Richtige Fernseher haben Röhren!

Richtige Fernseher haben Röhren!

In Brief: On this site you will find pictures and information about some of the electronic, electrical and electrotechnical technology relics that the Frank Sharp Private museum has accumulated over the years .

Premise: There are lots of vintage electrical and electronic items that have not survived well or even completely disappeared and forgotten.

Or are not being collected nowadays in proportion to their significance or prevalence in their heyday, this is bad and the main part of the death land. The heavy, ugly sarcophagus; models with few endearing qualities, devices that have some over-riding disadvantage to ownership such as heavy weight,toxicity or inflated value when dismantled, tend to be under-represented by all but the most comprehensive collections and museums. They get relegated to the bottom of the wants list, derided as 'more trouble than they are worth', or just forgotten entirely. As a result, I started to notice gaps in the current representation of the history of electronic and electrical technology to the interested member of the public.


Following this idea around a bit, convinced me that a collection of the peculiar alone could not hope to survive on its own merits, but a museum that gave equal display space to the popular and the unpopular, would bring things to the attention of the average person that he has previously passed by or been shielded from. It's a matter of culture. From this, the Obsolete Technology Tellye Web Museum concept developed and all my other things too. It's an open platform for all electrical Electronic TV technology to have its few, but NOT last, moments of fame in a working, hand-on environment. We'll never own Colossus or Faraday's first transformer, but I can show things that you can't see at the Science Museum, and let you play with things that the Smithsonian can't allow people to touch, because my remit is different.

There was a society once that was the polar opposite of our disposable, junk society. A whole nation was built on the idea of placing quality before quantity in all things. The goal was not “more and newer,” but “better and higher" .This attitude was reflected not only in the manufacturing of material goods, but also in the realms of art and architecture, as well as in the social fabric of everyday life. The goal was for each new cohort of children to stand on a higher level than the preceding cohort: they were to be healthier, stronger, more intelligent, and more vibrant in every way.

The society that prioritized human, social and material quality is a Winner. Truly, it is the high point of all Western civilization. Consequently, its defeat meant the defeat of civilization itself.

Today, the West is headed for the abyss. For the ultimate fate of our disposable society is for that society itself to be disposed of. And this will happen sooner, rather than later.

OLD, but ORIGINAL, Well made, Funny, Not remotely controlled............. and not Made in CHINA.

How to use the site:

- If you landed here via any Search Engine, you will get what you searched for and you can search more using the search this blog feature provided by Google. You can visit more posts scrolling the left blog archive of all posts of the month/year,
or you can click on the main photo-page to start from the main page. Doing so it starts from the most recent post to the older post simple clicking on the Older Post button on the bottom of each page after reading , post after post.

You can even visit all posts, time to time, when reaching the bottom end of each page and click on the Older Post button.

- If you arrived here at the main page via bookmark you can visit all the site scrolling the left blog archive of all posts of the month/year pointing were you want , or more simple You can even visit all blog posts, from newer to older, clicking at the end of each bottom page on the Older Post button.
So you can see all the blog/site content surfing all pages in it.

- The search this blog feature provided by Google is a real search engine. If you're pointing particular things it will search IT for you; or you can place a brand name in the search query at your choice and visit all results page by page. It's useful since the content of the site is very large.

Note that if you don't find what you searched for, try it after a period of time; the site is a never ending job !

Every CRT Television saved let revive knowledge, thoughts, moments of the past life which will never return again.........

Many contemporary "televisions" (more correctly named as displays) would not have this level of staying power, many would ware out or require major services within just five years or less and of course, there is that perennial bug bear of planned obsolescence where components are deliberately designed to fail and, or manufactured with limited edition specificities..... and without considering........picture......sound........quality........

..............The bitterness of poor quality is remembered long after the sweetness of todays funny gadgets low price has faded from memory........ . . . . . .....
Don't forget the past, the end of the world is upon us! Pretty soon it will all turn to dust!

