Abstract:
A noise-cancellation scheme in a system receiving acoustic information signals, where the system is carried in a vehicle having self-generated engine noise. An engine rotation sensor generates pulses driving a synchronous type filter device which passes only the engine noise frequency and its harmonics. The latter signal is then subtracted from the total input signal having information and noise combined. One filter circuit comprises a phase-locked loop with a multiplying counter whose output goes to a multiplexer having a plurality of grounded capacitors connected thereto.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to noise elimination from acoustic signals, and more particularly, to means for cancelling variable-frequency self-generated noise from acoustic signal tracking systems. 
     Acoustic target seekers are in use for locating and tracking the sound of certain attacking weapons, for example. If the seeker system is in a mobile vehicle, which could be an aircraft, submarine, tank, or the like, self-generated vehicle noise interferes with target information signals. A common source of such noise is the propulsion engine of the seeker vehicle, which may be running at various speeds or idling under certain conditions. The present invention will also serve in other kinds of acoustic signal-handling systems. 
     2. Description of the Prior Art 
     A notch filter has been suggested in the past, but its frequency response would have to be so broad, due to variable engine speeds, that target frequencies would be lost. 
     U.S. Pat. No. 3,071,752 discloses an interference reduction apparatus wherein interference noise is pre-recorded and later, in operation, played back to subtract from the total signal plus noise. This method is unsatisfactory for real-time operation. 
     U.S. Pat. No. 3,599,142 is for the same general purpose as the present invention, showing a control apparatus having phase-locked loops following the frequency of each interfering noise signal. The output of each phase-locked loop is treated by amplitude and phase determining means and later subtracted from a total input signal containing both information and noise. This patent system needs such amplitude and phase determining means for each frequency before the noise subtraction will work, thus adding complications. 
     While it is possible that more pertinent prior art exists, applicants&#39; search is believed to have been conducted with a conscientious effort to locate and evaluate the most relevant art available at the time, but this statement is not to be construed as a representation that no more pertinent art exists. 
     SUMMARY OF THE INVENTION 
     Since the engine of the vehicle carrying the acoustic system from which the noise is to be eliminated is readily accessible, it is an object of this invention to provide a noise control making direct use of the rotational frequency of the engine. Any suitable rotation sensor can be used. 
     It is another object of the present invention to provide a noise cancellation system wherein only noise at the (variable) engine operating frequency and its harmonics are cancelled. 
     Briefly, our invention comprises a synchronous or commutating filter effectively &#34;rotated&#34; in synchronism with the engine whose rotation is being sensed, the total input waveform being fed to the filter input, and means for subtracting the filter output from the input waveform having both information and noise present. The commutating filter includes a group of storage capacitors respectively connected to the individual channels of a multiplexer device, which capacitors sample the input waveform so that the filter passes only the synchronous noise signals. Multiplexer control means comprise an engine rotation sensor or transducer generating an electrical pulse type signal, multiplying means to provide a number of pulses per engine revolution equal to the number of capacitors, and means for feeding these pulses to the channel selector control of the multiplexer. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing showing the concept of a commutating filter. 
     FIG. 2 is a block diagram showing a noise cancellation system according to the present invention. 
     FIG. 3 is a more detailed block diagram showing a particular multiplexer arrangement for the system of FIG. 2. 
     FIG. 4 is a pictorial diagram showing one alternate form of engine rotation sensor usable in the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One type of tracking or synchronous filter that is especially suitable is the commutating filter, shown conceptually in FIG. 1. Here, an input line 1 carries an acoustic signal from a microphone 2 through an input resistor 4. The output side of resistor 4 is connected to the pole of a rotatable switch 5 and to an output line 6. At each of a plurality of switch positions, a capacitor 7 is connected to ground, all capacitors being equal in capacity. If the switch 5 is made to rotate in synchronism with a given frequency in the input signal, i.e., one complete switch revolution per cycle of the given waveform, then only that frequency and its harmonics will be passed on the output line 6. Assume the input to microphone 2 is an engine sound signal 9 from a vehicle having this commutating filter therein plus a non-synchronous sound signal 10 consisting essentially of any target sound to be utilized by or operated on by a seeker system. If the output waveform 11 from such a filter is then subtracted from the total microphone input waveform, the filtered frequency and its harmonics will be very nearly cancelled out, leaving only the desired target signal. This is the basis of the present invention. 
     One embodiment of this invention is shown in FIG. 2. A rotation sensor 12 operates directly from the engine (not shown) and generates one pulse for each engine revolution. One of the engine spark plugs may be used, for example (multiplied by two if it is a four-stroke engine), or a magnetic bar element which rotates with the engine and produces a pulse in a magnetic sensor once each revolution. The rotation pulse is preferably fed to a pulse shaper 14 to produce a suitable pulse form for the following components and/or to give a constant pulse width regardless of engine speed. For the latter purpose, a one-shot multivibrator can be used, adjusting its &#34;on&#34; time just so that its output will not remain &#34;on&#34; at the highest engine speeds to be encountered. 
     Next, the pulses are fed to a phase-locked loop 15 having a modulo-N binary counter 16 in the feedback loop. N is 32 in the present example. The sixth (last) stage output 17 of the counter is connected to the feedback input of the phase-locked loop 15. Thus the output of the voltage controlled oscillator in the phase-locked loop is at a frequency Nf into the counter 16, where f is the engine frequency, while the counter output 17 at its sixth stage is at frequency f. 
