Radio bands

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Another thing, do carrier waves ever carry any "Useful information" or is the information only in the side bands?
Whether the carrier wave carries information depends on modulation method.

For example, in the DSB-C AM, the power in the carrier conveys no useful information. Therefore, DSB-SC AM is basically an amplitude modulation wave without the carrier for reducing power waste. In addition, since both sidebands in DSB-SC AM contain identical copies of the information, thus SSB-SC AM can further increase the efficiency by transmitting one sideband only.

DSB-C AM - Double Sideband Large Carrier AM
DSB-SC AM - Double Sideband Suppressed Carrier AM
SSB-SC AM - Single Sideband Suppressed Carrier AM
 
sophiecentaur
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Whether the carrier wave carries information depends on modulation method.
Not really . To 'carry information' it has to be modulated, which means it is no longer a single frequency. Splitting hairs, perhaps but the information has to be spread over a finite bandwidth (the channel). In AM and some other systems, there is a carrier present at all times. No modern system would use unwanted carrier power 'cos it's such bad value. The carrier does the job of self - demodulation for simple AM and, of course, the crystal set and its descendants needed a carrier. To demodulate even basic FM needs a 'frequency discriminator' circuit which outputs a Voltage related to the instantaneous carrier frequency. But the expression "instantaneous carrier frequency" is very dodgy for any other than very slow frequency sweep and wide deviation. (FM is really hard to understand, even from the start).
An effective bodge for demodulating FM with an AM receiver is to offset the tuning so that the received signal sits on one of the skirts of the receiver filter. As the carrier frequency varies, the output of the AM detector varies and it 'becomes AM'. This Slope Detection system only works when the FM channel bandwidth is similar to the AM channel bandwidth. No good for your living room HiFi, though.

SSB is great value for spectrum use and noise performance BUT it does need a special receiver which makes its own carrier. The demodulated signal can frequently sound like Donald Duck so it's only of use for Comms.
 
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Not really . To 'carry information' it has to be modulated, which means it is no longer a single frequency.
Of course, the carrier wave must be modulated in order to carry information, a constant carrier wave or anything unchanged can not provide any further information except the proof of existence of itself.

I just mean that the carrier frequency component in the spectrum of the modulated carrier of the traditional AM broadcasting does not carry information of the modulating signal.
 
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sophiecentaur
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Of course, the carrier wave must be modulated in order to carry information, a constant carrier wave or anything unchanged can not provide any further information except the proof of existence of itself.

I just mean that the carrier frequency component in the spectrum of the modulated carrier of the traditional AM broadcasting does not carry information of the modulating signal.
We have got to the stage of definitions and classifications which is always a dead end. We both know what's what, here, I think. People tend to over estimate the importance of a 'carrier'. I would say that the only mod system that has an actual carrier is AM or its near derivatives. In that system, the carrier actually has a job to do in simple receivers but simplicity of receiver circuits is no longer important. Anyone can sync up a local oscillator to allow demodulation these days.
BTW, I just recalled "Exalted Carrier Demodulation" which is a quaint name for a system that helps performance in some variations go AM. It involves filtering and boosting the level of a carrier that's purposely transmitted at low level and then following by simple envelope detection. The term has a rather self-important air about it - out of the past, I feel.
 
Paul Colby
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Another thing, do carrier waves ever carry any "Useful information" or is the information only in the side bands?
I think the answer is ideally yes. But the carrier would have to be on and unchanging for infinite time to have the least possible information.

People use CW (just a carrier) for communication using Morse code. The carrier is cycled on and off to form dots and dashes. The act of turning the carrier on and off spreads the frequency spectrum a little. In fact such transmitters must be constructed to turn on and off at a slow enough rate so as not to spread too far into neighboring channels.
 
sophiecentaur
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I think the answer is ideally yes. But the carrier would have to be on and unchanging for infinite time to have the least possible information.

People use CW (just a carrier) for communication using Morse code. The carrier is cycled on and off to form dots and dashes. The act of turning the carrier on and off spreads the frequency spectrum a little. In fact such transmitters must be constructed to turn on and off at a slow enough rate so as not to spread too far into neighboring channels.
CW is an oxymoron. The wave isn’t continuous when Morse keyed so where is the difference between that, AM and all other forms of mod?
Your post could easily Be taken to imply some hard boundary between modulation and non-modulation. Arbitrary exceptions can confuse.
 
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sophiecentaur
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In fact such transmitters must be constructed to turn on and off at a slow enough rate so as not to spread too far into neighboring channels.
Imagine you made a receiver for 'CW comms' with a very narrow band receive filter, (Bandwidth of 0.01Hz). If it were tuned spot on the carrier frequency, you would just see the carrier with no visible envelope. That's despite the fact that the carrier "isn't there" for half the time; the filter would have stored Energy at the carrier frequency for 100s or more. The modulation has been wiped off.

One great advantage of 'CW' is that you can, in fact use a receive filter with just a few Hz bandwidth which can drag the signal up from what would seem to be unusable noise levels. Of course, the Tx and Rx tuning would need to match well within a Hz or so. No problem at all these days. Great for covert comms and not requiring any fancy codulation systems.
 
Paul Colby
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Your post could easily Be taken to imply some hard boundary between modulation and non-modulation. Arbitrary exceptions can confuse.
Agreed, but only if you don't read it and think carefully about it. CW is what people call it. It is in fact an amplitude modulation.
 
sophiecentaur
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only if you don't read it and think carefully about it.
Ah well, that's it!! Who is the intended audience for these posts? Which of us is always thinking carefully? (Present company excepted of course)

The term "CW" means two different things, according to context. If you say you picked up a CW signal, that does not imply that some old geyser was using it to send Morse with. If you say you were chatting to someone using CW, the old geyser was definitely there on his key! In my head, I was including Morse in amongst all the other mod systems. (not carefully, perhaps)
 

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