LM1881
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SNLS384F –FEBRUARY 1995–REVISED MARCH 2013
*Components Optional, See Text
Figure 9.
ODD/EVEN FIELD PULSE
An unusual feature of LM1881 is an output level from Pin 7 that identifies the video field present at the input to
the LM1881. This can be useful in frame memory storage applications or in extracting test signals that occur in
alternate fields. For a composite video signal that is interlaced, one of the two fields that make up each video
frame or picture must have a half horizontal scan line period at the end of the vertical scan—i.e., at the bottom of
the picture. This is called the “odd field” or “even field”. The “even field” or “field 2” has a complete horizontal
scan line at the end of the field. An odd field starts on the leading edge of the first equalizing pulse, whereas the
even field starts on the leading edge of the second equalizing pulse of the vertical retrace interval. Figure 8(a)
shows the end of the even field and the start of the odd field.
To detect the odd/even fields the LM1881 again integrates the composite sync waveform (Figure 9). A capacitor
is charged during the period between sync pulses and discharged when the sync pulse is present. The period
between normal horizontal sync pulses is enough to allow the capacitor voltage to reach a threshold level of a
comparator that clears a flip-flop which is also being clocked by the sync waveform. When the vertical interval is
reached, the shorter integration time between equalizing pulses prevents this threshold from being reached and
the Q output of the flip-flop is toggled with each equalizing pulse. Since the half line period at the end of the odd
field will have the same effect as an equalizing pulse period, the Q output will have a different polarity on
successive fields. Thus by comparing the Q polarity with the vertical output pulse, an odd/even field index is
generated. Pin 7 remains low during the even field and high during the odd field.
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