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PRINCIPLES OF CINEMATOGRAPHY
parallel areas of opacity symmetrically placed with respect to the perforations. In position T the clear sections between the perforations is equal in width to the perforations themselves and, consequently, no signal will pass to the photo-cell. In position '2' the amount of light transmitted by the clear section between the perforations is approximately one-third of that which will be transmitted by the perforations themselves and, therefore, a signal will be reproduced by the photo-cell and ampifier circuit. Similarly, in position '3', the difference in transmission between the clear sections of film and the perforations themselves is even greater and, in consequence, a stronger signal will be reproduced. The frequency of this signal will always be controlled by the number of perforations which pass the scanning point each second and will, therefore, always be a 96 cycle frequency. The strength of the signal will be controlled by the relative difference between the amount of light transmitted through a perforation and that transmitted through the clear central aperture in the emulsion between the perforations. This constant low frequency of variable strength is then fed to a variable-gain amplifier which is used to extend the natural volume range of the original sound track. This is more easily understood from Figure 173, which shows a block diagram of the control track apparatus. The main amplification circuit of the system must have sufficient power capacity for the required output when the maximum control is in operation. When the control is zero that is, when the area between the perforations is transparent only the normal horn systems and the main amplifier arrangement will be in operation and, consequently, the volume range will be limited by the photographic properties of the sound track. The control and variable-gain amplifier is inserted between the stages of the voltage amplifier so that the volume of any signal generated by the control track will be used to change the amplification range of the main amplifier, and so increase the amplification of the sound track signals. It is important to realise that the variable-gain circuit is so designed that the degree to which it amplifies one signal is dependent upon the volume of a second control signal. The frequency of this control signal is unimportant since it is no way superimposed upon the final speech signals from the sound track and, in any case, is filtered out before the main amplication stages.
SOUND TRACK PHOTO CELL
/OV >» VOLT*
V^/ -" AMPLIF
\f i\
VARIABLE GAIN AND CONTROL AMPLIFIER
o
CONTROL TRACK PHOTO CELL
ADDED SiDE HORNS
NORMAL HORNS
ADDED SIDE HORNS
Fig. 173. — Block schematic of control track apparatus.