Introduction

　　Audio noise reduction systems can be divided into two basic approaches. The first is the complementary

　　type which involves compressing the audio signal in some well-defined manner before it is recorded

　　（primarily on tape）。 On playback， the subsequent complementary expansion of the audio signal which

　　restores the original dynamic range， at the same time has the effect of pushing the reproduced tape noise

　　（added during recording） farther below the peak signal level—and hopefully below the threshold of

　　hearing.

　　The second approach is the single-ended or non-complementary type which utilizes techniques to reduce

　　the noise level already present in the source material—in essence a playback only noise reduction

　　system. This approach is used by the LM1894 integrated circuit， designed specifically for the reduction of

　　audible noise in virtually any audio source.

　　While either type of system is capable of producing a significant reduction in audible noise levels，

　　compandors are inherently capable of the largest reduction and， as a result， have found the most favor in

　　studio based equipment. This would appear to give compandors a distinct edge when it comes to

　　translating noise reduction systems from the studio or lab to the consumer marketplace. Compandors are

　　not， unfortunately， a complete solution to the audio noise problem. If we summarize the major desirable

　　attributes of a noise reduction system we will come up with at least eight distinct things that the system

　　must do—and no system as yet does all of them perfectly.

　　? The reproduced signal （now free of noise） is audibly identical to the original signal in terms of

　　frequency response， transient response and program dynamics. The stereo image is stable and does

　　not wander.

　　? Overload characteristics of the system are well above the normal peak signal level

　　? The system electronics do not produce additional noise （including perturbations produced by the

　　control signal path）。

　　? Proper response of the system does not depend on phase/frequency or gain accuracy of the

　　transmission medium.

　　? System operation does not cause audible modulation of the noise level.

　　? The system enables the full dynamic range of the source to be utilized without distortion.

　　? The recorded signal sounds natural on playback—even when decoding is not used. This means that

　　the system is compatible with existing equipment.

　　? Finally， the system is universal and can be used with any medium; disc， FM broadcast， television

　　broadcast， audio and video tapes.