Source: http://www.google.com/patents/US5933506?dq=5987118
Timestamp: 2014-08-27 15:55:50
Document Index: 231943543

Matched Legal Cases: ['art 10', 'art 10', 'art 20', 'art 20', 'art 10', 'art 10']

Patent US5933506 - Transmitter-receiver having ear-piece type acoustic transducing part - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign in<nobr>Advanced Patent Search</nobr>PatentsEar-piece type acoustic transducing part is provided with a bone-conducted sound pickup microphone for picking up a bone-conducted sound, a directional microphone for picking up an air-conducted sound and an electro-acoustic transducer for transducing a received speech signal to a received speech sound....http://www.google.com/patents/US5933506?utm_source=gb-gplus-sharePatent US5933506 - Transmitter-receiver having ear-piece type acoustic transducing partAdvanced Patent SearchPublication numberUS5933506 APublication typeGrantApplication numberUS 08/441,988Publication dateAug 3, 1999Filing dateMay 16, 1995Priority dateMay 18, 1994Fee statusPaidAlso published asCA2149563A1, CA2149563C, DE69525987D1, DE69525987T2, DE69527731D1, DE69527731T2, DE69531413D1, DE69531413T2, EP0683621A2, EP0683621A3, EP0683621B1, EP0984660A2, EP0984660A3, EP0984660B1, EP0984661A2, EP0984661A3, EP0984661B1Publication number08441988, 441988, US 5933506 A, US 5933506A, US-A-5933506, US5933506 A, US5933506AInventorsShigeaki Aoki, Kazumasa Mitsuhashi, Yutaka Nishino, Kohichi Matsumoto, Chikara Yuse, Hiroyuki MatsuiOriginal AssigneeNippon Telegraph And Telephone CorporationExport CitationBiBTeX, EndNote, RefManPatent Citations (17), Referenced by (51), Classifications (11), Legal Events (4) External Links: USPTO, USPTO Assignment, EspacenetTransmitter-receiver having ear-piece type acoustic transducing partUS 5933506 AAbstract Ear-piece type acoustic transducing part is provided with a bone-conducted sound pickup microphone for picking up a bone-conducted sound, a directional microphone for picking up an air-conducted sound and an electro-acoustic transducer for transducing a received speech signal to a received speech sound. A transmitting-receiving circuit connected to the acoustic transducing part includes: a low-pass filter which permits the passage therethrough of low-frequency components in a bone-conducted sound signal from the bone-conducted sound pickup microphone; a high-pass filter which permits the passage therethrough of high-frequency components in an air-conducted sound signal from the directional microphone; first and second variable loss circuits which impart losses to the outputs from the low-pass filter and the high-pass filter, respectively; a comparison/control circuit which compares the output levels of the low-pass filter and the high-pass filter with predetermined first and second reference levels, respectively, and based on the results of comparison, controls losses that are set in the first and second variable loss circuits; and a combining circuit which combines the outputs from the first and second variable loss circuits into a speech sending signal.
What is claimed is: 1. A transmitter-receiver comprising:acoustic transducing means composed of a bone-conducted sound pickup microphone for picking up a bone-conducted sound and for outputting a bone-conducted sound signal, a directional microphone for picking up an air-conducted sound and for outputting an air-conducted sound signal, and a receiver for transducing a received speech signal to a received speech sound; a low-pass filter which permits the passage therethrough of those low-frequency components in said bone-conducted sound from said bone-conducted sound pickup microphone which are lower than a predetermined cutoff frequency; a high-pass filter which permits the passage therethrough of those high-frequency components in said air-conducted sound from said directional microphone which are higher than said cutoff frequency; first and second variable loss circuits which impart losses to the outputs from said low-pass filter and said high-pass filter; a comparison/control circuit which compares the output levels of said low-pass filter and said high-pass filter with predetermined first and second reference levels and, based on the results of comparison, controls the losses that are set in said first and second variable loss circuits; a combining circuit which combines the outputs from said first and second variable loss circuits and outputs a speech sending signal; and means for supplying said received speech signal to said receiver. 2. The transmitter-receiver of claim 1, wherein said acoustic transducing means includes an omnidirectional microphone for detecting noise components, and which further comprises a noise suppressing part which combines the outputs from said directional microphone and said omnidirectional microphone to suppress said noise components and supplies said high-pass filter with said noise component suppressed output.
