Could somebody explain the concept of modulating a signal? It's such a broad term I never really understood what it meant.

Are phase shift, frequency modulation, and amplitude modulation all different techniques doing the same thing? How are these used to encode information on a signal?

What other methods exist that I am not aware of?

Thanks everyone.


1 Answer 1


Modulation just means modifying one signal with another.

In the most fundamental kind of modulation, amplitude modulation (AM), the amplitude of a high frequency sinusoidal 'carrier' wave is made to follow the value of a much lower frequency signal. AM has been around probably longer than any of the others, but since it wastes a lot of power, other modulation techniques have evolved. Variations on AM include suppressed carrier AM and 'sideband', which are less wasteful of power.

Frequency modulation (FM) can be thought of as making the instantaneous frequency of a carrier wave to increase and decrease according to the value of the modulating signal. Phase modulation (PM) similarly adjusts the phase of the carrier. These generally require a bit more bandwidth, but tend to be more immune to noise.

There are plenty of other modulation techniques, more sophisticated than AM, FM and PM, which do a better job of getting more information across in the same bandwidth.

  • \$\begingroup\$ So in AM, does the modulating signal basically become a low frequency bias added to the carrier wave? \$\endgroup\$
    – NickHalden
    Commented Jun 15, 2011 at 0:52
  • \$\begingroup\$ JGord: No, it limits the voltage range of the carrier wave. See Amplitude modulation. \$\endgroup\$
    – mgkrebbs
    Commented Jun 15, 2011 at 1:23
  • \$\begingroup\$ @JGord - not added, but multiplied actually. When the signal is zero, the carrier has some amplitude A, so it swings between +A and -A. When the signal is positive, the carrier has more amplitude, so it goes from +A+s to -A-S. When the signal is negative, the carrier goes between +A-s and -A+s. It helps at first to think of the maximum 's' value as being much smaller than the 'A' value, although the relative strength of the carrier and the signal is a parameter can be adjusted. \$\endgroup\$
    – JustJeff
    Commented Jun 15, 2011 at 1:43
  • \$\begingroup\$ @JustJeff Oh of course, amplitude, adding something doesn't change the amplitude. Ok so I'm assuming demodulation cannot be done in real time? What I mean is if you are sending me a sine wave who's amplitude is a function of time, all I do is read an analog voltage at a single point in time I cannot know what the amplitude is at that instant. I.E. I don't know how you would know the instantaneous amplitude. Oh perhaps I just stumbled on a theorem, is the data rate limited by the frequency of the carrier wave? \$\endgroup\$
    – NickHalden
    Commented Jun 15, 2011 at 3:15
  • \$\begingroup\$ @JGord - the ancient radio engineers just rectified the AM and passed the result through a low pass filter. more recent radio engineers devised 'direct conversion', in which you modulate the AM by the carrier frequency a 2nd time, which recovers the original signal. I suppose that it's accurate to say that data rate is limited by carrier frequency, in that you can't generally modulate frequency f with a signal that approaches f, in AM. You won't be surprised to find out that AM has long since fallen from favor for data transmission. \$\endgroup\$
    – JustJeff
    Commented Jun 15, 2011 at 10:47

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