I am convinced that sidebands exist. But as for whether the carrier goes away at 100% negative modulation...
How's this: a carrier should be considered to exist during the period when it has recently been detected, and we are actually receiving modulation from it. It should not be considered to exist when it is clearly not being generated. So in the example of an AM signal at the instant of 100% negative modulation, we should consider the zero to represent zero percent of the carrier, averaged over time during recent history. We can see that this will result in appropriate demodulation of the actual signal. But when the AM transmission is over, either after the end of a transmission, or after the end of a broadcast day, or in most cases if the signal fades out due to propagation, etc, we should consider the carrier not to be present. This gets a little tricky, because we can not really be certain (Don's Heisenberg Uncertainty Principle thought here) that the transmitter is not sending us a very, very long zero. But we can be pretty sure, pretty quick. A synchronous detector is a good flywheel that tracks the known carrier frequency and holds the reference for us during zeros and noise bursts. We set up a synchronous detector to detect the carrier and hold the reference for a short period of time. This works pretty well for the signals we actually transmit. Now take the example of a 1 KHz sinewave transmitted as double sideband suppressed carrier. You get pulses of alternating carrier polarity. During any given pulse, for about 500 microseconds at a time, you get an AM signal with a carrier, transmitting one half of a sine wave. When the pulse is over and the signal passes through zero, it goes negative and the next pulse appears - pretty much identical to the first pulse, but with reverse carrier polarity. You would not know the carrier polarity reversed except for your flywheel reference. But demodulating with the flywheel reference gives you the 1 KHz sinewave. It is evident to the observer that the flywheel reference was correct. With a very low modulating frequency, a DSBSC signal would just look like a fading carrier. The transmit frequency stability and the reference flywheel precision would have to be very high to determione that the carrier polarity had reversed. At some point this becomes irrelevant, because the transmission path varies in length, there is drift and phase noise in TX and RX, and the signal is not received well enough to know for sure that the carrier polarity flipped. (Uncertainty again.) And although with DSBSC we keep getting pulses of carrier, they keep reversing, and on average they balance out to zero. We accept that the carrier is suppressed, we don't hear a heterodyne where we would usually hear one, etc, yet we see the carrier dancing on the oscilloscope. But over the appropriate integration time, its frequent polarity reversals cause it to balance out to zero. For most real signals, a very long time base is inappropriate. In some cases though, such as slow synchronous CW, a long time base is appropriate. So the receiver time scale should be appropriate to the signal being sought. So it's a reality check issue. Surely a carrier was not transmitted for all time, just because it existed for less than a second at some point. But just as surely, the instant of 100% negative modulation should not be reproduced as a glitch. The application of the appropriate time scale is the key, and it is up to the listener to determine what happened. Bacon, WA3WDR
