Vertical Pulse

Width vs Temperature

Supply Current vs

Supply Voltage

00915011

00915002

Application Notes

The LM1881 is designed to strip the synchronization signals

from composite video sources that are in, or similar to, the

N.T.S.C. format. Input signals with positive polarity video

(increasing signal voltage signifies increasing scene bright-

ness) from 0.5V (p-p) to 2V (p-p) can be accommodated.

The LM1881 operates from a single supply voltage between

5V DC and 12V DC. The only required external components

besides a power supply decoupling capacitor at pin 8 and a

set current decoupling capacitor at pin 6, are the composite

input coupling capacitor at pin 2 and one resistor at pin 6 that

sets internal current levels. The resistor on pin 6 (i.e. R

allows the LM1881 to be adjusted for source signals with line

scan frequencies differing from 15.734 kHz. Four major sync

signals are available from the I/C; composite sync including

both horizontal and vertical scan timing information; a verti-

cal sync pulse; a burst gate or back porch clamp pulse; and

an odd/even output. The odd/even output level identifies

which video field of an interlaced video source is present at

the input. The outputs from the LM1881 can be used to

gen-lock video camera/VTR signals with graphics sources,

provide identification of video fields for memory storage,

recover suppressed or contaminated sync signals, and pro-

vide timing references for the extraction of coded or uncoded

data on specific video scan lines.

To better understand the LM1881 timing information and the

type of signals that are used, refer to Figure 1(a-e) which

shows a portion of the composite video signal from the end

of one field through the beginning of the next field.

COMPOSITE SYNC OUTPUT

The composite sync output, Figure 1(b), is simply a repro-

duction of the signal waveform below the composite video

black level, with the video completely removed. This is ob-

tained by clamping the video signal sync tips to 1.5V DC at

Pin 2 and using a comparator threshold set just above this

voltage to strip the sync signal, which is then buffered out to

Pin 1. The threshold separation from the clamped sync tip is

nominally 70 mV which means that for the minimum input

level of 0.5V (p-p), the clipping level is close to the halfway

point on the sync pulse amplitude (shown by the dashed line

on Figure 1(a). This threshold separation is independent of

the signal amplitude, therefore, for a 2V (p-p) input the

clipping level occurs at 11% of the sync pulse amplitude. The

charging current for the input coupling capacitor is 0.8 mA,

Normally the signal source for the LM1881 is assumed to be

clean and relatively noise-free, but some sources may have

excessive video peaking, causing high frequency video and

chroma components to extend below the black level refer-

ence. Some video discs keep the chroma burst pulse

present throughout the vertical blanking period so that the

burst actually appears on the sync tips for three line periods

instead of at black level. A clean composite sync signal can

be generated from these sources by filtering the input signal.

When the source impedance is low, typically 75

Ω, a 620Ω

resistor in series with the source and a 510 pF capacitor to

ground will form a low pass filter with a corner frequency of

500 kHz. This bandwidth is more than sufficient to pass the

sync pulse portion of the waveform; however, any subcarrier

content in the signal will be attenuated by almost 18 dB,

effectively taking it below the comparator threshold. Filtering

will also help if the source is contaminated with thermal

noise. The output waveforms will become delayed from be-

tween 40 ns to as much as 200 ns due to this filter. This

much delay will not usually be significant but it does contrib-

ute to the sync delay produced by any additional signal

processing. Since the original video may also undergo pro-

cessing, the need for time delay correction will depend on

the total system, not just the sync stripper.

VERTICAL SYNC OUTPUT

A vertical sync output is derived by internally integrating the

composite sync waveform (Figure 2). To understand the

generation of the vertical sync pulse, refer to the lower left

hand section Figure 2. Note that there are two comparators

in the section. One comparator has an internally generated

voltage reference called V

other comparator has an internally generated voltage refer-

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