Tim Amyes
In order to fully understand the post production process, a basic knowledge of picture recording and operations is necessary, and in this chapter we look at the technology behind video recording and shooting on film.
Moving images, whether they be stored on film or videotape, are nothing more than a series of sta- tionary pictures, each slightly different from the previous one and each capturing a successive move- ment in time. When these pictures are shown or projected at speed, the brain interprets them as continuous motion, a phenomenon known as persistence of vision.
It takes approximately 1/20th of a second to visually register a change in an image. When the images change at greater speeds than this, we see pictures that move.
Film
A film camera records pictures photographically. This is achieved by using a claw which engages onto a sprocket and pulls down the film so that it is in a stationary position in front of the lens. A shutter operates for a fraction of a second to expose the image on the film and then, while the shutter is closed, the next frame of unexposed photographic film is positioned (pulled down) for the shutter to open again for exposure. The full camera aperture of a piece of standard (35-mm) film is embraced by engi- neers under the term 2K. It requires a resolution of 2000 lines (as opposed to the 625 or 525 lines used in analogue television) to emulate the film frame.
The image can be viewed on a film editing machine (usually a Steenbeck), in a telecine machine or on a projector. Projectors are usually equipped with an enclosed xenon lamp as the light source. Four interna- tional width formats are used: 16 and 35 mm for television use, and 35 and 70 mm for cinema projection. Seventy-millimetre prints are, in fact, shot using a 65-mm camera negative. Sixteen-millimetre is often exposed, particularly in Europe, using a larger than standard sized aperture called Super 16 mm designed for theatrical release and for 16 by 9 widescreen television. Super 35 mm is a similar format. On both, the soundtrack area is ‘incorporated’ into the picture area, offering an enlarged picture aperture.
The intermittent movement produced in a film camera at the picture aperture is unsuitable for sound recording, which requires a continuous smooth motion. So sound cannot be recorded in synchroniza- tion at the ‘picture gate’. Therefore, sound is usually recorded separately on an audio recorder, which runs in sync with the camera; this is known as the double system. Cinema release prints are printed with the soundtrack to the side of the picture and the track is replayed some frames away from the pic- ture, where the intermittent motion of the claw can be smoothed out (above the picture-head in 35 mm and below it in 16 mm). Film normally runs at a speed of 24 frames per second (fps) in the USA and 24 (or sometimes 25) fps in Europe. Film is almost always transferred to video for editing. This can create problems, for film shot at 24 fps does not synchronize easily with our video systems – which are based on the local AC power frequencies of 50 Hz (Europe) or 60 Hz (USA).
Telecine
Film editing is usually carried out in digital non-linear editing suites. This means the film has to be transferred to video prior to being loaded into the system. The film-to-video transfer is made using a telecine machine. These machines handle the film very carefully and can produce images of the highest possible quality. However, high-quality images are not always necessary if the transfer is only for editing and viewing purposes in the cutting room. In this case, a basic one-light telecine transfer is made, saving time and cost. Once editing is complete, the edit is remade using the original camera negative, which is carefully colour corrected or graded from shot to shot, so that lighting and colour imbalances can be graded out.
Video
The television system works on a similar principle to that of film, again exploiting persistence of vision. Here, the image is produced as an electrical waveform, so instead of focusing the image onto a photographic emulsion, the television camera focuses its image onto a light-sensitive charged- coupled device (CCD), made up of many thousands of elements or pixels. In a high definition system
Figure 5.1 The claw and shutter mechanism of a film camera: 1, exposure of film; 2, claw pull-down;
3, film advance; 4, shutter opens gate (courtesy of F. Berstein).
Figure 5.2 The conversion of a picture to a television signal. The picture ABCD is converted into
various signal voltages. In the ‘blanking’ period between the lines, the ‘electron beam’ flies back. The synchronization pulses are added at the end of each field to keep the camera and receiver in step (courtesy of Robinson and Beard).
there may be as many as 2 073 600 pixels. Each of these picks up variations in light, and breaks the picture down into its separate elements. These are sent to a recorder or to the transmitter for broad- casting. The pictures are, in fact, interlaced together every half frame or field, to reduce flicker. To reproduce the picture, an electron beam that is ‘controlled’ by the camera is directed onto a phos- phor screen that forms the front of the television monitor, tracing a picture. The beam is muted dur- ing its ‘fly-back’ journey from the top to the bottom of the screen in order to stop spurious lines being seen. The electron beam scans across the picture at a speed of 15 625 times per second. Each vertical
scan is called a ‘field’. The European analogue TV standard PAL 625-line system presents 25 com- plete pictures every second and the screen is scanned 50 times. In America, on the NTSC system, 525 lines are produced at almost 30 frames per second (actually 29.97 frames, a change which became necessary because of the introduction of colour to US black and white television), with scanning taking place 60 times. Since the picture has been split up by scanning, it is important that it is reassembled in step with the camera. Two pulses are sent to ensure this, one a line scan pulse and one a field scan pulse.
