The Cine Technician (1935-1937)

I'rhi'liary, 1936 The Jonriial oj the Assoeiatioii of Ciiie-Tecliiiicicuis 75 Some Aspects of Sound Photography B. C. SEWELL Precis of Lecture, arranged by The Association of Cine-Technicians and delivered to the Royal Photographic Society, November 22nd, 1935. It is well known that sound consists of pressure waves. Any audible sound can be represented b\ a wave-form, that is a graph in which pressure is plotted against time. Any continuous, or sustained sound, although this corresponding wave-shape may be very complex, can be represented by a number of pure tones, or sine waves, such as those given by a tuning fork. A single continuous sound, such as a vowel sound in speech or singing or a musical note, is composed in this manner of a number of frequencies of different amplitudes, or strengths, such that each frequency is a multiple of the lowest frequency present. This lowest frequency is the pitch of the note, and it is the others, the harmonics — or overtones — which lend to the sound its distinctive nature. Different musical instruments Fig. 1. are recognisable from one another l)y reason of the differences in the number, amplitudes and frequencies of the harmonics which they produce. The problem of recording, then, is to translate into terms of photographic density a range of frequencies of pressure vibration. The range to which the ear responds extends from about 16, to 16 or 20 thousand cycles per second, but this range is not fully covered in recording. About the best that is done in practice is to record without appreciable attenuation the range between 40 and 8 or 9 thousand. The majority of recordings do not go as high as this, and so fail in some measure to give a faithful rendering of the original. In order to produce exposure changes on film we must have a kind of camera, which includes a light modulator. All present-day modulators are worked by electrical impulses, these being obtained by a microphone and amplifiers giving an electrical output in which the current variations are an exact copy of the pressure changes at the microphone. By careful design this can be achieved within the practical limits of frequency already mentioned. The different kinds of light modulators can be classified imder two main headings. Firstly, the variable area, or variable width type. In this the exposure is in the form of a line down the track, the width of which varies in accordance with the impressed signal. The remainder of the track width is left unexposed. The other type is the variable density, in which the exposure of the track is constant across the width, but varies along its length. If a narrow slit of light is projected on to either of these tracks the amount of light passing through the film will depend upon the transmission value of the track at that point, and if the film is moved with uniform velocity past the slit, then the changes in light transmitted will depend upon the variations in exposure of the negative, and so upon the original signal input to the modulator. These changes in transmitted light are translated back into electrical terms by means of a photo-electric cell. The essence of photographic sound recording consists in ensuring that the electrical output from the photo-cell shall be an exact copy of the input signal to the light modulator in the recording camera. A diagram of the more usual type of variable area modulator is shown in Fig. 1. The lamp filament L is focussed on the vibrating mirror M by means of condenser A and objective B. In front of the condenser is a trian KANDEM STUDIO INCANDESCENT 500-10,000 WATTS IN USE IN ALL LEADING STUDIOS LIGHTS SILENT ARCS 5-300 AMPERES FOR SALE AND FOR HIRE KANDEM ELECTRICAL LTD. 711 FULHAM ROAD, Works: 10 PARSO/VS GREEN. S.W.6 LONDON, S.W.6 Plwiie: FULHAM 2387 S