LMH1981 Datasheet, PDF (13/24 Pages) National Semiconductor (TI) – Multi-Format Video Sync Separator

LMH1981
www.ti.com
SNLS214H – APRIL 2006 – REVISED MARCH 2013
2500
2000
1500
PAL
1000
500
1080I
0
0
2
4
6
8
10
C_IN (PF)
Figure 11. Typical HSync Jitter vs. C_IN Values
Sync Lock Time
In addition to settling time, the LMH1981 has a predetermined sync lock time, T_SYNC-LOCK, before the outputs are
correct. Once the AC-coupled input has settled enough, the LMH1981 needs time to detect the valid video signal
and resolve the blanking & sync tip levels for 50% sync slicing before the output signals are correct.
For practical values of C_IN, T_SYNC-LOCK is typically less than 1 or 2 video fields in duration starting from the 1st
valid VSync output pulse to the valid HSync pulses beginning thereafter. VSync and HSync pulses are
considered valid when they align correctly with the input's vertical and horizontal sync intervals. Note that the
start-up time may vary depending on the video duty cycle, average picture level variations, and start point of
video relative to the vertical sync interval.
It is recommended for the outputs to be applied to the system after the start-up time is satisfied and outputs are
valid. For example, the oscilloscope screenshot in Figure 12 shows a typical start-up time of about 13.5 ms from
when an NTSC signal is switched in (no previous input) to when the LMH1981 outputs are valid.
Signal
Before C_IN
AC Coupled
Signal (V_IN)
Settling
Time
Sync Lock Time
HSync
Start- up Time
VSync
Figure 12. Typical Start-Up Time for NTSC Input to LMH1981 via 1 µF Coupling Capacitor
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