TV AND VIDEO WAVEFORMS AND STANDARDS
‘Television’ and ‘video’ are wide-ranging words. For our purposes, television means seeing over long distances by means of an electrical link, and video (in the everyday usage of the word) means a recording and playback system with which TV programmes can be stored on disc or magnetic tape for subsequent replay via a TV set or monitor. In analogue systems the picture information is conveyed as an electrical waveform. Since a single link between TV sender and receiver can only handle one signal at a time, and because a TV picture consists of many hundreds of thousands of individual picture elements, a scanning system is required at each end. At the sending end it breaks down the composite picture into separate picture elements which are then sequentially transmitted. At the receiving end this ‘serial’ video signal is used to modulate the light output of the display in order to recreate the original scene. Provided that the scanning system at the receiver runs in perfect synchronism with that at the transmitter the positioning of each picture element in the display will be correct, and a complete two-dimensional picture is built up.
The fidelity of the reproduced picture depends on many things. The scanning process consists of analysing the picture in terms of horizontal lines: the duration and number of these lines is the basic arbiter of picture definition and quality. Many other factors are present, such as the bandwidth of the entire video path from camera to picture tube; the screen structure of a colour display tube; the method of encoding the colour signal, and so on.
SCANNING STANDARDS
The number of horizontal scanning lines used in TV picture analysis is a fundamental characteristic of a TV standard. So far as line standards are concerned there are now only two in general broadcast use – 525 lines in the Americas, Greenland and Japan, and 625 lines elsewhere, including Eastern and Western Europe, Africa and Australia. More specific details appear later in this chapter. Of course the number of lines only describes how each stationary picture is analysed. For moving pictures it is necessary to present a series of frames at a rate which will fool the human eye into believing that it perceives continuous movement; and which avoids noticeable flicker. This depends for its success on ‘persistence of vision’, that characteristic of the eye which retains an impression of an image for a fraction of a second after the object itself has disappeared. A series of still images presented at a rate of about 14 per second would provide an illusion of continuous move- ment, but would give rise to a very distracting flicker. Increasing the rate to 25 per second would reduce the effect but not eliminate it. A repetition rate of 50 per second is satisfactory for most purposes, though 60 is better, especially where the picture is bright. For historical reasons having to do with the frequency of the public electricity supply, 625 line systems generally have a 50 Hz field rate, while 525 line systems have a 60 Hz field rate.
Taking the European 625/50 standard as an example, then, the requirement is for the picture to ‘light up’ 50 times per second to avoid a bad flicker effect. Since 25 pictures per second are adequate to fulfil the continuous movement requirement, however, it would be wasteful of bandwidth and broadcast spectrum space to transmit 50 complete pictures per second. The problem is neatly solved by the adoption (universal for broadcast TV) of interlaced scanning. In this system, instead of transmitting each line of the picture in sequence (Fig. 2.1(a)) the first vertical scanning sweep is done at twice-speed, as it were. The left-to-right scan-line paths are double spaced as a result, and so only 3121⁄2 lines (half the total of 625) are traced out, corresponding to lines 1, 2, 3, 4 etc. in Fig. 2.1(b). The second verti- cal sweep, by virtue of a very precisely timed start point, scans the gaps left between the lines of the first field – lines A, B, C and D in Fig. 2.1(b). By this means, although only 25 complete pictures (frames) are presented per second, the entire screen is scanned 50 times (50 fields) per second. Since at normal viewing distances individual scanning lines are not perceptible, the effect is to secure a 50 Hz flicker rate while using no more video or spectrum bandwidth than required for a 25 fields/second sequentially scanned system.
THE VIDEO SIGNAL
A standard video signal is an electrical analogy of the brightness of the TV picture at the point on the screen being described at that instant. The brighter the picture-point the higher the voltage, with ‘peak-white’ – corresponding to maximum drive – being standardised at the level of +1 V. Black is standardised at 0.3 V (300 mV). All the levels of grey therefore fall between these two voltages, and where a lot of detail is present in the scene the video voltage will very quickly alternate between different levels, giving rise to high frequencies in the video waveform. The range of possible frequencies goes from zero
(d.c.) when an even tone (i.e. all black, all grey, all white etc.) is being transmitted, up to about 5.5 MHz for a fine network of vertical black and white bars. Three lines of a TV waveform are shown in Fig. 2.2. Along with the video signal itself synchronising pulses must be sent to keep the scanning at the receiver in step with that in the camera. The ‘blacker than black’ area between 0 V and 0.3 V is reserved for sync pulses – two types are sent, one at 64 μs intervals to trigger the line scan generator, and one at 20 ms intervals to synchronise field scanning sweeps. In the receiver these pulses are stripped off the video signal by an amplitude limiter (sync separator) and then split into line- and field-rate pulses by frequency (time)-conscious circuits.
A section of this basic waveform, showing one complete line period of 64 μs is shown in Fig. 2.3. It is made up of 52 μs of picture information and a 12 μs line blanking period. The time-reference point for the whole waveform is the beginning of the 4.7 μs line sync pulse. Following the pulse is a ‘back porch’ period of 5.8 μs during which the waveform remains at black level. At the finish of picture information comes a ‘front porch’ of 1.55 μs. This short blanking
interval is introduced to ensure that (regardless of the voltage level on which the line ends) the signal level has dropped fully to black level at the instant of the crucial leading edge of the line sync pulse. Because of the path of the video signal does not offer infinite bandwidth it takes a finite time for the signal voltage to change state, hence the need for the front porch. For precise triggering of line and field scan generators in the receiver it is important that the steep leading edges of synchronising pulses are maintained.
At intervals of 20 ms it is necessary to insert a triggering pulse for the field time base. This will initiate flyback at the end of line 625 and halfway through line 313. The field triggering signal is in fact a series of five broad pulses as shown in Fig. 2.4. To give sufficient time for retrace or flyback of the scanning spot to take place in the picture tube (and to give some useful ‘spare’ lines for various forms of data transmission) picture information is suppressed for some 20 lines after the field sync pulse train. Since the broad field sync pulses occupy 21⁄2 lines and the preceding equalising pulses a further 21⁄2 lines, the picture is suppressed for a total of 25 (20 + 21⁄2 + 21⁄2) lines in each field period. So our total of 625 lines is thus reduced by 50, and close examination of an actual TV picture would reveal it to be composed of 574 complete lines and two half-lines.
The fact that the broad field sync pulses have a component at line rate ensures that synchronisation of the line oscillator is maintained throughout the field sync period. This is less relevant in modern TV design where flywheel line synchronisation (fully covered later) is used, as opposed to the direct sync of the earliest TV designs. Cur- rent receiver technology also permits the use of a counter to directly
derive field trigger pulses from line sync, so that in theory there is no longer any need for field sync pulses – in practice they will always be there to ensure compatibility with all types and ages of receiver.
The parameters of world TV standards are given in Tables 2.1 and 2.2.