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APPLICATION NOTE

STMicroelectronics 

TEA2028 - TEA2029 APPLICATION NOTE

V.3.7 - Phase comparator inhibition

The phase comparator is disabled under two con-

ditions :

- During frame sync pulse (see Figure 30)

Inverting the line sync pulse contained within the

video signal will provide the frame sync pulses

required for the synchronization of vertical scan-

ning.

Since the current supply to comparator φ1 is

controlled by the line sync pulse, the comparator

must be inhibited at the time of line sync inver-

sions so as to avoid occurence of phase errors at

the beginning of each frame.

This inhibition is activated during FRI (Frame

Retrace Inhibition) issued by frame logic circuitry.

If φ1 is locked before the vertical scanning syn-

chronization occurs, (e.g. when switching be-

tween channels), and since FRI phase is not yet

correctly positioned, the ϕ1 must be further inhib-

ited by FS signal which is the extracted frame

Figure 30 : On Screen Display of Time Constants

+ 4µs

- 4µs shift

N

sync pulse.

- During line scanning (see Figures 31 and 32)

This inhibition will eliminate the occurrence of all

possible phase errors due to a noisy sync signal

or parasitics during the line scanning phase. It

yields excellent display stability at noisy video

signals.

• f1 Inhibition in long time constant mode

(VCR = 0)

SINH(LONG) = Mute . (FRI + FS + BLK . LINEINH)

and SINH(SHORT) = 1

Inhibition is activated during, frame sync, FRI

and each time line trace interval - except at

frame beginning between lines 8 and 21.

• φ1 Inhibition in short time constant mode

(VCR = 1)

SINH(SHORT) = Mute . (FRI + FS) = SINH(LONG)

In VCR mode, inhibition is disabled during line

trace since phase or frequency variations are

not taken into account instantenously.

+ 4µs

- 4µs

N

n

Figure 31

Normal Mode

Long Time Constant

VIDEO

COMPOSITE

SYNC.

FRAME

SYNC. (FS)

FRAME ϕ1

INHIBITION (FR)

FRAME

BLANKING

S INHIBITION

(normal mode)

ϕ1 COMPARATOR

CURRENT

ϕ1 INHIBITION

SIGNAL (FRI + FS)

[VCR mode]

VCR Mode

Short Time Constant

Inverted Pulses for Frame Sync.

Inhibition

In the abscence of Frame Inhibition

17/46

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