Biblio5

REFERENCES 411

13. Signaling System No. 7—Functional Description of the Signaling Connection Control Part (SCCP), ITU-T Rec. Q.711, ITU, Helsinki, 1993.

14. Formats and Codes (Telephone User Part), CCITT Rec. Q.723, Fascicle VI.8, IXth Plenary Assembly, Melbourne, 1988.

15. Signaling System No. 7—Signaling Connection Control Part Procedures, ITU-T Rec. Q.714, ITU, Geneva, 1996.

16. Signaling System No. 7—General Function of Telephone Messages and Signals, CCITT Rec. Q.722, Fascicle VI.8, IXth Plenary Assembly, Melbourne, 1988.

17. Signaling System No. 7—Signaling Performance in the Telephone Application, ITU-T Rec. Q.725, ITU, Helsinki, 1993.

18. Roger L. Freeman, Telecommunication System Engineering, 3rd ed., Wiley, New York, 1996.

414 TELEVISION TRANSMISSION

1. Perception of the distribution of luminance or simply the distribution of light and shade;

2. Perception of depth or a three-dimensional perspective;

3. Perception of motion relating to the first two factors above; and 4. Perception of color (hues and tints).

Monochrome TV deals with the first three factors. Color TV includes all four factors. A video transmission system must convert these three (or four) factors into electrical equivalents. The first three factors are integrated to an equivalent electric current or voltage whose amplitude is varied with time. Essentially, at any one moment it must integrate luminance from a scene in the three dimensions (i.e., width, height, and depth) as a function of time. And time itself is still another variable, for the scene is changing

in time.

The process of integration of visual intelligence is carried out by scanning. The horizontal detail of a scene is transmitted continuously and the vertical detail discontinuously. The vertical dimension is assigned discrete values that become the fundamental limiting factor in a video transmission system.

The scanning process consists of taking a horizontal strip across the image on which discrete square elements called pels or pixels (picture elements) are scanned from left to right. When the right-hand end is reached, another, lower, horizontal strip is explored, and so on, until the whole image has been scanned. Luminance values are translated on each scanning interval into voltage and current variations and are transmitted over the system. The concept of scanning by this means is illustrated in Figure 14.1.

The National Television Systems Committee (U.S.) (NTSC) practice divides an image into 525 horizontal scanning lines.1 It is the number of scanning lines that determines the vertical detail or resolution of a picture.

When discussing picture resolution, the aspect ratio is the width-to-height ratio of the video image (see Section 14.2.1).2 The aspect ratio used almost universally is 4 : 3.3 In other words, a TV image 12 in. wide would necessarily be 9 in. high. Thus an image divided into 525 (491) vertical elements would then have 700 (652) horizontal elements to maintain an aspect ratio of 4 : 3. The numbers in parentheses represent the practical maximum active lines and elements. Therefore, the total number of elements approaches something on the order of 250,000. We reach this number because, in practice, the vertical detail reproduced is 64–87% of the active scanning lines. A good halftone engraving may have as many as 14,400 elements per square inch, compared to approximately 3000 elements per square inch for a 9-in. by 12-in. TV image.

Motion is another variable factor that must be transmitted. The sensation of continuous motion, standard TV video practice, is transmitted to the viewer by a successive display of still pictures at a regular rate similar to the method used in motion pictures. The regulate rate of display is called the frame rate. A frame rate of 25 frames per second will give the viewer a sense of motion, but on the other hand he/ she will be disturbed by luminance flicker (bloom and decay), or the sensation that still pictures are “flicking” on screen one after the other. To avoid any sort of luminance flicker sensation, the image is divided into two closely interwoven (interleaving) parts, and each part is presented in succession at a rate of 60 frames per second, even though complete pic-

1With most European TV systems this value is 625 lines.

2Resolution deals with the degree to which closely spaced objects in an image can be distinguished one from another.

3High-definition television (HDTV) is a notable exception.

