Several circuits have been suggested for solving the problem of AC line-correlated noise being superimposed on the signals of interest. Some systems rely upon the use of notch filters which are adjusted to filter-out the noise. While this is a satisfactory solution to the problem of the AC line-correlated noise, it also removes audio information which is of interest. In addition, it also introduces distortion in the form of undesirable phase-shift artifacts to the frequencies that remain. An additional problem with this technique is that it does not adequately address field-powered (i.e. local power generation) applications where the line frequency may vary somewhat from moment to moment. In this case, the solution would be to widen the notch, which worsens the effect of the inherent drawbacks.
Another device commonly used to address this problem in musical instrument and professional sound applications is the noise gate. These can be fairly sophisticated devices whose function is to alter the output amplitude of one or more frequency bands of a signal based on the input amplitude. These devices, however, introduce their own amplitude distortion artifacts.
Other solutions include expensive adaptive canceling devices based on digital signal processors, and passive or active line-balancing devices which address only some mechanisms by which this noise is introduced.
The McCormick patent titled "Signal Cancelling Circuit" (U.S. Pat. No. 4,093,923) specifically discloses and claims a method and system for canceling a noise signal while preserving a desired signal of the same frequency. The method and system, however, contemplate transducers and signal levels which are very predictable, namely, geophones used in seismic surveys. Because of this, the need to provide gain/attenuation scaling at the input and output of the waveform acquisition/playback block in other applications was overlooked. This scaling helps the operator realize the maximum performance of the device with a variety of transducers and signal levels with a minimum investment in the waveform acquisition/playback block. In addition, the McCormick patent does not contemplate the elimination of phase and amplitude distortions typical of common solutions used in musical instrument and professional audio applications. Nor does it appear to recognize the ground loop mechanism, uncommon in geophone applications, as one which can be overcome by the device. Several circuits have been suggested for solving the problem of 50- or 60-cycle power supply signals being superimposed on the audio signals of interest. Some systems rely on the use of notch filters or comb filters which are adjusted to filter out the 50- or 60-cycle signals and their harmonics. While this is a satisfactory solution to the problem of the 50 or 60 Hz related noise, it also removes 50 or 60 Hz audio signals and their harmonics which are of interest. It can also introduce undesired phase shifts in the audio frequency range.
