'PIC-based Closed Caption Decoder'
|By popular demand, the source code for the closed caption decoder that
Rich Ottosen and I designed is now available on the web. As with my other
PIC projects, it is covered by the GNU General Public License, Version 2.
The URL is
A copy of the text of the web page is attached below for your convenience.
This is a closed-caption decoder with serial output, based on a PIC16C71, an
Elantec EL4581C sync separator, and an LM393 dual comparator (for data slicing
with automatic threshold).
This project was partially inspired by the article "Build the Text Grabber" by
Kelly McArthur in the November 1994 issue of Electronics Now, but where Kelly
used an 8031 based design requiring a total of 16 chips, we found it possible
to implement essentially the same functionality with four by using a PIC.
The code for this project is available under the terms of the Free Software
Foundation's General Public License, Version 2. If you agree to the terms of
the license, you may obtain a copy via FTP.
The code won't do you much good if you don't have the schematic. I don't yet
have it available in electronic form, but if you want it send me email and
I'll arrange to get it to you.
We originally used a common National Semiconductor LM1881 sync separator (as
did Kelly), but it seemed fairly unreliable so we switched to the Elantec
EL4581C, which is a pin-compatible improved performance part. It works much
Because the field output of the LM1881 was particularly unreliable, we ended
up using only the composite sync output of the sync separator, and deriving
the horizontal sync, vertical sync and the field in software. The Elantec
part probably generates perfectly good sync and field outputs, but it would
be more trouble than it is worth to switch back.
Using other PICs
Rich has recently converted the code to run on a PIC16C56. The only advantage
of the '56 is that Rich had a Parallax downloader for the '5x family, and that
the '56 is cheaper than the '71. Microchip now offers a '61 (which is a '71
without the A/D converter), which probably has comparable price to the '56.
Rich points out that the '56 version of the code isn't very maintainable and
the RAM is completely full.
Microchip's recently announced PIC16C62X series parts may be an even better
fit for this application, since they have two built-in comparators which might
replace the separate LM393.
Another good article regarding capturing digital data from video signals is
"Exploring the Vertical Blanking Interval" by Mark Barnes, published in the
April 1994 issue of Circuit Cellar Ink. Mark's design is also 8031 based and
requires 24 chips, but in addition to closed captioning it also decodes
network time stamps, World System Teletext (WST), and North American Basic
Teletext Specification (NABTS).
Circuit Cellar also published in their May 1993 issue an article on the
Motorola MC68HC05CC1 chip, which is a single chip microcomputer specifically
designed for closed-caption decoding.
"Recommended Practice for Line 21 Data Service", ANSI/EIA-608-94, Sept. 1994
Electronic Industries Accociation
2500 Wilson Boulevard
Arlington VA 22201
American National Standards Institute
11 West 42nd Street
New York, NY 10036
"Television Captioning for the Deaf: Signal and Display Specifications"
Engineering Report No. E-7709-C
Public Broadcasting Service, May 1980
Public Broadcasting Service
1320 Braddock Place
Alexandria, VA 22314
"Telecaption II Decoder Module Performance Specification"
National Captioning Institute, May 1985
National Captioning Institute, Inc.
5203 Leesburg Pike
Falls Church, VA 22041
47 CFR 15.119
47 CFR 73.699
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