High definition television (HDTV) has radically improved quality in television pictures. Using this system, video images can be recorded and distributed to emulate the resolution quality of film. The number of scan lines is increased to between 720 and up to 1250 from the minimum of 525, although there is no agreement on a common standard as yet; 1080/24p is most commonly used in the USA and Europe. The 1080 defines the number of lines and 24p the speed and scanning pattern, which is pro-
gressive rather than interlaced, resulting in a sharper image. High definition systems create an immense
amount of data, which requires a much greater amount of storage space to be available in an editing system, compared to that required by conventional analogue formats. This also means that equipment requires higher data interconnection speeds and processing capacity.
Once there were a few standard video recording formats, now there are more than 10 in current usage. Audio post production suites, serving both the television and film markets, have to be ready to accept many of the current formats used in the field, although the Beta SP format has become somewhat of a standard for off-line worktapes in the industry. It is usually not cost-effective to have all VTR decks permanently available to replay all the formats required (although some are backwards compatible, e.g. a DigiBeta machine will play Beta SP), so these are often hired in as needed. They must be able to interface with equipment already available in terms of audio, picture, synchronization and transport control – so it is therefore important to check that the machines interface with existing equipment already installed. Most professional video recorders follow the Sony P2 (officially called DB-9) protocol for interfacing with editing suite controls using a nine-pin socket. The DV formats, however, tend to use the more modern ‘Firewire’ computer interface, both for control and exchanging data.
Figure 5.3 Sony nine-pin interface P2, used in many professional editing interfaces, picture and sound.
Pin No. 1 2 3 4 5 6 7 8 9 Controlling Device Frame Ground Receive A Transmit B Transmit Common Spare Receive Common Receive B Transmit A Frame Ground Controlled Device Frame Ground Transmit A Receive B Receive Common Spare Transmit Common Transmit B Receive A Frame Ground External view Remote in (9P) Remote out (9P) 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 Remote-1 in (9P)/Remote-1 out (9P)
A standard analogue colour video signal has a bandwidth of 5.5 mHz and specialist techniques are needed to record this signal. An analogue tape recorder such as a domestic cassette recorder can only reproduce a range of 10 octaves whereas, to record video successfully, an 18-octave range is required. The only practical way of achieving this speed is by using a rotating head, which scans the tape at a high speed, while the tape itself runs at a slow speed to allow a reasonable amount of data to be stored on the tape. This is known as helical scanning. A control track is recorded to precisely define the posi- tion of the tracks written by the recording drum on the tape. This allows the head to be synchronized correctly to replay the recorded picture. Most digital audio tape recorders use the scanning head tech- nology to allow their high-speed digital data streams to be successfully recorded.
The scanning head was first successfully used by Ampex in 1956, when it launched its 2-inch quadru- plex – the first commercially successful video recorder. Video recording was developed further when 1-inch tape was scanned at a greater angle, allowing still images to be produced at tape standstill. Then, to protect the tape it was placed in a cassette. These form the basis of tape-driven video recorders today.
In the 1970s, videotape began to replace film in the broadcasting industry. A small format was devel- oped that could match the quality and versatility of 16-mm film equipment in both size and quality. This was the helical scan U-matic ¾-inch tape cassette system, but with a linear speed of only 3.75 inches/second the audio quality was poor, as was picture quality. The U-matic format is now occa- sionally found as a viewing format. It formed the basis of our present VHS system.
In the mid-1980s, the Sony Betacam and the Matsushita/Panasonic MII camcorder video cassettes were introduced. Rather than recording combined (composite) signals, which included colour and brightness (or luminance), Betacam recorded the components separately in component form, a technique which resulted in better quality pictures. Betacam SP revolutionized location shooting in Europe and offered a viable alternative to high-quality 16-mm film production. The Beta SP format has been superseded by a variety of digital video formats from various manufacturers, each of which aims to serve a particular part of the market at a particular cost. Digital video ensures a higher quality recording, and on copying little generation loss. The digital video (DV) formats range from the ‘amateur’ Mini DV up to the Panasonic DVPRO50 format, which is used extensively in ENG work.