US power system

Presumably at the time there *was* no national grid.

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.

I wasn't sure when we got it.

Now that's what I call accurate.

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.

G.Harman

Right. And you're saying the studios couldn't be run at 525/60 because of strobing from the lights?

I'm not well up on the types of lighting used on real film.

BBC studios at the time in question were virtually pure tungsten. Only common discharge type was the CSI follow spot.

Strobing ? a 10Hz flicker yes.

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.

I'm not a physician. Could the brain sense flicker at less than 0.1Hz?

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.

It also depends to some extend on the persistence of the phosphors used in the tubes - with basic white being worse than decent ones in this respect.

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).

NTSC is that accurate. It's the mains that isn't.

And, yes, the mains is accurate over 24 hours.

Andy

So how did TVs work properly when they used the mains as a guide to frequency?

The same way as they did after they got rid of the mains lock.

The video signals were locked to the mains reference in the studio and the receiver locked its internal signals to the broadcast reference.

I'm not that old.

"Current" ones (ie solid state) lock to the flyback pulses in the transmission, and don't take any notice of mains.

Andy