BACKGROUND AND SUMMARY OF THE INVENTION The present invention relates generally to active cancellation systems for repetitive phenomena, and more specifically to a selective active cancellation system in which the cancellation takes place within an electronic circuit, i.e., in-wire.

Prior art analog notch filters are difficult to tune as the frequencies to be rejected change, require tight tolerances of components and often require non-standard values for components to select precisely the frequency to be rejected and have a small bandwidth.

Prior art active cancellation systems have required a sensor, such as a microphone, to detect the noise and an actuator, such as a speaker, to produce the cancelling antinoise. The present invention is directed toward eliminating the sensor and actuator for the cancellation of repetitive noise within communications, surveillance and related systems.

Linear flow, air duct systems, for example, Chaplin U.S. Patent 4,122,303; Warnaka U.S. Patent 4,473,906 and Eriksson U.S. Patents 4,677,676 and 4,677,677, take advantage of directional flow in linear, one dimensional flow to utilize an upstream sensor, followed by a cancellation actuator and downstream error sensor in sequence. These systems cancel repetitive and random noise. Chaplin characterizes the control as a general convolution process, including a "programme of time-related operational steps." Warnaka uses adaptive filters

to speed adaptation time and allow greater spacing between the speaker and the duct. Erikkson specifies recursive least means square (RLMS) and least means square (LMS) adaptive filters to perform the convolutions and measure the system transfer function sin the presence of noise.

These systems require both an upstream sensor and a downstream sensor and an actuator.

Systems for canceling repetitive noise and vibration, for example, Chaplin U.S. Patents 4,153,815 and 4,417,098, describe the use of a synchronizing timing signal to provide selective cancellation of repetitive noise or vibration. Additionally a controller, actuator and error sensor are used. The method presented by Chaplin in these patents divides the noise or vibration period into a number of intervals and adjusts the amplitude of the canceling signal within each interval in response to the sign or amplitude of the error sensor within the same or a delayed interval.

In U.S. Patent 4,490,841, Chaplin describes the use of Fourier transforms to process signals in the frequency domain. While this method might be used for random signals, processing time requirements generally limit its application to repetitive signals.

These systems require an error sensor and an actuator.

Cancellation of unwanted components within electronic signals generally is applied to communication signals. Renneck et al. in U.S. Patent 4,232,381, use a commutation filter synchronized to the rotation of an engine to cancel

self-generated engine noise within an electronic circuit. The level of the canceling signal is adjusted manually and no method is provided to adapt to phase shifts or varying amplitudes of different harmonics.

Garconnat et al. in U.S. Patent 4,594,694 use two sensors, one sensing both the wanted and unwanted signals and the other sensing only the unwanted signals. Narrow band filters or Fourier transforms are used to eliminate the unwanted signals from the combined signal.

Widrow in "Adaptive Noise Canceling Principles and Applications", Proceedings of IEEE, Vol. 63, No. 12, December, 1975, describe two forms of active adaptive cancelers. The first, as illustrated in Figure 1, uses a multi-tap adaptive FIR filter with a reference signal correlated with the noise to be cancelled. The reference signal is required to be within 90° in phase of the error signal. Consequently, the reference signal used by the adapter itself often requires filtering; the resulting approach is referred to as the "filtered-x algorithm."

The second form described by Widrow, as illustrated in Figure 2, provides a single frequency notch filter and requires only two single tap filters. Again, a reference signal correlated with the noise is used and is phase shifted 90° for one of the filters. Glover, in "Adaptive Noise Canceling of Sinusoidal Interferences," Stanford University, Stanford, California, May 1975, Ph.D. dissertation, extended this technique to multiple frequencies.

Each of Widrow's techniques requires a sensor for the

reference signal, a sensor for the primary signal and a sensor for the error signal and requires that the cancellation take place numerically within the processor. Thus, the processor and associated elements must have sufficient bandwidth for all frequency components of the signal, not just those components to be cancelled.

Thus it is an object of the present invention to provide a selective active cancellation system for repetitive phenomean that eliminates sensors for the noise and the error and eliminates the actuator.

