[Boatanchors] Short Wave Broadcast Folks:

[Sheldon Daitch]

Richard is so right.  The idea of CCM is nothing new.  I thought I had a QST article from maybe the 1930s, but I can't seem to locate it at the moment.

One of the major differences between the operational concepts of years gone by and today's several systems is the ability to have delay circuits in the audio chain of the transmitter so the transmitter will have look ahead circuitry and adjust the power level for the appropriate level of modulation.  

As to the effectiveness of CCM, all depends on what is the primary goal.  With most of the VOA transmitters, we are running CCM, where the output power of the transmtters goes back to full power at 100% modulation.

AMC uses the opposite tact, full power with zero modulation and some level of RF output cutback as the modulation level increases.

I mention J. Fred Riley's work on this and here is an interesting position discussion on AMC as it pertains to use on the MW bands in the US:

http://www.radioworld.com/article/riley-criticizes-use-of-amc-/211667

Riley indicates there may have been some use of CCM back in the early 1930s and references a QST article in January, 1935 issue.  I think that is the one I printed out, but can't find.

Riley's paper can be summed up quite easily:

"The conclusion was clear. If AMC works for you, you’re running too much power in the first place. But if you need to maintain coverage, you use a technique that maintains the full PEP capability of your transmitter."

The savings on power consumption really depend on the type of programming.  I'd submit that some station running a fair amount of compression, rockers, as in the days of the modulation wars years back on the AM bands, CCM isn't going to save more than about a dimes' worth of power.

On the other hand, if an MW broadcaster is running CCM with a lot of talk programming, lots of audio with longer pauses, then sure CCM will save some money.

73
Sheldon
WA4MZZ



_______
From: Richard Knoppow <1oldlens1 at ix.netcom.com>
Sent: Thursday, April 17, 2014 10:47 PM
To: Sheldon Daitch; Glenn Little; boatanchors at mailman.qth.net
Subject: Re: [Boatanchors] Short Wave Broadcast Folks:

----- 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