5-2
2. OUTLINE OF SYNC SIGNAL PROCESSING CIRCUIT
As mentioned above, signal input at D-SUB connector from personal computer, sometimes shows various shape of sync signal.
Representative signals are described in Table-2.
Kind Resolution fH Fv Hor. Sync polarity Ver. Sync polarity
IBM PGC 640 x 480 30.5kHz 60Hz Composite Sync
IBM VGA480 640 x 480 31.5kHz 60Hz Negative Negative
IBM VGA400 640 x 400 31.5kHz 70Hz Negative Positive
IBM VGA350 640 x 350 31.5kHz 70Hz Positive Negative
SVGA 800 x 600 35.2kHz 56Hz Positive Positive
Macintosh 13Ó 640 x 480 35.0kHz 67Hz Sync on Green or Composite Sync
VESA VGA 640 x 480 37.9kHz 72Hz Negative Negative
Table-2
Roughly classified, they are of two shapes; one is, like Macintosh, SYNC ON GREEN which is imposed on video signal, and
the other is the output in TTL level separated from video signal. The TTL level method is classified to Composite Sync which
combines horizontal and vertical sync, and to Separate Sync which separates respectively.
And besides, in Separate Sync method, polarity is different by kind of signal.
Even though these various sync signal are input, always positive polarity of hor and ver sync signal is supplied to horizontal
and vertical sync osc circuit. This is the role of this circuit.
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