Sound Person's Guide to Video
eBook - ePub

Sound Person's Guide to Video

David Mellor

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eBook - ePub

Sound Person's Guide to Video

David Mellor

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About This Book

An essential guide to all aspects of video technology for sound technicians wishing to broaden their knowledge. It explains in a highly readable and engaging way, the key technologies and issues, as well as the terms, acronyms and definitions. Although intended for the sound professional, this book will also appeal to anyone involved in working with video. Everything is covered: from how television and video cameras work to digital video recording, electronic news gathering, nonlinear editing, video effects as well as telecine, widescreen technology and the home cinema. The book also takes a look at the impact of digital technology on production methods and examines the technology and rationale behind digital television, High Definition Television, and DVD. It concludes with the use of video in multimedia and the internet.Based on a series of popular articles in Audio Media magazine, this a vital introductory work for students and professionals wishing to broaden their knowledge of video.

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Information

Publisher
Routledge
Year
2013
ISBN
9781136120770
Chapter 1
The Origins of Television and Video
In the Beginning
Contrary to the body of popular general knowledge that goes to make up the questions on Trivial Pursuit cards, there was no one person who can be said to have invented television. One particular person, John Logie Baird, was the first to get it to work and make the world aware of what he had done, but he was building on the important achievements of others, without which probably he would hardly have been remembered at all. It was a very logical progression after the invention of radio to think about how nice it would be to transmit pictures over the airwaves, or even along a length of wire. But electronics had not progressed to a sufficiently sophisticated level to make this possible. The cathode ray tube, around which most television receivers and video monitors are based, was first developed by William Crookes in the 1860s, and that important device the triode vacuum tube was invented by Lee De Forest in 1906. The main stumbling block was a detector for the television camera, and if one of these could have been time-warped back to the beginning of the century we would have had a satisfactory television system in operation very much sooner. Because of the impracticality of electronic television in the early days, the only other option was to do it mechanically, and this was developed into a system that was actually used for early experimental television broadcasts.
Scanning
It is no problem to convert a sound signal to electricity and pass it along a wire because one microphone can pick up all of the sound present at a particular point in space, and that is good enough for our ears to recognize the reconstructed sound emanating from a loudspeaker as being a passable imitation of the original. With pictures, the situation is more complex because a picture is two-dimensional, with height and width. It is not possible to transmit this picture along a wire all in one piece because the wire effectively only has a single dimension – length. What is needed is to split the picture up into components and send them down the wire one after the other and then reassemble them at the other end. For instance, in Figure 1.1, you could tell a friend over the telephone which squares were light and which were dark and if you were both working on the same grid they could assemble the same picture. All you would have to do is call out light-dark-light-dark, etc. and periodically say that you had reached the end of a line and that you were starting a new one. If you were transmitting a moving picture in this way then you would also have to say when you had finished one frame and were about to start on the next.
image
Figure 1.1 Example of scanning.
The first device that could do this automatically was invented by Paul Nipkow in Germany in 1884 and was called the Nipkow disc. Figure 1.2 shows how simple it was. It could scan a scene in a very similar way to a modern camera. The scene would have to be very brightly illuminated and an image focused onto the disc. The disc was rotated and light from only one hole at a time was allowed to fall upon the photocell. This scans the scene into a number of lines, one per hole, and the varying brightness of the scene as the scan progresses causes the output voltage to vary in an exactly similar way. When the disc has made one complete rotation, that completes a frame and a new scan starts. This can also be done in reverse with light being projected onto the scene through the holes.
image
Figure 1.2 Nipkow disc.
The only problem Paul Nipkow had with his disc is that he never got around to making it. It remained an interesting idea until John Logie Baird started work on his system, using the Nipkow disc scanner, in 1923. His first system was not very sophisticated and had only eight lines, resulting in a very coarse-grained picture. But it worked and it worked well enough for Baird to have to think of a name for the company he wanted to set up to exploit his invention. He called the company Television Ltd. By 1929, Baird had developed his invention to a point where the BBC had become interested and begun regular experimental broadcasts. The number of lines had increased to thirty, but the frame rate was only 12 Hz, which resulted in a very noticeable flickering of the image, although it did have the advantage that the bandwidth was low enough for transmission over a normal sound channel.
