[Boatanchors] Short Wave Broadcast Folks:

[Richard Knoppow]

----- Original Message ----- 
From: "Sheldon Daitch" <SDAITCH at bbg.gov>
To: "Glenn Little" <glennmaillist at bellsouth.net>; 
<boatanchors at mailman.qth.net>
Sent: Thursday, April 17, 2014 12:04 PM
Subject: Re: [Boatanchors] Short Wave Broadcast Folks:


> Glenn,
>
> re the ERP aspect you discussed below, that is much more 
> the norm in the FM and TV world, but on the MW and HF 
> broadcasting side, a 100kW transmitter should be capable 
> of having an RF output of 100kW.

     FWIW, controlled carrier AM is far from new although I 
can't tell you where and when it originated. As an example 
the Drake T4XB transmitter uses it.  The advantage there is 
that it can be amplified by a linear amplfier intended 
mainly for SSB without exceeding its dissipation ratings. 
The carrier level at times of no modulation is a matter of 
choice but if too low may cause problems on the receiving 
side.  The ultimage CC system runs at effectively 100% 
modulation for all levels of audio input. When the audio is 
low the carrier is low and etc. For both unprocessed voice 
and music the average level is very low compared to the 
short term peaks. Processors, i.e., peak limiters, 
compressors, clippers, all function to raise the average 
level for a given peak limit. The limit is set by either a 
power limit as in any variation of AM, or a frequency 
bandwidth limit as in any variation of PM or FM. In the case 
of digital encoding there is still a limit; namely the 
number of bits used in encoding and the fact that nearly 
always the digital signal must be translated to analogue at 
the other end.  In a way CC can be seen as a method of 
compression since it changes the time related power function 
of the signal.
     On the receiving end the amplitude of the recovered 
audio depends on the amplitude characteristics of the 
receiver. The use of an automatic volume control will change 
the gain of the receiver with the level of the received 
signal. It will be affected mostly by the carrier level but 
also by the modulation envelope. So, the receiver will tend 
to act as a volume compressor for CC signals.  If the AVC is 
turned off the recovered modulation will be much closer to 
the original signal.  However, the receiver will not be able 
to compensate for variations in overall signal strength.  In 
fact, this compression effect takes place for single 
sideband signals as well and is the reason for long time 
constant AVC.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com