"Roy Lewallen" wrote
Surely as a former broadcast engineer you're acquainted with a circuit
called a "DC restorer". This is a circuit which is always present in a TV
receiver. In its simplest form, it's just a diode clamping circuit,
although I've made very good ones with an FET switch and hold capacitor.
What it does is to set the sync pulse tip to a fixed DC value, which then
causes the rest of the TV waveform to be at a fixed DC value. This is how
the DC information is "transmitted". The actual TV waveform is AC coupled,
as it must be, and its DC values are established in the receiver by the DC
restorer....
_________________________

I must respectfully disagree with that, Sir. The baseband video signal is *
DC coupled * through analog TV transmitters . The peak power of a
transmitted TV RF waveform is a fixed value, at the power corresponding to
the licensed ERP of the station, occurring at the peak of sync pulses, and
independent of program video. Transmitted _average_ power is a function of
the video envelope.

Video is transmitted with negative polarity; 75% modulation when video is
black, and 12-1/2% modulation when it is white. The video waveform AS
TRANSMITTED can contain a steady state (DC) value throughout the video field
for any amplitude at or between those values (actually the color subcarrier
can exceed these for some conditions).

The circuits in a TV receiver cannot pass the DC component, thus the need
for a DC restorer following the video demodulator. But the point remains
that the transmitted TV waveform can, and often does contain a DC component,
and that this DC component is required for accurate reproduction of the
original video on the TV display.

RF

PS: I still AM a broadcast engineer.