Electrical audio signal processing systems and devices – Directive circuits for microphones
Reexamination Certificate
2001-01-19
2004-11-23
Ramakrishnaiah, Melur (Department: 2644)
Electrical audio signal processing systems and devices
Directive circuits for microphones
C381S111000, C381S122000
Reexamination Certificate
active
06823073
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a directional microphone assembly comprising a front and a rear microphone for application in electronic equipment, particularly compact communication devices, such as hearing aids, mobile phones etc. The present directional microphone assembly integrates most or all electronic processing circuits required to form a directional microphone output signal from a front and a rear microphone signal. Accordingly, the present directional microphone assembly occupies less volume within the compact communication devices than traditional directional microphones and further permits an equipment manufacturer to save discrete or integrated components or die area of an integrated circuit processing the microphone signals.
BACKGROUND OF THE INVENTION
Electronic equipment manufacturers continuously strive toward developing smaller communication devices, such as mobile phones, hearing aids, head-sets and other head-worn or body-worn devices, with more and more features. This development is helped by a continuous reduction in size of all components commonly utilised in such communication devices and by providing these components with an increased number of integrated features. In the field of hearing aids or instruments, the size of today's hearing aid microphones causes these to occupy a significant volume of a hearing aid housing. Accordingly, it is advantageous to minimise the size of the microphone or microphones employed within the hearing aid.
Hearing aid users often experience difficulty in understanding conversational speech in the level of background noise present in everyday situations such as conversations in homes, office buildings, department stores etc. It is well-known that this difficulty to some extent can be helped by employing a directional hearing aid on the hearing impaired individual. Such a directional hearing aid will commonly be adjusted so as to attenuate signals arriving from a side and back hemisphere of the individual with respect to frontally arriving signals, since is must be assumed that the individual faces a person or sound source with whom/which to communicate.
Such directional hearing aids have traditionally been equipped with a directional microphone that comprises two sound inlet ports spaced a distance apart and conveying the sounds from both inlet ports to a common microphone element responding to the difference between sound pressures of the inlet ports. The rear sound inlet is further provided with an acoustical delay element providing a fixed time delay to the rear sound signal that approximately equals the acoustic time delay between the inlet ports or equals a certain fraction of the acoustic time delay between the inlet ports. Accordingly, the signals arriving from the back are attenuated compared to signals arriving from the front in accordance with the directional pattern of the directional microphone, and the desired attenuation of interfering sounds
oise arriving from the side and rear may be provided.
However, such directional microphone constructions inherently attenuate low frequency signals independent of their direction of arrival and this often leads to an unnatural and fatiguing sound quality of the signal provided to the hearing impaired individual. Thus, it would be advantageous to provide a microphone assembly wherein the directional operation could be switched off and omni-directional operation employed in listening situations with little or no background noise.
Furthermore, traditional directional microphones occupy a significant amount of volume due to the need for the two inlet ports and their associated tubing carrying the sound signals from the inlet ports to a microphone element.
U.S. Pat. No. 5,524,056 discloses a hearing aid that comprises an omni-directional microphone and at least one directional microphone and an associated switching system which, manually or automatically can switch between utilising the omni-directional microphone or the at least one directional microphone. The automatic switching between the microphones may be controlled in response to a sensed level of background noise.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a directional microphone assembly comprising a front and a rear microphone that occupies less space than traditional directional microphones or combinations of directional microphones and omni-directional microphones.
It is also an object of the invention to provide a directional microphone assembly wherein an electronic time delay circuit and a summing circuit or unit is fully or partly integrated in one of the microphones.
In a first aspect, the present invention relates to a directional microphone assembly comprising:
a front microphone comprising a first electronic processing circuit, said front microphone being adapted to receive and convert sound waves into a front microphone signal,
a rear microphone comprising a second electronic processing circuit, said rear microphone being adapted to receive and convert sound waves into a rear microphone signal,
wherein the first or second electronic processing circuit comprises an electronic time delay circuit adapted to time delay the rear microphone signal with respect to the front microphone signal, and to receive a microphone signal so as to generate a directional microphone output signal.
The directional microphone assembly preferably comprises a pair of matched microphones in the form of the front and rear microphones. Since the attenuation of sound signals arriving from the rear direction in the directional microphone output signal is caused by the cancellation of two oppositely phased microphone signals of substantially equal amplitude provided by the front and the rear microphones, the matching of the microphones frequency responses must be considered. To provide a satisfactory attenuation of rear sound signals in the hearing aid, it has been found that the microphones amplitude responses must be matched to about 1 dB or less in the frequency range from 200 Hz-4000 Hz and the phase responses must be matched to about 2 degrees in the same frequency range. To achieve a satisfactory directionality in the low frequency range, below 500 Hz, it may be required to match the microphones amplitude responses to less than about 0.5 dB such as about 0.2 dB. Furthermore, since the microphone signals are processed by their respective electronic processing circuits before the directional microphone signal is formed in one of the electronic processing circuits, strict matching requirements may also be needed between these electronic processing circuits in the pair of matched microphones.
The electronic processing circuit of the front microphone or of the rear microphone must be adapted to receive a signal from the other microphone to generate the directional microphone signal. Accordingly, the rear microphone and its electronic processing circuit may be adapted to delay the rear microphone signal and to receive and combine the front microphone signal with a delayed rear microphone signal to generate the directional microphone signal. This directional microphone signal is preferably conveyed to an externally accessible output terminal of the rear microphone. Alternatively, the front microphone and its electronic processing circuit may be adapted to receive and delay the rear microphone signal and combine the delayed rear microphone signal with the front microphone signal to generate the directional microphone signal.
According to a preferred embodiment of the invention, the first electronic processing circuit of the front microphone comprises the electronic time delay circuit and is further adapted to receive the rear microphone signal from the rear microphone. The electronic time delay circuit of the first electronic processing circuit may furthermore be adapted to receive the rear microphone signal. Preferably, a summing circuit or unit is also provided in the first electronic processing circuit to combine a delayed rear microphone signal from the time delay circuit an
Harness & Dickey & Pierce P.L.C.
Ramakrishnaiah Melur
SonionMicrotronic Nederland B.V.
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