6

Multimedia ICs

BA7230LS

(1) Matrix circuit

The R, G and B inputs are clamped to 3.2V by the

compensate for the signal's 6dB attenuation in the delay

(2) Balanced modulator circuit

Color difference signals are modulated (rectangular 2-

phase balanced modulation) with color subcarriers

(3.58MHz) having a 90° phase difference. This is

called the carrier color signal.

The carrier color signal is mixed with color burst signals

chronization signal is added) to create the NTSC com-

(3) Switch circuit

Signal Ys (pin 23) switches between video input and

RGB composite signals. Performing this switching at

high speeds results in superimposition.

(4) Color subcarrier oscillator circuit

The subcarrier oscillator circuit for RGB input. This cir-

cuit is synchronized with the video input color burst sig-

nal extracted by BFP (burst flag pulses) during super-

imposition, preventing any unnatural color disturbance

due to switching between RGB and video input.

This oscillator circuit generates the RGB color burst

signal. An attached variable resistor can be used to

change the amplitude of the color burst signal and to

adjust its phase relative to the video color burst signal.

This oscillator circuit remains in the free-running state

when there is no video input.

(5) During superimposition, video input can be lowered

by about 5dB using an HDP (half-down pulse), darken-

ing the background and making RGB input easier to

see.

13

R

14

G

15

B

10k

Ω

5.1k

Ω

27k

Ω

BA7230LS

Y

Fig. 4

OUT

RGB

COMPOSITE

VIDEO IN

Fig. 6

9

20pF

1.2k

Ω

3.58MHz

87

VCXO

Fig. 7

VIDEO IN

HDP

(Half Down

Pulse)

OUT

RGB

COMPOSITE

– 5dB

Fig. 8

R - Y

R - Y

B - Y

3.58MHz (0

°)

C carrier color signal

(combining of R-Y and B-Y)

B - Y

θ

µs

90

°

θ

90

°

Time

1

3.58

Balanced

modulator

Carrier

color signal

3.58MHz (90

°)

Balanced

modulator

+

Fig. 5

cos2

πfst

1.14

+

sin2

πfst

2.03