Digital Signal Processing Reference
In-Depth Information
In the case of a sine three quantities can be varied: amplitude,
frequency and phase. Accordingly it is possible to superimpose
information on a sinusoidal “carrier” by means of a change in
the amplitude, frequency or phase or by a combination of these.
Note:
From a physical point of view - see Uncertainty Principle - an amplitude, frequency
or phase change always implies frequency uncertainty in the frequency domain; so
the sudden switching of the frequency of a sine in the time domain in reality implies
frequency uncertainty in the frequency domain which is expressed in a frequency
band.
A modulated signal with a sinusoidal carrier thus only has something like an
“instantaneous sine” or an “instantaneous frequency” in the time domain, in the
spectrum we always see a bundle of frequencies. This bundle of frequencies is wider
the shorter the existence of the “instantaneous frequency” (
Δ
f > l/
Δ
t) in the time
domain.
In classical modulation processes only one of the three quantities amplitude, frequency or
phase is varied continuously “in the rhythm” of the source signal.
The classical modulation processes are thus
•
Amplitude modulation AM
•
Frequency modulation FM
•
Phase modulation PM
In traditional transmission technology - e.g. radio and television - AM and FM are mainly
used.
AM, FM and PM are used to displace or transfer the source
signal into the desired frequency range. All modulation processes
represent non-linear processes because the modulated signals
occupy a different frequency range from the source signal.
Modulation and demodulation in the traditional sense
Any modulated signal must be demodulated in the receiver, i.e. it must be returned as
accurately as possible into the form of the original source signal.
In the case of a radio station the signal is modulated in the transmitter, i.e. only one
modulator is required. But each of the many thousands of receivers need a demodulator.
In the early days of radio technology it was only possible to use a modulation process
which required a simple and cheap demodulator in receivers.
