Datasheet

ADV7390/ADV7391/ADV7392/ADV7393 Data Sheet
Rev. J | Page 78 of 107
SD CLOSED CAPTIONING
Subaddress 0x91 to Subaddress 0x94
The ADV7390/ADV7391/ADV7392/ADV7393 support closed
captioning conforming to the standard television synchronizing
waveform for color trans-mission. When enabled, 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 can be enabled using Subaddress 0x83, Bits[6:5].
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 a Logic 1 start bit. Sixteen bits of data follow
the start bit. The data consists of two 8-bit bytes (seven data bits
and one odd parity bit per byte). The data for these bytes is
stored in SD closed captioning registers (Subaddress 0x93 to
Subaddress 0x94).
The ADV7390/ADV7391/ADV7392/ADV7393 also support the
extended closed captioning operation, which is active during
even fields and encoded on Line 284. The data for this operation is
stored in SD closed captioning registers (Subaddress 0x91 to
Subaddress 0x92).
The ADV7390/ADV7391/ADV7392/ADV7393 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 Line 284 if closed captioning is enabled.
The FCC Code of Federal Regulations (CFR) Title 47 Section
15.119 and EIA-608 describe the closed captioning information
for Line 21 and Line 284.
The ADV7390/ADV7391/ADV7392/ADV7393 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.
D0 TO D6 D0 TO D6
10.5 ± 0.25µs
12.91µs
7 CYCLES OF
0.5035MHz
CLOCK RUN-IN
REFERENCE COLOR BURST
(9 CYCLES)
FREQUENCY = F
SC
= 3.579545MHz
AMPLITUDE = 40 IRE
50 IRE
40 IRE
10.003µs
27.382µs
33.764µs
BYTE 1BYTE 0
TWO 7-BIT +PARITY
ASCII CHARACTERS
(DATA)
S
T
A
R
T
P
A
R
I
T
Y
P
A
R
I
T
Y
06234-101
Figure 103. SD Closed Captioning Waveform, NTSC
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