Video modulation

In Amplitude Modulated (AM) broadcast analogue television systems it is possible to modulate the video signal two ways.

Peak white can be made to correspond to peak transmitter power and the synchronisation (sync) pulses to zero transmitter power. This is known as positive modulation.

Alternatively, peak white can be made to correspond to zero transmitter power and the synchronisation pulses to peak transmitter power. This is negative modulation.

The first broadcast television systems used positive modulation, probably for no reason other than it seemed an obvious way to do it. Thus the UK 30, 240 and 405 line systems were all positively modulated. The French 819 line system (used also in Belgium) was similarly positively modulated, again probably for the same reasons.

Experience showed that positive modulation had some serious disadvantages.

Since the peak of the transmitted signal depended on video content, the AGC circuits in the receiver had to operate on the average level of the video signal. This resulted in the average light output of the cathode ray tube remaining more or less constant, regardles of picture content. Further any interference signals were likely to be of a similar amplitude to the sync pulses, which resulted in a poorly synchronised picture. This latter effect would occur before the interference became noticeable on the display.

When the Americans developed their own 525 line service, they saw the experience with positive modulation, and developed their service using negative modulation. This provided 2 important advantages. First, interference affected the peak whites of the display first where it was least noticeable. The interference had to acquire a significant amplitude before it affected the sync pulses (by which time the picture was unwatchable anyway).

Secondly, since the sync pulses represented peak transmitter power, there was a portion of the received signal that was of known and constant amplitude. It was merely necessary to gate the AGC circuit to only recognise this signal level. Consequently, for the first time, a completely black picture was completely black and a white picture white.

As television systems spread around the world, newer systems (now based on either the American 525 line or the newer 625 line systems) almost exclusively used negative modulation.

The exception to the rule was France which made the decision to positively modulate its colour UHF 625 line services. This decision was not made for any technical reason, but to make it impossible for French citizens to receive non-French originated programme material.

Note: It is widely believed that this was the reason for the French adopting their own SECAM colour system, but this is untrue. SECAM was developed to avoid the colour problems associated with the NTSC system. Besides SECAM was developed ahead of PAL and the French envisaged their system being adopted elsewhere. Were it not for positive modulation, French receivers would still have been able to receive foreign material, albeit in monochrome.

The Belgians initially adopted positive modulation on their first 625 line services broadcast on the VHF bands, but changed to negative modulation when their services migrated to the UHF bands.

The French television service still lives with a high susceptibility to interference to this day. However, modern electronic techniques have allowed receivers to gate the AGC circuits on the video signal's back porch giving a constant and predictable amplitude reference point.