1 _CRT Display Design_

CRT Display Design

©2000 Display Laboratories Inc.

Session 1Introduction

Featured Seminars Introduction to CRT Displays Video and Tube Biasing Deflection and High Voltage Micro-Control and Waveform Generators Special Topics and Miscellaneous

Circuits

Introduction to CRT Displays Early development General description Block diagram Principles of operation Input signals Timing and resolutions Front of screen

CRT Display

General Description Cathode Ray Tubes use a fundamental

method for producing a light emitting image

Two signals control X and Y location One to three signals control the intensity

and or color of the image Basically an analog device

Block Diagram (simplified)

Power Supply

Cathode Ray Tube

Deflection Amplifier

Video AmplifierRGB

H V

Principles of Operation Video Image Created by Vectored Beam

Direct X-Y positioning of beam Video Image Created by Scanning Beam

Left to Right and Top to Bottom Convention Progressive Scan, one frame per vertical

Image appears more stable on text Interlaced Scan, one field per vertical

Reduces video bandwidth, good for broadcast No fixed number of vectors, lines or

pixels defined

Video Display Components CRT Structures

Shadow Mask & Purity Gun Design and use Color Perception

X-Y Addressing Deflection System

Vertical Deflection Amplifier Horizontal Deflection

Amplifier Horizontal Linearity Horizontal ‘S’ correction Yoke

Z Axis Video Control RGB Video Amplifiers Blanking Amplifier

Operating Modes Other Services

Focus Modulation Convergence

Correction Sync. Processing Micro-Controller

Operating Modes Fixed Frequency Multi-Mode Multi-Sync Digital CRT Displays Timing and resolutions

Fixed Frequency Spot size set for ‘Merged Raster’. Geometry optimized at one frequency. Fixed horizontal and variable vertical

rate. Fixed horizontal and vertical rates. Fewest parts and lowest in cost.

Multi-mode Designed to operate at predefined

Frequencies or pairs of frequencies. Few and fixed horizontal rates. Low in cost. Compatibility issues. Will not adapt to new modes without

user adjustment.

Multi-Sync Automatically sets reasonable raster

size for any in range signal. Can be factory preset for prime modes. User adjustable for new modes. Automatic or manual ‘save’ of settings. Micro-controller based. Higher in cost.

Digital CRT Display All digital video and sync. signals. Precise ‘Distortion Free’ image

reproduction under all conditions. Elimination of all ‘Arcane’ controls.

Automatic adjustment of the rest. User adjustments free of interactive

artifacts. Yet to be fully defined. Cost pending.

Input Signals

Typical sync on green input signal.

Input Signals RGB Video Separate Sync. Composite Sync.

Deflection Amplifier

Video AmplifierRGB

H V

Video AmplifierRGB

H+V Sync.Sepr.

Sync. Types Sync. on Video/Green Separate Sync.

Composite 1 wire Separate 2 wire

Composite Sync. Types Serrations Half line Interlace Doubles Equalizing pulses Missing sync. pulses

Block DiagramPower Supply

Cathode Ray Tube

Vertical & HorizontalDeflection Amplifiers

Video Amplifier& Blanking

RGB

H+V Sync.Sepr.

Focus andConvergence

Future Seminars Video and Tube Biasing Deflection and High Voltage Micro-Control and Waveform Generators Special Topics and Miscellaneous

Circuits

Video System Block diagram Preamp Input select Termination Contrast Sync Separator

Video System Sync tips Black level Back Porch Clamp Output Amp Cathode voltage swing T-rise/T-fall Cathode capacitance

Video System Beam current CRT Bias Cutoff Brightness Black level Arc suppression

Video System Color Tracking White balance Preset Temp Variable Temp White to Black color tint White uniformity Circuit considerations

Deflection System & High Voltage

Horizontal Deflection Vertical Deflection High Voltage

Vertical Deflection Block diagram Principles of fly-back scanning design Multi-scan considerations Frequency considerations Timing considerations Inductive load conditions Power dissipation

Horizontal Deflection Block diagram Principles of fly-back scanning design Multi-scan considerations Frequency considerations Timing considerations Horizontal drive conditions Power dissipation Conventional vs. separate deflection

High Voltage System Block diagram Fly-back Multiplier FBT construction and operating

principles Static and dynamic regulation Beam current Power dissipation

Microprocessor & Jungle Micro-Controller AFC circuit Geometry correction principles Factory adjustment DPMS

Miscellaneous Circuits Digital convergence Focus (static and dynamic) Special Topics Single-sided board layout Jitter Beat noise EMI etc.

END END

The Early Years

How We Got Into This Mess and Those That Had a Part in It.

