NTSC's weakness is well known. Colour is carried as the phase of a subcarrier, so any phase error anywhere in the transmission path rotates the hue. A long path, a badly aligned receiver, a bit of weather, and faces go green. That's why NTSC is also known as Never Twice the Same Colour.

PAL's answer was to flip the phase of one of the two colour components on every other line. If a phase error rotates the hue one way on line 1 and the opposite way on line 2, the eye averages the two lines together and the error cancels. That is the whole idea, and it works.

Early PAL sets did exactly that, relying on the viewer's eye to do the averaging. They were called PAL-S, S for simple. The averaging is not perfect, and with a large phase error you can see the alternate lines pulling in opposite directions as coarse horizontal banding. That banding is Hanover bars, named after where it was first demonstrated.

Later sets did the averaging properly in hardware with a glass delay line. It stores one line as an ultrasonic wave travelling through a block of glass, roughly 64 microseconds of delay, and the decoder averages the current line against the stored one electronically instead of hoping the viewer does it. That is PAL-D, and it removes the bars.

Which is why Hanover bars are a fault of the fix rather than a fault of the system. You only get them because PAL is doing its phase cancellation trick, and you only see them when the averaging is either absent or mistimed.

Mistimed is the interesting case. If the delay line is not exactly one line long, the decoder averages the current line against a slightly shifted version of the previous one, and the cancellation goes partial. You get the bars back, at a strength that depends on how far out the timing is. A delay line drifting with temperature or age does exactly this.

Video shows a delay line going gradually out of alignment, so the bars fade in as the timing error grows and then fade out again as it comes back.

The app has settings that let you exaggerate these settings.

If you look at the video, these bars are most apparent in the blue sky.



Hanover bars, the PAL fault that only exists because PAL fixed NTSC's fault