[Boatanchors] Whither SWL'ing...?

[Sheldon Daitch]

Is this modulation vs S-meter reading issue noticeable on
HF broadcast signals as well?

Note that many broadcasters use several types of modulation
techniques which change the carrier power with modulation.

We use generally a 6 dB carrier drop back on our amplitude
modulation transmitters, that is, the carrier power at 100% modulation
will be full carrier but if there is no modulation, the carrier power will
drop back 6 dB.  Typically, this scheme is called Controlled Carrier
Modulation or Dynamic Amplitude Modulation  or Dynamic Carrier
Control.  One or more may be trade names.

The BBC, on the other hand uses a slightly different technique,
where they run full power during modulation pauses and reduced
carrier power upon modulation, AMC.

"The design and development of a modulation-controlled system known as
amplitude modulation companding (AMC) is described. In this method both
carrier and sideband levels are progressively reduced as the modulation 
level
is increased, this effect being compensated in the receiver by the 
normal action
of the automatic gain control circuit. Extensive trials of a fully 
developed system
for 50-kW MW transmitters have achieved input power savings of between
20% and 40%, depending on program content. This is achieved with 
imperceptible
loss of coverage, and further savings may be possible."


A Google search on

*BBC modulation carrier control

*nets some interesting hits.*

73
Sheldon


*DJED1 at aol.com wrote:
> To keep on topic, there was one aspect of the SP-600 that drives me  nuts.  
> The S-meter is affected by modulation on the signal, moving downward  with 
> increasing modulation.  I went crazy trying to set the meter  calibration until I 
> noticed that the reading depended on whether the generator  was modulated or 
> not.  A 14 dB difference in reading.  Once I figured  that out, I used CW and 
> all is now good.  Anyone know of a fix for  this?
> Ed
>
>