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Data Sheet

ADV7340/ADV7341

Rev. C | Page 81 of 108

SD CLOSED CAPTIONING

Subaddress 0x91 to Subaddress 0x94

The ADV7340/ADV7341 support closed captioning conforming

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 the even fields.

Closed captioning consists of a seven-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 the Logic 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

the SD closed captioning registers (Subaddress 0x93 to

Subaddress 0x94).

The ADV7340/ADV7341 also support the extended closed

captioning operation, which is active during even fields and

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

in the SD closed captioning registers (Subaddress 0x91 to

Subaddress 0x92).

The ADV7340/ADV7341 automatically generate all clock run-

in signals and timing that support closed captioning on Line 21

and Line 284. All pixels inputs are ignored on Line 21 and on

Line 284 if closed captioning is enabled.

The FCC Code of Federal Regulations (CFR) 47 Section 15.119

and EIA-608 describe the closed captioning information for

Line 21 and Line 284.

The ADV7340/ADV7341 use a single buffering method. This

means that the closed captioning buffer is only 1-byte deep.

Therefore, there is no frame delay in outputting the closed

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

The data must be loaded one line before it is output on Line 21

and Line 284. A typical implementation of this method is to use

VSYNC to interrupt a microprocessor, which in turn loads the

new data (two bytes) in every field. If no new data is required

for transmission, 0s 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. Otherwise, a TV does not

recognize them. If there is a message such as “Hello World”

that has an odd number of characters, it is important to add a

blank character at the end to make sure that the end-of-caption,

2-byte control code lands in the same field.

Figure 98. SD Closed Captioning Waveform, NTSC

D0 TO D6

10.5 ± 0.25µs

12.91µs

7 CYCLES OF

0.5035MHz

CLOCK RUN-IN

REFERENCE COLOR BURST

(9 CYCLES)

FREQUENCY = FSC = 3.579545MHz

AMPLITUDE = 40 IRE

50 IRE

40 IRE

10.003µs

27.382µs

33.764µs

BYTE 1

BYTE 0

TWO 7-BIT + PARITY

ASCII CHARACTERS

(DATA)

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