Early Development Electro-Mechanical P. Nipkow First Proposed Scanning

System (Germany 1884) J. Baird First Transmittion of TV Image

(England 1925) J. Baird Field Sequential Color Demo

(England 1928)

Early Development Electronic K. Braun Invented Cathode Ray Tube

(Germany 1897) V. Zworykin Demo of Crude Television

(USA 1923) for Westinghouse K. Takayanagi First Electronic Transmittion

(Tokyo 1926) P. Farnsworth Transmitted 60 Line Image

(USA 1927) of ‘$’ A. Schroeder Simultaneous Color CRT

(USA 1946)

Paul Nipkow’s Camerawith mechanically scanned raster. Patented in 1884

John L. Baird’s DisplayBased on the Nipkow disk, 1925

Karl Braun’s CRT with electron scanned raster, Proposed in 1884

Vladimir Zworkin’s Camerawith electron scanned raster, 1929

Al Schroeder’s Color TubeSimultaneous Color System, 1946

1/3

Proposed Beam Masking Structure

2/3

Proposed Mask and Screen Structure

end

Monochrome CRT Single

Beam Electron Gun

One Color Phosphor

Simple Vacuum Tube

Shadow Mask with Delta Gun

Paul Nipkow In 1884, university student Paul Nipkow of

Germany proposed and patented the world's first electromechanical television system.

Nipkow proposed a disc camera, that contained a disc which was perforated. To capture a moving image the disc was rotated before an image and had the effect of dividing the picture into lines.

Light sensitive selenium behind the perforated disk would capture the moving image. The camera became known as the Nipkow disk.

1/2

Paul Nipkow The Nipkow disk was a mechanical scanning

system and became the best known for its time. Nipkow could not build a working system. He could not amplify the electric current created by the selenium to drive a receiver.

It was not until 1907 and the development of an amplification tube that serious development in mechanical television would start.

2/2

John Logie Baird John Logie Baird, of Great Britain, has his place

in history as one of the champions of the development of mechanical television. Known as an idea-rich inventor, John Logie Baird was said to have had trouble changing a fuses by himself, but had a flair for publicity. Prior to his development efforts at television he had failed at making artificial diamonds and had attempted a cure for hemorrhoids that left him in severe pain for a week.

1/3

John Logie Baird Despite his history on October 30, 1925, John Logie

Baird of London was successful in transmitting his first picture: the head of a dummy. Looking for publicity he visited the Daily Express newspaper to promote his invention. The news editor was terrified. Later he was quoted by one of his staff as saying:" For God's sake, go down to reception and get rid of a lunatic who's down there. He says he's got a machine for seeing by wireless! Watch him-- he may have a razor on him."

In 1928, Baird extended his system by transmitting a signal between London and New York.

2/3

John Logie Baird In 1929 the British Broadcasting Service (BBC)

adopted the Baird mechanical system. By 1932 John Logie Baird had developed the first

commercially viable television system and had sold 10,000 sets.

Baird's electromechanical system consisted of a light sensitive camera behind a rotating disc. It delivered a crude picture consisting of thirty lines at twelve frames per second to a television receiver that displayed an uneven and tiny orange and black image.

3/3

Vladimir Zworykin The person most often associated as the father

of television is Vladimir Zworykin. Teamed with David Sarnoff, Zworykin lead the

development at RCA of electronic television. Zworykin was cursed with living in interesting

times, born in Russia in 1889, he studied at the St. Petersburg institute of technology.

He was hired by one of his instructors, Boris Rosing, who was seeking ways of extending mans vision.

1/10

Vladimir Zworykin By 1907 Rosing had developed a television

system which employed a mechanical disc system as a camera and a glass tube (cathode ray tube) as a receiver. The system was primitive but it was more electronic than mechanical.

With the Russian revolution, Rosing went into exile and died.

Zworykin carried on his work.

2/10

Vladimir Zworykin With the outbreak of world war I, Zworykin

decided to leave Russia for the United States. Zworykin found work at Westinghouse. Based on their pioneering efforts in radio, he

tried to convince them to do research in television. Turning down an offer from Warner Brothers.

Zworykin worked nights, fashioning his own crude television system.

3/10

Vladimir Zworykin In 1923, Zworykin demonstrated his system

before officials at Westinghouse and applied for a patent.

All future television systems would be based on Zworykin's 1923 patent.

Zworykin described his 1923 demonstration as "scarcely impressive". Westinghouse officials were not prepared to base an investment in television on such a flimsy system.

The company suggested that Zworykin devote his time to more practical endeavors.

4/10

Vladimir Zworykin Undeterred, Zworykin continued in his off

hours to perfect his system. He was so persistent that the laboratory guard was instructed to send him home a 2:00 in the morning if the lights of the laboratory were still on.

During this time, Zworykin managed to develop a more sophisticated picture tube called the kinescope, which serves as the basis for today's CRT display tubes.

5/10

Vladimir Zworykin In 1929, Vladimir Zworykin invented the all

electric camera tube. Zworykin called his tube the iconoscope

(literally "a viewer of icons"). He demonstrated both the iconoscope and

kinescope to IRE* the Institute of Radio Engineers. (*Now known as IEEE.)

6/10

Vladimir Zworykin Zworykin's all electronic television system

surpassed the limitations of the mechanical television system.

In attendance was David Sarnoff who eventually hired Zworykin to develop his television system for RCA.

Under Sarnoff's watchful eye, Zworykin continued to develop the electronic system.

7/10

Vladimir Zworykin When Zworykin started at RCA his system was

scanning 50 lines. Experimental broadcasts started in 1930 first

using a mechanical camera transmitting at 120 lines.

By 1933 a complete electronic system was being employed with a resolution of 240 lines.

8/10

Vladimir Zworykin Zworykin had originally told Sarnoff it would

cost $200,000 to develop a television system. The final cost was estimated to cost RCA about

$50,000,000.

