A Dynamic Noise Filter For Mastering

N OISE IS A PROBLEM! YOU can't escape it. It is in the environment, electronics used for recording, records, tapes, AM, FM, TV, microwave, and other audio communication media. Noise in studio tape recorders is too high for much of the musical program material being recorded today. Studios without noise reduction equipment usually overcome noise by such undesirable methods as (1) recording at such a high level that peaks far exceed the 1% or even the 10% distortion level of the tape, (2) limiting the signal before recording or (3) monitoring through a speaker system that attenuates the very high frequencies. The rather serious limitations of these methods have been overcome by electronic noise reduction. Among the three principal methods of noise reduction available for studio mastering two of them, the Dolby Laboratories Noise Reduction System' and the Burwen Laboratories Noise Eliminator2, involve compression of the signal before recording. As a result the signal recorded on the tape is nonstandard and a separate noise reduction channel must be used for each track in multitrack recording. 7% or 15 ips unweighted tape noise the reduction measured typically 10 to 11 dB. Because noise reduction is attainable without any special preprocessing of the signal, the Dynamic Noise Filter is useful not only for a multitrack mix or a master tape playback but also for prerecorded tape, cartridges, cassettes, records, F M programs, or video tape sound. This versatility comes, of course, at a sacrifice-the bandwidth is restricted for low level signals. However, as will be shown later full bandwidth is attained at such low signal levels and in such a short time that the effect of the Dynamic Noise Filter on most good quality program material is completely inaudible except for the reduction of noise. When musical instruments are played at such a low level that the bandwidth is restricted, they usually produce much less overtone output and little is lost by the attenuation of the high frequencies.