In article ,
Phil Allison wrote:

"Geoffrey S. Mendelson"

The 50 and 60 fields per second (a field being half an interlaced
frame) were chosen not because they needed to be that fast (48 would
have done), but to eliminate interefence effects from electrical
lights.


** But the lights concerned were those being used to illuminate the TV
studio.

Studio luminaries are commonly filament lamps. To allow easy control of
level, and because of their continuous spectrum light output.

When frame rates are not locked to the AC supply frequency, faint
shadows can be seen moving up or down studio images on a monitor or
home TV set - due to the twice per cycle dip in brightness of
incandescent lamps.

In the UK TV hasn't been mains locked for about 40 years. I'd guess other
countries the same. The mains frequency varies too much for modern
requirements.

Other fixes include using lamps with sufficient thermal inertia or
groups of lamps on different phases to eliminate the light modulation.

Fluorescent types are used on location these days, but use high frequency
ballasts. HID types don't run at mains frequency either.

Only time I've seen a phased array used was for a boxing ring - before
high frequency ballasts became common.

--
*I pretend to work. - they pretend to pay me.

Dave Plowman London SW
To e-mail, change noise into sound.