Have big FUN ! !
-----------------------

©2010, 2011, 2012, 2013, 2014 Frank Sharp - You do not have permission to copy photos and words from this blog, and any content may be never used it for auctions or commercial purposes, however feel free to post anything you see here with a courtesy link back, btw a link to the original post here , is mandatory.
All sets and apparates appearing here are property of
Engineer Frank Sharp. NOTHING HERE IS FOR SALE !

Thursday, March 3, 2011

SABA ULTRACOLOR " Festival " SABA 150 Anniversary Limited edition CHASSIS ICC5 (5103) UNITS VIEW.













































- VIDEO CHROMA UNIT 550162.01.000 WITH TDA4556 + U4647

- DIGITAL SOUND UNIT 550168 00.000 301743 0 000 592.750 85918 418 00
WITH ITT APU2400 + ADC2300



















TDA4556 Multistandard decoder


GENERAL DESCRIPTION
The TDA4555 and TDA4556 are monolithic integrated
multistandard colour decoders for the PAL, SECAM,
NTSC 3,58 MHz and NTSC 4,43 MHz standards. The
difference between the TDA4555 and TDA4556 is the
polarity of the colour difference output signals (B-Y)
and (R-Y).
Features
Chrominance part
· Gain controlled chrominance amplifier for PAL, SECAM
and NTSC
· ACC rectifier circuits (PAL/NTSC, SECAM)
· Burst blanking (PAL) in front of 64 ms glass delay line
· Chrominance output stage for driving the 64 ms glass
delay line (PAL, SECAM)
· Limiter stages for direct and delayed SECAM signal
· SECAM permutator
Demodulator part
· Flyback blanking incorporated in the two synchronous
demodulators (PAL, NTSC)
· PAL switch
· Internal PAL matrix
· Two quadrature demodulators with external reference
tuned circuits (SECAM)
· Internal filtering of residual carrier
· De-emphasis (SECAM)
· Insertion of reference voltages as achromatic value
(SECAM) in the (B-Y) and (R-Y) colour difference output
stages (blanking)
Identification part
· Automatic standard recognition by sequential inquiry
· Delay for colour-on and scanning-on
· Reliable SECAM identification by PAL priority circuit
· Forced switch-on of a standard
· Four switching voltages for chrominance filters, traps
and crystals
· Two identification circuits for PAL/SECAM (H/2) and
NTSC
· PAL/SECAM flip-flop
· SECAM identification mode switch (horizontal, vertical
or combined horizontal and vertical)
· Crystal oscillator with divider stages and PLL circuitry
(PAL, NTSC) for double colour subcarrier frequency
· HUE control (NTSC)
· Service switch.




U4647 - TEA5040 (TELEFUNKEN) WIDE BAND VIDEO PROCESSOR

An automatic contrast control circuit in a color television receiver for stabilizing the average DC level of the luminance information at a desired level and preventing focus blooming. The control circuitry, which is suitable for fabrication as a monolithic integrated circuit, contemplates the provision of a gain-controlled luminance amplifier stage for driving an image reproducer with luminance information having a stabilized black level. An average detector coupled to the amplifier stage output develops a control signal representative of the average DC level of the luminance information and applies it to the amplifier stage, varying its gain inversely with changes in the average luminance level. A peak limiter circuit is also provided for modifying the control signal to reduce the amplifier stage's gain whenever an AC brightness component comprising the luminance information exceeds a defined threshold level, regardless of the average DC level of the luminance information.

1. In a television receiver having a luminance processing channel for translating instantaneous luminance signals derived from received broadcast transmissions to an image reproducer, said luminance signals including black level reference information, an automatic contrast control circuit comprising in combination:

2. An automatic contrast control circuit in accordance with claim 1, wherein adjustable level shifting means are interposed between said amplifier stage and said average detector means, said adjustable level shifting means providing a contrast control for manually varying the average DC level of said luminance signals.

