eternal-september.org> 94230fff   
   XPost: rec.radio.amateur.equipment, uk.radio.amateur   
   From: g3vki@turner-smith.co.uk   
      
   "FranK Turner-Smith G3VKI"  wrote:   
   > "Michael Black"  wrote in message   
   > news:alpine.LNX.2.02.1501252044230.32234@darkstar.example.org...   
   >> 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   
   >>   
   > I believe there were some experiments a few years ago with stereo AM   
   > broadcasts using different sidebands for each channel. Automatic carrier   
   > reinsertion sounds like a problem, or was the carrier simply reduced and not   
   suppressed?   
      
   Sorry, pissed again. It was AM, full carrier, no BFO. Time for a another   
   drink.   
   --   
   73 de Frank Turner-Smith G3VKI - mine's a pint.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)