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Wire and Wireless Communication Applications Chap. 8
carrier term in the spectrum. This pilot tone at the carrier frequency, f c , provides synchronization
for the carrier oscillator circuits in the DTV receiver. A vestigial sideband filter
(described in Sec. 5–5 and Fig. 5–6) is used to remove the lower sideband of the AM signal
and produce the vestigial sideband signal. The VSB filter is designed such that the band edges
roll off with a square root raised-cosine rolloff characteristic (at the transmitter and the
receiver). As discussed in Sec. 3–6, this rolloff is used to filter off the sidelobes of the sin
(x)x spectrum (Fig. 5–33) that occur for rectangular-shaped data symbols and yet not introduce
ISI on the filtered symbols. Fig. 8–39 shows the spectrum of the resulting 8VSB signal
where the carrier frequency, f c , is 309.44 kHz above the lower band edge.
The complete U.S. DTV standards are available on the Advanced Television System
Committee (ATSC) Web site at www.atsc.org. In Europe, the Digital Video Broadcasting (DVB)
standard has been adopted for digital TV. DVB uses Coded Orthogonal Frequency-Division
Multiplexing (COFDM) for modulation instead of 8-VSB. See www.dvb.org for details.
Because the number of pixels in a SD frame (480 × 704 = 337,920) are about 16 the
number of pixels in a HD frame (1080 × 1920 = 2,073,600), the DTV broadcaster has the
very interesting option of time-division multiplexing data for multiple SD programs,
since this can be achieved without exceeding the 19.39 Mbs payload data rate. That is, over
a 6-MHz TV channel, four to six SDTV programs may be transmitted simultaneously with
one 1080i HD program. This multiple SDTV option is so profitable for broadcasters that they
usually offer them along with the HDTV program.
Digital TV has the following advantages:
• Use of digital signal processing (DSP) circuits.
• Error-free regeneration of relayed and recorded TV pictures, since binary data represent
the picture.
• Multiple sound channels (four to six) of CD quality for stereo multilingual capability.
• Data may be multiplexed for text captioning screen graphics and for control of TV
recording and other equipment.
• Multipath (ghost) images and ignition noise can be canceled using DSP circuits.
• Lower TV transmitter power required because digital modulation is used.
f ∆
f ∆
f ∆
f ∆
1.0
0.707
Pilot
carrier
f ∆ =309.44 kHz
f 0 =D/2=5.381119 MHz
f c
f
Figure 8–39
6 MHz
Spectrum of 8-VSB DTV signal with square root raised cosine-rolloff.