The sync for analogue TV signals is derived from a crystal, so should be accurate to within at worst one part per million. The crystal frequency in the UK for PAL is specified as 4,433,618.75 Hz.
London Underground still have a power station at Greenwich. It is just used as a standby now though in times past it was a conventional power station,Gas turbines once installed were used in the morning and evening peaks as they could be brought online quickly . It's these that are retained to be brought into operation quickly if a power failure from the grid means that trains come to a stop in. tunnels . Such an evacuation has to be controlled as it hasn't the capacity to supply enough power to keep a full service running. It might yet be used in another role. Some details here.
So why don't they flicker when the studio is running at 625/50 - since it's not locked to the mains frequency? The eye would see a flicker at much lower speeds than 10 a second.
It's one of these things which seem to vary from person to person. Many complain about flicker from some 50 Hz fluorescent lights saying it gives them a headache. I can't say I've ever noticed it.
My wife complained about them even in the US, whereas they have never bothered me anywhere. Except in China, where the hotel lights, all of them, flickered at about 1.5 Hz.
No one can see 100Hz flicker - the limit is below 75Hz. Animals with smaller brains can see higher frequencies, and flies can see 1000Hz flicker.
With discharge tubes, a problem can be that they are partially rectifying, resulting in 50Hz (or 60Hz) flicker, and most people can see that. High intensity discharge lighting in shops was often run on DC, to prevent this (prior to electronic ballasts). There's a DC switching unit which alternates the DC flow every switch-on, so you still get most of the expected lamp life. This doesn't work with fluorescent tubes, because the mercury tends to migrate to one end after some hours, leaving the other end of the tube glowing only a very dim pink (argon gas).
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