LMH1981 Datasheet, PDF (17/24 Pages) National Semiconductor (TI) – Multi-Format Video Sync Separator

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LMH1981
SNLS214H – APRIL 2006 – REVISED MARCH 2013
11H
V_IN
LINE# 566 567 568 569 570 571 572 573 574 575 576 577 578
CSOUT
HSOUT
(CLOCK)
VSOUT
3^rd HSync after VSync leading edge
VFOUT
(DATA)
11-BIT BINARY DATA STREAM
0
1
0
0
0
1
0
1
1
1
START
01000101111b = 559 HORIZONTAL LINES PER FIELD
1
END
Figure 17. Video Format Output for Interlaced Format, 1080I Field 2
OPTIONAL CONSIDERATIONS
Optional Input Filtering
An external filter may be necessary if the video signal has considerable high-frequency noise or has large
chroma amplitude that extends near the sync tip. A simple RC low-pass filter with a series resistor (R_S) and a
capacitor (C_F) to ground can be used to improve the overall signal-to-noise ratio and sufficiently attenuate
chroma such that minimum peak of its amplitude is above the 50% sync slice level. To achieve the desired filter
cutoff frequency, it’s advised to vary C_F and keep R_S small (ie. 100Ω) to minimize sync tip clipping due to the
voltage drop across R_S. Note that using an external filter will increase the propagation delay from the input to the
outputs.
In applications where the chroma filter needs to be disabled when non-composite video (ie: ED/HD video) is
input, it is possible to use a transistor to switch open C_F’s connection to ground as shown in Figure 18. This
transistor can be switched off/on by logic circuitry to decode the lines-per-field data output (VFOUT). As shown in
Table 4, NTSC and PAL both have 1 (logic high) for the 3rd bit of VFOUT. If the logic circuitry detects 0 (logic
low) for this bit, indicating non-composite video, the transistor can be turned off to disable the chroma filter.
VIDEO IN
75:
R_S
C_F
C_{IN VIN}
LMH1981
10 k:
LOGIC
CIRCUIT
VFOUT
HSOUT
VSOUT
ED/HD = ³0´
NTSC/PAL = ³1´
Figure 18. External Chroma Filter with Control Circuit
AC-Coupled Video Sources
An AC coupled video source typically has a 100 µF or larger output coupling capacitor (C_OUT) for protection and
to remove the DC bias of the amplifier output from the video signal. When the video source is load terminated,
the average value of the video signal will shift dynamically as the video duty cycle varies due to the averaging
effect of the C_OUT and termination resistors. The average picture level or APL of the video content is closely
related to the duty cycle.
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