On Fri, 19 Jul 2002, Alan B. Pearce wrote:

>>There certainly are video transformers.  We use them occasionally
>>at work when we need to DC isolate a video signal.  It is just a
>>toroidial transformer carefully wound to give a good frequency
>>responce from about 30Hz to 5MHz.  The DC component is lost but I
>>have never noticed that to make a visable difference in the picture.
>>I think modern monitors di their own level shifting.  I will try
>>to find out who makes them tomorrow.
>
>OK it may be that the expectation is that the monitor will do the DC restore
>function anyway, and hence there is no attempt to do it with the
>transformer.
>
>IIRC the DC restoration is done by clamping the signal to ground during the
>sync pulse, so the worst case is that the transformer has to maintain a
>reasonably constant level signal for one line period. My TV & video theory
>is getting pretty rusty, but that is how I remember it.

You are right, hovewer all video and monitor equipment currently in use is
internally ac coupled, even though the lines are resistively matched at 75
ohms. So each and every unit restores the dc level using more or less
elaborate clamping.

Peter

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