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
12.91s
S
T
A
R
T
P
A
R
I
T
Y
P
A
R
I
T
Y
D0–D6
D0–D6
10.003s
33.764s
50 IRE
40 IRE
FREQUENCY = F
SC
= 3.579545MHz
AMPLITUDE = 40 IRE
REFERENCE COLOR BURST
(9 CYCLES)
7 CYCLES
OF 0.5035 MHz
(CLOCK RUN-IN)
10.5 0.25s
TWO 7-BIT + PARITY
ASCII CHARACTERS
(DATA)
27.382s
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.
