The Closed-Caption Decoder Section - Sanyo AVM-2550S Training Manual

The closed-caption decoder system used in the VB7C type chassis is capable
of processing and displaying all of the latest standard line 21 closed-caption
transmissions.
The system employed in the VB7C chassis is comprised of two blocks: the
Data Slicer and the Screen Display Controller (OSD). Notice from the block
diagram below that the Data Slicer and the Screen Display Controller are
integrated into the CPU. The other components necessary for displaying the
caption data are shared circuits already in use for normal TV video.
Data Slicer
The Data Slicer extracts the caption data encoded on line 21 field 1 of the
composite video signal. The data is limited, shaped to a digital signal and
output to the OSD block through the Internal Bus. The Data Slicer also
generates the clock signal, the 21H detection pulse, and the odd/even field
discrimination pulse required for decoding the caption data.
THE CLOSED CAPTION DECODER SECTION
On Screen Display
The OSD block interprets the digital data signal input from the Data Slicer
through the Internal Bus. Specifically, the Data Slicer applies error detection
and correction to the incoming data and evaluates the data for display format
and character attributes. The OSD then directs the operation of the Display
RAM and Character ROM of the OSD. In addition, the OSD block controls the
mode selection (Caption/Text, C1/C2 etc.) based on the on-screen menu
directives. The OSD generates the R, G, B and blanking signals and controls
the character type (upper/lower case, italics and color attributes) based on the
commands from the OSD Controller. Included in the OSD section is a Display
RAM for storage and display of the recovered data and a Character
Generator. Serial data input from the Data Slicer is written to the Display RAM
and input to the Character Generator. The Character Generator contains the
Character ROM which holds the dot pattern for all the characters. The
Character Generator outputs the characters corresponding to the display
data. The character display area is a 26 x 16 dot matrix surrounding a 18 x
13 character. The additional area provides for spacing between characters
and underlining.
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