416 TELEVISION TRANSMISSION

Figure 14.2 Development of a sinusoid wave from the scan of adjacent squares.

nization. Accordingly, only about 53.3 ms are left per line of picture to transmit information.

What will be the bandwidth necessary to transmit images so described? Consider the worst case, where each scanning line is made up of alternate black-and-white squares, each the size of the scanning element. There would be 652 such elements. Scan the picture, and a square wave will result, with a positive-going square for white and a negative for black. If we let a pair of adjacent square waves be equivalent to a sinusoid (see Figure 14.2), then the baseband required to transmit the image will have an upper cutoff frequency of about 6.36 MHz, providing that there is no degradation in the intervening transmission system. The lower limit will be dc or zero frequency.

14.2.1 Additional Definitions

14.2.1.1 Picture Element (pixel or pel). By definition, “The smallest area of a television picture capable of being delineated by an electrical signal passed through the system or part thereof” (Ref. 1), a picture element has four important properties:

1. Pv, the vertical height of the picture element; 2. Ph, the horizontal length of the picture element; 3. Pa, the aspect ratio of the picture element; and

4. Np, the total number of picture elements in an entire picture.

The value of Np is often used to compare TV systems.

In digital TV, a picture consists of a series of digital values that represent the points along the scanning path of an image. The digital values represent discrete points and we call these pixels (pels).

The resolution of a digital image is determined by its pixel counts, horizontal and vertical. A typical computer picture image might have 640 × 480 pixels.

14.2.1.2 Aspect Ratio. This is the ratio of the frame width to the frame height. This ratio is defined by the active picture. For standard NTSC television and PAL television and computers the aspect ratio is 4 : 3 (1.33 : 1). Wide-screen movies have a 16 : 9 aspect ratio, and HDTV is expected to also use a 16 : 9 aspect ratio.

Figure 14.3 Breakdown in time of a scan line.

14.3 COMPOSITE SIGNAL

The word composite is confusing in the TV industry. On one hand, composite may mean the combination of the full video signal plus the audio subcarrier; the meaning here is narrower. Composite in this case deals with the transmission of video information as well as the necessary synchronizing information.

Consider Figure 14.3. An image that is made up of two black squares is scanned. The total time for the scan line is 63.5 msec, of which 53.3 msec are available for the transmission of actual video information and 10.2 msec are required for synchronization and flyback.4

During the retrace time or flyback it is essential that no video information be trans-

4Flyback is defined by the IEEE (Ref. 1) as “the rapid return of a beam in a cathode-ray tube in the direction opposite to that of scanning.” Flyback is shown in Figure 14.1, where the beam moves left returning to the left side of the screen.

418 TELEVISION TRANSMISSION

mitted. To accomplish this, a blanking pulse carries the signal voltage into the reference black region. Beyond this region in amplitude is the blacker than black region, which is allocated to the synchronizing pulses. The blanking level (pulse) is shown in Figure 14.3.

The maximum signal excursion of a composite video signal is 1.0 V. This 1.0 V is a video/ TV reference and is always taken as a peak-to-peak measurement. The 1.0 V may be reached at maximum synchronizing voltage and is measured between synchronizing “tips.”

Of the 1.0-V peak, 0.25 V is allotted for the synchronizing pulses and 0.05 V for the setup, leaving 0.7 V to transmit video information. Therefore the video signal varies from 0.7 V for the white-through-gray tonal region to 0 V for black. The best way to describe the actual video portion of a composite signal is to call it a succession of rapid nonrepeated transients.

The synchronizing portion of a composite signal is exact and well defined. A TV/ video receiver has two separate scanning generators to control the position of the reproducing spot. These generators are called the horizontal and vertical scanning generators. The horizontal one moves the spot in the X or horizontal direction, and the vertical in the Y direction. Both generators control the position of the spot on the receiver and must, in turn, be controlled from the camera (transmitter) synchronizing generator to keep the receiver in step (synchronization).

