Magazine article American Cinematographer

Cinema Workshop

Magazine article American Cinematographer

Cinema Workshop

Article excerpt

AUDIO BASICS-NOISE REDUCTION

Over the past few years, it has become more and more common to hear lhe term "noise reduction" mentioned by professional recordists. Almost every professional recording studio employs some form of signal processing to reduce noise, and even consumer items, such as hi-fidelity cassette decks, almost exclusively employ Dolby B circuitry. Brand names such as "dbx" and Dolby have become household words, and recently dbx has introduced two models of noise reduction units designed specifically for the Nagra SL and Nagra 4.2 respectively. The reasons for this current popularity are not as simple as the term "noise reduction" may suggest.

Each application has a different purpose for such circuitry, and, moreover, these circuits do more than just reduce noise. Any discussion of noise reduction must begin with the concept of "dynamic range". Cinematographers should be familiar with this concept as it is identical to the latitude or luminance ratio of a film stock. Most film stocks have a luminance ratio of 6 or 7 stops. This is in essence a "window" - only objects within this range will be recorded on the film. All objects brighter than this range will be burned out and have no detail, and all objects below this range will be black and indistinguishable on the film. By changing ASA or "T" stop, this "window" can be moved up or down to favor lighter or darker objects; but the width of the "window", the 6 or 7 stops it can distinguish, remains constant. This is the "dynamic range" of the film.

Magnetic recording tape is identical in this concept. The upper end is limited by tape saturation. Once the signal exceeds this level, it will be distorted. On the lower end, the tape recording process has a residual noise level usually referred to as "tape hiss". A signal must be substantially above this inherent noise to be useable.

Once again we have a "window" situation: a ceiling of distortion and a floor of noise. The signal must be restricted to fit within these limits, the dynamic range of the tape.

The cinematographer is always confronting the problem of "dynamic range". Quite often a scene exceeds the luminance ratio of the film, and reflectors and fill light must be employed to bring up the low end or compress the range of the scene to that of the film. The same problem exists with sound recording.

Figure 1 represents a typical situation. The subject being recorded might be a symphony orchestra. This is an extreme situation where the difference between a flute solo and a fortissimo of the entire orchestra can be on the order of 100 dB. The problem is the tape has a range of only 50 dB. The recordist can set the level to favor either the loud or soft portions, but the range remains at 5OdB. From Figure 1, it can be seen that the loud peaks will be clipped off, causing considerable distortion and the softest passages will be totally lost in the residual noise. In addition, the reproduced signal will only have a range of 50 dB between the loudest and softest passages, hardly the equivalent of the original 100 dB.

Like the cinematographer, the recordist would like to employ some means of reducing the dynamic range of his subject so that it will fit within the limits of his tape. Moreover, even if the subject had had a range of 50 dB, the softest passages would still be so close to the residual noise, that this noise would be quite prominent during playback. …

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