LF: "Downward modulation"

["Dick Rollema" <d.w.rollema@freeler.nl>]

To All from PA0SE

Apparently  my remark that the carrier of Droitwich goes downwards
during modulation  has caused confusion. I certainly did not mean
"downward modulation" as we remember it from our AM transmitter
days, an effect caused by insufficient drive, weak power supply or other
shortcomings of the transmitter (or the operator...)

The carrier of Droitwich and BBC World Service is normal as long as there
is a modulating signal. But during moments of no modulation the carrier
rises to a higher level. As soon as modulation reappears the carrier
returns to its normal level.  This can be clearly seen in the
attachment to the reflector posting of 31/7 by Jim, M0BMU.
I am sure this is done on purpose and not a matter of poor design, bad
maintenance or incompetence of the BBC engineers.  
Dave, G3YXM, in his posting of 30/7 has told us that it is a feature of
modern transmitters that can be used to  preserve a silent
background in the face of interference.
I can understand that in connection with the BBC World Service but why it
is applied to the 198kHz transmissions escapes me.

73, Dick, PA0SE