Audio signal processing circuit for reducing noise in an audio signal
An audio signal processing circuit reduces noise in an incoming audio signal and is particularly useful in telephone communication systems utilizing one or more hands-free microphones. A preferred embodiment of the audio signal processing circuit includes a pre-emphasis circuit receiving the audio signal from a microphone or other transducer, an amplifier circuit receiving the pre-emphasized audio signal from the pre-emphasis circuit and a de-emphasis circuit receiving the amplified signal from the amplifier circuit. An output of the de-emphasis circuit provides the processed audio signal having an improved signal to noise ratio with minimum audible distortion. A preferred embodiment of the amplification circuit includes an amplifier defining a non-linear transfer function therethrough which provides low gain to the lower amplitude noise signals and higher gain to the higher amplitude audio signals.
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The present invention relates generally to audio signal processing circuits, and more specifically to audio signal processing circuits for reducing noise in audio signals.
BACKGROUND OF THE INVENTIONTelephone communications are made easier and sometimes safer through the use of so-called “hands-free” microphones. Such microphones are typically located remote from the talker or talkers and thus pick up many times more noise and echo than with typical a hand-held unit. This problem is typically compounded in cellular telephone communications systems located in a moving vehicle due to added sources of noise such as road noise, for example.
Noise reduction units are known and are operable to enhance the sound quality and intelligibility of voice recognition systems by providing up to 15-20 dB of effective noise reduction. However, known noise reduction systems have drawbacks associated therewith. For example, some such noise reduction systems using digital signal processing technology create an artificial quality to the reproduced voice and are costly to implement. Known dynamic noise reduction (DNR) circuits, on the other hand, do not appear to work as well in very noisy environments, as “fuzzballs” of modulated noise tend to appear in the received signal.
What is therefore needed is an improved noise reduction system operable to enhance sound quality and intelligibility in telephone communication systems employing hands-free microphones, wherein such a noise reduction system is inexpensive and easy to implement, and provides a significant improvement in the signal to noise ratio while minimizing audible distortion.
SUMMARY OF THE INVENTIONThe present invention addresses the foregoing drawbacks of prior art distortion limiting amplifiers. In accordance with one aspect of the present invention, a signal processing circuit for reducing noise in an audio signal comprises a pre-emphasis circuit having an input receiving an audio signal and an output providing a pre-emphasized representation of the audio signal, the audio signal including a higher amplitude audio component and a lower amplitude noise component, an amplifier including a first input coupled to the pre-emphasis circuit output and an output, the amplifier defining a smooth transfer function providing higher gain to the higher amplitude audio component and lower gain to the noise component of the audio signal, and a de-emphasis circuit having an input connected to the amplifier output and an output providing a de-emphasized representation of the audio signal processed by the amplifier in accordance with the transfer function.
In accordance with another aspect of the present invention, an amplifier circuit for reducing noise in an audio signal comprises a non-inverting input receiving an audio signal, the audio signal including a higher amplitude audio component and a lower amplitude noise component, an inverting input, an output, and feedback circuitry connected between the output and the inverting input. The feedback circuit defines an amplifier transfer function having a smooth non-linear characteristic providing higher gain to the higher amplitude audio component and lower gain to the lower amplitude noise component.
In accordance with a further aspect of the present invention, an amplifier circuit for reducing noise in an audio signal comprises a non-inverting input connected to ground potential, an inverting input, an output, and feedback circuitry connected between the output and the inverting input, wherein the feedback circuitry receives an audio signal including a higher amplitude audio component and a lower amplitude noise component. The feedback circuitry defines an amplifier transfer function having a smooth non-linear characteristic providing higher gain to the higher amplitude audio component and lower gain to the lower amplitude noise component.
One object of the present invention is to provide an audio signal processing circuit providing a non-linear gain proportional to signal amplitude.
Another object of the present invention is to provide such a circuit wherein the non-linear gain is smooth and bi-directional to thereby minimize audible distortion of the processed audio signal.
Yet another object of the present invention is to provide such a circuit further having the audio signal pre-processed by a pre-emphasis circuit and post-processed by a de-emphasis circuit to thereby further minimize audible distortion of the processed audio signal.
