"Michael A. Terrell" wrote in message
m...

??? How can the reference signal correct a differential phase
error?
PLL circuits allowed the signals to remain in sync.

Things will remain in sync but that does NOT fix a differential
phase error in the transmission path. The VIR would adjust the
phase at a mid luminance point, but the differential error still
existed above and below that level.

Even when they built crystal controlled generators, the line
frequency remained close enough that it would take minutes or
hours to
roll though a frame. Also, by the '50s the power supplies were
better
filtered. The hum bars were faint, but visible on older TVs,
and one of
the first signs of trouble when they became more pronounced. A
lot of
US monochrome TVs were transformerless, and used a voltage
doubler in
the power supply. pairs of 300 uF 160 volt electrolytics, where
some
early TVs had 8 or 16 uF filtering.

Color TV changed the field rate to 59.94 Hz and a 120 Hz hum bar
would roll through the picture in under 15 seconds. Even
monochrome broadcasts adhered to the new frame rate.

David