27-03-2017, 11:40 PM
From my reading of RDR 1934/36, it looks as if "floating carrier" might have represented the BBC's early thoughts on what much later was known as "dynamic carrier control" (DCC), as described in BBC RDR 1988/15, "Implementation of Amplitude Modulation Companding in the BBC MF National Networks".
In normal AM, carrier amplitude and so energy actually remains constant. Sideband energy varies, and depending upon its phase relative to the carrier, either adds to or subtracts from carrier energy to provide the net energy variation with modulation.
On the other hand, in FM (and PM), the carrier amplitude varies, and the energy put into the sidebands varies more-or-less inversely so that the net radiated energy remains constant.
With DCC, where the carrier amplitude varies somewhat according to the modulation, it is as if a an element of PM had been added to AM. This would create additional sidebands, but the time constants are probably so that the PM sideband energy is mostly in the sub-AF region.
Cheers,
Steve
In normal AM, carrier amplitude and so energy actually remains constant. Sideband energy varies, and depending upon its phase relative to the carrier, either adds to or subtracts from carrier energy to provide the net energy variation with modulation.
On the other hand, in FM (and PM), the carrier amplitude varies, and the energy put into the sidebands varies more-or-less inversely so that the net radiated energy remains constant.
With DCC, where the carrier amplitude varies somewhat according to the modulation, it is as if a an element of PM had been added to AM. This would create additional sidebands, but the time constants are probably so that the PM sideband energy is mostly in the sub-AF region.
Cheers,
Steve
