Abstract:
An audio filter for a breathing apparatus uses active filtering in a multi-wire system where one or more electrical conductors contain bi-directional signals using multiple stages of active isolation to separate direct current (DC) power, which is then used to bias an active filtering element. Using active signal conditioning or processing elements, the audio filter directionally separates the power and audio components to allow active conditioning or processing of the audio signal.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims priority from U.S. Provisional Application No. 62/040,383, filed Aug. 21, 2014, the contents of which are incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Emergency breathing apparatus are used in situations where fire, smoke, dust and debris, or other impediments prevent normal breathing during a rescue. These breathing apparatus provide oxygen to the user and prevent smoke or other pollutants from entering the device. For safety reasons, many of these devices also include two-way communication devices to assist in rescue or coordination of efforts to fight a fire, etc. Two way communication devices typically comprise microphones powered by direct current batteries, where the voltage is used to amplify a voice for transmission via a transceiver to a remote receiver. An issue that plagues communication in emergency situations is that the microphone picks up and amplifies the heavy breathing and pronounced movement of air, leading to a transmission that is difficult to interpret and makes critical communication challenging. 
         [0003]      FIG. 1  depicts a prior art two wire system for communicating audio signals. In this circuit, a first wire carries both the audio signal and a direct current. A second wire is provided that serves as a ground/return path. The problem with this circuit is that it is impossible to isolate the audio signal and filter it effectively without interrupting the power signal. This results in a noisy audio signal that has poor quality and can lead to dangerous repercussions when communication is critical in an emergency situation. As constructed, the microphone tends to be very sensitive and picks up every minute sound while active. During normal modes of oxygen mask operation, the microphone is active when the wearer is not inhaling (and thus active for speaking) and not active when the wearer inhales. However, during certain modes of oxygen mask operation the microphone is continually active, and the continuous sounds of air rushing over the microphone are captured. This continuous unwanted “noise” is obtrusive and severally impedes effective communication. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention addresses the foregoing by establishing a microphone circuit that can filter out higher frequency audible noise created by air rushing over an oxygen mask microphone without a disruption of the DC power signal. 
         [0005]    The audio filter of the present invention may be used for both commercial and consumer products that utilize dual-wire bidirectional audio applications. Note that the term “dual” is not intended to be limiting, and that more than two wires can also be used. The invention channels active filtering in a multi-wire system where one or more electrical conductors contain bi-directional signals using two stages of active isolation (in certain cases, specifically created with op-amps) to separate direct current (DC) power, which is then used to bias active filtering elements. Using active signal conditioning or processing elements, the present invention directionally separates the DC and AC components to allow active conditioning or processing of the AC signal. The present invention can be applied to any application where it is advantageous to actively condition an AC signal that is present on the same wire as a DC voltage. 
         [0006]    These features as well as other advantages will best be understood with reference to the following figures in conjunction with the detailed description of the best mode for carrying out the invention set forth below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic of the prior art dual wire system; 
           [0008]      FIG. 2  is a schematic of a multi-wire system with active filtering; 
           [0009]      FIG. 3  is an exemplary detailed circuit diagram of a first embodiment of an audio filter of the present invention; 
           [0010]      FIG. 4  is a graph comparing an unfiltered and filtered audio response using the present invention; 
           [0011]      FIG. 5  is a plot of a speaking waveform versus time comparison of the present invention; and 
           [0012]      FIG. 6  is a plot of a breathing waveform versus time comparison of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]      FIG. 1  illustrates a prior art two wire system for communicating audio signals where a first wire carries both the audio signal and a direct current, and the second wire is a ground/return path. In this situation, it is impossible to isolate the signal and filter it effectively without interrupting the power signal. This leads to unfiltered or poorly filtered audio signals and the opportunity for ineffective communication. 
         [0014]      FIG. 2  illustrates a solution to the problem of  FIG. 1 , where a second path for the audio signal is established in parallel with the power signal, such that the audio signal can be isolated and filtered or otherwise processed without disturbing the accompanying power signal on the same path. The filtering of the audio signal takes place in an isolated zone where the audio is separated from the power signal. The filter  20  is represented by dashed lines and receives the dual wire inputs as with the example of  FIG. 1 , namely the power wire  22  and the ground wire  24 . The output consists of the power wire  26  and the ground/return wire  28 . Within the filter  20 , the DC power signal is represented by arrow  10  and traveling in a first direction. The DC power signal  10  has a path that can include power filters  12 ,  14  to process the power supply if necessary. Within the filter  20 , an audio signal represented by arrows  30  are parallel to the DC power signal  10 , and can include an audio filter  32  powered by the DC power signal  10  via connection  34 . That is, the DC power signal can be used to drive the audio filter  32  although separated from the power signal path. The DC power path  10  and the audio signal path  30  are connected to the ground/return wire  24 , 28  at connection  36 . 
