ADV7177 Analog Devices, ADV7177 Datasheet - Page 30

• Manufacturer Part Number: ADV7177
• Description: Integrated Digital CCIR-601 to PAL/NTSC Video Encoder
• Manufacturer: Analog Devices

ADV7177/ADV7178
The ADV7177/ADV7178 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. 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 Registers 0 and 1.
The ADV7177/ADV7178 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 captioning
on Lines 21 and 284 are generated automatically by the ADV7177/
ADV7178. All pixels inputs are ignored during Lines 21 and
284.
40 IRE
50 IRE
REFERENCE COLOR BURST
FREQUENCY = F
AMPLITUDE = 40 IRE
10.5
(9 CYCLES)
10.003 s
Figure 44. Closed Captioning Waveform (NTSC)
0.25 s
SC
= 3.579545MHz
27.382 s
CLOSED CAPTIONING
(CLOCK RUN-IN)
APPENDIX 2
OF 0.5035 MHz
7 CYCLES
12.91 s
–30–
FCC Code of Federal Regulations (CFR) 47 Section 15.119
and EIA608 describe the closed captioning information for
Lines 21 and 284.
The ADV7177/ADV7178 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 imple-
mentation of this method is to use VSYNC to interrupt a micro-
processor, which will in turn load the new data (two bytes) 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 will not recognize them. If
you have a message like “Hello World,” which has an odd num-
ber 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.
S
T
A
R
T
D0–D6
BYTE 0
TWO 7-BIT + PARITY
ASCII CHARACTERS
33.764 s
(DATA)
P
A
R
Y
T
I
D0–D6
BYTE 1
P
A
R
T
Y
I
REV. 0