3. An automatic contrast control circuit in accordance with claim 1, wherein said average detector means includes a capacitor having an output terminal coupled to said amplifier stage and a second terminal coupled to a plane of reference potential, said capacitor being charged by luminance signals from said amplifier stage and developing control signals representative of the average DC level of said luminance signals.

4. An automatic contrast control circuit in accordance with claim 3, wherein said control signals with respect to a plane of reference potential are equal to the potential at which black level is stabilized minus the potential drop between black level and the average DC level of said luminance information, said control signal increasing with respect to said plane of reference potential responsive to decreasing average DC levels of said luminance signals and decreasing responsive to increasing average DC levels.

5. An automatic contrast control circuit in accordance with claim 3, wherein said peak detector means includes a semi-conductor arrangement for providing said capacitor with a low impedance discharge path whenever said brightness components exceed a predetermined threshold level, the impedance of said discharge path being dependent on the amplitude of said brightness components and the discharge interval of said semiconductor arrangement being the time period during which said brightness components exceed said threshold level, said semiconductor arrangement further decreasing said control signals with respect to said plane of reference potential irrespective of the average DC level of said luminance signals.

6. An automatic contrast control circuit in accordance with claim 5, wherein said semiconductor arrangement comprises first and second transistors, said luminance signals from said amplifier stage being coupled to the input base electrode of said first transistor, said first transistor further having an emitter electrode coupled to said capacitor output terminal and a collector electrode coupled to the base electrode of said second transistor, said second transistor having a collector electrode coupled to said capacitor output terminal and an emitter electrode coupled to said plane of reference potential, said semiconductor arrangement being conductive to provide said capacitor with a low impedance discharge path whenever said brightness components exceed the base-emitter junction breakdown voltage of said first transistor.

7. An automatic contrast control circuit in accordance with claim 6, wherein said gain-controlled luminance amplifier stage includes a pair of transistors arranged in a differential amplifier configuration, the gain of which is dependent on the bias applied to the base electrodes of said transistors.

8. An automatic contrast control circuit in accordance with claim 7, wherein inverter means invert and couple said control signals to said base electrodes in said amplifier stage, the inverted control signals increasing the gain of said amplifier stage whenever the average DC level of said luminance signals decreases and decreasing the gain of said amplifier stage whenever the average DC level of said luminance information increases or whenever said brightness components exceed said threshold level.

9. An automatic contrast control circuit in accordance with claim 3, wherein said beam current limiter means provide a low impedance discharge path for said capacitor whenever the beam current exceeds a predetermined level.

10. An automatic contrast control system in accordance with claim 9, wherein said beam current limiter means monitors pulses from a voltage multiplier high-voltage system, said pulses being proportional to the beam current generated during the previous horizontal scan line.

11. An automatic contrast control circuit in accordance with claim 10, wherein said beam current limiter means comprises a transistor having a base electrode coupled to said voltage multiplier high-voltage system, an emitter electrode coupled to a plane of reference potential and a collector electrode coupled to said capacitor, said transistor providing a low impedance discharge path whenever said pulses exceed the base-emitter junction breakdown voltage of said transistor.

Description:
BACKGROUND OF THE INVENTION

This invention relates in general to control circuitry for color television receivers and more particularly to an automatic contrast control circuit incorporated in the luminance processing channel. In accordance therewith, a variable DC control signal is derived from the luminance signal information as a function of the average luminance level. The DC control signal is applied to a gain-controlled amplifier stage in the luminance channel, varying its gain and thereby insuring that excessive beam currents will not be generated due to high average luminance levels. Conversely, the circuit is effective to increase the gain of the amplifier stage when under-modulated signals are received thereby providing the desired contrast level. When the white content of the instantaneous received signal exceeds a predetermined level, however, the DC control signal is modified to reflect the excessive white content even though the average luminance level may be low. Accordingly, the amplifier stage's gain is reduced to prevent defocusing.

