Patent EA-002094-B1 (EA)
Title: Digital and analog directrional microphone
Inventor: GREEN JACQUELYNN
IPC: H04R3/00
Filed: 1998-12-21
Abstract: 1. A directional microphone, comprising a shotgun microphone having an elongated tube which is designed to control the directivity of said directional microphone at frequencies above a predetermined frequency, at least two reference microphones spatially arranged about said shotgun microphone, a signal processor electrically connected to said shotgun and reference microphones, said signal processor generating interference canceling signals from the output signal of said reference microphones having frequencies generally below said predetermined frequency, said signal processor combining said canceling signals with the output signal from said shotgun microphone to generate an output signal in which signals originating from the location in front of the directional microphone in a direction along the longitudinal axis of said tube are enhanced and signals originating from locations other than in front of the directional microphone in a direction along the longitudinal axis of said elongated tube are suppressed. 2. The directional microphone of claim 1 wherein the directional microphone includes at least four reference microphones. 3. The directional microphone of claim 2 wherein said signal processor combines the output signals of said at least four reference microphones to form at least two reference microphone difference signals, said signal processor generating said cancelling signals from the portions of said difference signals which have frequencies generally below said predetermined frequency. 4. The directional microphone of claim 1 wherein said signal processor includes a preamplifier and limiter circuit electrically connected to each one of said shotgun and reference microphones and an analog to digital conversion circuit electrically connected to each one of said preamplifier and limiter circuits, each one of said preamplifier and limiter circuits having gain and limiter parameters which are balanced to allow a noise floor and dynamic range of said shotgun and reference microphones to matched to a noise floor and dynamic range of said analog to digital conversion circuits. 5. The directional microphone of claim 1 wherein said signal processor includes a filter circuit and an analog to digital conversion circuit electrically connected to each one of said shotgun and reference microphones, said filter circuits allowing aliasing type noise to be reduced to a level below a noise threshold of said analog to digital conversion circuit corresponding thereto. 6. The directional microphone of claim 5 wherein each of said filter circuits comprise an anti-aliasing filter and an over-sampling Sigma-Delta converter. 7. The directional microphone of claim 1 wherein said signal processor includes an adaptive beamformer. 8. The directional microphone of claim 1 wherein said signal processor creates at least two sets of cancelling signals from individual portions of said reference microphone signals which have frequencies generally below said predetermined frequency. 9. The directional microphone of claim 1 wherein said predetermined frequency is approximately 3 kHz. 10. The directional microphone of claim 1 wherein said signal processor includes an output level limiter circuit coupled to each one of said shotgun and reference microphones and an analog to digital converter circuit coupled to each one of said output level limiting circuits, said analog to digital conversion circuits providing a predetermined maximum dynamic range, wherein said output level limiter circuits reduce the level of the output signals from said shotgun and reference microphones by a predetermined amount to allow the apparent dynamic range to be increased. 11. The directional microphone of claim 10 wherein said maximum dynamic range is approximately 95 dB and said limiter circuits reduce signal levels by approximately 17 dB to provide an apparent dynamic range of 112 dB. 12. The directional microphone of claim 1 wherein a shelving filter circuit is coupled to each one of said at least two reference microphones, said shelving filter circuits boosting a portion of the output signal from the reference microphone corresponding thereto which is below a certain frequency. 13. The directional microphone of claim 12 wherein each of said shelving circuits boosts a portion of the output signal from the reference microphone corresponding thereto by reducing the portion of said output signals above said certain frequency. 14. The directional microphone of claim 1 wherein said elongated tube is approximately 12,5 cm in length.