[QuadList] Helicals, Quads

[Dennis Degan]

		On Feb 8, 2010, at 8:55 PM, <bill.spencer at northstarstudios.tv> wrote:

 > The fuss over blanking had to do with the amount of power the 
transmitter put out while in black as the minimum was 12.5% carrier.  
In the US we used negative modulation for video so black was minimum 
carrier.

		I say:

	Actually, with negative modulation, the highest modulation component 
would come from the lowest level baseband video signal.  The lowest 
baseband level is always the tip of sync, which produces the highest RF 
AM modulation.  Black level produces slightly lower modulation than the 
tip of sync, but still much more modulation than white level.  Peak 
white level video produces the lowest level of modulation with the 
minimum (at maximum white) being 12.5%.
	Additional information:  The reason 12.5% was chosen as a standard 
minimum modulation level was to prevent zero carrier.  If modulation 
peaks were allowed to reach zero carrier, the result would be buzz in 
the audio due to the complete loss of video carrier.  Nearly all TVs 
use the video carrier to 'locate' the audio carrier and demodulate it.  
Without the video RF, even for the moments of peak white, there's no 
reference and the TV would buzz terribly.  Using 12.5% as a minimum 
level allows a bit of a cushion to ensure that the video carrier is 
never cut off.
	This is also why negative modulation is used in the first place.  If 
positive modulation was used, the tip of sync would cause buzzing 
because sync occupies about 40% of the total power produced by the 
modulated video RF.  If that percentage of the total RF power was 
transmitted with the carrier close to cutoff (even if it was no less 
than 12.5%), every TV set would have large amounts of buzz in the 
audio.  Even though it uses more power to do so, negative modulation 
was what allowed the audio transmission quality to be acceptable.

			Dennis Degan, Video Editor-Consultant-Knowledge Bank
	  					NBC Today Show, New York