563489578934

Sec. 8–9 Television 651
Fig. 5–33, B T ≈ D = R = 203 = 6.67 MHz.) Consequently a data-compression factor of
about 75 (1,50020) is needed. As shown in Table 8–14, this compression factor is achieved
by using a Motion Pictures Experts Group (MPEG) encoding technique. This technique consists
of taking the discrete cosine transform (DCT) of 8 × 8 blocks of pixels within each
frame and digitizing (using) only the significant DFT coefficients of each block for transmission.
Furthermore, for each new frame, data are sent only when there is a change
(motion) within the 8 × 8 pixel block frame to frame.
Frame-to-frame redundancy is removed by motion estimation and motion compensation.
There are three types of encoded frames. The interframe (I-frame) is coded in isolation
without prediction and is used as a reference frame. The predictive frame (P-frame) is forward
predicted from the last I-frame. The bidirectional frame (B-frame) is predicted for either past
or future I and P frames. A sequence of one I-frame and one or more P and B frames form a
MPEG group of pictures (GOP) [Gibson, 1997].
As indicated in Table 8–14, this produces a payload (compressed) data rate of 19.39
Mbs. This payload data plus parity bits for the FEC codes are fed into a 3-bit digital-to-analog
converter (DAC) to produce an 8-level baseband line code that has a symbol (baud) rate of
D = 10.76 Msymbolssec. As shown in Fig. 8–38, the 8-level data are partitioned into segments
consisting of 832 symbols with a 4-symbol synchronization (sync) pulse included at
the beginning of each segment to provide a reference for the transmitted data. Additional
training data is inserted over a whole segment after 312 segments. The additional data can be
used to train the receiver to adjust for channel fading and cancel multipath interference. The
composite 8-level baseband signal is amplitude modulated onto the carrier. In the modulation
process, a DC bias voltage is added to the baseband signal so that there will be a discrete
One segment
Permitted data levels
7
5
3
1
1
3
5
8-level
data
Sync
8-level data
Sync
8-level
data
7
Levels before
addition of
1.25 bias to
produce pilot
carrier
Sync
4 symbols
Figure 8–38
828 symbols
832 symbols
8-level baseband signal with segment sync.
Sync
4 symbols