AD720/AD721

REV. 0

–7–

COLOR BURST

NTSC: 5.30

µs

PAL: 5.46

µs

COMPOSITE SYNC PULSE

NTSC: 2.51

µs

PAL: 2.25

µs

NTSC: 2.51

µs

PAL: 2.25

µs

NTSC: 2.79

µs

PAL: 3.21

µs

IF THE TRAILING EDGE OF A COMPOSITE SYNC PULSE IS WITHIN

THIS WINDOW, THE PULSE IS TREATED AS A HORIZONTAL SYNC PULSE.

IF THE TRAILING EDGE IS OUTSIDE THIS WINDOW, THE PULSE IS TREATED

AS AN EQUALIZING OR BLANKING PULSE.

APPLYING THE AD720/AD721

Figure 9 shows the application of the AD720 and Figure 10

shows the application of the AD721. Note that the AD720 and

AD721 differ from other analog encoders because they are dc

coupled. This means that, for example, the expected RGB

inputs are 0 mV to 714 mV in NTSC and 0 mV to 700 mV in

PAL. The luminance, chrominance, and composite outputs

are also dc coupled. These outputs can drive a 75

Ω reverse-

terminated load. Unused outputs should be terminated with

150

Ω resistors.

The RGB data must be supplied to the AD720/AD721 at

NTSC or PAL rates, interlaced format. Various VGA chip set

vendors support this mode of operation. Most computers supply

RGB outputs in noninterlaced format at higher data rates than

NTSC and PAL, which means that “outboard” encoders must

supply some form of timing conversion before the RGB data

reaches the AD720/AD721.

Note also that the AD720/AD721 does not have internal dc res-

toration and does not accept sync on green. The composite sync

input is a separate, CMOS logical-level input and must be syn-

chronized with the 4FSC input, which serves as the master clock

for the AD720/AD721.

The AD720/AD721 does not implement two elements of the

PAL and NTSC standards. In NTSC operation, it does not

support the 7.5 IRE unit setup (1 IRE unit = 7.14 mV)—this

must be added via software using the RGB inputs. Many RAM-

DACs, such as the Analog Devices ADV471 and ADV478, offer

a logic-selectable setup mode. In PAL operation, the AD720/

AD721 does not implement a 25 Hz subcarrier offset.

Decoupling and Grounding

Referring to the pin descriptions, the AD720/AD721 uses mul-

tiple analog grounds, digital grounds, digital positive supply in-

puts, analog positive supply inputs, and analog negative supply

inputs in order to maximize isolation between analog and digital

signal paths.

The most sensitive input of the AD720/AD721 is the 4FSC pin:

any noise on this pin directly affects the subcarrier and causes

degradation of the picture. Digital and analog grounds should

be kept separate and brought together at a single point.

All power supply pins should be decoupled using 0.1

µF ceramic

capacitors located as close to the AD720/AD721 as possible. In

addition, ferrite beads may be slipped over the power supply

leads to reduce high frequency noise.

If a high speed RAM-DAC is used (e.g., capable of 80 MHz op-

eration with subnanosecond rise times), care must be taken to

properly terminate the input printed-circuit-board traces to the

AD720/AD721. Otherwise, ringing on these traces may occur

and cause degradation of the picture.

APPLICATIONS HINTS

In applying the AD720/AD721, problems may arise due to in-

correct input signals. A few common situations follow.

Fade to Black or White—Invalid Horizontal Sync Pulses

Some systems produce sync pulses that are longer or shorter

than the NTSC and PAL standards specify. The digital sync

separator in the AD720/AD721 ignores horizontal sync pulses

that are too long or too short. Figure 11 shows the timing win-

dows for valid NTSC and PAL horizontal sync pulses.

Figure 11. NTSC and PAL Timing for Valid Horizontal

Sync Pulses

When the horizontal sync pulses are too long or too short, a dc

offset voltage (due to charge storage) increases on the output of

the sampled data delay line’s auto-zero amplifier. Normally, this

offset voltage is removed at the beginning of every line, as signi-

fied by the horizontal sync pulse. Without the horizontal sync

pulse, the dc offset on the auto-zero amplifier increases over

time (usually about three to five minutes) until it overrides the

luminance information. The end result is a slow fade to black or

white.

Color Flickering—Asynchronous Operation

The AD720/AD721 requires that its 4FSC and composite sync

signals be synchronized. In most systems, when the two signals

are synchronized, the composite sync signal is generated using a

4FSC signal as the reference. After every four frames, the

AD720/AD721 resets the phase quadrature generator. When the

CSYNC and 4FSC are synchronized, this reset is transparent to

the system because the reference phase does not change. When

the CSYNC and 4FSC are not synchronized, the difference

between the reference phase and its new value upon reset causes

an instantaneous color shift, which appears as a flickering in the

color.

Adding NTSC Setup

The easiest way to add the 7.5 IRE unit

ADV471/478 or ADV477/475 or ADV473 type RAM-DAC,

which have a logic-selectable setup (called “pedestal” on some

data sheets and “setup” on others).

Color Fidelity

A source impedance other than 37.5

Ω (75 Ω 75 Ω—a

reverse-terminated 75

Ω input) can cause errors in the YUV

encoding matrix, which is basically resistive and depends on the

correct source impedance for accuracy. Figures 9 and 10 show

the correct interface between a RAM-DAC and the AD720 and

AD721 respectively, using 75

Ω reverse-terminated connections.

NOTE