eternal-september.org> 94230fff
XPost: rec.radio.amateur.equipment, uk.radio.amateur
From: et472@ncf.ca
On Mon, 26 Jan 2015, Frank Turner-Smith G3VKI wrote:
> "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.
And it was two independent sidebands, not DSB. So you couldn't get
information from the two sidebands to place the carrier right in the
middle since the sidebands carried different information.
That was one of the various schemes for AM stereo. I remember reading
about early experiments, you could use two am portables, tuning each a bit
off the centre frequency so each radio received one of the channels.
Since the FCC in the US changed their mind and allowed all methods to be
used, it's probably legal for AM broadcasters, but it wasn't the most
common system used. Actually I guess it is, just a modified version.
One reason Sony put a synchronous detector in their classic 2010 shortwave
portable was because they'd designed an IC for this method of AM stereo,
and since AM stereo never took off, they had the IC. And it just
happenened to be what wsa needed to demodulate DSB, the modern
equivalent of a phasing type "sideband slicer" with circuitry to lock the
carrier to the incoming signal. There has to be a method of turnig off
the locking, since the IC also works for demodulating SSB (and if the
carrier is set right, you could get LSB out of one channel and USB out of
the other). It's the same thing with the Motorola series of AM stereo
detectors.
I think for shortwave broadcasting, they've tried just about everything
except DSBsc. I gather our time station, CHU, uses a single sideband
signal with carrier, though I can't remember if it's reduced or not.
Michael
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