9/10

Vladimir Zworykin RCA and Zworykin were not alone. By 1934

two British electronic firms, EMI and Marconi, created an all-electronic television system.

They used the orthicon camera tube invented by an American company, RCA.

This electronic system was officially adopted by the BBC in 1936. It consisted of 405 scanning lines, changing at twenty five frames per second.

10/10

Kenjito Takayanagi The Japanese say the honor of the first working

electronic television system goes to Kenjito Takayanagi of Tokyo.

On Christmas Day, 1926, he used a cathode-ray tube to display an image of Japanese writing.

Japan now honors Mr. Takayanagi, as the inventor of TV.

1/1

Philo T. Farnsworth In 1922, American Philo T. Farnsworth, who

was then a 15-year-old Idaho farm boy born in a log cabin, described to his friends and teachers how an electronic TV system might work.

Later in 1927, he would transmit his first TV image based on his system.

1/1

Cathode Ray Tube The CRT Display Monitor evolved from

electronic television which is based on the principal of the cathode ray tube.

Cathode Rays were first identified in 1859 by Julius Plucker, a German mathematician and physicist.

In 1878 William Crookes, a British chemist, confirmed their existence by building a tube that displayed them.

1/3

Cathode Ray Tube Later , English physicist Ambrose Flemming,

working with Crookes tube, discovered that cathode rays could be deflected and focused.This was accomplished by wrapping the tube with wire and creating a magnetic field by passing electric current through the wire. A technique still used today.

2/3

Cathode Ray Tube In 1897, German physicist Karl Braun

developed the first cathode ray oscilloscope. Braun made the cathode rays visible by

placing fluorescent materials at the end of the tube.

Braun built the oscilloscope to demonstrate how cathode rays could be controlled by a magnetic field.

3/3

Chronology Of Television Technology

1817 - Swedish Baron Jons Berzelius isolates the element selenium.

1839 - Edmond Becquerel discovers the electrochemical effects of light.

1842 - Alexander Bain proposes facsimile telegraph transmission that scans metal letters and reproduces image by contact with chemical paper. Synchronized scanning is part of proposed transmission system.

1847 - F. Bakewell improves facsimile by creating rotating scanning drums.


1859 - German mathematician and physicist Julius Plucker experiments with invisible cathode rays.

1861 - Italian priest, Abbe Caselli, uses tin foil on facsimile to transmit handwriting and pictures.

MAY 1873 - British scientists, Willoughby Mith and Joseph May noted that the electrical conductivity of the element selenium changes when light falls on it. This property, called photoconductivity, is used in camera tubes.


1878 - M. Senlacq proposes the use of selenium in facsimile machines to transmit paper documents.

1878 - Sir William Crookes develops a tube that confirms the existence of cathode rays.

1881 - British pioneer Shelford Bidwell demonstrates his scanning photo telegraph that establishes both scanning and the use of selenium in transmitting still pictures.


1884 - German scientist Paul Gottlieb Nipkow patented a device for scene analysis that consisted of a rapidly rotating disk placed between a scene and a light sensitive selenium element. It became known as the Nipkow disk. Although this was a mechanical design (not in use today), it was the first television scanning system, outlining the principle of scanning a moving image. If the Nipkow disk was turned fast enough, it theoretically created a scanning system capable of showing a moving picture. It is believed a working model was never built by Nipkow himself. It would take the development of the amplification tube before the Nipkow Disc would become practical.


1888 - German physicist Wilhelm Hallwachs noted that certain substances emit electrons when exposed to light. Hallwachs demonstrated the possibility of using photoelectric cells in cameras. This property called photoemission was applied in the creation of image orthicon tubes allowing the creation of the electronic television camera.


1897 - German Karl Braun invents the Cathode Ray Tube (CRT).

1904 - First color television system is proposed based on the principle of scanning three primary colors.

1907 - American engineer Lee De Forest invented the triode electron tube. This made amplification of video signals created by photoconductivity and photoemission possible.


1907 - English inventor A.A. Campbell-Swinton and Russian Boris Rosing independently suggested using a cathode ray to reproduce the television picture on a phosphorous coated screen. This suggested that the electronic scanning system used in the CRT could replace the mechanical Nipkow disk.

1911 - English inventor A.A. Campbell-Swinton proposed an electronic scanning system using a charge-collecting screen and an electron gun to neutralize the charge to create a varying current. The electronic scanning system used in the CRT could then be adapted as an electronic scanning system to replace the mechanical Nipkow disk.


1923 thru 1926 - American Charles F. Jenkins developed a working television system based on the Nipkow disk. In England, Scottish engineer John L. Baird demonstrated a working television system that was based on the Nipkow disk, with improved resolution. The Baird system used infrared rays and could take pictures in the dark. Both systems produced a small crude orange and black recognizable image.

1923 - Westinghouse, General Electric, RCA, and AT&T entered into television research.


1923 - Vladimir K. Zworykin, a Russian immigrant to the United States, patented the "iconoscope" an electronic camera tube based on A.A. Campbell-Swinton's proposal of 1911.

1923 - Philo T. Farnsworth (13 years old) developed an electronic camera tube, similar tube to Zworykin's named the "kinescope".