The horizontal scanning generator in the video receiver is synchronized with the camera synchronizing generator at the end of each scanning line by means of horizontal synchronizing pulses. These are the synchronizing pulses shown in Figure 14.3, and they have the same polarity as the blanking pulses.

When discussing synchronization and blanking, we often refer to certain time intervals. These are described as follows:

•The time at the horizontal blanking pulse, 2–5 in Figure 14.3, is called the horizontal synchronizing interval;

•The interval 2–3 in Figure 14.3 is called the front porch;

•The interval 4–5 is called the back porch.

The intervals are important because they provide isolation for overshoots of video at the end of scanning lines. Figure 14.4 illustrates the horizontal synchronizing pulses and corresponding porches.

The vertical scanning generator in the video/ TV receiver is synchronized with the camera (transmitter) synchronizing generator at the end of each field by means of vertical synchronizing pulses. The time interval between successive fields is called the vertical interval. The vertical synchronizing pulse is built up during this interval. The scanning generators are fed by differentiation circuits. Differentiation for the horizontal scan has a relatively short time constant (RC ) and that for the vertical a comparatively long time constant. Thus the long-duration vertical synchronization may be separated from the comparatively short-duration horizontal synchronization. This method of separation of synchronization, known as waveform separation, is standard in North America.

In the composite video signal (North American standards) the horizontal synchronization has a repetition rate of 15,750 frames per second, and the vertical synchronization has a repetition rate of 60 frames per second (Refs. 2, 3).

Figure 14.4 Sync pulses and porches.

14.4 CRITICAL VIDEO PARAMETERS

14.4.1 General

Raw video baseband transmission requires excellent frequency response, in particular, from dc to 15 kHz and extending to 4.2 MHz for North American systems and to 5 MHz for European systems. Equalization is extremely important. Few point-to-point circuits are transmitted at baseband because transformers are used for line coupling, which deteriorate low-frequency response and make phase equalization very difficult.

To avoid low-frequency deterioration, cable circuits transmitting video have resorted to the use of carrier techniques and frequency inversion using vestigial sideband (VSB) modulation. However, if raw video baseband is transmitted, care must be taken in preserving its dc component (Ref. 4).

14.4.2 Transmission Standard Level

Standard power levels have developed from what is roughly considered to be the input level to an ordinary TV receiver for a noise-free image. This is 1 mV across 75 Q . With this as a reference, TV levels are given in dBmV. For RF and carrier systems carrying video, the measurement refers to rms voltage. For raw video it is 0.707 of instantaneous peak voltage, usually taken on synchronizing tips.

The signal-to-noise ratio is normally expressed for video transmission as

The Television Allocation Systems Organization (TASO) picture ratings (4-MHz bandwidth) are related to the signal-to-noise ratio (RF) as follows (Ref. 5):

420 TELEVISION TRANSMISSION

Figure 14.5 Video amplitude–frequency response. (From Ref. 6, EIA-250C. Courtesy of Electronic Industries Association/ Telecommunication Industry Association. Reprinted with permission.)

14.4.3 Other Parameters

For black-and-white video systems there are four critical transmission parameters:

1. Amplitude-frequency response (see Figure 14.5).

2. EDD (group delay);

3. Transient response; and

4. Noise (thermal, intermodulation distortion [IM], crosstalk, and impulse).

Color transmission requires consideration of two additional parameters:

5. Differential gain; and

6. Differential phase.

A description of amplitude–frequency response (attenuation distortion) may be found in Section 3.3.1. Because video transmission involves such wide bandwidths compared to the voice channel and because of the very nature of video itself, both phase and amplitude requirements are much more stringent.

Transient response is the ability of a system to “follow” sudden, impulsive changes in signal waveform. It usually can be said that if the amplitude–frequency and phase characteristics are kept within design limits, the transient response will be sufficiently good.

Noise and signal-to-noise ratio are primary parameters for video transmission. Of course, noise is an impairment and is described in Section 3.3.3. Signal-to-noise ratio is the principal measure of video signal quality. Signal-to-noise ratio is defined in Section 3.2.1.