3. A transmitter-receiver comprising:acoustic transducing means composed of a bone-conducted sound pickup microphone for picking up a bone-conducted sound, a directional microphone for picking up an air-conducted sound, an omnidirectional microphone for detecting noise, and a receiver for transducing a received speech signal to a received speech sound; a low-pass filter which permits the passage therethrough of those low-frequency components in the output from said bone-conducted sound pickup microphone which are lower than a predetermined cutoff frequency; a noise suppressing part which combines the outputs from said directional microphone and said omnidirectional microphone to suppress a noise component; a high-pass filter which permits the passage therethrough of those high-frequency components in the output from said noise suppressing part which are higher than said cutoff frequency; a combining circuit which mixes the outputs from said low-pass filter and said high-pass filter and outputs a speech sending signal; and means for supplying said received speech signal to said receiver. 4. The transmitter-receiver of claim 1, which further comprises: third and fourth variable loss circuits connected to the output side of said combining circuit and the input side of said received speech signal supplying means, for controlling the levels of said speech sending signal and said received speech signal, respectively; and a second comparison/control circuit which compares the level of said speech sending signal to be fed to said third variable loss circuit and the level of said received speech signal to be fed to said fourth variable loss circuit with predetermined third and fourth reference levels, respectively, and on the basis of the results of comparison, controls the losses that are set in said third and fourth variable loss circuits.
5. The transmitter-receiver of claim 2 or 3, wherein said noise suppressing part comprises: a first amplifier for amplifying said air-conducted sound signal from said directional microphone; a second amplifier for amplifying said noise components from said omnidirectional microphone; and a noise suppressor circuit which adds together the outputs from said first and second amplifiers in a 180� out-of-phase relation to each other to generate an air-conducted sound signal with said noise components suppressed and applies it to said high-pass filter.
6. A transmitter-receiver comprising:acoustic transmitting means composed of a bone-conducted sound pickup microphone for picking up a bone-conducted sound and for outputting a bone-conducted sound signal, air-conducted sound pickup microphone means for picking up an air-conducted sound and for outputting an air-conducted sound signal, and a receiver for transducing a received speech signal to a received speech; comparison/control means which estimates a level of ambient noise, compares said estimated level with a predetermined threshold level and generates a control signal on the basis of the results of comparison; and speech sending signal generating means which responds to said control signal to control additive mixing of said air-conducted sound signal from said air-conducted sound pickup microphone means and said bone-conducted sound signal from said bone-conducted sound pickup microphone to generate a speech sending signal; wherein said comparison/control means generates, as said control signal, a signal indicating whether said estimated noise level is higher or lower than said threshold level; said speech sending signal generating means includes signal select means responsive to said control means to select either one of said bone-conducted sound signal and said air-conducted sound signal; and said speech sending signal generating means generates said speech sending signal from said selected signal. 7. A transmitter-receiver comprising:acoustic transmitting means composed of a bone-conducted sound pickup microphone for picking up a bone-conducted sound and for outputting a bone-conducted sound signal, air-conducted sound pickup microphone means for picking up an air-conducted sound and for outputting an air-conducted sound signal, and a receiver for transducing a received speech signal to a received speech; comparison/control means which estimates a level of ambient noise, compares said estimated level with a predetermined threshold level and generates a control signal on the basis of the results of comparison; and speech sending signal generating means which responds to said control signal to control additive mixing of said air-conducted sound signal from said air-conducted sound pickup microphone means and said bone-conducted sound signal from said bone-conducted sound pickup microphone to generate a speech sending signal; wherein said comparison/control means is a means which, when said estimated noise level is within an area of a fixed width defined about said threshold level, supplies said speech sending signal generating means with a control signal for mixing said air-conducted sound signal and said bone-conducted sound signal at a ratio corresponding to said estimated noise level; and said speech sending signal generating means includes a means responsive to said control signal to mix said air-conducted sound signal and said bone-conducted sound signal at said ratio. 8. The transmitter-receiver of claim 6, or 7 wherein said comparison/control means includes means for holding a relationship between the ambient noise level and at least the level of said air-conducted sound signal in non-talking states; and said comparison/control means is a means which obtains, as said estimated noise level, a noise level corresponding to the level of the air-conducted sound signal during the use of said transmitter-receiver based on said relationship, compares said estimated noise level with said threshold level and generates said control signal on the basis of the result of comparison.
