ADV7172KST datasheet(43/60 Pages) AD

43 / 60 page

REV. B

ADV7172/ADV7173

–43–

The ADV7172/ADV7173 supports closed captioning, conform-

ing to the standard television synchronizing waveform for color

transmission. Closed captioning is transmitted during the

blanked active line time of Line 21 of the odd fields and Line

284 of even fields.

Closed captioning consists of a 7-cycle sinusoidal burst that is

frequency and phase locked to the caption data. After the clock

run-in signal, the blanking level is held for two data bits and is

followed by a Logic Level “1” start bit. Sixteen bits of data

follow the start bit. These consist of two 8-bit bytes, seven data

bits and one odd parity bit. The data for these bytes is stored in

closed captioning Data Registers 0 and 1.

The ADV7172/ADV7173 also supports the extended closed

captioning operation, which is active during even fields, and is

encoded on scan Line 284. The data for this operation is stored

in closed captioning extended Data Registers 0 and 1.

All clock run-in signals, and timing to support closed caption-

ing on Lines 21 and 284, are automatically generated by the

ADV7172/ADV7173. All pixels inputs are ignored during

Lines 21 and 284. Closed captioning is enabled.

APPENDIX 2

CLOSED CAPTIONING

12.91 s

D0–D6

10.003 s

33.764 s

50 IRE

40 IRE

FREQUENCY = FSC = 3.579545MHz

AMPLITUDE = 40 IRE

REFERENCE COLOR BURST

(9 CYCLES)

7 CYCLES

OF 0.5035 MHz

(CLOCK RUN-IN)

10.5

0.25 s

TWO 7-BIT + PARITY

ASCII CHARACTERS

(DATA)

27.382 s

BYTE 0

BYTE 1

Figure 69. Closed Captioning Waveform (NTSC)

FCC Code of Federal Regulations (CFR) 47 Section 15.119

and EIA608 describe the closed captioning information for

Lines 21 and 284.

The ADV7172/ADV7173 uses a single buffering method. This

means that the closed captioning buffer is only one byte deep,

therefore there will be no frame delay in outputting the closed

captioning data, unlike other 2-byte deep buffering systems.

The data must be loaded at least one line before (Line 20 or

Line 283) it is outputted on Line 21 and Line 284. A typical

implementation of this method is to use

VSYNC to interrupt a

microprocessor, which will in turn load the new data (two bytes)

every field. If no new data is required for transmission, zeros must

be inserted in both data registers; this is called NULLING. It is

also important to load “control codes,” all of which are double

bytes, on Line 21, or a TV will not recognize them. If there is a

message like “Hello World,” which has an odd number of char-

acters, it is important to pad it out to an even number to get

“end of caption” 2-byte control code to land in the same field.

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