Electrical audio signal processing systems and devices – Directive circuits for microphones
Reexamination Certificate
1999-03-15
2003-02-18
Harvey, Minsun Oh (Department: 2644)
Electrical audio signal processing systems and devices
Directive circuits for microphones
C381S123000, C381S164000, C381S091000
Reexamination Certificate
active
06522756
ABSTRACT:
The present invention is generically directed on reception “lobe” shaping of a converter arrangement, which converts an acoustical input signal into an electrical output signal. Such a reception “lobe” is in fact a spatial characteristic of signal amplification, which defines, for a specific reception arrangement considered, the amplification or gain between input signal and output signal in dependency of spatial direction with which the acoustical input signal impinges on the reception arrangement. We refer to such spatial reception characteristics throughout the present description by the expression “spatial amplification characteristic”.
Such spatial amplification characteristic may be characteristically different, depending on the technique used for its shaping, for instance dependent from the fact whether the reception arrangement considered is of first, second or higher order.
As is well known from transfer characteristic behaviour in general, a first order arrangement has a frequency versus amplitude characteristic characterised by 20 dB per frequency decade slopes. Accordingly, a second order reception arrangement has 40 dB amplitude slopes per frequency decade and higher order reception arrangements of the order n, 20 n dB amplitude per frequency decade slopes. We use this criterion for defining respective orders of acoustical/electrical transfer characteristics.
The order of a reception arrangement may also be recognised by the shape of its spatial amplification characteristic.
In
FIG. 1
, there are shown three spatial amplification characteristics in plane representation of a first-order acoustical/electrical converting arrangement. The spatial amplification characteristic (a) is said to be of “bi-directional”-type. It has equal lobes in forwards and backwards direction with respective amplification maxima on one spatial axis, according to
FIG. 1
the 0°/180° axis and has amplification zeros on the second axis according to the +90/−90° axis of FIG.
1
.
The second characteristic according to (b) shows an increased lobe in one direction as in the 0° direction according to
FIG. 1
, thereby a reduced lobe characteristic in the opposite direction according to 180° of FIG.
1
. This characteristic is of “hyper-cardoid”-type. The lobe of the spatial amplification characteristic may further be increased in one direction as in the 0° direction of
FIG. 1
, up to characteristic (c), where the lobe in the opposite direction, i.e. the 180° direction of
FIG. 1
disappears. The characteristic according to (c) is named “cardoid”-type characteristic. Thus, “bi-directional” and “cardoid”-types are extreme types, the “hyper-cardoid”-type is in between the extremes.
At second and higher order reception arrangements the spatial amplification characteristics become more complicated having an increasing number of side-lobes.
FIG. 2
shows one example of a second order amplification characteristic of cardoid-type.
In the EP 0 802 699 of the same applicant as the present application and which accords to the U.S. application Ser. No. 09/146 784 and to the PCT/IB98/01069, it is described in detail how a reception arrangement for acoustical/electrical signal conversion may be realised, with a desired spatial amplification characteristic. Thereby, two spaced apart acoustical/electrical converters, microphones, are of multi- or omni-directional spatial amplification characteristic. They both convert acoustical signals irrespective of their impinging direction and thus substantially unweighted with respect to impinging direction into their respective electrical output signals. To realise from such two-microphone arrangement a desired spatial amplification characteristic the output signal of one of the two microphones is time-delayed —&tgr;—, the time-delayed output signal is superimposed with the undelayed output signal of the second microphone.
It is further described, with an eye on
FIG. 1
of the present application, how the time-delay &tgr; is to be selected for realising bi-directional, hyper-cardoid or cardoid-type spatial amplification characteristics: For the time-delay &tgr;=0 the characteristic becomes bi-directional (a), by increasing &tgr; the characteristic becomes hyper-cardoid, and finally becomes cardoid (c) if &tgr; is selected as the quotient of microphone spacing —p— to speed of sound, c. This technique, which has been known for long is referred to as “delay and superimpose” technique.
In this literature, which is to be considered as an integral part of the present invention by reference, it is further described how spatial amplification characteristic shaping may be improved, following the concept of electronically i.e. “virtually” controlling the effective spacing of the converters without influencing their physical “real” spacing.
First-order reception arrangements for acoustical input signals and especially when realised with a pair of omni-directional converters, as of microphones and as described in detail in the above mentioned literature, have several advantages over higher order reception arrangements. These advantages are especially:
simple electronic structure and small constructional volume, which is especially important for miniaturised applications as e.g. for hearing aid applications,
low cost,
low sensitivity to mutual matching of the converters used, as of the microphones,
small roll-off, namely of 20 dB per frequency decade.
Nevertheless, such a reception arrangement, as mentioned construed of two multi- or omni-directional converters has disadvantages, namely:
The maximum theoretical directivity index DI is limited to 6 dB, in practise one achieves only 4 dB to 5 dB. With respect to the definition of the directivity index DI please refer to speech communication 20 (1996), 229-240, “Microphone array systems for hand-free telecommunications”, Garry W. Elko.
It is an object of the present invention to quit with the disadvantages mentioned above, thereby keeping the advantages. Although the present invention departs from advantages and disadvantages of first order reception arrangements directed on acoustical signal treatment, it must be emphasised that once the inventive concept has been recognised, principally it may be applied to other types of reception arrangements, as to higher order reception arrangements.
To resolve the above mentioned object the present invention proposes a method for shaping the spatial amplification characteristic of an arrangement which converts an acoustical input signal to an electrical output signal and wherein, as was mentioned above, the spatial amplification characteristic defines for the amplification with which the input signal impinging on the arrangement is amplified, as a function of its spatial impinging angle, to result in the electrical output signal.
The inventive method thereby further comprises the following steps:
There are provided at least two sub-arrangements with at least one converter which sub-arrangements each convert an acoustical input signal to an electrical output signal, but which sub-arrangements have different spatial amplification characteristics.
There are generated at least two first signals which are proportional to the output signals of the sub-arrangements, in frequency domain and with a number of spectral frequencies.
There are further generated at least two second signals which are proportional to the output signals of the sub-arrangements, in frequency domain, and with said number of said spectral frequencies. Thus, the first and second signals may, but need not be equal.
The magnitudes of spectral amplitudes of the at least two first signals at equals of said spectral frequencies are compared, there results for each spectral frequency mentioned one comparison result. By these “spectral” comparison results one controls, which of the spectral amplitudes of the second signals at respective ones of the spectral frequencies mentioned is passed to the output of the arrangement.
Thereby, it principally becomes possible to combine the advantages of either of the at least two
Hottinger Werner
Maisano Joseph
Harvey Minsun Oh
Pearne & Gordon LLP
Phonak AG
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