AD722 View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

AD722

RGB to NTSC/PAL Encoder

Analog Devices 

AD722

H TIMING MEASUREMENT RS–170A (NTSC)

FIELD = 1 LINE = 22

5.49µs

4.59µs

9.72µs

9.0 CYCLES

H TIMING (PAL)

LINE = 25

4.60µs

5.59µs

2.28µs

100ns

124ns

AVERAGE 256 ≥ 256

39.7 IRE

33.8 IRE

94ns

102ns

AVERAGE 256 ≥ 256

293.5mV

249.0mV

Figure 12. Horizontal Timing, NTSC

Figure 13. Horizontal Timing, PAL

SUB-

CARRIER

FSC

4FSC

NTSC/PAL

HSYNC

VSYNC

XOSC

PHASE

DETECTOR

CHARGE

PUMP

FILTER

LOOP

4FSC

VCO

4FSC

XNOR

SYNC

SEPARATOR

FSC

BURST

CSYNC

NTSC/PAL

4FSC

CSYNC

QUADRATURE

+4

DECODER

FSC 90°

FSC 0°

±180°

SC 90°/270°

(PAL ONLY)

BURST

POWER AND GROUNDS

+5V

LOGIC

+5V

ANALOG

AGND

ANALOG

DGND

LOGIC

NTSC/PAL

CLOCK

AT 4FSC

NOTE:

THE LUMINANCE, COMPOSITE, AND

CHROMINANCE OUTPUTS ARE AT

TWICE NORMAL LEVELS FOR DRIVING

75Ω REVERSE-TERMINATED LINES.

RED

DC

CLAMP

GREEN

DC

CLAMP

BLUE

DC

CLAMP

Y 3-POLE

LP PRE-

FILTER

RGB-TO-YUV U

ENCODING

4-POLE

MATRIX

LPF

U

CLAMP

V

4-POLE

LPF

V

CLAMP

CSYNC

INSERTION

SAMPLED-

DATA

DELAY

LINE

5 MHz

2-POLE

LP POST-

FILTER

NTSC/PAL

BALANCED

MODULATORS

3-POLE LPF

3.6MHz (NTSC)

4.4MHz (PAL)

LUMINANCE

X2

OUTPUT

X2

COMPOSITE

OUTPUT

CHROMINANCE

X2

OUTPUT

Figure 14. Functional Block Diagram

THEORY OF OPERATION

The AD722 was designed to have three allowable modes of ap-

plying a clock via the FIN pin. These are FSC (frequency of

subcarrier, 3.579545 MHz for NTSC or 4.433618 MHz for

PAL) mode with CMOS clock applied, FSC mode using on-

chip crystal oscillator, and 4FSC mode with CMOS clock ap-

plied. To use FSC mode SELECT is pulled low and then either

a CMOS FSC clock is applied to FIN, or a parallel-resonant

crystal and appropriate tuning capacitor is placed between FIN

pin and GND to utilize the on-chip oscillator. The on-chip

Phase Locked Loop (PLL) is used in these modes to generate an

internal 4FSC which is divided to perform the digital clocking

as well as to create the quadrature subcarrier signals for the

chrominance modulation. In 4FSC mode the PLL is bypassed.

Referring to the AD722 block diagram (Figure 14), the RGB in-

puts (each 714 mV p-p max for NTSC or 700 mV p-p max for

PAL) are ac-coupled and then pass through dc clamps. These

clamps allow the user to have a black level which is not at 0 V.

The clamps will clamp to a black input signal level between 0 V

and 3 V. The clamping occurs just after the falling edge of

HSYNC.

The RGB inputs then pass into an analog encoding matrix

which creates the luminance (“Y”) signal and the chrominance

color difference (“U” and “V”) signals. The RGB to YUV en-

coding is performed using the following standard transformation:

Y = 0.299 × R + 0.587 × G + 0.114 × B

U = 0.493 × (B–Y)

V = 0.877 × (R–Y)

After the encoding matrix, the AD722 has two parallel analog

paths. The Y (luminance) signal is first passed through a 3-pole

4.85 MHz/6 MHz (NTSC/PAL) Bessel low-pass filter to pre-

vent aliasing in the sampled-data delay line. This first low-pass

filter is also where the unclocked analog sync signal is injected

into the Y signal (more on the creation of this sync signal to fol-

low). The Y signal then passes through the sampled-data delay

line, which is clocked at 4FSC. The delay line was designed to

give an overall chrominance to luminance delay of –170 ns. Fol-

lowing the sampled-data delay line is a 5.25 MHz/6.5 MHz

(NTSC/PAL) 2-pole low-pass Bessel filter to smooth the recon-

structed luminance signal.

REV. 0

–7–

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