XPost: rec.radio.amateur.equipment, uk.radio.amateur   
   From: et472@ncf.ca   
      
   On Sun, 25 Jan 2015, gareth wrote:   
      
   > "G3XBM via rec.radio.amateur.moderated Admin"   
   >  wrote in message   
   > news:047d7b5d612a205e9c050d737a33@google.com...   
   >>   
   >> Although not impossible, it is quite difficult to demodulate a DSB signal   
   >> on a simple direct-conversion receiver. DSB rigs are ideal as simple   
   >> transceivers to communicate with SSB rigs.   
   >   
   > The reason being that to resolve DSB, the injected carrier must have a   
   > very close phase relationship with the original carrier, and at a certain   
   > phase relationship (don't remember but probably 90 degrees) there's   
   > no output at all!   
   >   
   It doesn't even have to get to the point where phase comes into play.   
      
   If you don't have it "zero beat" the two sidebands will not only sound odd   
   (because they aren't converting down to their original baseband) but the   
   two sidebands will clash with each other.  If the carrier is in the right   
   place, both sidebands will convert to the exact same audio frequency.  If   
   the carrier isn't right in the middle of those sidebands, the two   
   sidebands will convert to different audio frequencies, so it will sound   
   awful, at the very least.   
      
   Single sideband can be mistuned somewhat, all it sounds like is osmeone   
   talking in a higher or lower tone than "normal".  But if an DSB signal is   
   demodulated with the carrier not right in the centre, the two sidebands   
   converted to audio interfere with each other.  If the carrier is right in   
   the middle, they combine properly.   
      
   Synchronous detectors were originally created because of this, to properly   
   demodulate DSB with no carrier.  Which is kind of odd, since in more   
   recent times synchronous detectors have become relatively common in   
   shortwave receivers, yet some or many of those designs likely don't work   
   with DSB no carrier.  It depends on where the circuit is getting the   
   information to control the BFO.   
      
   There was a time when DSBsc demodulators were shown in the ham magazines,   
   the simple ones would square up and then double the incoming signal and   
   then divide it back down to the IF signal.  Which always resulted in the   
   carrier being right in the middle between the sidebands.   
      
   But a synchronous detector was too much trouble, might as well use an SSB   
   receiver and convert the DSBsc signal to SSB inside the receiver.  By the   
   time synchronous detectors were known, the move to receivers for SSB had   
   already started.   
      
   Of course, DSBsc has other advantages beyond the simple transmitter, a   
   proper demodulator makes use of the redundancy the two sidebands to   
   improve reception.  If nothing else, it gets information from both   
   sidebands, yet allows one to switch between the two if interference is on   
   one sideband.  A proper DSBsc system is more complicated at the receiver,   
   but has some advantages over SSB (though narrower bandwidth isn't one of   
   them).   
      
   Various shortwave broadcast stations have played with SSB for   
   transmitting, and often have had to fall back to SSB with reduced carrier,   
   since if they are playing music, they need the carrier to tell people   
   where the local BFO should be set; otherwise the music converts to the   
   wrong audio frequencies, and sounds way worse than a mistuned voice SSB   
   signal.  Or they could put the other sideband back, but eliminate the   
   carrier.  That way the unneeded carrier isn't transmitted (so more power   
   for the sidebands) but the extra sideband provides the information needed   
   to insert the local BFO right in the middle of the sidebands for proper   
   demdulation.   
      
      Michael   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)