1926 - Canadian experiments with mechanical television start in Montreal.

1927 - First long distance television broadcast from Washington to New York performed by AT&T.


1928 - John L. Baird demonstrates a color television system using a modified Nipkow disk.

1928 - American inventor E. F. W. Alexanderson demonstrates the first home television receiver in Schenectady, New York. It consisted of a 3" screen and delivered a poor and unsteady picture. On May 28, 1928 the first television station WGY began broadcasting in Schenectady. Sets were built and distributed by General Electric in Schenectady.


1929 - John L. Baird starts transmissions using BBC radio towers in off hours.

1929 - Zworykin demonstrates the all electronic television camera and receiver.

1930 - American Philo Farnsworth patents electronic television.

1930 - NBC is granted an experimental broadcast license.

1931 - Television broadcasting starts in Canada by CKAC of Montreal.


1933 - 33 Radio Stations Are Broadcasting In Canada.

605 Radio Stations Are Broadcasting In United States.

8 Radio Stations Are Broadcasting In Newfoundland.

1935 - Germany begins world's first public broadcasting service.

1935 - RCA pledges millions of dollars towards the development of TV.


1936 - Public broadcasting begins in England. 1936 - Germany broadcasts Olympic Games. 1939 - RCA displays TVs at World's Fair. 1940 - American Peter Goldmark introduces a

refined color television system in New York City.

1941 - NBC and CBS are granted commercial broadcast licenses.


1941 - United States adopts a 525 line black and white system as the standard for broadcasting.

1941 - A total of 400 television receivers had been sold in the United States.

1945 - There are an estimated 10,000 television sets in the US.

1946 - 6,500 television receivers are sold in the United States.


1947 - World Series (Baseball) is broadcasted, attracting an audience in excess of four million in the United States.

1948 - CBS announces development of color television system.

1949 - NBC announces development of color television system.

1951 - Television broadcasting in color began and ended in United States using the Peter Goldmark color system that was not compatible with the 525 line black and white standard.


1952 - CBC is licensed in Montreal and Toronto.

1952 - Television sets in American homes pass the 22 million mark.

1953 - Half the homes in the United States have television sets.

1953 - NTSC television standard is adopted in the United States allowing for color television that is compatible with existing black and white TV sets.


1954 - Commercial color broadcasting begins in United States using the NTSC standards.

1955 - Videotape is introduced. 1959 - CBC linked by microwave from Victoria

to St. Johns. 1960 - Television sets in American homes

pass the 60 million mark. 1961 - CTV receives television broadcast

license.


1962 - Telestar 1 satellite launched, thus opening doors for television satellite transmission and allowing intercontinental transmission when in proper position.

1962 - Television sets in American homes pass the 70 million mark.

1965 - Commercial satellite Early Bird launched in fixed orbit allowing continuous intercontinental transmission.


1965 - With only two exceptions, NBC announces all prime time programs to be in color.

1966 - Color television tests were conducted in Canada.

1967 - Canadian color television standards are set and color transmission begins.

1969 - Apollo 11 moonwalk is transmitted and broadcast live from the moon.


1974 - 97% of American homes have at least one TV set and it is on at least five hours per day.

1984 - Stereo television authorized. 1987 - CBC shoots the world's first large scale

commercial HDTV production, "Chasing Rainbows".

1990 - 1446 television stations broadcasting in United States.

END END

Notes

Beam Masks & Color Purity

Shadow Mask This is a very popular technology. It is made up

of a screen laying just behind the phosphors. The electrons from three guns pass through the

mask at different angles to strike the desired phosphor.

Aperture Grill This technology is used by Sony in Trinitron

tubes and Mitsubishi in Diamondtron tubes. The aperture grill consists of vertical wires. The mask has less metal, more of the

electrons hit the phosphor.

Aperture Grill The mask is flat and under tension in the

vertical direction. The phosphor is also placed in vertical stripes. A higher percentage of the area is covered by

phosphor. The result is a brighter picture & good color

saturation. Darker tinted glass can be used to give a

higher contrast ratio.

Slot Mask This technology is used by NEC under the trade

name “CromaClear”. It is a combination of Shadow Mask and Aperture Grill technologies. The phosphor dots are rectangular (tall and thin).

This increases the phosphor area and reduces moiré in the vertical direction. .

Flat Tension Mask Pioneered by Zenith and now manufactured by

LG in Korea. Perfectly flat face plate and mask. Mask is under tension on a flat frame.

Doming The shadow mask inside the CRT

is a thin sheet of steel or InVar positioned about a half an inch behind the phosphor screen.

The shadow mask is susceptible to thermal expansion.

In areas of high beam current the shadow mask will deform, shifting the position of the holes or slots in the shadow mask.

Individual phosphor dot may be spaced as little as .13 mm apart (for a .22 mm dot pitch CRT).

Doming Very little movement in the

shadow mask will cause the electron beam to strike the wrong color phosphors.

The result is poor color purity and discoloration.

InVar shadow masks can sustain two to three times higher current density than steel shadow masks without noticeable problems. .

Trinitrons and FTMs are resistant to small area doming because the wires are under tension. The suspension components can still expand result in color purity with an overall bright picture.

