BACK PORCH OUTPUT (pin 5)
CIRCUIT DESPCRIPTION
The block diagrams for the GS1881, GS4881 and GS4981,
are shown in Figures 17 through 19, with timing diagrams for
the devices shown in Figure 20.
InanNTSCcompositevideosignal,horizontalsyncpulsesare
followed by the back porch interval. The device generates a
negative going pulse on pin 5 during this time. It is delayed
typically 500 ns from the rising edge of sync and has a typical
width of 2.5µs. Both of these times are set by the external RSET
resistor.
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
todefineinternalcurrentsallowingthedevicestoaccommodate
horizontal scan rates from 15 kHz to 130 kHz.
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 14.
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 1.5 V ( Vclamp ) and then slices at 77 mV above the
clamp voltage ( Vslice ). The resultant signal, provided at
pin 1, is a reproduction of the input signal with the active video
portion removed. As Vclamp and Vslice are supply and input
signal independent, for 0.5 V p-p signals (sync height of 143
mV) slicing will occur at just above the 50% point and for 2 V
p-p signals (sync height of 572 mV) slicing will occur at
approximately 13% of sync height.
Video
Input
Impulse
Noise
Back
Porch
Output
GS4881
GS4981
The video signal path and composite sync slicing circuitry
have been optimized and compensated to achieve a low
propagationdelaythatisstableovertemperature. Thetypical
delay is 60 ns with less than 3 ns drift over the commercial
temperature range.
Fig. 14 Back Porch Noise Immunity
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 11/2 horizontal lines.
When this flag is raised the discharge current is increased by
85 µA so that the recovery time is sped up by nearly 10 times.
Figure 13 shows a comparison between the recovery times
with and without the increased discharge current.
VIDEO INPUT
The GS1881 does not gate the Back Porch which allows for
total pin compatibility with the LM1881.
VERTICAL SYNC OUTPUT (pin 3)
The vertical sync interval is detected by integrating the
composite sync pulses. The first broad vertical sync pulse
causes an internal capacitor to charge past a fixed threshold
and raises an internal vertical flag. Once the vertical flag is
raised, the positive edge of the next serration clocks out the
vertical output. When the vertical sync interval ends, the first
post equalizing pulse is unable to charge the capacitor
sufficiently, causing the internal vertical flag to go high. The
rising edge of the second post-equalizing pulse then clocks
out the high flag to end the vertical sync pulse. The vertical
output is clocked in and out and therefore is a fixed width of
197.7µs (3H + 4.7 µs + 2.3 µs). In the case of a non-standard
vertical interval that has no serrations, a second internal
capacitor is charged and clocks the vertical pulse out after
typically 65µs. In this case the end of the vertical pulse will still
betherisingedgeofthesecondpost-equalizingpulse. Asthe
vertical detector is designed as a true integrator, it provides
improved noise immunity.
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
Fig. 13 Impulse Noise: Recovery Time Comparison
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