The development of electronic scanning was the last piece in the jigsaw that made electronic television systems a practicality. A collaboration between EMI and the Marconi Wireless Telegraph company produced a system that was capable of 405 lines at a frame rate of 25 frames per second. Baird, too, had been busy and had a mechanical system with 240 lines at 25 frames per second, and also a system which involved filming the subject and quickly developing that film before scanning to produce almost live pictures. Apparently this last system was just a little bit unreliable! Known far and wide for their fairness and impartiality, the BBC implemented the Baird and EMI systems and began broadcasting. Not surprisingly, EMI’s electronic system was found to be so superior to Baird’s systems that the tests, originally planned for two years, were cut short after three months. The 405 line, 25 frames per second standard lasted a long time up to the 1960s when the BBC introduced their new channel, BBC2, on the new 625 line standard only. This meant that to receive the new channel, you had to get a new set, but 405 line broadcasting continued into the 1980s by which time, presumably, all the old 405 line sets – or their owners – had worn out.
Meanwhile in the USA, an engineer called Vladimir Zworykin was inventing a device known as the iconoscope – the forerunner of today’s camera tube. This was working by 1932 and although there were intermediate developments, it was still in use up until 1954 when the vidicon tube came into being. The development of electronic scanning was the last piece in the jigsaw that made electronic television systems a practicality. In 1929 the FCC (Federal Communications Commission) licensed a number of stations to make experimental mechanical television broadcasts but over the next few years it became apparent that mechanical TV was not the way to go. In 1935 David Sarnoff of RCA allocated a budget of $1 million to develop a complete electronic television system – an extraordinarily large sum of money during the economic depression. Tests commenced using an iconoscope-based camera using 343 lines at 30 frames per second. 1939 saw an increase in the number of lines to 441 and a more sensitive camera. RCA’s system was apparently workable but the FCC were torn between their role in promoting new technology and in controlling the giant near-monopoly of RCA. They determined that full commercial broadcasting would not be allowed until a standard was agreed by the whole of the industry, including Dumont and Philco who had devised rival systems. To resolve this problem a committee representative of the industry as a whole was established to report to the FCC on a system suitable for television broadcast. That committee was called the National Television Standards Committee, or NTSC, and devised the set of standards including the 525 lines and 30 frames per second that is now familiar. This was accepted by the FCC in 1941 and commercial television broadcasting was ready to begin.
The Coming of Colour
Detailed explanations of how colour television works can wait until later, but in principle it is necessary to transmit three pictures, one for each of the three primary colours red, green and blue. However this is achieved, there are three major problems. The first is that you can only fit so many transmission channels into a given amount of airspace, or bandwidth to use the correct term. If three full-bandwidth pictures were to be transmitted then this would obviously reduce the maximum number of channels available to a third. The other twin problems are of compatibility. In the early stages of colour television, at least, it was very important that it was possible to receive a colour television transmission on a monochrome set, and also that a colour set would be able to receive a monochrome transmission.
Colour television was seen as early as 1928 when Baird devised a Nipkow disc with three sets of holes for the three primary colours, but the first serious proposal for a colour system suitable for broadcasting came from CBS in 1940. To overcome the difficulty of bandwidth, the system was compromised in three ways from the existing monochrome standard of 525 lines at 30 fps and a video bandwidth of 4 Megahertz: the bandwidth was increased from 4 MHz to 5 MHz; the frame rate was reduced to 20 Hz; and the number of lines was reduced to 343. This gave a less detailed picture which would have had a noticeable flicker, but it had the all-important ingredient of colour.
The CBS system was known as a ‘field-sequential’ system in which the first frame of the programme included red and green information, the second frame carried blue and red, the third green and blue, etc. The problem of flicker might have been bad enough but this was compounded by the problem that a white object moving across the screen would break up into a sequence of coloured images. People wanted colour television, but not that badly! The system also used a mechanical disc to filter the images, which seemed like a nasty hangover from the days of Nipkow discs. Nevertheless, the CBS system was remarkable in that it was television in colour, and that in itself was a significant achievement. They were able to improve the system and by 1949 it had 405 lines at 25 fps and would still fit in a standard broadcast channel.
RCA, perhaps mindful of the mechanical versus electronic debate in earlier days, had decided to wait until it could produce a fully electronic system and was not ready to compete. But CBS had a system and RCA didn’t, yet. The FCC were not in the business of blocking working systems at the behest of a powerful rival company because they said that they could do better given time. The CBS colour television system was approved for broadcasting in the USA in 1951, after a decade of debate and arguments that went all the way up to the Supreme Court.
