 1 "Superstation" from Atlanta Georgia. Then other superstations and C-SPAN popped up. C-SPAN is the broadcast equivalent of water torture. Joe
Yup. Cable TV is common usage for CATV or Community Antenna Television. CATV was just MATV (Master Antenna Television) on a grander scale. A MATV system would serve a building or group of buildings under common ownership from a single antenna or antenna location. A CATV system took this a step further and served whole towns. It had its roots back in the 50's in areas that were blocked by hills or mountains and couldn't receive off-air reception. Somebody had the idea to put an antenna on top of the mountain and run a cable down to the town and then to each house. This worked better than a repeater and didn't require an FCC license.
Then with the advent of satellite delivered programing that debuted HBO and Showtime and a bit later MTV and Playboy in the 80's CATV became the "golden egg" for a lot of entrepreneurs who saw the opportunity to make a lot of money. And make money they did because in about 10 years all these small operators were bought up by the few MSO's (multiple system operators) that we have today.
Technologically CATV went from at most 12 channels (or whatever was available off-air)to 35 and now to hundreds. As Joe mentioned, when the channel capacity got up there so did the frequency spectrum that it required. Even though this frequency spectrum was "sealed" within the cables of the system leakage does occur. Because some of these same frequencies are used over the air for such things as aircraft navigation cable engineers were forced to deviate from the standard channel assignments so that carriers on the cable were not near these critical terrestial frequencies. HRC and IRC offsets allowed amplifiers to operate with less harmonic distortion.
One of the biggest advances was when fiber replaced coaxial trunks. Trunk lines can be thought of as like primaries that feed step down transformers that in turn serve neighborhoods. The reason for this was not a higher quality signal but rather it allowed set top boxes to to send a signal back to the head end for pay-per view. This was never really successful with the old coaxial trunks because no matter how well you built the plant, sooner or later things would loosen up and ingress from RF sources outside the cable system stomped all over your return signal.
So, now that cable operators had the higher bandwidth of fiber they began to think of other revenue producing avenues besides more channels of drivel. Digital cable along with broadband internet and telephone is what they have come up with so far. But keep in mind, as far as the signal on the fiber and coax, it's still good old analog.
Just like satellite or even DSL on a phone line. The information is "digital" that is compressed and modulated on an RF carrier in order to get from it's source to the customer. Once it's demodulated it's 1's and 0's again.
Note too that all cable systems I am aware of only carry a portion of their programming digitally. Cablevision here, for instance starts the digital channels at 101. That means there are 99 or 100 channels that still use the original format and are prone to the same problems such as interference, ghosting and low signal levels as always.
Hooking the cable feed directly to our cable ready TVs or VCRs, will certainly reveal normal signals, displaced or not. But broadcast channels make very inefficient use of spectrum. I used to tune our 5 internal cavity klystrons using a tracking generator and a spectrum analyzer. This is how you tweak the bandwith Vs power compromise. It's the only time the output is anywhere close to filling out a given channel's spectrum. The rest of the time, there is a huge spike at sync and a minor one at chroma subcarrier.
I believe that digital cable design geeks, much smarter than me, have found ways to even out the spectrum. I have not been able to see this first-hand but would expect to see a much flatter display on a spectrum analyzer tuned to the "digital cable" channels. Does anyone have any scope pictures of this or a quick link??? Joe
Joe, Hal, Thanks a million for your great explanations!. I've designed and installed a few larger MATV systems in hotels and motel complexes here and I have to say that I really enjoyed working on them. I just wasn't really that sure what a CATV installation was, or in fact, how it compared to the MATV system. It shows in your posts though, just how far New Zealand is behind the US, with regard to TV technology, satellite Pay-TV is a relatively new thing here and the majority of people here still receive their TV signals through terrestrial VHF and UHF aerials. CATV will be a long time off, if we ever get it at all.
Hal, I can't wait for that day to come. No more TVI complaints from the neighbours about my Ham gear. You think Redsy has problems with one neighbour, try complaints from 5 different neighbours. The RSM (Radio Spectrum Management) came here and made sure that I was on frequency. They then told all the complainants that there was no problem and then left town, that made things even worse, people don't like being told they are wrong after filing a complaint. I don't get on too well with my neighbours, apart from the guy over the fence, who is also a Ham. (They checked his radio gear as well BTW).
Of course you two could have clean TXs but a corroded nut & bolt on your tower or Mr. Complainer's gutters and downspout could be like a diode mixer spewing forth garbage. But I'm sure you've heard those stories before. Joe