These and other objects are obtained by providing an electronic selective active cancellation controller and electronic mixer circuitry with inputs for the primary signal containing both noise and intelligence and for the cancellation signal from the controller and an output of the signal containing the intelligence with the repetitive noise cancelled

In preferred embodiments, the cancellation controller contains all necessary filters, including antialiassing filters and reconstruction filters. Since the primary signal containing both the noise and intelligence is not passed through the cancellation controller, the intelligence signal component is not degraded by the controller filters and other circuitry. The primary signal is thus only affected by the repetitive noise cancelling signal fed to the electric mixer.

Other object, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction

with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 schematically depicts a prior art adaptive noise cancellation system;

Figure 2 is a schematic circuit diagram of a single frequency adaptive notch filter of the prior art Figure 1 system.

Figure 3 is a block diagram of a selective active cancellation system constructed according to a preferred embodiment of the present invention; and

Figure 4 is a schematic diagram of a circuit combining the level shifters and electron mixer of the Figure 3 system, constructed to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 depicts a prior art approach to eliminating repetitive noise from electronic signals using analog notch or comb filters to remove all signals within bands around each frequency component to be eliminated. In Figure 1, the signal from a signal source and an unwanted noise source is combined as a primary input to an analog filter circuit. The analog filter circuit includes an adaptive filter supplied with a reference input signal n from the noise source and an error signal error at the system output E. The output y from the filter is added (subtracted) at the adder or summer. Figure 2 depicts the circuit, for the filter circuit outlined in dash

lines in Figure 1.

These above mentioned cancellation controllers produce the effects of notch widths down to a few Hertz. Furthermore, these controllers adjust the phase and amplitude of the cancelling signal to match only the frequency components of the primary signal that are correlated with the frequencies to be cancelled. Thus, the impact of the cancellation on the intelligence in the primary signal is minimized.

Figures 3 and 4 depict preferred embodiments of the present invention. The timing signal 102 sets the fundamental frequency, f _Q , to be cancelled by the selective active cancellation controller 103 and the optional operator interface 104 is used to select the range of frequencies within which the harmonics of f _Q will be cancelled.

The primary signal 101 containing both the undersired repetitive noise and the desired intelligence is shifted in level by level shifter 105 to match the level of the cancellation output of controller 103. The mixer 106 combines the primary signal and the cancellation signal to produce an output signal containing only the intelligence of the primary signal. The level shifter 107 sets the desired output level for the modified primary signal and the level shifter 108 matches the output level to the required level for the residual, error, input to the active cancellation controller 103.

Figure 4 shows a circuit combining the functions of the level shifters and the mixer. Level shifter 105 is manually

adjusted in this circuit and is composed of Ul, CR1, Cl, SW1, Rl thru R4 and R12 thru R19. CR1, Rl and Cl serve to protect Ul and AC couple the primary signal. SW1 selects one of R12 through R19 along with R2 to set the gain of Ul which is fine adjusted by R3 and R4. Ul is an instrumentation amplifier such as the AMP-01 by Precision Monolithics, Inc.

U2 and R5 thru R8 form the mixer 106 combining the level shifted primary signal and the cancellation output signal from the active cancellation controller 103. U2 is general purpose or other Op-Amp as required by the primary signal. Level shifter 107 is provided by R9 while RIO and Rll provide level shifter 108.

The active cancellation controller 103 contains all necessary filters, including anti-aliasing filters and reconstruction filters. Thus, the primary signal does not pass thru these filters and is unaffected except for cancellation of the repetitive noise.

Furthermore, the sampling rates, processing requirements and other requirements on the active cancellation controller are minimized since it only processes the frequency components of the signal within the range of frequencies to be cancelled. Thus, primary signals containing both low and high frequency components can have low frequency repetitive noise cancelled without affecting other low frequency components or high frequency components by a controller processing only the low frequencies which minimizes the bandwidth and cost of the controller.

Multiple harmonics and multiple fundamental frequencies (not natural harmonics of one another) can be cancelled in a simple manner by providing multiple synchronitation rates according to preferred embodiments of the invention. Copending commonly assigned PCT U.S. patent application International Application No. PCT/US89/0397, filed August 30, 1989, disclosed method for determining and providing such cancellation signals.

Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example, and is not to be taken by way of limitation. The spirit and scope of the present invention are to be limited only by the terms of the appended claims.