FIG. 10A is a block diagram showing the construction of a signal mixing circuit which is used as a substitute for each of signal select circuits 331 to 33n in the FIG. 5 embodiment;
Reference numeral 16 denotes an omnidirectional microphone for detecting noise, which has a sound pickup aperture or opening in the direction opposite to the directional microphone 15. Reference numeral 17 denotes an electro-acoustic transducer (hereinafter referred to as a receiver) for transducing a received speech signal into a sound, and 18 designates lead wires for interconnecting the acoustic transducing part 10 and a transmitting-receiving circuit 20 described later; the transmitting-receiving circuit 20 has its terminals TA, TB, TC and TD connected via the lead wires 18 to the directional microphone 15, the bone-conduction sound microphone 14, the receiver 17 and the omnidirectional microphone 16, respectively.
In FIG. 2 there is shown in block form the configuration of the transmitting-receiving circuit 20 which is connected to the acoustic transducing part 10 exemplified in FIG. 1. In FIG. 2 terminals TA, TB, TC and TD are connected to TA, TB, TC and TD in FIG. 1, respectively.
Reference numeral 21B denotes an amplifier for amplifying a bone-conduction sound signal from the bone-conduction sound microphone 14, and 21A is an amplifier for amplifying an air-conduction sound signal from the directional, air-conduction sound microphone 15. The gains of the amplifiers 21B and 21A are preset so that their output speech signal levels during a no-noise period are of about the same order at the inputs of a comparison/control circuit 24 described later. Reference numeral 21U denotes an amplifier which amplifies a noise signal from the noise detecting omnidirectional microphone 16 and whose gain is preset so that its noise output during a silent period becomes substantially the same as the noise output level of the amplifier 21A in a noise suppressor circuit 23 described later. The amplifiers 21A and 21B and the noise suppressor circuits 23 constitute a noise suppressing part 20N. The noise suppressor circuit 23 substantially cancels the noise signal by adding together the outputs from the amplifiers 21A and 21U after shifting them 180� out of phase to each other.
The directional microphone 15 and the omnidirectional microphone 16 bear such a relationship of sensitivity characteristic that the former has a high sensitivity within a narrow azimuth angle but the sensitivity of the latter is substantially the same in all directions as indicated by ideal sensitivity characteristics 15S and 16S in FIG. 3, respectively. Then, assuming that the ambient noise level is the same in any directions and at any positions, and letting the total amount of noise energy per unit time applied to the omnidirectional microphone 16 from all directions be represented by the surface area NU of a sphere with a radius r, the noise energy per unit time applied to the directional microphone 15 is represented by an area NA defined by the spreading angle of its directional characteristic on the surface of the sphere. Hence, their energy ratio NA /NU takes a value sufficiently smaller than one. Now, assume that the amounts of speech energy SA and SU applied to the directional microphone 15 and the omnidirectional microphone 16 take the same value S, and let the gains of the amplifiers 21A and 21U be represented by GA and GU, respectively. By setting that a value GA NA is nearly equal to a value GU NU, noise is substantially canceled by the noise suppressor circuit 23 but the speech signal level at the output of the noise suppressor circuit 23 becomes GA S-GU S=GA S(1-NA /NU), and since the energy ratio NA /NU is sufficiently smaller than one, the speech level is nearly equal to GA S--this indicates that a speech signal in the air-conduction sound signal can be effectively extracted therefrom ideally. The noise suppressing effect that could be achieved by the directional microphone 15, the omnidirectional microphone 16 and the noise suppressing part 20N actually used was typically in the range of 3 to 10 dB.
