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Posted by D Sherman [72.47.9.228] on Thursday, August 05, 2010 at 16:27:47 :
In Reply to: Re: O.T. Three-tooth phillips bits? posted by mannyc [68.165.89.2] on Thursday, August 05, 2010 at 11:00:18 :
The early "scrambled" cable pay channels were "scrambled" simply by stripping the sync signal out of the NTSC video, so that your TV wouldn't lock into it. The result, on the pron channels, was a tantalizing view of various fleshy bits swirling around on the screen. If the TV's horizontal and vertical oscillators were more stable than average and you kept one hand on the vertical sync and one on the horizontal sync adjustments, you could tweek them to get glimpses of actual pictures from time to time.
Soon the hobby magazines had schematics for "sync restorer" circuits (calling them "cable de-scramblers" resulted in trouble from the cable companies and the FCC) whose ostensible purpose was to "improve" the sync pulses on "weak" video signals. The whole thing was a bit like "medical" marijuana.
The circuit itself was actually pretty simple.
The cable companies also had a tricky way of tracking down people who were stealing/borrowing a cable signal from their neighbors or even off the pole. On the programming they sent down their cable, they'd shift the horizontal scan frequency slightly off from the NTSC standard of 15.75 kHz. Every TV has a horizontal output transformer, also known as the flyback transformer, that runs at the horizontal scan rate and generates both the horizontal sweep signal to the deflection yoke and the ~25,000 volt 2nd anode voltage for the CRT. That thing radiates a fair amount of electromagnetic radiation at the horizontal scan rate. By shifting the cable's horizontal scan frequency just slightly away from the broadcast standard, the cable companies could have a guy drive around the streets in a van equipped with an antenna that would pick up that nominal 15.75 kHz frequency, and if he found anybody with a TV that was radiating at the "cable" frequency rather than the "broadcast" frequency, but who wasn't a cable customer, they'll start looking for his "pirate" cable running to the neighbor's house or up the pole.
There was a also an an.alogous method for surreptitiously monitoring what channels people were watching, by looking for spurious radiation from the local oscillator in their TVs. When you change channels on a TV, you change the tuning coils in the RF front end, and you change the frequency of the L.O., which downconverts the RF to the IF frequency. The L.O., like the flyback transformer, emits some spurious radiation, but the L.O. frequency tells the snooper what station the TV is tuned to.
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