Clamp

In record mode, signals are AC-coupled into IN3, IN4,

and IN5 through small 0.1µF capacitors. The clamp cir-

cuit internally DC-restores the signal for optimal perfor-

mance of the circuit so that the OUT4 blanking level sits

at GND and the OUT5 signal is centered at GND. Do

not connect any resistors in series or parallel with

clamp capacitors.

Analog Inputs/Outputs

Figures 4 and 5 show the signal flow diagrams for play-

back and record modes. P/R controls the mode of oper-

ation, while CVBSON and YCON select which outputs

are enabled. Table 1 shows the various combinations of

inputs and outputs that can be selected.

In playback mode, inputs IN1 and IN2 are active and

the outputs are selected by driving CVBSON and

YCON. IN1 receives the composite or luma (Y) signal

and level-shifts the signal by approximately 350mV at

the output such that the sync tip is below GND and the

active video is above GND. IN2 receives the chroma (C)

signal and level-shifts by approximately 500mV at the

output such that the signal is centered at GND. Drive

YCON high for outputs OUT1 and OUT2 to be active

and/or drive CVBSON high to combine IN1 and IN2 into

composite video and output on OUT3. Pull both control

inputs low to disable outputs and power down the output

buffers. OUT4 and OUT5 are always disabled in play-

back mode.

In record mode, drive YCON high and CVBSON low to

output Y/C format video on OUT4 and OUT5 or drive

CVBSON high and YCON low to output composite

video on OUT4. Pull both control inputs low to disable

OUT4 and OUT5 and power down the output buffers.

Driving both CVBSON and YCON high in record mode

results in an invalid state and all output buffers power

down. The Y signal inputs on IN3, C signal inputs on

IN4, and composite signal inputs on IN5. OUT4 is

shared between the composite signal and Y signal. The

composite or Y signal that outputs on OUT4 has the

blanking level set to GND with the sync tip below GND

and active video above GND. The C signal that outputs

on OUT5 is centered at GND. The outputs get level-

shifted by the bias of the video processor input after

the AC-coupling capacitors.

Bidirectional Video Filters/Buffers for Composite

and S-Video Portable Media Devices

8

_______________________________________________________________________________________

CVBSON

YCON

P/R

LPF

LPF

VIDEO

ADC

VIDEO

DAC

VIDEO

ADC

CONTROL

LOGIC

VIDEO

DAC

VIDEO

PROCESSOR

MAX7481

MAX7482

S4

S5

S1

S2

S3

S6

75

Ω

0.1

μF

75

Ω

75

Ω

0.1

μF

0.1

μF

S7

S8

Figure 3. Signal Flow Diagram