         [0015]      FIG. 3  illustrates an exemplary detailed circuit diagram of an audio filter  20  of the present invention. The DC supply wire  22  and the ground/return wire  24  are connected to the ground  42  and the audio signal output  40  of the filter. The input is the wire  44  from the mask microphone  46 , which should also be connected to the ground  48 . The filter  20  establishes a first path  10  that includes at least a pair of filters  12 ,  14  and provides a flow of current (the DC power path  10 ) along an upper path. A tunable resistor  50  controls the current through the DC power path. Resistor  52  precedes the division of the DC power and audio paths, where capacitors  56  and  58  regulate the current through the audio path  30 . A tunable filter  32  is placed in the path of the audio signal to filter out noise and unwanted signal frequencies. The tunable filter  32  allows only the optimal frequencies to be passed through the filter while undesirable frequencies are blocked by the filter  32 , as determined by the circumstances. Capacitors  56  and  58  are tunable as well to improve the output and adjust the noise to output signal ratio. 
         [0016]      FIG. 4  is a graph illustrating a comparison of the filtered versus unfiltered audio signal plot as a function of signal frequency. As can be seen, the reference wave form is steady at −8 dB, and the phase data varies as shown between 20 degrees and −140 degrees. The resultant audio signal shows a high filtering at frequencies above 2 KHz, corresponding with a second order filtering. In this example, the processing of the audio signal is low-pass filtered with a cut-off frequency near 5 kHz. The amplitude roll-off of this filter is consistent with a first order filter. Also, while  FIG. 4  denotes a second order filter, the plot only demonstrates a 6 dB/octave of roll-off, as one would expect with a single order filter. In general, the amplitude roll-off is consistent with that of a low-pass filter. 
         [0017]    The filter  20  may utilize Op-Amps as the active elements. However, it would also be possible to establish the filter using transistors connected in a diode configuration. For example, using a BJT the base and collector would be connected together, and the emitter would be the active device output; for a FET, the gate and drain would be connected together and the source would be the active device output. This is an example of other active device configurations that could be used; it is understood that there are other active device configurations possible. 
         [0018]      FIG. 5  depicts a graph of a waveform plot versus time illustrating the effect of the present invention using speech as the input. It can be seen that the unfiltered portion of the output includes a large amount of unwanted noise, whereas the filtered output effectively eliminates the unwanted noise, thereby better enabling communication to occur. That is, the speech waveform suffers minimal degradation using the present invention and the filtered and unfiltered speech waveforms are nearly identical. This results in the desired signal having zero to minimal degradation. 
         [0019]      FIG. 6  illustrates a graph of an emergency breathing waveform (as opposed to speech waveform) versus time. The graph of  FIG. 6  shows how significantly the amplitude of the breathing contribution may be eliminated by the filter by the present invention. In situations where noise from breathing can overwhelm the audio signal, the repression of the audio signal due to the breathing contribution demonstrates the benefit of the present invention. The pronounced reduction in noise associated with the user&#39;s breathing paves the way for easier and better communication by the user and the listener. The graphs of  FIGS. 5 and 6  show that the filter of the present invention can transmit an audio signal where the speech portion of the audio signal is largely intact while the breathing contribution of the audio is significantly filtered, preserving the communication portion of the audio and significantly reducing noise. 
         [0020]    In this circuit, it should be understood that the “filter” represents an active signal conditioning circuit which requires DC power, where this power is transmitted over the same wire as the active signal. Moreover, the invention doesn&#39;t have to be limited to single wire bidirectional DC power and AC signals. Rather, the AC signal could be traveling the same direction as the DC power. The invention surrounds the separation of the DC and AC components so that signal conditioning/processing may be performed on either component. Thus, while the foregoing descriptions have been made with reference to a breathing apparatus microphone, the invention is not so limited and may take many forms and applications.