Floating carrier system.

[colly0410]

Been reading BBC R&D report 1934/36 called 'Effect of floating carrier system on short wave receivers.' What is/was a floating carrier system? Never heard of one myself..

[ppppenguin]

Could it be an early term for spread spectrum or frequency hopping? Look up the work done by George Antheil and Hedy Lamarr using piano roll technology to implement pseudo random sequences for frequency hopping.

[colly0410]

Presume it's to do with broadcasting as was in a BBC document... I'm wondering if the carrier power varies in relation to the sidebands somehow i.e. The power of the carrier reduces as more power goes into the sidebands on loud sounds, or something like that. I've been racking my brain trying to think what a 'floating carrier' is, I've googled it & it just takes me to the BBC report..

[Synchrodyne]

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

[colly0410]

Thanks for that 1988 BBC link... Seems there are 2 types of so called 'floating carrier' systems: DAM that reduces the carrier power when low volume or no sound is transmitted, & AMD where the carrier is reduced when high volume sound is transmitted. To me they seem to work in opposite ways. Both are very clever ways of reducing the electric bill. It says that a disadvantage of DAM is that on weak signals when the modulation is low the reduced carrier power can increase noise a bit, would Mr/Mrs Average Listener notice though? Probably not. To me the AMD system seems very elegant as it uses the receiver AGC to make it work effectively, how it would sound on a crystal set or old TRF set without AGC I don't know...