▼ Menu

    
Extras ▼

Technical details of the nature of the high-definition television transmissions which will be radiated from the Alexandra Palace have long been awaited. At the request of the BBC, the Baird and the Marconi-EMI companies, who are responsible for the development of the apparatus, have now issued statements defining the nature of the transmissions. These official statements are reproduced in full in these pages, and a summary of the chief points of importance contained in them appears in this article.

Aerial Photograph of Alexandra Palace circa 1931. Courtesy of London Metropolitan Archives.

The eagerly awaited details of the transmitting arrangements which will be employed on the opening of the Alexandra Palace ultra-short-wave station have at last appeared and the full statements issued by the Baird and Marconi-EMI companies are printed at the end of this article. The details given are chiefly about the form of the synchronising signals and are essential to the design of a receiver. Their practical interpretation in receiver design is, of course, a matter which is left to the designers of the equipment, and there is no doubt that many different solutions to the problems involved will be found.

In spite of the differences between the Baird and Marconi-EMI transmissions many points of similarity are to be found. Owing to the irregular nature of the modulation it is hardly possible to picture the transmission in the familiar way as a carrier having a constant average amplitude but an absolute amplitude which varies from moment to moment about the mean. With television it is much more convenient to work in terms of the maximum amplitude during 100% modulation; this is an amplitude just double that of an unmodulated carrier.

The Picture Signal

The picture itself is conveyed by changes in the carrier amplitude over a range of 40% to 100% in the case of the Baird transmissions, and over a range of 30% to 100% in the case of those transmitted by the Marconi-EMI system. Thus Baird uses 60% of the total available range of amplitude for conveying the picture, and Marconi-EMI 70%.The remaining portion of the carrier amplitude range is not wasted but used for synchronising. In each case a rise in carrier amplitude corresponds to a brightening of the picture, and the gear should be so adjusted that black occurs for a 40% amplitude for Baird transmissions and 30% for Marconi-EMI. A fall in the carrier below these values can then cause no change in the picture, for the light-spot is already extinguished. The 40 and 30% amplitudes can be thought of as a sort of dividing line between the picture impulses and the synchronising, for at suitable intervals the carrier amplitude falls from this line to zero to give synchronising pulses.

Now in the Baird system there are 25 pictures a second and 240 lines to a picture, so that there are 25 synchronising impulses a second for the frames and 6,000 for the lines. The line synchronising pulses occur at the start of each line and each occupies 8% of the total time of traverse for one line. In addition, a further 2% is occupied by a black edging to the picture. A total of 10% of the line time is thus occupied by the synchronising pulse and the edging. and the receiving gear should be so arranged that the return stroke of the time-base occurs within the 2% devoted to the border in order that it may be invisible.

A similar procedure is adopted in the case of the frame pulses. Here the synchronising pulse occupies 12 lines and the edging 8 lines, so that out of the total of 240 lines forming a complete frame, only 220 appear in the actual picture.

Now in the Marconi-EMI system the arrangements are rather more complex,because in an effort to reduce flicker, interlaced scanning is used. There are actually 50 frames a second and 405 lines, but this statement must be interpreted differently from usual, for there are neither, 50 complete pictures a second nor 405 lines to each frame. Actually, there are 550 half-pictures a second, each containing 202.5 lines. In effect, in each half-picture, each line is spaced from its neighbours by twice the normal amount so that there is an appreciable gap between the lines. The next half-picture scans the bits missed by the first and fits into the gaps left by the first. Thus the second picture is similar to the first but is staggered slightly so that its lines fall into the gaps between the lines of the first.

As in the Baird system, a portion of each line is used for the synchronising impulses, and 15% of the total time occupied by one line is allotted to this, 10% of the line time being devoted to the actual synchronising impulse and 5% to the provision of a black edge to the picture during which the return stroke of the cathode-ray gear can take place. Thus a total of 85% of the line is used to provide the picture.

Certain lines are also devoted to the synchronising, and between frames there is an interval of 10 lines, so that the number of lines in each frame actually effective in producing the picture is 192.5. The frame synchronising consists of two pulses for each line, each pulse being equal in length to 0.4 line and separated by an interval of 0.2 line. At least 3 lines (6 pulses) will be transmitted in this way, but up to 6 lines (12 pulses) maybe used, the remaining 7 to 4 lines being black with the normal line synchronising signal at the commencement of each.

The precise position of the frame synchronising impulses depends upon the picture being transmitted. In the case of the first frame, the first synchronising pulse occurs exactly one-half line after the commencement of a line, so that whereas an ordinary line is made up of 10% line pulse, 5% black and 85% picture, the first synchronising line of the first (and all odd numbered) pictures is made up of 10% line pulse, 5% black, 35% picture, 40% frame pulse, 10% black. The next line, however, starts off with 40% frame pulse, 10% black, 40% frame pulse, 10% black, and so on.

