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The size of the television image is determined by the width and height. Image size must satisfy the optimum conditions of observation in which the eye can see the whole image as a whole and its parts at the same time.
Geometric similarity is preserved when the image format when receiving consistent format for transmission. Violation of geometric similarity is due to the distortions introduced by light and electron optics systems developments. Clarity describes the maximum number of distinguishable small details of the image, when within the size of one part can be considered an average brightness constant. Sharpness of the image depends on resolution television system. However, the excessive increase in the number of lines leads to expansion of the frequency spectrum of the television signal and requires an increase in bandwidth television channel, which is associated with the complication of the apparatus.
. But the subjective sharpness of this is good enough, and the width of the frequency spectrum signal is relatively small.
Brightness - the most important parameter is the television system. Lack of brightness or too big, almost equally unpleasant to the eyes and tiring him.
Contrast ratio is determined by the brightness of the bright area to the brightness of the darkest area. In case of insufficient contrast the number of light tones - shades of brightness is reduced and naturalness of the image decreases. In a televised image of satisfactory quality should be at least 10 gradations of brightness.
Video signal (image signal), contains information about the brightness of the transmitted image, so it is also called luminance.
The presence of abrupt changes in image brightness determines the pulse character of video. The smaller rise time pulse signal transmission abrupt transitions from one brightness value to another, the more these transitions are transferred and the greater clarity.
Horizontal and Frame Scan creates a frequency signal in the transmission of moving images, since the signal is formed at discrete intervals from line to line and from frame to frame. Flat tops of pulses correspond to the transfer of areas of the image of constant brightness. The smaller the bevel flat top pulse of long duration, the more accurately reproduces the constant brightness of the major parts of the image and the less the lower frequency range of the video.
The width of the frequency spectrum of the video depends on the nature of the transmitted images and the parameters of the sweep.
Maximum possible number of differences of brightness along the lines contained, for example, in the image uniformly alternating black and white stripes of width equal to the diameter of the unfolding of the beam. Video, obtained by transfer of such images is a periodic sequence of pulses with a minimum of repetition.
Interlacing allows a factor of 2 to narrow the frequency band video signal by reducing by half the frame rate in the absence of flicker brightness. The essence of the interlaced scanning is that each image frame, as well as in the technique of cinema, passed twice. Full frame is composed of two half-frames (fields). The first field is deployed, all odd
line and half of the last row. Then the electron beam makes a reverse quickly in the field, starting an even half-frame from the middle of the top line. After the scan beam polustroki consistently draws all the even lines. Due to the inertia of view, playback image of the two half-field impression frame with the total number of rows.
Video signal transmission of still images is strictly periodic. Its spectrum has a discrete line spectrum in nature and is composed of frequencies - harmonics frame and line scan frequency when transmission of moving? images of each frequency component of the spectrum becomes lower and upper sidebands, but empty-sections of the spectrum is a relatively large portion of the band, the frequencies occupied by the video. This feature of the spectrum of the brightness signal is used to seal its frequency components of signals in the chroma-compatible color TV system.
Envelope of the spectrum (dotted line) characterizes the law of energy distribution in the spectrum. The initial portion of the spectrum of G1-3 Hz corresponding to frequencies average video called "DC component". Signal black-and-white television should contain information not only about the changes the brightness of individual sections of the transmitted images in the scan, but on the average of its brightness. When transmitting moving images mean brightness varies over time, so the average level of the video. Since the change in average brightness occurs with very low frequencies - from 0 to 3 Hz, and the average level of the video signal varies with the same frequencies.
Full TV signal (TCP), prepared for transmission over a communication channel, includes video, sync and blanking interval receivers.
The video signal contains information on the distribution of brightness transmitted image along the rows and the average of its brightness to indicate how light or dark a picture as a whole. Blanking interval are used for locking the picture tubes and screens blanking intervals reverse moves horizontal and vertical refresh is not used for transmitting the image signals. If you do not shut up, then the image will be visible to the line flyback in rows and shot, illuminating the dark details of the image and reduces contrast. Vertex blanking pulse transmitted at a level slightly higher black level, which ensures blocking of CRT.
Clock pulses generated sync at the transmitting end a television system, designed to produce a synchronous and in phase of deploying devices at the reception and transmission of images. Synchronization pulses is at the top blanking interval and are separated from the video signal in the receiver amplifier-limiter level.
The big difference in the duration of row and human clock provides ease of their separation at the receiver differentiating and integrating circuits.
The difference in the form of polukadrovyh pulses obtained at the output of integrating circuit, can create an extra time delay between the time synchronization master oscillator field sweep and, therefore, between the beginning of return strokes sweep across the fields.