GS1881_09 Datasheet, PDF (15/29 Pages) Gennum Corporation – Monolithic Video Sync Separators

VIDEO INPUT
IMPULSE NOISE
COMPOSITE SYNC RECOVERY TIME without INCREASED DISCHARGE CURRENT (LM1881)
RECOVERY TIME T1
COMPOSITE SYNC RECOVERY TIME with INCREASED DISCHARGE CURRENT (GS1881, GS4881, GS4981)
RECOVERY TIME
T1 / 10
Figure 4-1: Impulse Noise: Recovery Time Comparison
4.2 Back Porch Output (Pin 5)
In an NTSC composite video signal, horizontal sync pulses are followed by the back
porch interval. The device generates a negative going pulse on Pin 5 during this time. It
is delayed typically 500ns from the rising edge of sync and has a typical width of 2.5μs.
Both of these times are set by the external R_SET resistor.
During the pre-equalizing, vertical sync, and post-equalizing periods, composite sync
doubles in frequency. The GS4881 and GS4981 maintain the back porch output at the
horizontal rate due to Back Porch Enable (BPEN), generated by the internal windowing
circuit, which forces back porch to be asserted at the horizontal rate. This gating circuit
is also the reason for the excellent impulse noise immunity of the back porch output as
shown in Figure 4-2.
Video
Input
Back
Porch
Output
GS4881
GS4981
Impulse
Noise
Figure 4-2: Back Porch Noise Immunity
GS1881, GS4881, GS4981 Monolithic Video Sync
Separators
Data Sheet
6926 - 5
November 2009
15 of 29
Proprietary & Confidential