Digital Signal Processing Reference
In-Depth Information
Figure 1.1.
Left:
the
first
commercial
modem
(AT&T,
1962).
Right:
V.92 laptop modem, 56 kbit/s, 85
×
54 mm.
Example: Bell 103 modem
A good example is the Bell 103 (V.21) modem system
1
(Figure 1.1).
In the transmission section of this modem, 1s and 0s are translated in
two separate tones (1,070 Hz and 1,270 Hz), which are sent over the
telephone line. At the receiver side, a pll-based demodulator attempts
to track the frequency of the received fsk signal. The decoded signal
is recovered at the output of the loop filter.
Digital processing was not present in these early digital communication sys-
tems, simply because it could not match the speed requirements at that time. It
was only from the early 1990s, with the appearance of more, faster and cheaper
digital circuits that the required processing power became commercially avail-
able. It is very important to recognize the true impact of this shift from analog
towards digital processing. Whether or not digital circuits would be superior
to their analog counterparts is not relevant for this discussion, but it is indis-
putable that digital computing power has brought signal processing to a higher
level of abstraction. In a digital world, signal processing is no longer performed
by projecting a mathematical model on an analog circuit implementation. Dig-
ital signal processing (dsp) involves that the signal itself is virtualized and
brought to the same abstraction level as the mathematical description of the
system. The result is a much higher flexibility, not only because more complex
1
Running at the mind-boggling speed of 300 bit/s [Swe05].
