LM1881, LM1881-X
Application Notes
(Continued)
00915004
*Components Optional, See Text
FIGURE 2.
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 appli-
cations 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.
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
A capacitor is
charged during the period between sync pulses and dis-
charged when the sync pulse is present. The period between
normal horizontal sync pulses is enough to allow the capaci-
tor 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
7
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.
BURST/BACKPORCH OUTPUT PULSE
In a composite video signal, the chroma burst is located on
the backporch of the horizontal blanking period. This period,
approximately 4.8 µs long, is also the black level reference
for the subsequent video scan line. The LM1881 generates a
pulse at Pin 5 that can be used either to retrieve the chroma
burst from the composite video signal (thus providing a
subcarrier synchronizing signal) or as a clamp for the DC
restoration of the video waveform. This output is obtained
simply by charging an internal capacitor starting on the
trailing edge of the horizontal sync pulses. Simultaneously
the output of Pin 5 is pulled low and held until the capacitor
charge circuit times out — 4 µs later. A shorter output burst
gate pulse can be derived by differentiating the burst output
www.national.com
NSC [ NATIONAL SEMICONDUCTOR ]