In color television receivers, the various elemental areas of differing brightness levels, or shades, in the televised image correspond to the amplitude levels of the instantaneous brightness components of the luminance signals which, together with the chrominance signal, reproduce the transmitted picture information on the image display tube. The intensity of the electron beams developed in the receiver's image display tube are varied, for the most part, according to the detected amplitude levels of the instantaneous luminance signals. Accordingly, progressively higher amplitude levels generate higher intensity electron beams and, consequently, progressively lighter shades. In addition, suitable viewer-adjustable controls are customarily provided in the television receiver whereby a particularized contrast and brightness setting may be selected according to viewer preference.

It is desirable that the level of the luminance signal component corresponding to black in the televised image be maintained at the cut-off of the image reproducer. But even in those instances where there is a measure of DC coupling, the DC components of the luminance signal coupled from the video detector to the luminance channel may be degraded or otherwise restricted due to the nature of the processing circuitry as well as to other factors. Moreover, the luminance processing channel itself may well permit a degradation or undesirable shift in the desired DC characteristics. The result is that the DC level in the processed luminance signal is not properly maintained, such that, upon application to the image display tube, the black level is shifted to some undesirable reference. This leads to less than faithful half-tone reproduction on the screen of the image display tube. Gray tones can be lost simply because they are beyond the cut-off of the display tube. In other instances, blacks may appear as grays on the image display tube screen.

Thus, it is desirable to make provision for the maintenance of black level in the televised image at some stabilized reference. Various systems are of course known in the art for accomplishing this objective and take various forms and configurations. For example, an arrangement commonly known as a DC restorer circuit which includes a clamping device may be employed. However, when the black level is effectively stabilized at the image reproducer's cut-off bias point, the average level of the luminance signal information may reach the point where excessive average beam currents capable of severely damaging the image reproducer are generated. In addition, the high voltage power supply during instances of high beam current may be incapable of delivering the required beam current. Such overloading reduces the power supply output voltage and results in undesirable "focus blooming." That is, there will be a loss of brightness, reduction of horizontal widths and severe defocusing of the reproduced image. The problem in this regard has been further compounded by the "new generation" high-brightness cathode-ray tubes which require higher beam currents in order to illuminate the tube to its fullest capability during high-modulation (white) scenes. In view of the added demands on the high voltage power supply and the danger of damaging the image display tube, some method for effectively limiting the beam current is required.

Accordingly, automatic contrast control systems have been developed which reduce the gain of the luminance amplifier stage to prevent the generation of excessive beam currents or increase the gain when under-modulated signals are received. Most of these prior art automatic contrast control systems, however, measure only the average level of the luminance signals to derive the control signal utilized to vary the gain of the luminance amplifier. Consequently, when all or a major portion of the luminance signal's white content is of a high amplitude level and is concentrated on a very small portion of the image reproducer's screen, the control signal derived from the average luminance level is low, permitting the luminance amplifier stage to operate at nearly maximum gain. By concentrating the high-amplitude white content into a small area of the screen, the image display tube is likely to be overdriven during that period of time and "focus blooming" will result. Some automatic contrast systems, on the other hand, derive a control signal based on the peak amplitudes of the instantaneous luminance signals without regard to the average luminance level. Thus, while preventing blooming on high-amplitude white content, such systems are susceptible to luminance signals which have a dangerously high average level, but do not have any peak white signal content of a level where the system would take corrective action.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the present invention to provide a color television receiver having black level stabilization with a new and improved automatic contrast control circuit which effectively overcomes the aforenoted disadvantages and deficiencies of prior circuits.

A further object of the invention is to provide an improved automatic contrast control circuit which develops control signals effectively varying the gain of a luminance amplifier stage to maintain an optimum contrast, while preventing the generation of excessive beam currents in the cathode-ray tube.

