9

FN7018.1

Applications Information

Video In

A simplified block diagram is shown following page.

nominally 0.1µF. Clamp charge current will prevent the

signal on pin 2 from going any more negative than Sync Tip

Ref, about 1.5V. This charge current is nominally about

1mA. A clamp discharge current of about 10µA is always

lost between sync pulses that must be replaced during sync

pulses. The droop voltage that will occur can be calculated

from IT = CV, where V is the droop voltage, I is the discharge

current, T is the time between sync pulses (sync period -

An NTSC video signal has a horizontal frequency of

15.73kHz, and a sync tip width of 4.7µs. This gives a period

of 63.6µs and a time T = 58.9µs. The droop voltage will then

be V = 5.9mV. This is less than 2% of a nominal sync tip

amplitude of 286mV. The charge represented by this droop

is replaced in a time given by T = CV/I, where I = clamp

charge current = 1mA. Here T = 590ns, about 12% of the

enough so that the droop voltage does not approach the

switching threshold of the internal comparator.

Fixed Gain Buffer

The clamped video signal then passes to the fixed gain

buffer which places the sync slice level at the equivalent

level of 70mV above sync tip. The output of this buffer is

presented to the comparator, along with the slice reference.

The comparator output is level shifted and buffered to TTL

levels, and sent out as Composite Sync to pin 1.

Burst

A low-going Burst pulse follows each rising edge of sync,

FIGURE 4. STANDARD (NTSC INPUT) H. SYNC DETAIL

EL1881C