22-10-2017, 08:48 AM
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.
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.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
