TEA2028 datasheet(29/47 Pages) STMICROELECTRONICS

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16 .6 6ms

20ms

50 H z

Sync

60H z

Sync

15. 7 73m s

1 7. 696ms

1 9.744m s 2 0. 12 8ms

L 247

L 27 7

L309

L315

WR(60Hz)

WR(50Hz)

Figure 57

V.6.3.1 - 50/60Hz Standard recognition

This function is performed by two shift registers

which are loaded by sync pulses (if present) and if

these pulses fall within the time interval specific to

each standard. These intervals are called ”Register

Windows” and labeled ”WR(50)” and WR(60).

A. 50Hz STANDARD RECOGNITION

This identification is considered valid if two suces-

sive sync pulses applied to 50Hz shift register fall

within the 50Hz window ”WR(50)”. At the time of

synchronization capture, the first pulse will reset

the counters. The second pulse, if present, will

then trigger the 50Hz identification 20ms later

[ID(50) = 1].

The identification is not valid if two sucessive 50Hz

pulses are not detected. Identification signal is also

used to reduce the vertical synchronization window

in 50Hz standard thereby offering excellent noise

immunity against noise susceptible to be present

in sync signal and hence good display stability.

B. - 60Hz STANDARD RECOGNITION

This identification is validated after three sucessive

sync pulses at 16.6

µs period have been detected.

Three pulses are necessary to ascertain the iden-

tification prior to switching the saw-tooth amplitude.

The identification signal [ID(60) = 1] is also used to

reduce the synchronization window and, in case of

one or two missing pulses close to 60Hz, to set the

free-running frequency.

V.6.3.2 - Vertical synchronization window -

Free-running period

In the absence of sync pulse various free-running

periods are specified. Since vertical scanning must

be always active, these free-running periods must

be higher than those of 50 and 60Hz standards so

as to ensure synchronization.

An other window, allowing synchronization only at

the end of scanning, is also necessary. Upon syn-

chronization, this window will allow vertical flyback

only at the bottom of screen. This window should

be narrow for good noise immunity but also wide

enough to yield, upon synchronization, a capture

time unperceptible on screen.

In our case, as long as no standard identification

takes place the window will remain wide, and once

one of the standards has been identified, the win-

dow will be considerably reduced.

In VCR mode, this window will be always wide since

frame frequencies delivered in high-speed search,

slow review and picture pause modes are very

much variable and must be taken into considera-

tion.

In the absence of transmission (Mute = 0), synchro-

nization is disabled (so as to avoid incorrect syn-

chronization due to noise) and the free-running

frequency is around 50Hz. This will eliminate the

occurrence of picture overlay at the end of trace at

a lower free-running frequency.

Window

L247

L277

L309 L315

L361

50Hz

60Hz

Free-running Period

Reset

WR(60)

WR(50)

WR(WIDE)

WR =0

Mute = 1

No Transmission

(Mute = 0)

50Hz Standard

60Hz Standard

ID60 =1

ID50 =1

ID50 =0, ID60 =0

or VCR Mode

Figure 58 : Definition of Synchronization Windows and Free-running Periods

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