MPEG applications - The MPEG - page 224

Figure 7.16 shows a block diagram of a DVB receiver. The off-air RF signal is fed to a mixer driven by the local
oscillator. The IF output of the mixer is bandpass filtered and supplied to the ADC which outputs a digital IF signal
for the FFT stage. The FFT is analysed initially to find the higher- level pilot signals. If these are not in the correct
channels the local oscillator frequency is incorrect and it will be changed until the pilots emerge from the FFT in the
right channels. The data in the pilots will be decoded in order to tell the receiver how many carriers, what inner
redundancy rate, guard band rate and modulation scheme are in use in the remaining carriers. The FFT magnitude
information is also a measure of the equalization required.
Figure 7.16: DVB receiver block diagram. See text for details.
The FFT outputs are demodulated into 2K or 8K bitstreams and these are multiplexed to produce a serial signal.
This is subject to inner error correction which corrects random errors. The data are then de-interleaved to break up
burst errors and then the outer R-S error correction operates. The output of the R-S correction will then be de-
randomized to become an MPEG transport stream once more. The de-randomizing is synchronized by the
transmission of inverted sync patterns.
The receiver must select a PID of 0 and wait until a Program Association Table (PAT) is transmitted. This will
describe the available programs by listing the PIDs of the Program Map Tables (PMT). By looking for these packets
the receiver can determine what PIDs to select to receive any video and audio elementary streams.
When an elementary stream is selected, some of the packets will have extended headers containing Program
Clock Reference (PCR). These codes are used to synchronize the 27 MHz clock in the receiver to the one in the
MPEG encoder of the desired program. The 27 MHz clock is divided down to drive the time stamp counter so that
audio and video emerge from the decoder at the correct rate and with lip sync.
It should be appreciated that time stamps are relative, not absolute. The time stamp count advances by a fixed
amount each picture, but the exact count is meaningless. Thus the decoder can only establish the frame rate of the
video from time stamps, but not the precise timing. In practice the receiver has finite buffering memory between the
demultiplexer and the MPEG decoder. If the displayed video timing is too late, the buffer will tend to overflow
whereas if the displayed video timing is too early the decoding may not be completed. The receiver can advance or
retard the time stamp counter during lock-up so that it places the output timing mid-way between these extremes.
7.5 CD-Video and DVD
CD-Video and DVD are both digital optical disks which store video and audio program material. CD-Video is based
on the Compact Disc, with which it shares the same track dimensions and bit rate. MPEG-1 coding is used on
video signals adhering to the common intermediate format (CIF - see Chapter 1 ). The input subsampling needed
to obtain the CIF imaging and the high compression factor needed to comply with the low bit rate of a digital audio
disk means that the quality of CD-Video is marginal.
DVD is based on the same concepts as Compact Disc, but with a number of enhancements. The laser in the
pickup has a shorter wavelength. In conjunction with a larger lens aperture this allows the readout spot significantly
to be reduced in size. This in turn allows the symbols on the tracks to be shorter as well as allowing the track
spacing to be reduced. The EFM code of CD is modified to EFMPlus which improves the raw error rate.
The greatly increased capacity of DVD means that moderate compression factors can be employed. MPEG-2
coding is used, so that progressively scanned or interlaced material can be handled. As most DVDs are mastered
from movie film, the use of progressive scan is common.
The perceived quality of DVDs can be very high indeed. This is partly due to the fact that a variable bit rate is
supported. When difficult frames are encountered, the bit rate can rise above the average. Another advantage of