The second picture (and all even numbered pictures) is different, for the first line devoted to the framing synchronisation does nothing else. The first 40% line pulse occurs at the start of this line and is followed by a 10% black, another 40% pulse, and concludes with 10% black just as do the other lines in all the pictures, whether even or odd.

In the case of receiving equipment designed to operate from both types of transmission, one of the chief requirements is obviously that the time bases shall respond correctly to the different synchronising signals employed. Means must also be provided for changing the frequencies generated. For Baird reception, one time base must give 25 strokes a second and the other 6,000, whereas for Marconi-EMI transmissions one time base must give 50 strokes and the other 10,125. Moreover, the relative outputs of the two time bases must be altered, for the picture ratio of Baird transmissions is 4:3, whereas that of the Marconi-EMl is 5:4. The Marconi-EMI transmissions involve higher modulation frequencies than the Baird, and the highest frequencies involved are about 2 MHz instead of only 1 MHz.

The statements issued provide the essential data upon which the design of receiving equipment must be based, and are consequently invaluable to all designers and experimenters in this field. It should be emphasised, however, that the information is only basic and an enormous amount of work is waiting to be done in the development of simple and reliable gear which can hardly be started until the commencement of regular television transmissions. It is to be hoped, therefore, that these will not be much longer delayed.

The Baird System

Fig. 1. - The picture modulation and synchronising impulses of the Baird transmissions. The drawings are made from tracings taken off the vision radio transmitter monitoring oscillograph.

Details of the Signal Radiated. The drawing, Figure 1, gives complete details of the waveform for picture modulation and synchronising impulses. From this it will be seen that, using the arbitrary aerial current units of zero to 100, the total modulation tor synchronising (black) extends between the tolerance limits of zero to 5 and 37.5 to 42. 5, while the picture modulation (black to white) extends between the tolerance limits of 37.5 to 42.5 and 100.

It will be noted that the high-frequency synchronising impulse is rectangular in shape and is maintained for 8% of the total time taken in tracing the line, and occurs between the line traversals. The low-frequency synchronising impulse, which is also rectangular in shape, is maintained during the time that 12 lines are traced, and occurs between the frame traversals. These traversals, as seen by an observer looking at the received image from the front, scan from left to right (line) and from top to bottom (frame).

The drawing also shows that, in addition to the above 8% of the line traversal time occupied by the high-frequency synchronising impulse, a further 2% is masked off to form a black edging. Similarly, an additional 8 lines are masked of in the case of the low-frequency synchronising impulse for the same purpose.

The total number of lines in the complete picture is 240, scanned sequentially and horizontally at 25 picture traversals per second and 25 complete frames per second. The line frequency is thus 6,000 impulses per second and the frame frequency 25 impulses per second. The dimensions of the observed picture have the ratio of 4 horizontal to 3 vertical.

Amplitude modulation is employed, which results in light intensity modulation in the observed picture, the transmitter carrier increasing towards the white. The line synchronising signals and the frame synchronising signals are in the sense opposite to increasing picture modulation. The maximum frequency band involved in the transmission is 2 MHz, and the average component of light in the picture is transmitted, a black in the picture being transmitted as black and a white transmitted as white, in accordance with the modulation percentages referred to above.

The Marconi-EMI System

Fig.2. - The waveform of the transmissions by the Marconi-EMI system.

Specification of Radiated Wave-form The Marconi-EMl.television system transmits 25 complete pictures per second, each of 405 total lines. These lines are interlaced so that the frame and flicker frequency is 50 per second. The transmitter will radiate signals with sidebands extending to about 2 MHz either side of the carrier frequency. Good pictures can be received utilising only a fraction oi the radiated band, but, naturally, the quality of the received picture will depend upon the degree to which the receiver makes use of the transmitted band width. The transmitted wave-form is shown in Figure 2.