A more particular object of the invention is to provide an improved automatic contrast control circuit for continuously monitoring the average (DC) level of the luminance signal information and providing a control signal representative thereof to vary the gain of a luminance amplifier stage while remaining sensitive to the amplitude levels of brightness components exceeding a threshold level and modifying the control signal in accordance therewith.

Another object of the invention is to provide an improved automatic contrast control circuit which increases the gain of a luminance amplifier stage during reception of undermodulated luminance signals.

A further object of the present invention is to provide an automatic contrast control circuit of the foregoing type for deriving a variable DC control potential from applied luminance signals which, upon application to the luminance channel, adjusts the gain of a luminance amplifier stage in accordance with the varying luminance signal requirements.

Still another object of the invention is to provide a luminance processing channel including automatic contrast control circuitry which may be fabricated as a monolithic integrated circuit to provide an output luminance signal having stabilized black level and optimum contrast without producing excessive beam currents.

SUMMARY OF THE INVENTION

In accordance with the present invention, an improved automatic contrast control circuit is provided for varying the gain of an amplifier stage in the luminance processing channel of a color television receiver whenever the average DC level of the input luminance information varies from a desired level, or whenever the peak amplitudes of the AC brightness components of the luminance information exceed a predetermined threshhold level. In a preferred embodiment, the automatic contrast control circuit includes a gain-controlled luminance amplifier stage in a luminance processing channel for translating instantaneous luminance signals derived from received broadcast transmissions to an image reproducer. The amplified luminance signals found at the output of the amplifier stage have a stabilized black level. There are also provided detector means coupled to the amplifier output for developing control signals that are representative of the average DC level of the instantaneous luminance signals. The control signals are then applied to the gain-controlled amplifier stage to vary its gain inversely with changes in the average luminance level. Finally, peak limiter means are coupled between the amplifier output and the detector means to modify the control signals whenever the instantaneous luminance signals exceed a threshhold level. The modified control signals are similarly utilized to effect inverse gain variations in the gain-controlled amplifier stage regardless of the average level of the luminance signals.

DESCRIPTION
The U4647 - TEA5040S is a serial bus-controlled videoprocessing
device which integrates a complex architecture
fulfilling multiple functions.


.
DIGITAL CONTROL OF BRIGHTNESS,
SATURATION AND CONTRAST ON TV SIGNALS
AND R, G, B INTERNAL OR EXTERNAL
SOURCES .BUS DRIVE OF SWITCHING FUNCTIONS .DEMATRIXING OF R, G, B SIGNALS FROM
Y, R-Y, B-Y, TV MODE INPUTS .MATRIXING OF R, G, B SOURCES INTO
Y, R-Y, B-Y SIGNALS .AUTOMATIC DRIVE AND CUT-OFF CONTROLS
BY DIGITAL PROCESSING DURING
FRAME RETRACE .PEAK ANDAVERAGE BEAM CURRENT LIMITATION
.ON-CHIP SWITCHING FOR R, G, B INPUT
SELECTION .ON-CHIP INSERTION OF INTERNAL OR EXTERNAL
R, G, B SOURCES

GENERAL DESCRIPTION
Brief Description
This integrated circuit incorporates the following
features :
- a synchro and two video inputs
- a fixed video output
- a switchable video output
- normal Y, R-Y, B-Y TV mode inputs
- double set of R, G, B inputs
- brightness, contrast and saturation controls as
wellon aR,G, B picture ason a normalTVpicture
- digital control inputs by means of serial bus
- peak beam current limitation
- average beam current limitation
- automaticdrive and cut-off controls
Block Diagram Description
BUS DECODER
A 3 lines bus (clock, data, enable) delivered by the
microcontroller of the TV-set enters the videoprocessor
integrated circuit (pins 13-14-15). A control
system acts in such a way that only a 9-bit word is
taken intoaccount by the videoprocessor.Six of the
bits carry the data, the remaining three carry the
address of the subsystem.