In FIG. 2 the bone-conduction sound signal and the air-conduction sound signal, which have their frequency characteristics equalized by the low-pass filter 22B and the high-pass filter 22A, respectively, are applied to the comparison/control circuit 24, wherein their levels VB and VA are compared with predetermined reference levels VRB and VRA, respectively. Based on the results of comparison, the comparison/control circuit 24 controls losses LB and LA of variable loss circuits 25B and 25A, thereby controlling the levels of the bone- and air-conducted sound signals. A mixer circuit 26 mixes the bone-conducted sound signal and the air-conducted sound signal which have passed through the variable loss circuits 25B and 25A. The thus mixed signal is provided as a speech sending signal ST to a speech sending signal output terminal 20T via a variable loss circuit 29T. A comparison/control circuit 28 compares the level of a speech receiving signal SR and the level of the speech sending signal ST with predetermined reference levels VRR and VRT, respectively, and, based on the results of comparison, controls the losses of variable loss circuits 29T and 29R, thereby controlling the levels of the speech sending signal and the speech receiving signal to suppress an echo or howling. The speech receiving signal from the variable loss circuit 29R is amplified by an amplifier 27 to an appropriate level and then applied to the receiver 17 via the terminal TC.
FIG. 4 is a table for explaining the control operations of the comparison/control circuit 24 in FIG. 2. The comparison/control circuit 24 compares the output level VB of the low-pass filter 22B and the output level VA of the high-pass filter 22A with the predetermined reference levels VRB and VRA, respectively, and determines if the bone- and air-conducted sound signals are present (white circles) or absent (crosses), depending upon whether the output levels are higher or lower than the reference levels. In FIG. 4, state 1 indicates a state in which the bone-conducted sound signal (the output from the low-pass filter 23B) and the air-conducted sound signal (the output from the high-pass filter 23A), both frequency-equalized, are present at the same time, that is, a speech sending or talking state. State 2 indicates a state in which the bone-conducted sound signal is present but the air-conducted sound signal is absent, that is, a state in which the bone-conducted sound pickup microphone 14 is picking up abnormal sounds such as wind noise of the case 11 and frictional sounds produced by the lead wires 18 and the human body or clothing. State 3 indicates a state in which the air-conducted sound signal is present but the bone-conducted sound signal is absent, that is, a state in which no speech signal is being sent and that noise component of the ambient sound picked up by the directional microphone 15 which has not been canceled by the noise suppressor circuit 23 is being outputted. State 4 indicates a state in which neither of the bone- and air-conducted sound signals is present, that is, a state in which no speech signal is being sent and no noise is present. The control operations described in the right-hand columns of the FIG. 4 table show the operations which the comparison/control circuit 24 performs with respect to the variable loss circuits 25B and 25A in accordance with the above-mentioned states 1 to 4, respectively.
Next, a description will be given of the operation of an embodiment of the above construction. When a user of this transmitter-receiver utters a vocal sound with the ear-piece type acoustic transducing part 10 of FIG. 1 put on his or her ear, vibration of the skull as well as aerial vibration are created by the vibration of the vocal chords. The vibration of the skull is picked up as a bone-conducted sound signal by the bone-conducted sound pickup microphone 14, from 11 which the signal is provided via the terminal TB to the amplifier 21B. The aerial vibration of the speech is picked up by the directional microphone 15, from which the signal is provided as an air-conducted sound signal to the amplifier 21A via the terminal TA.
