GS1881_09 Datasheet, PDF (14/29 Pages) Gennum Corporation – Monolithic Video Sync Separators

4. Circuit Description
The block diagrams for the GS1881, GS4881 and GS4981, are shown in Figure 4-5
through Figure 4-7, with timing diagrams for the devices shown in Figure 4-8.
When stimulated by a composite input signal, the GS1881 and GS4881 sync separators
output composite sync, vertical sync, back porch, and odd/even field information. The
GS4981 substitutes the odd/even output of the GS4881 with a horizontal output. An
external resistor on Pin 6 is used to define internal currents allowing the devices to
accommodate horizontal scan rates from 15kHz to 130kHz.
4.1 Composite Video Input (Pin 2) and Composite
Sync Output (Pin 1)
Composite video is AC coupled via an external coupling capacitor to Pin 2. The device
clamps the sync tip of the input video to 1.5V (V_clamp) and then slices at 77mV above the
clamp voltage (V_slice). The resultant signal, provided at Pin 1, is a reproduction of the
input signal with the active video portion removed. As V_clamp and V_slice are supply and
input signal independent, for 0.5Vp-p signals (sync height of 143mV) slicing will occur
at just above the 50% point and for 2Vp-p signals (sync height of 572mV) slicing will
occur at approximately 13% of sync height.
The video signal path and composite sync slicing circuitry have been optimized and
compensated to achieve a low propagation delay that is stable over temperature. The
typical delay is 60ns with less than 3ns drift over the commercial temperature range.
The typical input clamp discharge current is 11μA. This current is optimal under normal
operating circumstances but needs to be increased when the clamp is trying to recover
from negative going impulse noise. The device improves the recovery time by raising a
NOSYNC flag when there has not been a sync pulse for approximately 1 1/2 horizontal
lines.
When this flag is raised the discharge current is increased by 85μA so that the recovery
time is sped up by nearly 10 times. Figure 4-1 shows a comparison between the recovery
times with and without the increased discharge current.
GS1881, GS4881, GS4981 Monolithic Video Sync
Separators
Data Sheet
6926 - 5
November 2009
14 of 29
Proprietary & Confidential