"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.

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.

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



...... Phil