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. Fig-

ure 1(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 2). 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

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

00915004

*Components Optional, See Text

FIGURE 2.

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