[opendtv] Re: 405 line pictures

  • From: flyback1 <flyback1@xxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Fri, 15 Dec 2006 15:13:46 -0500

Allen Le Roy Limberg wrote:

I suppose transmitters may have been purposely adjusted not to
maintain correct black level to avoid murky pictures on cheap receivers.

I visited a B&W TV transmitter site near Orlando in the mid 1960s and saw first hand how non-linear the modulators
in those old behemoths can be.

To transmit a signal which, when received on a consumer TV, would lock the sweep circuits properly and have approximately 714 mV of video and 286 mV of sync, it was necesary to 'pre-distort' the composite signal feeding the exciter to levels of 550 mV of sync and 450 mV of video. The engineer on duty showed me the waveform monitors displaying the signal going in
and then coming back from the transmitter diode demod port.

It's amazing the pictures looked as good as they did.

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