Purity,4,6 Pole Magnets 2 Pole, Purity 4 Pole, Red-Blue 6-Pole, Magenta-

Green

Purity Magnet The 2-Pole ‘Purity’ magnet is analogous with

the beam centering magnet on Monochrome Yokes.

Its purpose is to move the beam/beams to the ‘optical’ center line of the deflection yoke.

This will produce orthogonal vertical and horizontal lines through the center of the faceplate and the best overall balance of spot landing and purity.

Magnet Placement The purity magnet is

placed over a gap in the gun so all three beams are affected.

The 4 and 6 pole magnets are placed over magnetic shunt structures to optimize their effect.

The 4 and 6 pole magnets may be used in combination with dynamic correction coils.

2-Pole Magnet Effect

Flux

Magnet

Movement

The ‘purity’ magnet can move all beams in any direction together.

4 & 6 Pole Magnets


Magnetic shunt

Flux

Magnet

Movement


Magnetic shunt

Flux

Magnet

Movement

Degaussing The shadow mask or aperture grill must be

made of magnetic material! This leads to the need for degaussing.

The shadow mask and internal shielding hood form a closed chamber where there is a near zero field.

Degaussing is not to demagnetize the metal, but to create a magnetization that compensates for the earth's magnetic field.

The sum of the two fields must be near zero. This theory was put forward by Philips in a

recent paper.

END END

Notes

Gun Design and Use

The Electron Gun (Mono-Chrome) The uni-potential gun can be broken down

into two sections, the triode and the lens section.

The triode is the electron emitting and shaping section formed by the electron source (cathode), the control grid (G1) and the accelerator (G2).

The lens of the uni-potential design has three elements G3, G4 and G5.

The Triode A 6.3 or 12-volt filament heats the cathode.

Operating at an elevated temperature causes electrons to "boil off" the surface of the cathode and form a cloud.

The electron-emitting surface of the cathode is often impregnated with tungsten and barium to lower the work function of the surface.

The effective area of the cathode is determined by the aperture size of G1 (first control grid).

The Triode The video signal is typically applied to the

cathode along with video blanking. This permits the G1 and G2 elements to be biased relative to the cathode for uniform operation from CRT to CRT.

The physical relationship of G1, G2 and the Cathode, influence the lower beam angle, center focus voltage, and the spot size for a given gun design.

The Triode Electrons carry a negative charge. They are

attracted to positive elements within the tube like G2 and the faceplate (1st and 2nd anodes) and are repelled by the relative negative charge of G1.

The more negative G1 appears to the cathode, the greater the reduction in the flow of electrons.

The Triode Beam current is attracted through a small

hole in G1, called the aperture by a positive voltage applied to grid two (G2, the first accelerator or anode).

The bias voltage applied to G2 is set in conjunction with G1 and Cathode to establish “cut-off” of electron flow, at black levels in the picture.

The bias voltage between the cathode (+) and G1 (-) has the largest influence on spot size after physical gun design.

Triode Cross Section

Cathode

Heater

G1G2

0 v -100v 500-800v

+30v-30v

+30v

0v

The Lens Electrostatic lenses are used to focus the

electron bean onto the phosphor screen in the front of the tube.

The combination of G3, G4, and G5 form the lenses. G3 and G5 are connected to the fixed high voltage of the anode (originally called the 2nd anode).

The voltage on G4 is varied to control the focal distance. Static focus voltage and any dynamic voltage if required are applied to G4 through the base connector.

The Lens High-resolution displays require dynamic focus

to maintain pixel quality into the corners of the CRT. This is even more critical on flat profile CRTs.

Electron guns can be optimized for dynamic focus or flat focus applications. A flat focus gun will provide a compromise of focus quality from center to edge and is appropriate for many low to medium resolution applications.

Displays requiring higher resolution such as those in desktop publishing, graphics terminals, document processing, and medical imaging, require dynamic focus.

The Lens The function of each of these gun elements

and their interaction is critical to the overall performance of the CRT.

From cathode to the face of the CRT, it is the relationship between each of these individual elements that determines the final appearance of the un-deflected spot in the center of the tube face.

END END

Notes

The Electron Gun (Color PIL)

Example of Hitachi Elliptical Aperture, Dynamic focus (A-EADF) Electron Gun

Hitachi Elliptical Aperture, Dynamic focus (A-EADF) Electron Gun At the heart of Hitachi's high performance

monitors is the EADF electron gun which ensures the sharp focus, high definition, distortion free image.

The elliptical aperture lenses produce maximum focusing control while minimizing distortion effects due to centerline offset.

Hitachi Elliptical Aperture, Dynamic focus (A-EADF) Electron Gun Hitachi's dynamic focus capability means

that even FST screens have consistently sharp focus, right into the corners where the beam path length is substantially greater than at the center.

An electro-static Quadra-pole lens which makes constant adjustments to the cross sectional shape of each beam ensures that the landing spot is precisely circular whatever the deflection of the beam or the position on the screen surface.

Beam Shape & Focus The electron beam leaving an electron gun will

normally be circular in shape. If the gun were at the radius of curvature of the faceplate then the spot at the faceplate would be round.

The problem is that most CRTs are short, the gun is very close to the faceplate.

This causes the electron beam to strike the phosphor at a non right angle.

The corners will have an elliptical shaped spot-landing pattern.

