This invention relates generally to audio input circuits for use with a sound transducer and more particularly to audio input circuits for interfacing with host devices such as voice recognition host devices. The invention provides a universal interface such that the audio input circuit may be attached to a variety of voice recognition host devices through various switch settings. Additionally, the audio input circuit conditions the output of an inexpensive microphone such that the output response of the microphone is ideally suited to the input requirements of the voice recognition host device.
Voice recognition systems are being used with increased regularity in industry, the home, and the office. Such systems are typically computer based and are either free standing units or are incorporated within a computer system. Currently, there is no standardized format governing the electrical or physical connections between an input sound transducer, such as a microphone, and differing and various voice recognition host devices. Some systems are self-contained and include a microphone while other systems connect to an externally supplied microphone. Users wishing to attach an external microphone to a voice recognition host device may find that each individual system may require a unique interconnection format such as a differential input signal or a common mode input signal.
Additionally, there are various configurations for supplying power to the microphone. Thus, the use of a particular voice recognition host device selected from a wide array of available systems is cumbersome and inconvenient due to the wide variety of input requirements and interconnection formats available.
Typically, the host device receives audio signals directly from the microphone or receives the signals from a microphone circuit. Often, a two or three wire connector and a plug are used to couple the audio signal to the host device. Additionally, the host device may supply an external DC bias voltage component on one or more of the wires. The DC bias voltage may be used to supply power to the microphone or microphone circuit, or may unique to the host device. In one particular configuration, the external DC bias voltage may be referred to as phantom power when it is used to supply electrical power to the audio input circuit in a balanced, differential mode format. However, the voltage level of the DC bias voltage may be inadequate for use by the audio input circuit. Hence, there is a need to be able to utilize or block the external DC bias voltage supplied by the host device.
Voice recognition host devices accept input from a microphone or similar device and typically process the input signal in accordance with specialized algorithms and signal processing hardware. However, these systems are typically very sensitive to the frequency response characteristics of the signal received. If the microphone signal received has a nonlinear frequency response, or falls off sharply in a particular frequency band, voice recognition performance may be degraded. Typically, voice recognition host devices perform optimally when the input signal received is essentially linear or has a uniform energy spectrum throughout the frequency range of approximately 200 Hz to 10,000 Hz. Usually, high quality, expensive microphones provide this optimal output response. However, the increased cost of such microphones reduces the marketability of many voice recognition systems. In addition, such expensive microphones typically cannot connect directly to more than one type of system. On the other hand, inexpensive microphones typically attenuate sharply below 500 Hz, and respond nonlinearly above 500 Hz exhibiting undesirable variations in amplitude. This nonlinear response of inexpensive microphones impairs voice recognition performance.
Thus, it is an object of the present invention to provide an audio input circuit that substantially overcomes the above problems.
It is another object of the present invention to provide an audio input circuit that universally interfaces to a wide variety of voice recognition host devices.
It is a further object of the present invention to provide an audio input circuit including a universal interface, and filtering to enhance the frequency response characteristics of an inexpensive microphone.
It is an additional object of the present invention to provide an audio input circuit that includes a center-tap transformer for providing an inductive element as part of a filtering circuit.