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222 PRINCIPLES OF CINEMATOGRAPHY
triangle is inverted with respect to the other so that, as one beam of light becomes wider the other beam will be closing down towards the centre. Of course, both triangles are cut in one aperture plate and, since they are both reflected by one galvanometer, they both oscillate vertically and in phase. Referring now to the actual track produced by this system, we see that, when one track is almost closed, the other is opened to a maximum. Thus we have produced the required conditions and, assuming that one track can always be related to one of the two output valves, (which, it will be remembered, were connected in parallel in the amplifier), the signal applied to this valve will be at a maximum when that applied to the other is at a minimum. This type of recording is known as the 'Class A, Push-Pull' method.
It should be remembered that 'Push-Pull, Class A' tracks may also be produced by the Variable Density method of sound recording, and in this case, two pairs of light-valve ribbons are employed and so connected that, when one pair is at a maximum 'open' position, the other will be at a minimum.
Ground Noise Reduction
The importance and definition of 'Ground Noise' in relation to sound recording can only be fully understood if we are able to measure the relative volume of various sounds. This relative volume is usually expressed in terms of the 'Decibel'. The decibel is the unit of electrical transmission and is best defined as follows: Firstly, the power output obtained from a circuit must be divided by the power put into it. The quotient so obtained must then be converted to a logarithm. When this logarithm has been multiplied by ten the resultant figure will be the loss or gain in decibel units. For example, if one sound has twice the energy of another, and it is required to find the difference between the two in terms of decibels the procedure is as follows: (1) The difference between the two sounds, in terms of energy, has been given as two. (2) The logarithm of two will be found to be 0-3. (3) By multiplying 0*3 by ten a quotient of 3 is obtained. This quotient 3 is then in decibel units and it can therefore be said that a sound having twice the energy of another is 3 decibels louder than the other. Again, if one sound has five times the energy of another, it is first necessary to find the logarithm of five, which is approximately 0-7, and then to multiply this by ten, which produces a quotient of 7-0. Thus, a sound having five times the energy of another will be 7 decibels louder than the other. The decibel is also used to measure the performance of amplifiers. If the amplifier in a sound projector