Beam Shape & Focus A Dynamic Quadra-pole Lens found in some

electron gun enables the beam to be made elliptical as it leaves the gun. Naturally the Quadra-pole lens will distort the beam in a direction that is at a right angle to the normal elliptical effect. The addition of the two elliptical effects will result in a round but larger spot. The beam’s spot shape, and therefore picture sharpness, remain the same all over the screen.

Beam Shape & Focus Dynamic Focus voltages may be applied to

the gun of the CRT to optimize the spot size. A complex 3- D waveform is often needed on

very flat and/or large CRT’s. Several hundreds of volts of drive are used

to effect the beam at this point in the gun. The voltage may be different for the four

corners of the tube.

Thermionic Emission Thermionic emission is to electrons what

evaporation is to the water molecules in a hot cup of coffee.

It is a process by which some of the electrons inside a piece of metal can 'boil away', leaving the surface of the metal into the surrounding space.

Thermionic Emission Inside a metal the electrons are not

stationary, but are constantly moving, with an average speed that is controlled by the temperature of the metal.

It is important to realize that this is only the average speed of the electrons, that some of them will have speeds that are significantly larger than this.

It is these higher speeds, and therefore higher energy electrons, which have enough energy to escape from the metal.

Thermionic Emission Since the speed of the electrons increases

with temperature, the number of electrons with sufficient energy to escape also increases with temperature, in fact exponentially so.

At room temperature (300 K) the number is very small, but if the wire is heated to 1000K the number of electrons escaping is dramatically increased.

END END

Color Perception

Additive Properties of Light

Color Colors are rays of light, i.e. electromagnetic

waves with wavelengths between 380 nm and 780 nm.

We perceive them with our eyes and our brain translates them into what we call "colors". In other words: colors are products of our brain.

This means that one person may perceive colors slightly differently from another.

Color To display colors, monitors use what is called

"additive color mixing", using red, green and blue light. If we mix red, green and blue light together, we get white light.

When white is required on the screen, three electron guns hit the red, green and blue dots, or different shapes, of phosphor that coat the inside of the screen, which in turn glow together and produce white light.

Color Metrics The idea of tri-receptor

vision was worked out far before the physical mechanism of retinal pigments was understood.

A common diagram for describing human color perception was developed by the International Commission on Illumination (CIE).

The CIE diagram is an attempt to precisely quantify the tri-receptor nature of human vision.

Color Phosphors

END END

Notes

Vertical Deflection

Vertical Power Amplifier A vertical power amplifier is related to an

audio power amplifier. Audio amplifiers are voltage amplifiers (voltage in

voltage out). Vertical amplifiers are current amplifiers (voltage in

current out). Feedback comes from a current sensing point.

This is done because current is proportional to the amount of deflection.

Vertical Power Amplifier Audio

amplifier

Vertical amplifier

B+

B-

B+

B-

Audio amplifier

Vertical Power Amplifier Low noise is critical. Open loop unity gain needs to extend to 1 -

10mhz. A small monitor may need only ± 0.5 amps p-p

of vertical yoke current using a 12 volt supply. Large color monitors may require ± 3 to 4

amps p-p and use a 35 –50 volt supply during vertical trace and 70 –100 volts during retrace.

Voltage Doublers In order to obtain sufficiently short fly-back

times, a voltage greater than that required during scanning must be applied to the yoke.

During vertical retrace time a large voltage is needed across the yoke to cause a fast retrace. A voltage doubler boosts the positive supply voltage only during vertical retrace.

The vertical power amplifier can then run from a low supply voltage when little output voltage is needed and from a high supply voltage for the short time that a high output voltage is needed. This results in 1/3 the power loss and 2 to 3 times faster retrace.

Voltage Doublers The top trace is the output voltage of the power

amplifier. The second trace is the supply voltage.

Anti-ringing Resistor Many power amplifiers have instability in the 1

to 3Mhz region. An anti-ringing resistor & capacitor dampens out oscillations. See the manufacture’s data sheet for proper values.

Generally the resistor is in the 1 to 5 ohm range. It is chosen to load down the amplifier at the oscillation frequency. The time constant for the RC is often in the .2 to 1uS range. The impedance of the capacitor, at the oscillation frequency, should be ½ to ¼ that of the resistor.

If the value of the capacitor is too large, the resistor and amplifier will get hot.

Vertical Damping

In many vertical amplifier designs a damping resistor is placed across the yoke.

One method to determine the resistor value is to select a power resistor in the 100 to 500 ohm range and adjust the value for best results.

As can be seen, too large a value of resistance leaves oscillation.

Too small of a value slows the amplifier.

Vertical Damping The second method of determining the

damping resistor value involves knowing the power amplifier’s gain/phase plot.

The gain and phase of the resistors, capacitors and yoke inductance must also be known and plotted on the same graph. Watch for adequate gain and phase margin.

END END

Horizontal Deflection

Horizontal Wave Forms A horizontal deflection circuit

makes a saw tooth current flow through a deflection coil.

The current will have equal amounts of positive and negative current.

The horizontal switch transistor conducts for the right hand side of the picture.

The damper diode conducts for the left side of the picture.

Current only flows through the fly back capacitor during retrace time.