A demultiplexer directs the data towards latches
which drive the appropriate control. More detailed
information about serial bus operation is given in
the following chapter.
Video Switch
The video switch has three inputs :
- an internal video input (pin 39),
- an external video input (pin 37),
- a synchro input (pin 41),
and two outputs :
- an internal video output (pin 40),
- a switchable video output (pin 42)
The 1Vpp composite video signal applied to the
internal video input is multiplied by two and then
appears as a 2Vpp low impedance composite
video signal at the output. This signal is used to
deliver a 1Vpp/75W composite video signal to the
peri-TV plug.
The switchable video output canbe any of the three
inputs.When the Int/Ext one active bit word is high
(address number 5), the internal video input is
selected. If not, either a regeneratedsynchro pulse
or the external video signal is directed towards this
output depending on the level of the Sync/Async
one active bit word (address number 4). As this
output is to be connected to the synchro integrated
circuit, RGB information derived from an external
source via the Peri-TV plug canbedisplayed on the
screen, the synchronization of the TV-set being
then made with an external video signal.
When RGB information is derived from a source
integrated in the TV-set, a teletext decoder for
example, the synchronization can be made either
on the internal video input (in case of synchronous
data) or on the synchro input (in case of asynchronous
data).
R, G, B Inputs
There are two sets of R, G, B inputs : one is to be
connected to the peri-TV plug (Ext R, G, B), the
second one to receive the information derived from
the TV-set itself (Int R, G, B).
In order to have a saturation control on a picture
coming from the R, G, B inputs too, it is necessary
to getR-Y, B-Y and Y signals from R, G, B information
: this is performed on the first matrix that
receives the three 0.9Vp (100% white) R, G, B
signals and delivers the corresponding Y, R-Y, B-Y
signals. These ones are multiplied by 1.4 in order
to make the R-Y and B-Y signals compatible with
the R-Y and B-Y TV mode inputs. The desired R,
G, B inputs are selected by means of 3 switches
controlled by the two fast blanking signal inputs. A
high level on FB external pin selects the external
RGB sources. The three selected inputs are
clamped in order to give the required DC level at
the output of this firstmatrix. Thethree not selected
inputs are clamped on a fixed DC level.
Y, R-Y, B-Y Inputs
The 2Vpp composite video signal appearing at the
switchable output of the video switch (pin 42) is
driven through the subcarrier trap and the luminance
delay line with a 6 dB attenuation to the Y
input (1Vpp ; pin 12). In order to make this 1Vpp
(synchro to white) Y signal compatible with the
1Vpp (black to white) Y signal delivered by the first
matrix, it is necessary to multiply it by a coefficient
of 1.4.

