Thread: Zx81
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Old 11th Mar 2020, 1:22 pm   #42
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 9,291
Default Re: Zx81

The thing to realise about the mini transmitters (modulators) used in tune-in tech from this era (like the ZX81) is that the little signal generator inside is quite crude and simple. The transmitter / channel frequency is determined by the adjustment of a coil, nothing more, and as they warm up the frequency tends to drift a little.

TV tuners of the time were also quite prone to drift so they incorporated a feature called AFC (Automatic Frequency Control) which would automatically keep the TV's tuner centred on any signal it was tuned to, so, as the frequency of the incoming signal slowly drifted, the TV would self-retune to keep itself centred on the signal.

Modern TVs have incredibly stable tuners and they expect any incoming signal to which they are tuned to be the same, so they don't track drifting signals the way older TVs do.
They stay precisely tuned to whatever frequency you tune them to.

The upshot of all this is that if you tune a modern TV to the ZX81's output while it is cold you may have to retune the TV in little steps from time to time as the ZX81 warms up.

That's why an older TV with a simple analogue tuning knob on the front is the best type to use for the ZX81. Up to a point the AFC will try to maintain a lock on the signal, and then if it drifts out too far it's just a simple matter to reach out and retweak the tuning for best picture.

You could bypass all of this by converting the output to composite video , as you did on your Spectrum. Only you will know whether you feel confident enough to do that.
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