Golborne Vintage Radio

Why +ve mod for 405 line ?

Lawrence.

This topic has been discussed elsewhere before but AFAIK there are 2 main reasons for using +ve modulation.

You can efficiently use the very non-linear part of the transmitter's characteristics. You amplify the syncs before the modulator and the nonlinearity of the TX puts them back to normal size, all the way down to 0% carrier. The other reason was that impulse interference would mimic the syncs. In the days before flywheel sync this mattered.

+ve modulation with PAL/NTSC colour subcarrier going below black level would have meant much less efficient transmitters. Not so critical for SECAM which was used with +ve mod.

Thanks for the reply Jeffrey, I figured sync would be better off sitting down the bottom but hadn't considered the other factor you've mentioned.

Lawrence.

Although it may be noted that  Carnt and Townsend came out quite strongly in favour of positive modulation for NTSC colour systems.  See Volume 1 page 429ff.

Apparently NTSC I chose negative modulation in 1941 mostly because it made for easier for receiver black level AGC circuits.  But early on it was necessary to develop gated AGC systems because simple peak level systems had poor immunity to noise impulses.  Noise-gated sync separators were another early development for the same reason.

Cheers,

Steve  

Just had a quick look through the Carnt and Townsend reference. They say there's a great advantage to +ve mod with AM sound but I can't really see how that conclusion can be drawn from the evidence they present. Yes, +ve has advantages but so does -ve. The question of gated AGC is interesting. AFAIK it was little used in the UK in single standard 625 sets. For that matter it wasn't much used for 405 with "Always Grey Control" results. I don't know about US practice.

The Pye V4 was of course the notable exception, marred by its rather poor EHT regulation.

Been trying to think of any advantages of mean level AGC; here go's... Cheaper than back porch gated AGC; could get away with poor (cheaper) EHT stability as screen averages out to grey; err, can't think of anymore advantages, but there must be some...

Hi.
You can still have DC restoration on mean level AGC or for that matter no AGC at all. If the level of the video changes on either mean level or no AGC the contrast level changes and that can give the viewer the misconception the brightness is changing. That is one reason with no or mean level AGC there were virtually no sets with DC restoration or fully DC coupling from the detector to CRT.
If you think back very few sets had AGC of any type prior to the 50's, the contrast was purely an IF or IF/RF gain control, some sets had either an attenuator for band one and some separate adjustable gain controls for band 1 & 3. That was the cheapest option and even on a well engineered set like the Murphy V310 that's how it was done. Actually mean level AGC was in some respects worse, on some Ekco sets picture flutter was a problem and I have heard that other makes had other issues.
The Pye V4 of 1953 put paid to a lot of this and did a good job of showing how 405 could really be like. Of course now with no aircraft flutter, signal fading etc, with the Aurora you can easily have direct coupling from detector to CRT or add a restorer diode, the difference is quite outstanding and worth doing to impress your friends!

I suppose mean level AGC & pure AC coupling really messed with the black level & contrast. I have a 625 single standard Elizabethan 12 inches portable from the early 70's & the black level is all over the place so presume AC coupling, not sure if mean level or sync tip AGC though. Did any single standard 625 TV's use mean level AGC?

Once you've got AC coupled video, then having mean level AGC doesn't make things much worse. On -ve modulation it's dead easy to do sync tip AGC so there's no real economy in adopting mean level AGC.

One other point about AC coupling and mean level AGC is that they reduce the variation in EHT current. Hence EHT regulation can be worse without causing objectionable changes in picture size with content.