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Wire and Wireless Communication Applications Chap. 8
sync. The vertical interval is also used to send special signals, such as testing signals that can
be used by TV engineers to test the frequency response and linearity of the equipment on-line
[Solomon, 1979]. In addition, some TV programs have closed-caption information that can
be decoded on TV sets used by deaf viewers. These sets insert subtitles across the bottom of
the screen. As shown in Table 8–12, this closed-captioning information is transmitted during
line 21 of the vertical interval by inserting a 16-bit code word. Details of the exact sync structure
are rigorously specified by the FCC. (See Fig. 8–37.)
TABLE 8–12
U.S. ANALOG TELEVISION BROADCASTING STANDARDS
Item
FCC Standard
Channel bandwidth
Visual carrier frequency
Aural carrier frequency
Chrominance subcarrier
frequency
Aspect (width-to-height)
ratio
Modulation-type visual
carrier
6 MHz
1.25 MHz ; 1000 Hz above the lower boundary of the
channel
4.5 MHz ; 1000 Hz above the visual carrier frequency
3.579545 MHz ; 10 Hz
Four units horizontally for every three units vertically
AM with negative polarity (i.e., a decrease in light level
causes an increased real envelope level)
Aural carrier
FM with 100% modulation being ∆F = 25 kHz with a
frequency response of 50 to 15,000 Hz using 75-ms
preemphasis
Visual modulation levels
Blanking level
75% ; 2.5% of peak real envelope level (sync tip level)
Reference black level
7.5% ; 2.5% (of the video range between blanking and
reference white level) below blanking level; this is called
the setup level by TV engineers a
Reference white level
12.5% ; 2.5% of sync tip level a
Scanning
Number of lines 525 lines>
frame, interlaced 2:1
Scanning sequence
Horizontally: left to right; vertically: top to bottom
Horizontal scanning
15,734.264 ; 0.044 Hz (2>
455 of chrominance frequency);
frequency, f h
15,750 Hz may be used during monochrome transmissions
Vertical scanning
59.94 Hz (2>
525 of horizontal scanning frequency); 60 Hz
frequency, f v
may be used during monochrome transmissions; 21
equivalent horizontal lines occur during the vertical sync
interval of each field
Vertical interval signaling
a See Fig. 8–31b.
Lines 13, 14, 15, 16. Teletext
Lines 17, 18. Vertical interval test signals (VITS)
Line 19. Vertical interval reference (VIR)
Line 20, Field 1. Station identification
Line 21, Field 1. Captioning data
Line 21, Field 2. Captioning framing code ( 1 2 line)