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Wide Screen, Colour Television   CW100 Chassis

Sharp Electronics (UK) Ltd

Technical Support

February 99

Page 25

It is perceived that the way in which our eye view images is in a 16:9 ratio. It is for this reason why
16:9 has been chosen for wide screen CTV and not to enable viewing of off wide screen cinema films
without loosing any of the picture edge.

There are several picture formats used by the film industry, the more common ones are:

4:3

14:9
16:9
21:9

The 21:9 format is used by most of the Hollywood block-busting films, this means it is possible with
wide screen CTV to still loose picture information from the sides of the image or have black borders at
the top and bottom of the picture, depending on the mode of operation.

The CW100 has six modes of operation, which are shown in the table below:

Panorama

Converts the broadcast picture to 16:9 without
changing the centre view.

Ideal for news programs

Full

Stretches the whole picture including Centre

Ideal for sports programs

Normal

4:3 format.

Large boarders at the sides of
picture

Zoom 14:9

Adapts 14:9 broadcasts.

Small boarders at  the  sides  of
the picture

Cinema

Adapts 16:9 broadcasts to fill screen.

Automatic

 Selects format depending on the WSS signal.

No  WSS  or  4:3  detected
Panorama will be selected

The Automatic mode utilises a signal known as the 'Wide Screen Signalling' (WSS) this signal is
transmitted on line 28, When the CTV detects this data it will automatically adjust the screen format.
However, this WSS signal is normally not transmitted, therefore the end user will have to select the
correct screen format. For this reason the default condition for the CW100 chassis, is 'Panorama'.

The end user can make adjustments to the height & vertical shift when using Panorama, Zoom 14:9
or Cinema formats

When carrying out engineers, picture geometry adjustments, the CTV has to be operating in the Full
picture format mode.

When transmitting a 16:9 format picture to ensure that it is compatible with 4:3 CTV it is always
transmitted in a 4:3 format. This achieved by multiplying 576 (active picture lines) by 3/4 which = 432
lines. This is then stretch by a wide screen CTV to fill the screen. It can now be seen that line
resolution is actuarially decreased.

Sharp Electronics (UK) Ltd

Technical Support

February 99

Page 26

Sharp Electronics (UK) Ltd

Technical Support

February 99

Page 27

The Vertical output stage in the CW100 chassis are operated in a ‘Class D’ configuration; similar to
the CS chassis. The output transistors are acting as switches, and not amplifiers. The advantages of
this type of biasing system are minimal power consumption with minimal distortion, however a
disadvantage is the noise that is left on the signal.

It can be seen in Figure 1 that our vertical drive is fed into a switching amplifier along with line pulses.
The resultant of mixing these two signals is that at the switching amplifier output we have a PWM
signal with a fix frequency of 15.625kHz but the width of the pulses will vary depending on the
amplitude of the vertical drive signal.

From the output of the switching amplifier the PWM signal passes through a class 'D' amplifier circuit
and it is converted back to an analogue signal by means of a low pass filter before being applied to
the vertical scanning coils

Line frequency is chosen for the PWM frequency because each field is made up of 312.5 lines and
therefore, our PWM signal will have a different size pulse per line and the scan will move down the
screen one line by line. The CW100 chassis has 100Hz field scan therefore, the line frequency will
31.25kHz, but the principle is exactly the same.

From the output of the switching amplifier shown in Figure 2 (IC501) the PWM signal is fed to the
base of Q504 & Q505.Starting with a positive going PWM signal; Q504 will turn on & Q505 turns off.
Allowing current to flow from the +13V rail, through Q504 collector/emitter junction, D502 & D522
turning on Q508, allowing current to flow from ground to the –13V rail via R523, vertical scanning
coils, L501 & C514 (LPF), & Q508 collector/emitter junction.

Next when the PWM signal goes negative Q505 will turn on & Q504 turns off. Allowing current to flow
from the -13V rail, through Q505 collector/emitter junction, D502 & D522 turning on Q508, allowing
current to flow from +13V to ground via Q507 collector/emitter junction t vertical scanning coils, L501
& C514 (LPF), &. R523

To prevent Q507 & Q508 being switched on simultaneously damaging Q507 & Q508 due to
excessive current flow. Crossover distortion is introduced into the drive signal to Q507 & Q508 by the
action of C507, D502, D522, C508, D501 & D521.

Sharp Electronics (UK) Ltd

Technical Support

February 99

Page 28

How many variations in the width of the PWM signal are there?

+

-

-13V

-13V

+13V

+5V

+140V

+40V

Q512

Q510

R518

R508

R534

R512

R563

R533

Q511

R531

Q509

C515

D511

D513

R530

D505

D503

Scan Coils

R532

C514

L501

C508

Q508

C532

C512

D507

D512

C534

Q507

R549

D519

Q505

Q504

C508

C507

D521

C7V5

D518

R547

D501

C7V5

D502

C7V5

D522

C7V5

R526

IC501

R515

R505

C513

D504

C504

R503

C503

IC502

R513

Vertical Drive (A)

Vertical Drive

        (B)

R524

R510

R565

R511

C533

C519

C502

Q501

C501

R503

R501

C509

D510

D508

D509

C2V7

Horizontal

   Drive

VPROT

R523

R522