In general, as compared with the air-conducted sound, the bone-conducted sound has many low-frequency components, makes less contribution to articulation and contains, in smaller quantity, high-frequency components which are important for the expression of consonants. On the other hand, abnormal sounds such as wind noise caused by the wind blowing against the case 11 and frictional sound between the cords (lead wires) 18 and the human body or clothing are present in lower and higher frequency bands than the cutoff frequencies of the filters 22A and 22B. Such wind noise and frictional sounds constitute contributing factors to the lack of articulation of the speech sending sound by the bone conduction and the formation of abnormal sounds. On the other hand, "speech" passes through the sound pickup tube 13 and is picked up as an air-conducted sound signal by the directional microphone 15, from which it is applied to the amplifier 21A via the terminal TA. The air-conducted sound produced by a talker's speech is a human voice itself, and hence contains frequency components spanning low and high frequency bands.
The noise levels at the directional microphone 15 and the omnidirectional microphone 16 can be regarded as about the same level as referred to previously; but, because of a difference in their directional sensitivity characteristic, the directional microphone 15 picked up a smaller amount of noise energy than does the omnidirectional microphone 16, and hence provides a higher SN ratio. Since the gains GA and GU of the amplifiers 21A and 21U are predetermined so that their output noise levels become nearly equal to each other as mentioned previously, the gain GA of the amplifier 21A is kept sufficiently larger than the gain GU of the amplifier 21U. Hence, the user's speech signal is amplified by the amplifier 21A with the large gain GA and takes a level higher than the noise signal level.
The comparison/control circuit 24 compares, at regular time intervals (1 sec, for instance), the outputs from the low-pass filter 22B (for the bone-conducted sound) and the high-pass filter 22A (for the air-conducted sound) with the reference levels VRB and VRA, respectively, to perform such control operations as shown in FIG. 4. At first, the characteristic of the transmitter-receiver of the present invention immediately after its assembling is adjusted (or initialized) by setting the losses LB and LA of the variable loss circuits 25B and 25A to initial values LB0 and LA0 that the level of the air-conducted sound signal to be input into the mixer 26 is higher than the level of the bone-conducted sound signal by 3 to 10 dB when no noise is present (State 4 in FIG. 4). The reason for this is that it is preferable in terms of articulation that the air-conducted sound be larger than the air-conducted one under circumstances where no noise is present.
The comparison/control circuit 23 compares the output level VA of the high-pass filter 22A with the reference level VRA. When the output from the high-pass filter 22A is smaller than the reference level VRA (State 4), the comparison/control circuit 23 decides that noise is not present or small and that no talks are being carried out and sets the losses of the variable loss circuits 25B and 25A to the afore-mentioned initial values LB0 and LA0, respectively. When this state changes to the talking state (State 1), a mixture of the bone-conducted sound signal composed of low-frequency components and the air-conducted sound signal composed of high-frequency components is provided as the speech sending signal ST at the output of the mixer circuit 26.
Next, when the output level VB of the low-pass filter 22B is smaller than the reference level VRB and the output level VA of the high-pass filter 22A is larger than the reference level VRA (State 3), the comparison/control circuit 23 decides that no talks are being carried out and that ambient noise is large. In this instance, the comparison/control circuit 23 applies a control signal CA to the variable loss circuit 25A to set its loss LA to a value larger than the initial value LA0 in proportion to the difference between the output level VA of the high-pass filter 22A and the reference level value VRA as expressed by such an equation as follows:
LA =K(VA -VRA)+LA0                     (1)
where K is a predetermined constant. Alternatively, it is possible to increase the loss LA by a constant K on a stepwise basis each time the level difference (VA -VRA) increases by a constant VM, as expressed by the following equation:
LA =.left brkt-top.(VA -VRA)/VM .right brkt-top.K+LA0                                       (2)
When the output from the low-pass filter 22B becomes larger than the reference level VRB, that is, when this State 3 changes to the talking state (State 1), the losses of the variable loss circuits 25A and 25B are not changed but are kept at set values in the immediately preceding State 3. By this, the bone-conducted sound signal composed of low-frequency components and the air-conducted sound signal of the same level as or lower than the level of the bone-conducted sound signal and composed of high-frequency components are mixed by the mixer circuit 26 into the speech sending signal ST. In this case, it is also possible to hold the loss of the variable loss circuit 25A unchanged and control the loss of the variable loss circuit 25B so that the mixed output level of the mixer circuit 26 takes a predetermined value.
