Actividades de control

• The electron beam is steered by variable

electrostatic fields produced by two pairs of deflecting coils that wrap around the vidicon tube. "Time-varying voltages from a 'sweep generator'"

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and down to scan the target.

ƒ The other pair of coils moves the beam from side to side along a horizontal line. ƒ All four coils, working together, move the electron beam over the target in a

repetitive scanning motion. Video Signal

• When a globule absorbs light, photoelectrons are emitted (Fig. 13-5B). • The electrons are immediately attracted to

the anode and removed from the tube. • The globule, having lost electrons, becomes

positively charged.

• Since the globule is insulated from its surroundings it behaves like half of a tiny capacitor, and draws a current onto the conductive signal plate → this current that flows onto the signal plate is ignored, or clipped, and is not recorded (Fig. 13-5C). • Similar events occur over the entire surface

of the target.

• A brighter area in the light image emits more photoelectrons than a dim area, and produces a stronger charge on the tiny capacitors.

• The result is a mosaic of charged globules that store an electrical image that is an exact replica of the light image focused onto the target.

• The electron beam scans the electrical image stored on the target and fills in the holes left by the emitted photoelectrons (i.e. discharging the tiny globule capacitors).

• After the capacitors are fully discharged (no more positive charges are left), no additional electrons can be deposited in the globules.

• Excess electrons from the scanning beam drift back to the anode and are removed

from the tube.

• When the electrons in the scan beam neutralize the positive charge in the globules, the electrons on the signal plate (Fig. 13-5D) no longer have an electrostatic force to hold them on the plate. They will leave the plate via the resistor.

ƒ These moving electrons form a current flowing through a resistor → a voltage appears across the resistor which constitutes the video signal (Fig. 13-5E).

It was indicated earlier that the electrons in the electron beam were reduced to low energy electrons before they entered the target. There are two reasons for this. ƒ The first Reason is that we want no electrons to enter the target after the positive

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ƒ The second reason is that the electrons should not have sufficient energy to produce secondary electrons when they do enter the globules. "high energy secondary electrons would be able to neutralize the positive charge in other globules and degrade the image".

The globules are not all discharged at the same time.

ƒ Only a small cluster, a dot, is discharged each instant in time. Then the electron beam moves on to the next dot in an orderly sequence, discharging all the globules on the target.

ƒ The result is a series of video pulses, all originating from the same signal plate but separated in time.

ƒ Each pulse corresponds to an exact location on the target.

ƒ Reassembling these pulses back into a visible image is done by the camera control unit and the television monitor.

Television Monitor

• The last link in the television chain is the monitor.

It contains the picture tube and the controls for regulating brightness and contrast.

Figure 13-6 Television monitor

• A picture tube is similar to a vidicon camera tube (Fig. 13-6).

ƒ Both are vacuum tubes and both contain an electron gun, control grid, anode, focusing coil, and deflecting coils.

ƒ A picture tube, however, is much larger. • This evacuated glass envelope contains:

ƒ at the narrow end, an electron gun (c), which projects a pencil of electrons, shown as a dashed line in the figure,

ƒ A phosphor screen (e) coated on the inside of the wide end of the envelope, where the pencil produces a small dot of light.

• The focusing and deflecting coils are wrapped around the neck of the tube, and they control the electron beam in exact synchrony with the camera tube.

• The brightness of the individual dots in the picture is regulated by the control grid → modulate the brightness of spot of light on the monitor screen.

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ƒ To produce a bright area in the television picture, the grid allows a large number of electrons to reach the fluorescent screen. To produce a dark area, the grid cuts off the electron flow almost completely.

• The anode is plated onto the inside surface of the picture tube near the fluorescent screen. It carries a much higher positive potential (10,000 V) than the anode of the camera tube (250 V), so it accelerates the electron beam to a much higher velocity. • The electrons strike the fluorescent screen at the flared end of the tube, which

makes the screen emit a large number of light photons. The generation of light pho- tons over the entire surface of the tube is the visible television image.

• Many secondary electrons are set free by the impact of the electron beam with the screen, and they are attracted to the anode and conducted out of the picture tube. TELEVISION SCANNING

• The television image is stored as an electrical image on the target of the vidicon tube, and it is scanned along 525 lines by a narrow electron beam 30 times per sec-

ond.

• Each scan of the entire target is called a "frame".

• The electron beam scans the target in much the same manner that we read a page

in a book, only it does not have to turn pages. Instead, as the beam reads, it also

erases. As the electron beam discharges the globule capacitors, it erases their image.

• As soon as a line is read and erased, it is ready to record a new image, and it begins immediately.

• Because the electron beam scans the target 30 times each second → our eyes perceive a continuous motion as in a cine film.

• But, the eye can detect individual flashes of light, or flicker, up to 50 pulses per

second. A television monitor only displays 30 frames per second, so an electronic

trick, called interlaced horizontal scanning, is employed to avoid flicker.

ƒ Instead of scanning all 525 lines consequently, only the even-numbered lines are scanned in the first half of the frame, and only the odd-numbered lines are scanned during the second half (Fig. 13-7).

ƒ Each pass of the electron beam over the video target is called a field, and consists of 2621/2 lines.

∴

Although only 30 frames are displayed each second, they are displayed in 60 flashes of light (fields), and flicker disappears.

15 Synchronization

• It is necessary to synchronize the video signal between the camera and monitor to keep them in phase with each other.

• The camera control unit adds synchronization pulses to the video signal at the end of each scan line & scan field "horizontal and vertical synchronization pulses". • They are generated during the retrace time of the electron beam, while no video

signal is being transmitted.

ƒ First, the picture screen is blackened by a blanking pulse, and the synchronization signal is added to the blanking pulse.

ƒ If the synchronization pulses were added to the video signal while the screen was white → white streaks of noise.

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I- RESOLUTION

• Spatial resolution is the ability to detect a single small structure against its background or to distinguish two separate structures close together.

• It is tested by imaging a bar 'test tool' or 'resolution grid' (Fig. 4.5a). ƒ The bars are strips of lead affixed to a Perspex plate.

ƒ The bars are equally spaced & each space = the width of a bar. ƒ A bar and a space together make up a line pair.

ƒ The spatial frequency = the number of such line pairs per millimeter (lp\mm). • Figure 4.5a also plots the brightness of the screen image (or the video signal)

along a scan line.

ƒ The higher the spatial frequency → ↑ blurring & ↓ contrast.

∴ if the blurring is too large or the bars too narrow and too close together → the blur of the edges of each bar merges with that of the adjacent one → the gap between them cannot be distinguished.

• The effect of blurring is to worsen resolution

So, the smaller the blurring → the better the resolution.

• The spatial resolution of the system is defined as the spatial frequency of the finest pattern that can still be resolved.

Video Signal Frequency (Bandpass)

• Bandpass "bandwidth", is the frequency range that the electronic components of the video system must be designed to transmit without distortion. • The frequency of the video signal fluctuate from moment to moment, depending on

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• Figure 13-9 shows one scan line of an image containing 4 line pairs (4 lp) → So, video signal for this scan line consists of 4 cycles.

• We can calculate the frequency "number of cycles per second" of the video

signal generated by the four-line-pair image by multiplying the number of cycles

per scan line (four in this case) by the number of scan lines per frame by the number of frames per second:

cycles

X scan lines X frames = cycles/sec

scan line Frame seconds

4 x 525 x 30 = 63,000

• When the number of line pairs in the image changes, the frequency of the video signal also changes.

In document INFORME ANUAL DE GOBIERNO CORPORATIVO DE LAS SOCIEDADES ANÓNIMAS COTIZADAS (página 56-59)