     The first five counter stage outputs form binary select lines 19 going to the control input of a 32-channel analog multiplexer 20 having a bank of 32 equal capacitors 7a connected respectively in the channels. Each of these capacitors is grounded as in FIG. 1, the ungrounded capacitor ends being sequentially connected, one at a time, to the multiplexer output line 21, in synchronism with the phase-locked loop output frequency 32f. These capacitors 7a may each have a capacity of 0.1 microfarad, for example. 
     The output of microphone 2 (FIG. 2) is amplified, if desired, by a first amplifier 22 and applied simultaneously to one end of a filter input resistor 4a and a first difference amplifier input line 24. The other end of resistor 4a is connected to a second amplifier 25 and to the multiplexer output line 21, with the output of second amplifier 25 forming a second difference amplifier input line 26. First and second difference amplifier input lines 24 and 26 preferably have level adjusting means 27 and 28 before entering the sum and difference inputs of a conventional difference amplifier 30. The signal on difference amplifier output line 31 is the output of the present invention. This is the signal with respect to ground potential, of course, since that is the common reference point to which the capacitors 7 and 7a are connected. 
     It will be seen that the multiplexer 20, together with the filter input resistor 4a and grounded capacitors 7a, forms the commutating filter. The information signal mixed with engine noise is present at the microphone output and is passed through the first amplifier 22 to the filter input resistor 4a and to the first difference amplifier input line 24. At the input to the second amplifier 25 appears a filtered signal consisting of the engine noise only, due to filter operation as described above. The &#34;noise only&#34; then appears on the second difference amplifier input line 26. At the same time, the composite &#34;signal plus noise&#34; appearing on the first difference amplifier input line 24 results in the noise cancellation by the difference amplifier 30. 
     The single-block multiplexer 20 may actually comprise a number of commercial integrated circuit chips as shown in FIG. 3. Here, there are four 8-line multiplexer chips 32, which may each be Siliconix type DG508 CMOS multiplexers, for example. The three binary input address lines 34 of each are connected in parallel and to the first three stage outputs, respectively, of the counter 16. An enabling line 35 of each multiplexer chip 32 comes from a conventional binary decoder 36 which is controlled through decoder input lines 36 and 37 connected from the fourth and fifth stage outputs of counter 16. The output line of each multiplexer chip 32 is connected to a common multiplexer output line 21a which in turn is connected to the output side of the filter input resistor 4a as is done in FIG. 2. 
     In the circuit of FIG. 2, the phase-locked loop 15 may be a commercial type CD4046, for example. In the embodiment of FIG. 3, the counter 16 may be a commercial type CD4520, for example, and the decoder 36 may be a commercial type CD4028A, for example. These are all CMOS integrated circuits. 
     One particular alternate form of rotation sensor 12 in FIG. 2 could be the arrangement shown in FIG. 4. Here, an engine-driven shaft 40 has attached to it an opaque shutter disc 41 with N annular apertures 42 equally spaced around the disc 41 near the rim thereof. A stationary light source 44 directs its light toward the rotating path of apertures from one side of disc 41, while a light detector 45 having an electrical output is located directly on the opposite side of the disc. Approximately 20 apertures 42 are illustrated, for example only. It will be understood that 32 openings would be provided, to fit with the example described previously where N equals 32. But obviously N could be more than 32, if desired. Thus, N pulses per revolution of shaft 40 are produced by the detector 45, and these pulses are fed to the input of the counter 16. 
     The multiple-pulse rotation sensor type shown in FIG. 4 is advantageous where an engine-driven shaft is accessible for such connection, since it directly provides the pulse multiplication required for the number of capacitors used in the commutating filter. When such a rotation sensor as the apertured disc 41 is used, it could eliminate the need for the phase-locked loop 15, but a pulse shaper 14 is still preferably used. The constant width of the output pulses from shaper 14 must obviously be less than the shortest anticipated period of the pulses from the rotation sensor 12. Also, there are obviously other types of rotation sensor devices which are suitable for use with the present invention. 
     Our invention is not restricted to use with a propulsion engine, since obviously the invention can be applied to other noise sources of an equivalent or similar nature. Although N in the example given herein is 32, it may be more or less, but for practical purposes is preferred to be at least 8. Also, while the commutating filter actual implementation herein is a solid state device, it could in some instances be a stepping switch or other mechanical movement. 
     Thus it is seen that an effective noise control has been provided which is simple to manufacture and use. All noise sounds which follow the engine speed frequency, plus harmonics thereof, are substantially cancelled, as long as N is sufficiently large. Of particular advantage due to the capacitive sampling process is the fact that the phase and amplitude of the unwanted noise signal need not be determined, because they are automatically correct at the output of the commutating filter. It will also be noted that additional microphone inputs can be used for a single system and do not require duplication of the rotation sensor, pulse shaper, phase-locked loop and counter components. 
     While in order to comply with the statute, the invention has been described in language more or less specific as to structural features, it is to be understood that the invention is not limited to the specific features shown, but that the means and construction herein disclosed comprise the preferred mode of putting the invention into effect, and the invention is therefore claimed in any of its forms or modifications within the legitimate and valid scope of the appended claims.