LOGIC

CIRCUIT

LMH1981

VFOUT

HSOUT

VSOUT

75:

RS

CIN

CF

10 k:

VIN

VIDEO IN

ED/HD =

³0´

NTSC/PAL =

³1´

VIN

CSOUT

HSOUT

(CLOCK)

VSOUT

LINE#

VFOUT

(DATA)

567

568

569

571

570

572

574

573

576

575

577

578

566

1

0

0

0

1

1

0

1

1

1

0

01000101111b = 559 HORIZONTAL LINES PER FIELD

START

11H

11-BIT BINARY DATA STREAM

END

3

rd HSync after VSync leading edge

LMH1981

www.ti.com

SNLS214H – APRIL 2006 – REVISED MARCH 2013

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 (RS) and a

capacitor (CF) 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 CF and keep RS small (ie. 100Ω) to minimize sync tip clipping due to the

voltage drop across RS. 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 CF’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.

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 (COUT) 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 COUT and termination resistors. The average picture level or APL of the video content is closely

related to the duty cycle.

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