Or another view. ALL radio communication is done with an analog basic "carrier wave" of a set frequency. Then an information signal is used to vary that signal in some way - usually by altering the amplitude of the wave. That is AM or Amplitude Modulation. (Other options are to vary the frequency of the carrier slightly (that's Frequency Modulation, or FM) or Phase Modulation (used infrequently, but essential for the Colour signal in older analog colour television).) AM is a bit easier to understand and is widely used. Almost all DIGITAL communication by radio signals is done by modulating the Amplitude of the carrier wave with digital signals.
In AM the frequency of the information signal is much lower than that of the carrier. For example, for an older AM Radio station broadcasting at 1000 KHz (1 MHz) the time for ONE cycle of that carrier wave is 1 millionth of a second (1 µs), while the time for one cycle of an analog audio tone at 1KHz in the middle of normal hearing is 1 millisec (1 ms), or 1000 µs. So over the time of one cycle of the analog information signal at 1 KHz, the amplitude of the thousand cycles of the carrier wave changes smoothly from very small to full amplitude and back to minimum.
But what if the information signal is a digital one? That means the info is not a smoothly varying sine wave. Instead it is a signal that is either full-on of full-off for some specified short time period. If that short time is 10 µs for one bit of data, then we are transmitting digital info as a series of binary bits at the rate of 0.1 Megabits per second or 0.1 Mb/s or 100 Kb/s. Now, we use digital bits to encode whole numbers, often in groups of 8 bits which allows us to encode and send 256 possible whole numbers (from 0 to 255) as a single group of 8 binary bits, and a whole long series of such numbers in groups of 8 bits or one Byte per number. If that data at 0.1 Mb/s is being sent on the original 1 MHz carrier frequency, each binary bit is10 µs long in time and that is 10 cycles of the carrier frequency. Those 10 cycles will have an amplitude of either full or zero for that signal. At the receiving end it is easy for the digital circuits to determine whether each bit is ON or OFF, and small variations from full signal or zero signal due to noise or "static" can be ignored. AFTER that is done, if the original signal was actually a digitally-encoded analog audio signal, the original information can be re-created exactly. But if the original signal was digitally-encoded written alphabetic characters, or some form of video signal, that also can be re-constructed from the digital bits received. There just needs to be some prior agreement on what the original info was and how to re-construct it from the bits.
So the difference is not in the carrier wave system. It is in the type of information used to modulate that carrier wave in order to convey information from transmitter to receiver.
en.m.wikipedia.org
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