Digital Signal Processing Reference
In-Depth Information
controlled independently from the amplitude information, despite some brave
attempts that came in the form of single-sideband (am-ssb) and vestigial side-
band modulation (am-vsb). Another ubiquitous example is fm radio, where
the analog audio signal modulates the frequency of the carrier. The amplitude
of the signal defines the degree in which the rf-carrier deviates from its center
frequency. The amplitude of the carrier signal, however, maintains a constant
level.
At first sight it might look as if fm has better control over the characteristics
of the transmitted signal. After all, the bandwidth of the transmitted signal is
defined by the modulation index,
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while the amplitude of the carrier is fixed.
The drawback of fm is that, although the total transmitted power level remains
constant, there is no control over how this power will be spread over the spec-
trum. Furthermore, it is particularly interesting to see how the single analog
(l+r audio) channel, for which fm broadcast was originally developed, was
later on abused to accommodate a second audio channel (l-r) for stereo re-
ceivers (Figure 1.2) and again, about 30 years later, to incorporate a low-rate
information data channel (rds [CEN97]) at a 57 kHz subcarrier.
All those tricks and twists were brought into life in order to retain compatibil-
ity with older fm radio receivers. From these examples, it should be clear that
before cheap digital processing made its entrance in the world of transceiver
systems, modulation systems were optimized towards their implementation
in analog hardware. It was only much later, when portable communications
equipment became widespread, that the attention began to shift towards sec-
ondary parameters such as bandwidth efficiency or power consumption.
Figure 1.2.
Stereo demultiplexer circuit with two vacuum tubes [DeV61] (Zenith
Electronics Corp., 1961).
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The fm modulation index indicates the maximum deviation of the rf-carrier from the center frequency.
