On Sun, 20 Apr 2014 13:15:26 +0100, tony sayer
wrote:


I see. So the locking in the studio was so the cameras didn't show flicker on
lighting? Or couldn't they just use DC lighting?

You're following a red herring with that line of thought. The reason
for locking to a wandering 50 or 60 Hz grid supply reference was to
stop the hum bars moving which rendered them invisible unless you
watched the TV screen at the end of the program day to detect the
vertical change of shading in the mid grey tone of the raster scan.

If the TV broadcasters hadn't locked the camera scan rates to the
mains frequency, the resulting moving hum bars would have detracted
noticably from the picture content.

By the time colour transmissions were introduced the LOPT supply
smoothing had significantly improved so that the moving hum bars could
only be observed at the end of the programming day, the change in
level due to less than perfect smoothing being too slight to be
observable during reception of program content.

LOPT supply smoothing?. What do you mean by that Johny?...

Line OutPut Transformer - used to generate the 25 or so KV from an
overwind on said transformer using the horizontal flyback pulses to
feed the eht rectifier valve or HV silicon diode / diode string (half
wave rectification using the picture tube itself as the EHT smoothing
capacitor).

If the LOPT driver circuit is powered from an insufficiently smoothed
supply, the EHT will have this ripple superimposed upon it and show up
as a variation in picture brightness ('Hum Bars').

Of course, you will see similar effects if the supply to the video
amplifier channels is similarly afflicted. Indeed, it's likely to be
the sum effect of these two sources of ripple interference by a badly
filtered common source supply rail.

The early colour TV sets in the UK were hybrid valve/transistor
designs where the penultimate LOPT driver valve was the last to be
usurped by a very fast high voltage power transistor (the final
irreplacable valve being the picture tube itself).

The low voltage psu for the transistor stages could easily be
designed to eliminate mains ripple, leaving just the LOPT valve driver
HT perhaps still relying on an older style less than perfect filtered
supply.

I suppose the later hybrid designs could have made good use of high
voltage power transistor to regulate the 300 odd volt HT supply and
eliminate even this residual source of 'hum' (or perhaps, more likely
now I think about it, an SMPSU was used where the only high voltage
fast semiconductors required would be the fast switching rectifier
diodes).
--
Regards, J B Good