HORIZONTAL OUTPUT (pin 7 GS4981)
ODD/EVEN FIELD OUTPUT (pin 7 GS1881, GS4881)
As mentioned above, the odd/even field output of the
GS1881 and GS4881 is generated by comparing vertical
sync with an internal horizontal sync signal. This horizontal
sync signal is a true horizontal signal (i.e. maintained during
the vertical interval) and is outputted on pin 7 for the
GS4981. A delay of 190 ns from the video input and a width
of 6.5 µs are typically characteristics for this signal.
NTSC PAL and SECAM composite video standards are
interlaced video schemes and therefore have odd and even
fields. For odd fields the first broad vertical sync pulse is
coincident with the start of horizontal, while for even fields the
firstbroadverticalsyncpulsestartsinthemiddleofahorizontal
line. Thereforebycomparingtheverticalsyncwithan internally
generated horizontal sync the odd/even field information is
determined. This output is clocked out by the falling edge of
vertical sync. The odd/even output is low during even fields
and high during odd fields. This method of detecting odd and
even fields is very noise tolerant.
The windowing circuit which generates horizontal provides
excellent impulse noise immunity as shown in Figure 16. This
output buffer is an open collector stage with an internal
10 kΩ pull up resistor.
Noise during the pre-equalizing pulses does not affect the
output since the field decision is made at the beginning of the
vertical interval. This noise immunity is displayed in Figure 15
in which an extra pre-equalizing pulse has been added to the
video input with no negative effect on the odd/even field
information.
Video
Input
Impulse
Noise
Video
Input
Horizontal
Output
Impulse
Noise
Fig. 16 Horizontal Output
Even
Odd
Odd/Even
Output
Fig. 15 Odd/Even Output
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