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Using amplitude modulation instead of frequency modulation would theoretically allow infinite channels because each channel would require just a single frequency instead of a frequency band.
In a normal data stream you get pulses of all different widths, corresponding to 00000100000 or 1010101010 type patterns, where the pulses are narrow and 11111100001111000 where the pulses are wide. The filtering will end up making the 1010101 pulses look more or less like a sinewave and the 1111100000 look like rounded square waves. All the information gets through and saves oodles of bandwidth. (Basically)
Incidentally, dealing with signals such as 00000010000000 can be more of a problem if your circuitry does not handle very low frequencies. Some coding systems ensure that there is at least one signal transition within every so many bits in order to ensure you don't get the ultra low frequency problems.
David Spencer asked the Naked Scientists:One I was pondering a while ago is, with countrywide data services such as 3G, Wi-Fi, bluetooth, radio, television broadcasting, and radio communications, are we going to eventually run out of useable frequencies? Or will we keep refining our technology to use more frequencies in between the old ones?