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42
MOTION PICTURE SOUND ENGINEERING
in noise reduction, a further increase is sometimes obtained due to the removal of restrictions applying to the noise reduction equipment as discussed below.
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FREQUENCY C.P.S.
Figure 23 — Equalizer circuits used in complementary recording.
Breathing is caused by the action of the noise reduction system in placing the modulator in a position to handle the signal. Normal operation of noise reduction systems consists of a bias on the modulator or a reduction in track width, so that the minimum amount of static surface noise is present for the no-signal condition. Opening up of the recording mechanism or track width by the rectified bias signal increases the surface noise, causing the effect known as breathing. The greater the volume of the recorded signal, the greater the breathing effect.
As already mentioned, most of the signal energy lies in the lowfrequency range which means that much of the breathing is produced by the lower frequency components of the signals. Hence, use of the above type of pre-equalizer greatly decreases breathing because the recording mechanism is not modulated nearly as much by the lowfrequency intent. Tests have shown that breathing is practically eliminated by this method even on piano and organ tracks. It may be mentioned that in normal recording, breathing caused by the highfrequency content of signals is not nearly as objectionable as that from low-frequency components because of the low-energy content and masking effects.
Some other beneficial effects derived from this type of pre-equalized rpcording are: Possibility of the use, in some cases, of a greater amount