Horizontal Trace Right Side For time 1 the transistor is

turned on. Current ramps up in the

yoke. The beam is moved from

the center of the picture to the right edge.

Energy is stored on the inductance of the yoke.

E=I2L/2

Horizontal Retrace Right Side For time 2 the transistor is

turned off. Energy transfers from the

yoke to the fly-back capacitor.

At the end of time two all the energy from the yoke is placed on the fly-back capacitor.

There is zero current in the yoke and a large voltage on the capacitor.

The beam is quickly moved from the right edge back to the middle of the picture.

Horizontal Retrace Left Side During time 3 the energy

on the capacitor flows back into the yoke.

The voltage on the fly-back capacitor decreases while the current in the yoke builds until there is no voltage on the capacitor.

By the end of time 3 the yoke current is at it's maximum but in the negative direction.

The beam is quickly deflected form the center to the left edge.

Time 4 represents the left hand half of the picture.

Yoke current is negative and ramping down

The beam moves from the left to the center of the picture.

Horizontal Trace Left Side

END END

Notes

Horizontal Linearity

Horizontal Linearity In the yoke current path there is

a saturable coil. Just like a size coil, any inductance in series with the yoke will reduce the size of the picture.

This saturable coil will change inductance depending on the amplitude and direction of current flow.

At the start of a trace the linearity coil has an inductance of 20 percent of that of the yoke.

By the center of the trace, the linearity inductance has decreased to about 4 percent of the yoke where it remains for the rest of the trace.

Adjust this variable inductor so the right and left sides of the picture are the same size.

Voltage from two turns of wire added around the linearity coil. When the coil saturates the voltage drops to near zero

Horizontal Linearity Trace A is the yoke voltage

at about 1000 volts peak to peak. Trace B is the yoke current. Trace C is the voltage across the total of all resistance in the horizontal loop. Trace D is the voltage loss due to the semiconductors in the loop. Trace E is the voltage across the S capacitor. Trace F is the voltage across the linearity coil.

The linearity coil should have a waveform like the inverse of trace C+D. Thus the loss seen in traces C+D+F should equal a straight line.

Horizontal Losses Horizontal deflection

schematic to show losses that cause linearity problems.

The ‘R. total’ is the combination of deflection yoke resistance + resistance of the linearity coil + resistance of any size coil + yoke wires + printed circuit traces + ESR of the S-cap.

END END

Notes

Horizontal ‘S’ Correction

‘S’ Capacitor The ‘S’ capacitors corrects

outside versus center linearity in the horizontal scan.

The voltage on the ‘S’ cap has a parabola plus the DC horizontal supply.

Reducing the value of ‘S’ cap increases this parabola thus reducing the size of the outside characters and increasing the size of the center characters.

‘S’ Capacitor value: Too low: picture will be

squashed towards edges. Too high: picture will be

stretched towards edges.

‘S’ Capacitor By simply putting a

capacitor in series with each coil, the saw-tooth waveform is modified into a slightly sine-wave shape.

This reduces the scanning speed near the edges where the yoke is more sensitive.

Generally the deflection angle of the electron beam and the yoke current are closely related.

T?

Bsase Drive C1Fly Back cap

C2S cap

D1

Damper Diode

Q1H. Switch

DYH. Yoke

T2Fly Back Transformer

B+

L1

Size Coil

Deflection Angle .vs. ‘S’ Linearity

In this example an electron beam is deflected with nine different current values. (4,3,2,1,0,-1,-2,-3,-4 amps)

A current in the range of 0 to 1 amp causes the beam to move 4cms.

Current changing from 3 to 4 amps causes 6.5cm movement.

The yoke appears to be 1.5 times more sensitive at the edge of the picture.

4

5

5.5

6.5

4

5

5.5

6.5

High Deflection Angles and Flat Tubes

The amount of ‘S’ correction needed is related to the flatness of the tube and the deflection angle.

If the yoke is at the radius of the curvature of the tube then no ‘S’ correction is needed.

As the yoke is pushed toward the face of the tube deflection angles get large.

This problem is compounded on very flat tubes.

Inner Pin-cushion Many CRTs, especially

flatter ones, need geometry correction that goes beyond simple ‘S’ correction.

Most tubes need inner pin-cushion correction, which is also called "dynamic ‘S’ correction".

Some tubes need more ‘S’ correction only at the extreme edges, this is called "higher-order ‘S’ correction".

END END

Notes

Yokes

Yokes Saddle / Toroidal Saddle / Saddle Stator Pin Wound

Saddle / Toroidal Saddle horizontal

winding Toroidal vertical

winding Split core Inconsistent

winding control High leakage Low cost high

volume

Saddle / Saddle Great flexibility in

correcting geometry, focus, and convergence.

Minimal radiation, windings contained within the ferrite core.

Reasonable sensitivity.

Medium cost High volume

Stator Reliable and precise. The windings and

placed between the slots in the ferrite.

Consistent unit to unit quality.

Low inductance windings.

Good for Stroke Displays.

High cost low volume.

Pin Wound

Pin Winding is a technique developed to precisely control placement of wire in the saddle.

‘Pins” are inserted in the fixture as winding progresses.

