Introduction
Audio, for all its artistic power and technological intricacy, is just one element in today’s multimedia entertainment phalanx. Today’s recording engineers are more likely to find themselves working in a MIDI studio locked to SMPTE time code than they are to be rigging microphones. Today’s composer has a better chance of making his/her living by proactively seeking work for television and film (for which he/she will require an understanding of the medium) than to wait for a commission for a symphony from a rich patron! This chapter contains a description of the principles and concepts that form the technical foundations of an understanding of analogue and digital television.
Persistence of Vision
The human eye exhibits an important property that has great relevance to the film and video industries. This property is known as the persistence of vision. When an image is impressed upon the eye, an instantaneous cessation of the stimulus does not result in a similarly instantaneous cessation of signals within the optic nerve and visual
processing centers. Instead, an exponential “lag” takes place with a relatively long time required for total decay. The cinema has exploited this effect for over 100 years. Due to the persistence of vision, if the eye is presented with a succession of still images at a sufficiently rapid rate, each frame differing only in the positions moving within a fixed frame of reference, the impression is gained of a moving image. In a film projector each still frame of film is drawn into position in front of an intense light source while the source of light is shut off by means of a rotating shutter. Once the film frame has stabilized, the light is allowed through—by opening the shutter—and the image on the frame is projected upon a screen by way of an arrangement of lenses. Experiments soon established that a presentation rate of about 12 still frames per second was sufficiently rapid to give a good impression of continuously flowing movement but interrupting the light source at this rate caused unbearable flicker. This flicker phenomenon was also discovered to be related to the level of illumination; the brighter the light being repetitively interrupted, the worse the flicker. Abetted by the low light output from early projectors, this led to the first film frame-rate standard of 16 frames per second (fps). A standard well above that required simply to give the impression of movement and sufficiently rapid to ensure flicker was reduced to a tolerable level when used with early projection lamps. As these lamps improved, flicker became more of a problem until an ingenious alteration to the projector fixed the problem. The solution involved a modification to the rotating shutter so that, once the film frame was drawn into position, the shutter opened, then closed, and then opened again before closing a second time for the next film frame to be drawn into position. In other words, the light interruption frequency was raised to twice that of the frame rate. When the film frame rate was eventually raised to the 24-fps standard, which is still in force to this day, the light interruption frequency was raised to 48 times per second, a rate that enables high levels of illumination to be employed without causing flicker.