ADV7170/ADV7171
FCC Code of Federal Regulations (CFR) 47 Section 15.119 and
EIA608 describe the closed captioning information for Line 21
and Line 284.
APPENDIX 2—CLOSED CAPTIONING
The ADV7170/ADV7171 support closed captioning, conform-
ing to the standard television synchronizing waveform for color
transmission. Closed captioning is transmitted during the
blanked active line time of the odd fields Line 21 and the even
fields Line 284.
The ADV7170/ADV7171 use a single buffering method. This
means that the closed captioning buffer is only one 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 at least one line before (Line 20 or
Line 283) it is outputted on Line 21 and Line 284. A typical
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. 16 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 Register 0 and Closed Captioning Data
Register 1.
VSYNC
implementation of this method is to use
to interrupt a
microprocessor, which in turn loads the new data (two bytes) in
every field. If no new data is required for transmission, you
must insert zeros in both the 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 does not recognize them.
If you have a message like “Hello World,” which has an odd
number of characters, 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.
The ADV7170/ADV7171 also support 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 Register 0 and Closed
Captioning Extended Data Register 1.
All clock run-in signals and timing to support closed captioning
on Line 21 and Line 284 are automatically generated by the
ADV7170/ADV7171. All pixel inputs are ignored during
Line 21 and Line 284.
10.5 ± 0.25μs
12.91μs
7 CYCLES
OF 0.5035MHz
(CLOCK RUN-IN)
TWO 7-BIT + PARITY
ASCII CHARACTERS
(DATA)
P
A
R
I
T
Y
S
T
A
R
T
P
A
R
I
T
Y
D0–D6
D0–D6
50 IRE
40 IRE
BYTE 1
BYTE 0
REFERENCE COLOR BURST
(9 CYCLES)
FREQUENCY = F = 3.579545MHz
SC
AMPLITUDE = 40 IRE
10.003μs
27.382μs
33.764μs
Figure 56. Closed Captioning Waveform (NTSC)
Rev. C | Page 40 of 64
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