The introduction of colour was a disaster. In the time between CBS developing its system and actually being given permission to broadcast, the number of black and white sets had increased enormously, and because of its incompatibility with these sets the new colour service could only be slotted in at a time when most people were doing things other than watching TV. CBS was legally committed to manufacture sets which no-one wanted and faced severe financial problems. It is also worth mentioning that NBC did not have any particular interest in broadcasting CBS colour!
Fortunately for CBS, the Korean War intervened and the manufacture of colour TV sets was prohibited by the National Production Authority so they didn’t have to carry on flogging their dead horse for too long.
A New Standard
While CBS had been busy working on a set of compromises that would allow their field-sequential system to work satisfactorily (yet still not give the public what they wanted), RCA had made the important discovery that the eye was not as sensitive to detail in colour as it was to detail in a monochrome picture. RCA proposed that fine detail should be extracted from the red and blue components of the picture and added to the green signal. This would allow the bandwidth of the red and blue signals to be reduced. This developed into a system where monochrome information is transmitted as a full bandwidth signal and the colour as a pair of reduced-bandwidth signals called I and Q. The Q signal represents blue shades, in which the eye is particularly insensitive to fine detail, so that the bandwidth in this signal can be reduced still further. The I and Q signals are modulated onto a carrier in such a way that the colour information slots neatly into gaps that exist in the monochrome waveform and compatibility was maintained with existing monochrome sets. Consisting mainly of RCA’s technology, yet with important contributions from others, the NTSC finalized and approved what came to be known as the ‘NTSC system’ on July 21, 1953. It was accepted by the FCC in December the same year and commercial colour broadcasts were authorized from January 22, 1954.
Shortly afterwards in Europe, politicians were thinking that if they allowed colour television, they would have to use a different system or Europe would be flooded by imports. There was also the advantage that whatever problems had been overlooked in the NTSC system could be corrected in a new European system. The one disadvantage of the NTSC system is that the colour signals are transmitted on the same frequency and are separated only by their phase. This means that if there is any phase problem in the transmission path, the overall colour bias of the picture will change. This led to the NTSC system’s description as being ‘Never The Same Colour’! The European PAL system, developed by Telefunken in Germany, corrects this problem by alternating the phase of the colour carriers every line, hence Phase Alternate Line. In PAL, any phase errors tend to average out rather than change the overall colour of the picture.
SECAM stands for Sequential Colour with Memory, but translated into French, since it is a French system. Their reasons for having a different system to everyone else can only be guessed at, but it is hardly surprising that SECAM was adopted by Eastern Europe so that if people did tune in to Western TV broadcasts, with all the consumer temptations they offer, they would only receive black and white pictures. Whereas NTSC and PAL are very similar, SECAM operates in a very different way by transmitting only half of the colour information on each line of the picture. It holds that information in a delay line until the next line starts, when it is mixed with the other half of the colour, which is then delayed until the next line, and so on. This avoids having to transmit two colour signals at the same time, but raises complications in signal processing such as special effects.
Video
The difficulty of recording a video picture onto tape was long held to be on a par with finding the secret of eternal youth. RCA’s video recorder was developed by Harry F. Olsen who was thinking along the lines of an audio recorder, but able to record a signal with a bandwidth of 4 Megahertz, rather than the tiny-in-comparison audio bandwidth – a mere 20 kHz. His machine used five tracks to record red, green, blue, brightness and sync signals and, after a period of development, he managed to reduce the tape speed to twenty feet per second. Apparently the spools were so large, and their inertia so great, that the engineers were issued with leather gloves in case the brakes failed and they had to slow down the machine by hand! The problems of longitudinal recording were insuperable and the machine never made it into broadcast use. While other companies, including the BBC with their VERA (Video Electronic Recording Apparatus), continued with their impractical efforts, it was a much smaller company that developed the video recorder into a usable device – Ampex.
Ampex was founded in 1944 by Alexander M. Poniatoff, who gave his initials to the company name, plus ‘EX’, standing for excellence. One could say he was being a bit smug, considering that he started with six employees in a garage, but he showed the mighty RCA corporation more than a thing or two in 1956 when he demonstrated a video recorder that knocked the spots off the RCA demonstration machine. Charles Ginsberg was the head of a development team that included Ray Dolby of noise reduction fame. The idea that made video recording a practicality was transverse scanning. A rotary head is used to write a track from one edge of the tape to the other, which breaks up the continuity of the recording and so would be totally unsuitable for analogue audio, but which is entirely appropriate to the line structure of a video image. Four heads were mounted o...

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