Positive Video Modulation

[its ur aerial]

Does any one, know the reason behind why the old VHF 405 system used positive going Modulation for the Video signal, with its inherent problems from ignition interference.

Ken G6HZG VMARS

One reason was reduced interference to the syncs, another is it was hard to make the bottom end linear, didn't matter to the syncs.

[Radio Wrangler]

I think it was just the obvious way to do it, plus detector threshold effects would distort the syncs.

The switch to negative modulation was impelled by several advantages:

Signal to noise is better at black levels than white.... black snow on a white background isn't noticed, white snow on a black background is dead obvious.

Better detectors meant that low level signals were detected linearly and changes in shades close to peak white aren't easily seen, changes in shadow are more visible. Human sight is engineered for spotting prey and predators in shadows.

Even though detectors were easiy improved, the compression of transmitters running close to full power was still real. so negative modulation put the picture info in the linear region.

Finally, negative modulation gives the highest S/N ratio and therefore the greatest accuracy and security to the syncs. It's no good if you know exactly what the intensity/colour is if you don't know exactly where in the picture it goes.

David

[its ur aerial]

Thanks to both of you for the informed replies, the mention of the Sync pulse problem, jogged my memory, and I now vaguely recall , being told this at College back in the very early 70`s.
Many thanks again.

Ken, G6HZG.

[Graham G3ZVT]

They used to say that black "holes" in the picture were less intrusive than white spots, but the move from band I/III to IV/V made any type of impulse interference much less of an issue.