[Spooks] Re: On a related note

[spooks@mailman.qth.net]

The one sideband plus carrier was known in TV 
broadcasting (the way the video signal is broadcast) as 
vestigal sideband.  Whoops, did I spell that right?  
Doesn't look right.  

Overseas telephone used to use SSRC - Single Sideband 
Reduced Carrier.  (Now they use more sophisticated 
digital systems)  The carrier provided a steady 
(sometimes!) source for automatic frequency control.  
Carrier reduction was a standard 20 db.  Likewise 
multiplex AFSK also used a -20 db carrier for frequency 
lock-on, so that each station receiving could pick his 
own AFSK signal out of the multiplex and use the carrier 
offset as AFC.  

Steady carriers heard on the bands are usually simply 
because it is easier for many transmitter sites, which 
are often remote controlled from receiver sites, to just 
leave the carrier up all the time.  It provides a tuning 
point for those who might want to receive it, and it 
keeps the frequency clear, more or less.  Monitoring such 
a carrier for hours on end might result in it suddenly 
being keyed in RTTY, or even voice.  Or, when it is shift 
change, the new T-site op may flip the TX off if it isn't 
being used.  Or it may disappear when it is to be 
retuned, due to time of day and propagation, to a 
different freq.

Pete

> There are in fact three different "modes" of single-sideband (SSB) :
> 
> - fully suppressed carrier (SSB as we normally think of it)
> - reduced carrier (not fully suppressed)
> - full carrier (like half an AM signal)
> 
> Early SSB transmitters were capable, after suitable adjustments, of all
> three modes, but most commercial rigs now produce only SSSC
> (single-sideband suppressed carrier, as SSB used to be called) unless
> modifications are requested before they are built.  
> 
> As a variation, Station CHU uses full carrier but with reduced power in
> the lower sideband, so the signal sounds OK in both AM  and USB. 
> (Reception in LSB is poor.)  
> 
> Each of these three modes uses power more efficiently than AM and they
> are more economical of bandwidth.  Obviously, for most purposes, the
> fully suppressed carrier is best, but they each have their advantages.  
> 
> 73, 
> ... Martin    VE3OAT
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