MPEG applications - The MPEG - page 217

A key difference between analog and digital transmission is that the transmitter output is switched between a
number of discrete states rather than continuously varying. The process is called channel coding which is the
digital equivalent of modulation. [ 2 ] A good code minimizes the channel bandwidth needed for a given bit rate. This
quality of the code is measured in bits/s/Hz.
Where the SNR is poor, as in satellite broadcasting, the amplitude of the signal will be unstable, and phase
modulation is used. Figure 7.5 shows that phase-shift keying (PSK) can use two or more phases. When four
phases in quadrature are used, the result is quadrature phase shift keying or QPSK. Each period of the transmitted
waveform can have one of four phases and therefore conveys the value of two data bits. 8-PSK uses eight phases
and can carry three bits per symbol where the SNR is adequate. PSK is generally encoded in such a way that a
knowledge of absolute phase is not needed at the receiver. Instead of encoding the signal phase directly, the data
determine the magnitude of the phase shift between symbols.
Figure 7.4: Program Specific Information helps the demultiplexer to select the required program.
Figure 7.5: Differential quadrature phase shift keying (DQPSK).
In terrestrial transmission more power is available than, for example, from a satellite and so a stronger signal can
be delivered to the receiver. Where a better SNR exists, an increase in data rate can be had using multi-level
signalling instead of binary. Figure 7.6 shows that the ATSC system uses an eight-level signal (8-VSB) allowing
three bits to be sent per symbol. Four of the levels exist with normal carrier phase and four exist with inverted
phase so that a phase-sensitive rectifier is needed in the receiver. Clearly the data separator must have a three-bit
ADC which can resolve the eight signal levels. The gain and offset of the signal must be precisely set so that the
quantizing levels register precisely with the centres of the eyes. The transmitted signal contains sync pulses which
are encoded using specified code levels so that the data separator can set its gain and offset.