1. Line Frequency. 10,125 lines per second, scanned from left to right when looking at the received picture.
2. Frame Frequency. 50 frames per second, scanned from top to bottom of the received picture.
3. Type of Scanning. The scanning is interlaced. Two frames, each of 202.5 lines, are interlaced to give a total of 405 lines with a complete picture speed of 25 per second. The line component and the frame component of scanning are regularly re-current, the interlace being derived from the fractional relationship between line and frame frequencies. An explanation of the method of interlacing is given at the end of this specification.
4. Interval Between Lines. There will be intervals between the vision signals of successive lines, which intervals provide time for the transmission of a line synchronising signal and also provide time for the return of the cathode ray beam to the beginning of the next line. The minimum interval between the vision signal of successive lines will be 15% of the total line period (1/10,125 sec.), the first 10% of this interval between lines being occupied by the line synchronising signal and the remaining 5% by a signal corresponding to black in intensity. The remaining 85% of the total line period is available for transmitting vision signals.
5. Interval Between frames. There will be intervals between the vision signals of successive frames. The minimum interval between frames will be 10 lines, leaving a maximum of 192.5 active lines per frame, or 385 active lines per complete picture.
6. Picture Ratio. The picture ratio will be 5:4, that is to say, the distance scanned during the active 85% of the total line period will be 5/4 times the distance scanned during the 192.5 active lines of the frame.
7. DC Modulation. The picture brightness component (or the DC modulation component) is transmitted as an amplitude modulation so that a definite carrier value is associated with a definite brightness. This has been called DC Working, and results in there being no fixed value of average carrier, since the average carrier varies with picture brightness. The radio frequency transmitter output is specified in what follows as a percentage of the peak output. This percentage is in terms of current (or voltage) and not in terms of power.
8. Vision Modulation. The vision modulation is applied in such a direction that an increase in carrier represents an increase in picture brightness. Vision signals occupy values between 30% and 100% of peak carrier. The amount by which the transmitted carrier exceeds 30% represents the brightness of the point being scanned.
9. Synchronising Modulation. Signals below 30% of peak carrier represent synchronising signals. All synchronising signals are rectangular in shape and extend downwards from 30% peak carrier to effective zero carrier.
10. Line Synchronising Signals. The line synchronising signals are of one-tenth of a line duration, and are followed by a minimum of one-twentieth of a line of black (30% peak) signal.
11. Frame Synchronising Signals, The frame synchronising signals comprise a train of two pulses per line, each occupying four-tenths of a line and having one-tenth of a line interval of black (30% peak) signal between them. At the end of even frames the first frame impulse starts coincident with what would have been a line signal. At the end of odd frames the first frame pulse starts half a line alter the preceding line signal. At least six frame signals will be transmitted at the end of each frame, but the number may be increased to any number up to 12 pulses (6 lines). During the remainder of the intervals between frames normal line synchronising signals will be transmitted with black (30% peak) signals during the remaining nine-tenths of the line. It will be noted that throughout the interval between frames (as during the whole transmission) the carrier falls from 30% to zero regularly at line frequency and in phase with the beginning of the normal line synchronising pulses.
12. Variations in Transmitted Waveform. The 15% interval betweenvision signals of successive lines, and the 10 lines interval between successive frames are minimum intervals used at the transmitter. During the initial development of the transmitter certain transmissions may have longer intervals between lines and between frames, which lengthened intervals correspond to the transmission of a black border round the picture.

The 30% carrier is the 'black level' below which no vision signals exist and above which no synchronising signals extend. The mean black level of any transmission will be 30% of peak carrier. The black level during any one transmission will not vary by more than 3% of peak carrier from the mean value of that transmission.

The residual carrier during the transmission of a synchronising pulse will be less than 5% of the peak carrier.

The line frequency and the frame frequency will be locked to the 50 Hz supply mains, and therefore will be subject to the frequency variations of the mains.

Explanation of the Method of Interlacing.

Fig. 3. -This diagram shows the method of interlaced scanning to be used in the Marconi-EMI transmissions.

The method of interlacing is demonstrated in Figure 3, which represents the top and bottom portions on the scanned area with the distance between the lines very much enlarged. The lines show the track of the scanning spot, which moves under the influence of a regular downward motion (frame scan) with quick return and a regular left to right motion (line scan) with very quick return (not shown on drawing). The combination of these motions produces the slightly sloping scanning lines. Starting at A, not necessarily at the beginning of a line, the spot completes the line A B, returns to the left and traverses line C D, then E F, and so on down the 'dotted' lines on the drawing. At the bottom of the frame the spot travels along line G H and-then starts at J and travels to K. At this point the return stroke of the frame motion begins and returns the spot to L at the top of the frame. A complete frame scan has now been made since leaving A, so that 202-1/2 lines have been completed, and the point L is half a line away from A. The downward frame motion now starts again, causing the spot to travel along L M, completing a single line motion JKLM. The spot then returns to the left and traces out line N O, which, due to L being half a line ahead of A, will lie between lines AB and C D. Similarly, the next line P Q will lie half way between CD and EF. The spot now traces down the chain-dotted lines to R S and finally traces out T U, at which latter point the frame return causes the spot to rise again to the top. When the spot reaches the top it will have completed two frames since leaving A, and, as two frames occupy the time of exactly 405 complete lines, the spot will return exactly to A, after which the cycle begins again.

From the foregoing it will be seen that the complete picture is scanned in two frames, but as each frame contains an integer number of lines, plus a half, the two frames will interlace. The system does not require the short return times shown for the line and frame scans, nor need the lines begin in the positions shown. Provided the line and frame traversals are regularly recurrent and have the correct frequency ratio (two frames=odd number of lines), an interlaced picture will be obtained.