evertz 8084 Instruction Manual page 131

A APPLICATION NOTE 3: CONFIGURING EVERTZ CLOSED CAPTION
ENCODERS TO BRIDGE CAPTION DATA AROUND A DVE DEVICE
A.1 OVERVIEW
In a broadcast facility, it is common for video to pass through a piece of processing hardware that does
not accurately preserve the caption data stored on line 21. One of the features of Evertz 8084 and
8084AD Closed Caption Encoders is the ability to extract and re-encode closed caption data around
such devices. The configuration is typically referred to as a "VBI bridge".
describes several ways that a VBI bridge for caption data on SDI digital video can be implemented
using Evertz captioning hardware.
A.2 BACKGROUND
In NTSC analog video, closed caption data exists as a waveform that resides on video line 21. This
waveform carries the data that is extracted by closed caption decoders to produce the human-readable
text on the viewer's screen.
IRE H Sync Color Burst
50
25
0
-40
Figure A-1: Typical Line 21 Closed Caption Analog Waveform
Figure A-1 shows the typical waveform of the signal. The line 21 data signal consists of a sine wave
clock burst, start bits, and two data characters. Each character consists of seven data bits with odd
parity. The clock rate is 32 times the horizontal line frequency or 503.4 KHz. The signal amplitude is
50 IRE units peak to peak. The full specification for the caption data waveform is given in the EIA-608B
standard.
Caption encoders for SDI digital video duplicate this waveform in the digital domain, so that digital-to-
analog equipment at the point of transmission can process line 21 as if it were just another line of active
video. However, digital video processing equipment in a broadcast facility may also treat line 21 as
simply a line of active video, corrupting line 21 and damaging the caption data. An example would be a
Digital Video Effects (DVE) device that shifts the picture either horizontally or vertically to display a text
message or alternate video on the remainder of the screen.
The purpose of the "VBI bridge" is to extract data from the closed caption waveform before it is
corrupted by a piece of video processing hardware, then re-insert the captions after the output of the
offending hardware.
APPLICATION NOTE 2
Models 8084/8084AD/8085 Closed Caption Encoder Manual
Clock Run-In Start Bits
S1 S2 S3 b0
Revision 1.6.6
Character One
b1 b2 b3 b4 b5 b6 P1 b0
This application note
Character Two
b1 b2 b3 b4 b5 b6 P2
AN2-1