flip-flop is toggled by the default comparator, starting the

vertical sync pulse at pin 3 of the LM1881. If the default

vertical sync period ends before the end of the input vertical

sync period, then the falling edge of the vertical sync (posi-

tive pulse at the “D” flip-flop) will clock the high output from

the comparator with V

ger the oscillator, generating a second vertical sync output

pulse. The “Vertical Default Sync Delay Time vs R

shows the relationship between the R

delay time from the start of the vertical sync period before

the default vertical sync pulse is generated. Using the NTSC

example again the smallest resistor for R

Ω. The

vertical default time delay is about 50 µs, much longer than

the 30 µs serration pulse spacing.

A common question is how can one calculate the required

R

pulses during the vertical blanking. If the default vertical sync

is to be used this is a very easy task. Use the “Vertical

Default Sync Delay Time vs R

essary R

sync output signal. If a second pulse is undesirable, then

check the “Vertical Pulse Width vs R

the vertical output pulse will extend beyond the end of the

input vertical sync period. In most systems the end of the

vertical sync period may be very accurate. In this case the

preferred design may be to start the vertical sync pulse at the

end of the vertical sync period, similar to starting the vertical

sync pulse after the first serration pulse. A VGA standard is

to be used as an example to show how this is done. In this

standard a horizontal line is 32 µs long. The vertical sync

period is two horizontal lines long, or 64 µs. The vertical

default sync delay time must be longer than the vertical

sync period of 64 µs. In this case R

680 k

Ω.R

integrator to reach V

the input pulse. The first graph can be used to confirm that

R

vertical serration pulse separation, use the actual pulse

width of the vertical sync period, or 64 µs in this example.

This graph is linear, meaning that a value as large as 2.7 M

Ω

can be used for R

30 µs). Due to leakage currents it is advisable to keep the

value of R

Ω. In this example a value of 1.0

M

Ω is selected, well above the minimum of 680 kΩ. With this

value for R

pulse of the LM1881 is about 340 µs.

00915003

FIGURE 1. (a) Composite Video; (b) Composite Sync; (c) Vertical Output Pulse;

(d) Odd/Even Field Index; (e) Burst Gate/Back Porch Clamp

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