[Tig] AM Video Transmitter Modulation (2)
Wed Oct 23 16:19:43 BST 2002
> A video modulator inverts the video signal so that maximum power output is
> during sync tip
> and minimum power output is during white periods. (White + I & Q )
> The AM video transmitter modulator clamps to tip of sync as a reference.
> If you have a scope graticle meant for transmitter modulation
> it is calibrated with the acceptable modulation limits
> White (+ I & Q) at 100 IRE % = 12.5% modulation
> The tip of sync at -40 IRE = 100 % modulation
> (IRE stands for "Institute of Radio Engineers" now known as IEEE)
> This means that as one approaches video signal content in any given line
> greater than 100 %
> the power output of the transmitter goes to less than 12.5%.
> Any content that is greater than 112.5% video (+I & Q) is 0 % modulation
> not overmodulation.
> Various forms of amplitude compression are also used to really prevent
> this from occurring.
> This results visually in a forced white clip effect during that time.
> And all of this applies to any type of modulator used in the US either for
> over the air or cable .
> Walt Rauffer
More information about the Tig