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Name: Hazel
Status: student
Grade: 9-12
Location: CA 
Country: N/A
Date: 1/24/2005



Question:
Can we use non-periodic signals as carriers in radio transmission?


Replies:
You could if you were the only person transmitting radio waves, but the receiver normally is tuned to the frequency of the transmitting signal to exclude all other signals.

Of course, the transmitted signal is not perfectly periodic. If it were it could not send any information. The bandwidth of the signal is needed to carry information. For example, WOR broadcasts on a carrier frequency of 710 kilocycles = 710,000 Hertz. The bandwidth they are allowed to use is 10,000 Hz. This permits them to transmit signal frequencies up to 10,000 Hz. FM stations broadcast at higher frequencies (WNYC at 93.9 MHz, for example). Their allowed bandwidth is much larger and so they can transmit music with greater fidelity.

Best, Dick Plano


I know of one example, but it is really a periodic carrier that is changed often. So for a while the carrier frequency might be one thing, then jump to another, then another. How often it jumps can be chosen later. This is called "spread spectrum" for radio. One use for it is on something like cell phones, where you keep driving from one cell zone to another, so the radio has to change anyway. Another use for it is to cram more signals into the same bandwidth -- lots of people hopping around through the same set of carrier signals, but they do not interfere if the receivers know the jumping sequence. Another use if for secret communication -- any given signal may be detectable but then you jump to another and people (in this case) would not know how to follow you, except of course the person you are talking with.

Another way to look at things is that you need energy to send a signal from here to there. And this is what the carrier is for. You can modulate it to impress the information, but you need that basic energy. So you can flick on and off a flashlight, but you still need the flashlight, and it is periodic -- the light wave. A wave is just a very good way to send continuous energy, and as such makes a good carrier.

So I would bet that for any practical communication link, you do need this carrier. Hard to say "no" to you though, one never knows!

Steve Ross


If your receiver always has a way to predict which way the transmitter's carrier will wiggle next, yes, you can use a non-periodic carrier in radio communication.

The receiver needs a way to hear the transmitter's wave even though the wave has gotten weak from spreading out as it travels a long distance. For radio to work, there needs to be a way the signal can be "focused upon" amidst lots of other noise in the world.

Narrow-band modulation of a periodic carrier was merely the first and easiest way to do this. Actually, the other way is so hard, I am not sure we ever try it yet. To use a non-periodic carrier wave form, both transmitter and receiver need to know exactly when the starting time of the wave form is. Both receiver and transmitter need to carry the best possible clock, and make each bit in their communication slower than the growing difference between those two clocks. Only high-value, high-expense, small-content communications could use it. Only clandestine communications would really need it.

If there is a "go" signal obvious to everybody, that could be used as the agreed-upon starting time. But you don't usually have a recent supernova handy. Perhaps for some applications it would be OK to broadcast a time-signal on a periodic carrier. We have lots of those. Or broadcast it on a non-periodic carrier when transmitter and receiver are started close together, and as they go separate ways, have the receiver keep listening often enough to keep time synchronization. It the receiver neglected such checking-in for too long, it would lose the signal forever.

A periodic wave form is always starting over, so it is much easier for the receiver to synchronize its decoding waveform with the carrier wave form that the transmitter used. When we have a relatively friendly, open society, and lots of information to exchange, this seems by far the more efficient way to go.

As long as we have a "polite", well-regulated radio environment, perhaps even our non-periodic codes will mostly be transmitted on periodic carriers. This helps create nice little quiet frequencies between radio channels, so each radio can first hear its channel clearly and second think about the secret code. With no such separation between bands, it might get hard to use radio as everybody starts shouting a little louder to be heard over the noise from neighboring conversations. At least, that is the way we have regulated radio so far.

There are lots of options in between, and lots of games to play with this kind of thing. Ii might be fair to say we are gradually evolving towards less-periodic carriers, for purposes of private communications and exclusively-sold information streams. If you have a signal/carrier idea you would like to see tried, I'd encourage you to find an amateur radio or electronic person to help you try it for yourself. At low power over short distances, the legal regulations are not very difficult. You could probably do a non-periodic carrier demonstration at Morse-code rates (beep-beep) using ordinary quartz-crystal time keeping. You might then observe speech-bandwidth communications breaking up with time apart.

Jim Swenson



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