Digital Signal Processing Reference
In-Depth Information
17
VIDEO MODULATION
AND TRANSPORT
CHAPTER OUTLINE
17.1 Complex Modulation and Demodulation
147
17.2 Modulated Signal Bandwidth
150
17.3 Pulse Shaping Filter
152
17.4 Raised Cosine Filter
155
17.5 Signal Upconversion
163
17.6 Digital Upconversion
164
Video compression is often performed to reduce the data rate,
or bandwidth, necessary for transport. Video is sent over satellite,
fiber, coax or copper twisted-pair wires. In practice, multiple
video signals
are often sent over
the
same medium
simultaneously.
17.1 Complex Modulation and Demodulation
Modulation is the process of taking information bits and
mapping them to symbols.
This sequence of symbols is filtered to produce a baseband
waveform with the desired spectral properties. The baseband
waveform or signal is then upconverted to a carrier frequency,
which can be transmitted over the air, through coaxial cable,
through fiber or some other medium. At some point in this
process a DAC converts the signal to analog, which is amplified
and transmitted.
One of the most common and simplest modulationmethods is
known as QPSK (quadrature phase shift keying). With QPSK,
every two input bits will map to one of four symbols, as shown
below in
Figure 17.1
. The complex plane is used in order to
represent two dimensions.
The bitstream of zeros and ones input to the modulator is
converted into a stream of symbols. Each symbol is represented
