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MAX7474

Adaptive Equalizer for Video Over Twisted Pair

Maxim Integrated 

Adaptive Equalizer for

Video Over Twisted Pair

The chroma gain for the MAX7474 is automatically

adjusted between 0dB and +12dB, while the low-fre-

quency gain is automatically adjusted between -6dB

and +3.6dB, providing full equalization for cable losses

that can occur when using up to 300m of UTP cable to

transmit CVBS signal with color burst.

Automatic Gain Control (AGC)

The automatic gain control circuit corrects low-frequen-

cy signal losses such that the output signal obtains a

nominal sync level (VSYNC_OUT) of 293mV. Differential

input sync pulse amplitude in the 210mVP-P and

600mVP-P range is automatically adjusted to the nomi-

nal output sync level (VSYNC_OUT).

If the sync is lost for 32 consecutive horizontal lines, the

LOS output goes high and OUT is set to a high-imped-

ance state.

Automatic Equalization Control (AEQ)

The automatic equalization control circuit corrects the

chroma signal attenuation such that the output-signal

burst amplitude is 293mV (subcarrier frequency, fSC =

3.58MHz or 4.43MHz). Differential input color-burst

amplitude in the 76mVP-P and 300mVP-P range is auto-

matically adjusted to the nominal output color-burst

level (VBURST_OUT).

The AGC and AEQ equalization take approximately 1s

(16,384 lines) to change from the lowest gain to the

highest gain. During this time, the chroma amplitude is

gradually increased from minimum to maximum.

Fixed Equalization Mode (LOB Is High)

If the color-burst amplitude at the output of the

MAX7474 (OUT) is less than 80mVP-P with the equalizer

gain set to maximum for 32 consecutive horizontal

lines, the loss-of-burst output (LOB) goes high. LOB

returns low when color bursts with amplitude greater

than 80mVP-P are present at OUT for 32 consecutive

horizontal lines.

For applications where the input signal has no color

burst, or the input burst amplitude is lower than the LOB

threshold voltage (VLOB), the MAX7474 asserts LOB and

automatically applies a user-defined fixed equalization

set by logic inputs FEQ1 and FEQ0 (see Table 1).

Table 1. Fixed Equalization Level Settings

PART FEQ1

GND

MAX7474 GND

VCC

X = VCC or GND.

FEQ0

GND

VCC

X

TYP (dB)

CABLE LENGTH

RANGE (m)

0

0 to 75

4.5

75 to 225

10.5

≥ 225

When using cable lengths greater than 75m, set the

fixed equalization level according to Table 1, or higher,

to ensure the device comes out of the fixed equaliza-

tion mode. If the fixed equalization is set to a lower gain

setting than specified in Table 1, the amplitude of the

color-burst signal at the output may remain smaller than

the LOB threshold (VLOB) of 80mVP-P, keeping the

device in LOB mode (see the Automatic Equalization

Control (AEQ) section). If the conditions in Table 1 are

not met, set FEQ1 to high to ensure the device always

comes out of LOB mode for longer cable lengths.

Output Video Driver

The MAX7474 features a single-ended video output dri-

ver with 0dB gain. The typical output voltage swing of

the output driver is 1.2VP-P. The output driver is capa-

ble of driving two AC-coupled or DC-coupled 150Ω

video loads. The output back-porch DC level is set by

an internal back-porch clamp circuit (see the Back-

Porch Clamp section). OUT enters a high-impedance

state when sync is lost for 32 consecutive horizontal

lines, asserting LOS.

Back-Porch Clamp

The MAX7474 features a back-porch clamp to set the

output blanking level. This clamp shifts the DC level of

the video signal so that the back-porch level is approxi-

mately equal to the voltage level on the BPLVL input. The

voltage range allowed at the BPLVL input is 1V to 1.6V.

Applications Information

Differential UTP Input Interface

Applications with differential input interface require

proper input voltage levels, termination, and AC-cou-

pling (Figure 1). The maximum differential input voltage

swing allowed is 2.4V. Use active drivers to drive the

cable as shown in Figure 1. For Category 5e cables,

the proper termination resistance is 50Ω to ground at

INN and INP inputs. Apply a 0.022µF coupling capaci-

tor at each input.

Output Interface

The video output is capable of driving an AC-coupled

or DC-coupled load. Use a 220µF or larger coupling

capacitor for AC-coupling. Figure 1 shows applications

with AC- and DC-coupled output interface.

Security Systems Application

Security systems typically consist of many cameras that

are viewed and recorded on a relatively smaller number

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