These and other objects of the present invention will become more apparent from the following description of the preferred embodiment.
BRIEF DESCRIPTION OF THE DRAWINGS
For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated devices, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates.
Referring now to
Pre-emphasis circuit 16 and de-emphasis circuit 24 may be formed in accordance with known techniques and with known circuit components. It is known in the art that desired audio signals in the higher audio-frequency range typically have relatively low amplitude because they are harmonics of the fundamental tones. Therefore, it is desirable to pre-emphasize the amplitude of higher audio frequencies by increasing their relative values prior to amplification. Circuitry for providing such a pre-emphasis function is known and may be manifested as a high pass filter. The result of pre-emphasis, followed by subsequent de-emphasis, is to provide a processed signal having a higher signal to noise ratio. In order to restore the original relative amplitudes, the audio signal is de-emphasized after amplification by de-emphasis circuit 24. Circuitry for providing such a de-emphasis function is known and may be manifested as a low pass filter.
The present invention takes advantage of the fact that noise accompanying voice or other audible sound sensed by a suitable transducer is typically lower in amplitude than the desired audible components of the resultant signal. Referring to
Referring now to
The transfer function of the amplifier circuit 20′ illustrated in
instantaneous gain=1+[R2/(diode resistance+R1)] (1),
wherein the diode resistance is given by the equation:
diode resistance=[ln(diode current*2.5×1014)/(40×diode current)] (2).
Referring now to
The cathode of D1 is connected to the anode of D1 and to a first end of a third resistor R3. The opposite end of resistor R3 is connected to signal path 22 (
The transfer function of the amplifier circuit 20″ illustrated in
instantaneous gain=R2/{1/[1/(R3+diode resistance)+1/R1} (3),
wherein the diode resistance is given by equation (2).
Referring now to
Audible distortion of the audio signal 14 processed by the signal processing circuit 10 is minimized by the smooth low to high gain transition of the amplifier circuit transfer function 60 (
The following table contains typical values for some of the electrical components illustrated in
The present invention is illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.
Claims
1. A signal processing circuit for reducing noise in an audio signal, comprising:
- a pre-emphasis circuit having an input receiving an audio signal and an output providing a pre-emphasized representation of said audio signal, said audio signal including higher amplitude audio components and lower amplitude noise components when said higher amplitude audio components are not present;
- an amplifier including a first input coupled to said pre-emphasis circuit output, an input circuit in series with said first input, a negative feedback circuit, and an output, and one of said input circuit and said negative feedback circuit comprises a pair of parallel connected diodes, the pair of diodes having a cathode of each connected to an anode of the other so as to provide bi-directional, non-linear current flow therethrough, said amplifier further defining a non-linear transfer function providing higher gain to said higher amplitude audio components and lower gain to said lower amplitude noise components of said audio signal; and
- a de-emphasis circuit having an input directly connected to said amplifier output and an output providing a de-emphasized representation of said audio signal processed by said amplifier in accordance with said non-linear transfer function.
2. (Cancelled)
3. The signal processing circuit of claim 1 wherein said first input of said amplifier is a non-inverting input;
- and wherein said amplifier further includes an inverting input connected to one end of a first resistor and one end of a second resistor with an opposite end of said first resistor connected to said amplifier output and an opposite end of said second resistor connected to an anode of a first diode and a cathode of a second diode, said first diode having a cathode connected to an anode of said second diode and to ground potential.
4. The signal processing circuit of claim 1 wherein said first input of said amplifier is an inverting input connected to one end of a first resistor, one end of a second resistor, an anode of a first diode and a cathode of a second diode;
- and wherein an opposite end of said first resistor is connected to said amplifier output, and an opposite end of said second resistor connected to said output of said pre-emphasis circuit and one end of a third resistor, an opposite end of said third resistor connected to a cathode of said first diode and an anode of said second diode;
- and wherein said amplifier includes a non-inverting input connected to ground potential.
5. (Cancelled)
Type: Application
Filed: Oct 18, 2004
Publication Date: Mar 10, 2005
Applicant:
Inventors: Richard Stroud (Kokomo, IN), Gary McQuilling (Russiaville, IN)
Application Number: 10/967,782