Can be used in any saddlePrecise like Stator Yoke

Uniform Field Yoke

END END

Scanning Methods

Stroke Display In the stroke character CRT the image is

painted by the electron beam. There is no raster. This type of CRT is often used for computer

aided design and other applications where line drawn images are used.

The effect is much like a pen plotter.

Scanned Rasters Two orthogonal electromagnet coils are used

to deflect the electron beam. One coil is used for horizontal positioning. The other for vertical position. Current through each coil determines

position of beam. They act like an optical lens. They are subject to similar distortions.

Raster Scan CRT Television sets and most computer monitors

are raster scanned. The electron beam scans the screen from left

to right and top to bottom to create a raster on the screen.

Characters are formed by changing the brightness of dots at the required points on the raster.

Progressive Raster

Non-Interlaced Raster

Interlaced Rasters The next diagram shows an example of an

interlaced picture. The odd lines are scanned first omitting the even lines. Then the even lines are scanned to complete the picture frame.

Interlaced Raster

Interlaced Pictures The benefit of an interlaced picture is that the

horizontal and video rate can be cut in half. This makes the video card in the computer much

easier to build. The video amplifier and the horizontal deflection

circuits in the monitor are also simplifier. An interlaced picture as used in television, works

well for pictures of flowers and trees or action shots.

Interlaced Pictures Generally an interlace picture is not

excitable for data terminals or application where the viewer is close to the picture.

An example of where interlace does not work well is the letter "E". The vertical bar in the letter is drawn both in the odd and even fields and thus gets updated 60 times a second. The three horizontal lines in the letter "E" reside in the odd field and only get drawn 30 times a second. This makes the right side of the "E" flicker.

END END

Notes

Focus and Convergence

Focus and Convergence Focus and Convergence are interrelated. Adjusting for optimal focus and spot size

exaggerates the visibility of poor convergence.

Often focus is sacrificed for convergence.

Convergence Poor convergence is often

thought to be just bad focus by the uninformed.

Black characters on a light or white background can suffer from contrast loss and color fringing.

Very high density display formats obviously need excellent convergence.

Low resolution displays will show the greatest improvement to image quality because they suffer from the poorest convergence control.

Dynamic Focus All monitors have the

ability to set a focus voltage. The voltage is DC and will be the same for all areas of the picture.

The Top Focus control sets the focus voltage for the top and bottom of the picture. Adding a ‘1D’ dynamic vertical correction.

Dynamic Focus The Side Focus sets the

focus voltages along the side of the picture. Often called Horizontal Focus Modulation. This can be a ‘1D’ or ‘2D’ modulation.

When both top and side focus voltages are adjusted the focus voltage is a complex ‘2D’ or ‘3D’ waveform containing horizontal and vertical frequencies.

Horizontal Focus The horizontal focus voltage is a parabola at the

horizontal frequency. The typical place to get this waveform is from the ‘S’

capacitor. The amount of side focus voltage varies from tube to

tube. A variable gain amplifier is used to set the amount of

H. focus voltage needed. A power amplifier boosts the parabola to about 50-

volts. A transformer multiplies the 50-volt signal to several

hundred volts.

Dynamic Convergence Dynamic convergence has many of the same

waveform requirements as focus. Horizontal convergence has a ‘S’ shape leaning from

left to right, and reverses polarity from top to bottom. ‘Natural’ functions can compensate but not totally

correct convergence. High order distortions occur due to the variable

mechanical construction and assembly of tube and yoke.

Dynamic waveform generation, coil drivers and neck components are needed for dynamic correction.

Digital CRT solutions may eliminate coils and drivers by applying correction in a frame buffer.

(C) Copyright Display Labs 1996

192

Correction Waveform Measurements

Tile Correction Waveform Measurements.

(C) Copyright Display Labs 1996

193

Correction Waveform Measurements

Tile Correction Waveform Measurements.

END END

VESA Monitor Timing Specifications Version 1.0 Rev. 0.8

Standards and Guidelines Summary



END END

Moiré

Moiré At low resolution there will be no moiré

effect. Each pixel may extend over several holes in the shadow mask. As the resolution goes up the number of pixels may approach the number of shadow mask holes. Individual pixels no longer cover enough phosphor dots to ensure that they are constant brightness or constant color. The average of all the pixels in an area will still be expectable.

Moiré Moiré appears as wavy lines, contour lines,

or light and dark bands often seen in areas of constant brightness. These may be very fine or 2 cm large and changing across the screen. Tubes with good focus will have worse moiré.

Moiré There are two sources of moiré. Video moiré

is caused by the video pattern (horizontally) verses the dot pitch. Raster moiré is the spacing of scan lines verses the dot pitch. Trinitrons, which do not have a vertical dot structure should not have interference of this sort from the raster lines will have interference from the horizontal pixel structure.

Moiré You can test for moiré by slowly adjusting the

vertical size and horizontal size. If it is moiré, you should see the pattern change in location and frequency. Changes to vertical and horizontal position will change the moiré patterns very little. They will not remain locked to the moving image.

Some monitors have a Moiré control or switch. Generally this control causes every other picture to be moved very slightly. Generally the movement is 0 to .5 pixels. A similar effect is to slightly de-focus the picture.

Moiré An example of large beam with fine shadow mask.

Moiré An example of a beam that is 4% different

than the mask size.

END END

1 _CRT Display Design_

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