[Don Pearce]

On Thu, 20 Sep 2007 06:24:26 -0700, Richard Fry
wrote:

Other than brief portions of the evening news the question arises:
What's the whole point in restoration these days? If OTA-TV really goes
digital some distant day we might not even bother buying a new set.
_________

Accurate colour reproduction requires accurate transmission of the
luminance (brightness) value of each colour, which is set by a
specific, DC-coupled voltage. If the video signal was transmitted
using AC coupling, then luminance values would be a function of the
average voltage of the video waveform.

For that matter accurate monochome reproduction also requires DC
coupling, but it is not as objectionable if not used (cheap TV set,
etc).

Also - the purpose of inverting video for transmission is to transmit
the peaks of sync pulses at +100% modulation, which allows TV
receivers to show the most stable picture in the presence of noise
(eg, fringes of the coverage area of the TV station).

RF
RCA Broadcast Field Engineer, retired

You don't need to use DC coupling as long as your sync separator can
identify the back porch for you. Having done that you have a piece of
known black to which you can apply a gated voltage clamp. From that
point onwards you need DC coupling - but it is not far from there to
the tube.

d

--
Pearce Consulting
http://www.pearce.uk.com