I see tadpoles!

One interesting side-effect of reducing the number of bits to carry the video signal is that the impact of errors is magnified – in essence each transmitted bit of information assumes greater significance. Any error between the transmitter and receiver means that the predicted values at each end will be different. Since predicted values are used several times over to re-create the output samples, then the effect of a single error propagates to affect many samples! Indeed, unless the predictor is carefully designed, it is possible to build a system that behaves perfectly until the first error and is then completely unstable until the entire system is reset!

With the predictor values used in the 68PAL trial, the effect of each error was magnified into a roughly triangular shape with the appearance of a black point to one side gradually becoming lighter, horizontally and vertically, as the effect of the error worked its way through the system. System faults also generated the same effect. They were quickly christened ‘tadpoles’ for what should be obvious reasons and many’s the happy hour we spent trying to pinpoint the source of that day’s shoal (if that’s the right collective noun) of tadpoles!