atari2600land's Blog

The VCS does not put out a standard video signal. It's close, but a it's off enough to cause problems with things like VCRs.

Hmm... as supercat says, the VCS video signal is a little out of spec. I suspect the VCS is making the VCR's AGC circuit work a little harder (low signal levels, with low luma bias anyway).

 

Cable systems typically use the higher channel frequency blocks for digital channels. Making the TV tuner handle the higher channels is cheap, but it's expensive for the cableco (higher bandwidth amplifiers). The cablecos would rather stuff more digital channels (especially PPV which makes them money) into the same frequency space than use it for analog channels.

You should be able to swith the VCR to antenna or air mode vs. cable and the local uhf channel will be on the correct channel.

Hmm... as supercat says, the VCS video signal is a little out of spec. I suspect the VCS is making the VCR's AGC circuit work a little harder (low signal levels, with low luma bias anyway).

 

AGC is part of the problem, but also frame timing. Though VHS VCR's don't need or employ the same sort of timebase correction as uMatic (3/4") ones, they expect each frame to have two fields of 262.5 lines of 227.5 chroma clocks each. The VCS generally does not generate proper frame timing (though it could probably be programmed to do so it could be tricky if the start and end of horizontal sync pulse don't correspond with CPU cycle boundaries). VCR playback can have unstable warbly audio as a result.

 

BTW, it's interesting that Atari XOR'ed horizontal and vertical sync. While the vertical sync pulse should be 'subdivided' by horizontal pulses, the gaps should end when the normal horizontal sync would begin. Using an XOR gate throws that timing off; more accurate timing could have been obtained by having two spots in the scan line that turn off HSYNC, and have one of them gated by the VBLANK signal.