AD724

REV. B

–13–

The crystal should be a parallel resonant type at the appropriate

frequency (NTSC or PAL, 1FSC or 4FSC). The series combi-

nation of C1 and C2 should be approximately equal to the crys-

tal manufacturer’s specification for the parallel capacitance

required for the crystal to operate at its specified frequency. C1

will usually want to be a somewhat smaller value because of the

input parasitic capacitance of the inverter. If it is desired to tune

the frequency to greater accuracy, C1 can be made still smaller

and a parallel adjustable capacitor can be used to adjust the

frequency to the desired accuracy.

Resistor R2 serves to provide the additional phase shift

required by the circuit to sustain oscillation. It can be sized by

R2 = 1/(2

×

π × f × C2). Other functions of R2 are to provide a

low-pass filter that suppresses oscillations at harmonics of the

fundamental of the crystal, and to isolate the output of the in-

verter from the strange load that the crystal network presents.

The basic oscillator described above is buffered by U1B to drive

the AD724 FIN pin and other devices in the system. For a

system that requires both an NTSC and PAL oscillator, the

circuit can be duplicated by using a different pair of inverters

from the same package.

Dot Crawl

Numerous distortions are apparent in the presentation of com-

posite signals on TV monitors. These effects will vary in degree,

depending on the circuitry used by the monitor to process the

signal, and on the nature of the image being displayed. It is

generally not possible to produce pictures on a composite moni-

tor that are as high quality as those produced by standard qual-

ity RGB, VGA monitors.

One well known distortion of composite video images is called

dot crawl. It shows up as a moving dot pattern at the interface

between two areas of different color. It is caused by the inability

of the monitor circuitry to adequately separate the luminance

and chrominance signals.

One way to prevent dot crawl is to use a video signal with sepa-

rate luminance and chrominance. Such a signal is referred to as

S-video or Y/C video. Since the luminance and chrominance are

already separated, the monitor does not have to perform this

function. The S-Video outputs of the AD724 can be used to

create higher quality pictures when there is an S-Video input

available on the monitor.

Flicker

In a VGA conversion application, where the software controlled

registers are correctly set, two techniques are commonly used by

VGA controller manufacturers to generate the interlaced signal.

Each of these techniques introduces a unique characteristic into

the display created by the AD724. The artifacts described below

are not due to the encoder or its encoding algorithm as all en-

coders will generate the same display when presented with these

inputs. They are due to the method used by the controller dis-

play chip to convert a noninterlaced output to an interlaced

signal.

The first interlacing technique outputs a true interlaced signal

with odd and even fields (one each to a frame Figure 19a). This

provides the best picture quality when displaying photography,

CD video and animation (games, etc.). It will, however intro-

duce a defect, commonly referred to as flicker, into the display.

The VM700A has a special measurement mode that enables it

to directly measure the frequency of one subcarrier in a video

waveform with respect to an internally stored reference or a

simultaneously supplied reference. The instrument gives a read-

ing of the relative frequencies of the reference and test signals in

units of 0.1 Hz. This is not a direct reading of the subcarrier

frequency in MHz but a relative reading in Hz of the difference

in frequency between the two signals.

If the reference video source is supplied by a video generator

that has a CW subcarrier output, its CW subcarrier can be

measured with a frequency counter to accurately determine its

frequency. The AD724 circuit under test can then be measured

relative to this reference by using the built in color burst mea-

suring function of the VM700A, and the offset frequency

measured can be added to or subtracted from the measured

frequency of the CW subcarrier to determine the operating

frequency of the DUT.

It should be noted that the VM700A is a highly specialized

video measurement instrument. In order for it to synchronize

on a video signal, the synchronization pattern of the signal must

adhere very closely to the appropriate video standard. In par-

ticular, a video signal that is missing equalization and serration

pulses from the vertical blanking interval will cause the “Loss of

Sync” message to be displayed by the VM700A. Many such

signals might make a perfectly acceptable picture on a monitor,

but will not be recognized by the VM700A.

Low Cost Crystal Oscillator

If a crystal is used with the on-chip oscillator of the AD724,

there will be no CW clock available that can be used elsewhere

in the system: the only AD724 signals that output this fre-

quency are the chrome and composite that have only colorburst

and chrominance at the subcarrier frequency. These cannot be

used for clocking other devices.

A low cost oscillator can be made to provide a CW clock that

can be used to drive both the AD724 FIN and other devices in

the system that require a clock at this frequency. In addition,

the same technique can be used to make a clock signal at a

4FSC, which might be required by other devices and can also

be used to drive the FIN pin of the AD724.

Figure 18 shows a circuit that uses one inverter of a 74HC04

package to create a crystal oscillator and another inverter to

buffer the oscillator and drive other loads. The logic family

must be a CMOS type that can support the frequency of opera-

tion, and it must NOT be a Schmitt trigger type of inverter.

Resistor R1 from input to output of U1A linearizes the inverter’s

gain so it provides useful gain and a 180 degree phase shift to

drive the oscillator.

R1

1M

Y1

TO PIN 3

OF AD724

U1A

U1B

R2

1k

C2

60pF

C1

47pF

C3

(OPT)

TO OTHER

DEVICE CLOCKS

HC04

HC04

Figure 18. Low Cost Crystal Oscillator