Controls
The four brightness, contrastand saturationcontrol
functions are direct digitally controlled without using
digital-to-analog converters.
The contrast control of the Y channel is obtained
by means of a digital potentiometer which is an
attenuator including several switchable cells directly
controlled by a 5 active bit word (address
number 1). The brightness control is also made by
a digital potentiometer (5 active bit word, address
number 0). Since a + 3dB contrast capability is
required, the Y signal value could be up to 0.7Vpp
nominal. For both functions, the control characteristics
are quasi-linear.
In each R-Y and B-Y channel, a six-cell digital
attenuator is directly controlled by a 6 active bit
word (address number 6 and 7). The tracking
needed to keep the saturation constant when
changing the contrast has to be done externally by
the microcontroller. Furthermore, colour can be
disabledby blankingR-Y andB-Ysignals using one
active bit word (address number 2) to drive the
one-chip colour ON/OFF switch.
Second Matrix, Clamp, Peak Clipping, Blanking
The second matrix receives the Y, R-Y and B-Y
signals and delivers the corresponding R, G, B
signals. As it is required to have the capability of +
6dB saturation, an internal gain of 2 is applied on
both R-Y and B-Y signals.
A low clipping level is included in order to ensure a
correct blanking during the line and frame retraces.
Ahigh clipping level ensures thepeakbeamcurrent
limitation. These limitations are correct only if the
DC bias of the three R, G, B signals are precise
enough. Therefore a clamp has been added in
each channel in order to compensate for the inaccuracy
of the matrix.
Sandcastle Detector And Counter
The three level supersandcastle is used in the
circuit to deliver the burst pulse (CLP), the horizontal
pulse (HP), and the composite vertical and
horizontal blanking pulse (BLI). This last one is
regenerated in the counter which delivers a new
composite pulse (BL) in which the vertical part lasts
23 lines when the vertical part of the supersandcastle
lasts more than 11 lines.
The TEA5040S cannot work properly if this minimum
duration of 11 lines is not ensured.
The counterdelivers different pulses neededcircuit
and especially the line pulses 17 to 23 used in the
automatic drive and cut-off control system.
Automatic Drive And Cut-off Control System
Cut-off and drive adjustments are no longer required
with this integrated circuit as it has a sample
and hold feedback loop incorporating the final
stages of the TV-set. This system works in a sequentialmode.
For this purpose, special pulses are
inserted in G, R and B channels. During the lines
17, 18 and 19, a ”drive pulse” is inserted respectively
in the green, red and blue channels. The line
20 is blanked on the three channels. During the
lines 21, 22 and 23, a ”quasi cut-off pulse” is
inserted respectively in the green, red and blue
guns.
The resulting signal is then applied to the input of
a voltage controlled amplifier. In the final stages of
the TV-set, the current flowing in each green, red
and blue cathode is measured and sent to the
videoprocessorby a current source.
The three currents are added together in a resistor
matrix which can be programmed to set the ratio
between the three currents in order to get the
appropriate colour temperature. The output of the
matrix forms a high impedance voltage source
which is connectedto the integratedcircuit (pin 34).
Same measurement range between drive and cutoff
is achieved by internally grounding an external
low impedance resistor during lines 17, 18 and 19.
This is due to the fact that the drive currents are
about one hundred times higher than the cut-off
and leakage currents.
Each voltage appearing sequentially on the wire
pin 34 is then a function of specific cathode current
:
- When a current due to a drive pulse occurs, the
voltage appearing on the pin 34 is compared
within the IC with an internal reference, and the
result of the comparison charges or discharges
an external appropriate drive capacitor which
stores the value during the frame. This voltage is
applied to a voltage controlled amplifier and the
system works in such a waythat the pulse current
drive derived from the cathode is kept constant.
- During the line 20, the three guns of the picture
tube are blanked. The leakagecurrent flowing out
of the final stages is transformed into a voltage which is stored by an external leakage capacitor
to be used later as a reference for the cut-off
current measurement.
- When a current due to a cut-off pulse occurs, the
voltage appearing on the pin 34 is compared
within the ICto the voltagepresenton the leakage
memory. Anappropriate externalcapacitor is then
charged or discharged in such a way that the
difference between each measured current and
the leakage current is kept constant, and thus the
quasi cut-off current is kept constant.
AverageBeam Current Limitation
The total current of the three guns is integrated by
means of an internal resistor and an external capacitor
(pin 36) and thencompared with a programmable
voltage reference(pin 38). When 70% of the
maximum permitted beam current is reached, the
drive gain begins to be reduced ; to do so, the
amplitude of the inserted pulse is increased.
In order to keep enough contrast, the maximum
drive reduction is limited to 6dB. If it is not sufficient,
the brightness is suppressed.
SPECIFICATION FOR THE THOMSON BI-DIRECTIONAL
DATA BUS
This is a bi-directional 3-wire (ENABLE, CLOCK,
DATA) serial bus. The DATA line transmission is
bi-directional whereas ENABLE and CLOCK lines
are only microprocessor controlled. The ENABLE
and CLOCK lines are only driven by the microcomputer.

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