(b) When the output (the bone-conducted sound signal) level VB of the low-pass filter 22B is larger than the reference level VRB (State 1 or 2 in FIG. 4):
The comparison/control circuit 24 checks the output level VA of the high-pass filter 22A and, if it is smaller than the reference level VRA (State 2), determines that no talks are being carried out and that the bone-conducted sound pickup microphone 14 is picking up abnormal sounds. In such an instance, the comparison/control circuit 24 applies a control signal CB to the variable loss circuit 25B to set its loss LB to a value greater than the initial value LB0 in proportion to the difference between the output level VB of the low-pass filter 22B and the reference level VRA, as expressed by the following equation:
LB =K(VB -VRB)+LB0                     (3)
Alternatively, as is the case with the above, the loss LB may be controlled as expressed by the following equation:
LB =.left brkt-top.(VB -VRB)/VM .right brkt-top.K+LB0                                       (4)
When the output level VA of the high-pass filter 22A becomes larger than the reference level VRA, that is, when this State 2 changes to the talking state (State 1), the losses of the variable loss circuits 25A and 25B are held unchanged, and hence are kept at the set values in the immediately preceding State 2. An air-conducted sound signal composed of high-frequency components and a bone-conducted sound signal of a level set in accordance with the output level VB of the low-pass filter 22B and composed of low-frequency components are mixed together by the mixer circuit 26. In this instance, it is also possible to hold the loss of the variable loss circuit 25B unchanged and control the loss of the variable loss circuit 25A so that the output level of the mixer circuit 26 may assume the aforementioned predetermined fixed value.
Next, when the output level VA of the high-pass filter 22a is larger than the reference level VRA (State 1), the comparison/control circuit 24 decides that the state is the talking state, and causes the variable loss circuits 25B and 25A to hold losses set in the state immediately preceding State 1. As a result, bone- and air-conducted sound signals of levels controlled in accordance with the losses held unchanged are mixed by the mixer circuit 26, which provides the speech sending signal ST.
This problem could be solved by known techniques such as a method for the suppression of howling in the loudspeaking telephone system. The configuration by the comparison/control circuit 28 and the variable loss circuits 29T and 29R is an example of such a prior art. The comparison/control circuit 28 monitors the output level VT of the mixer circuit 26 and the signal level VR at a received speech input terminal 20R and, when the speech receiving signal level VR is larger than a predetermined level VRR and the output level VT of the mixer circuit 26 is smaller than a predetermined level VRT, the circuit 28 decides that the transmitter-receiver is in the speech receiving state, and sets a predetermined loss LT in the variable loss circuit 29T, reducing the coupling of the speech receiving signal to the speech sending system. When the output level VT of the mixer circuit 26 is larger than the predetermined level VRT and the input level VR at the speech receiving signal input terminal 20R is lower than the predetermined level VRR, the comparison/control circuit 28 decides that the transmitter-receiver is in the talking state, and sets a predetermined loss LR in the variable loss circuit 29R, suppressing the sidetone from the speech receiving system. When the output level VT of the mixer circuit 26 and the input level VR at the speech receiving signal input terminal 20R are higher than the predetermined levels VRT and VRR, respectively, the comparison/control circuit 28 decides that the transmitter-receiver is in a double-talk state, and sets in the variable loss circuits 29T and 29R losses one-half those of the above-mentioned predetermined values LT and LR, respectively. In this way, speech with great clarity can be sent to the other party in accordance with the severity of ambient noise and the presence or absence of abnormal noise.
FIG. 5 illustrates in block form the transmitter-receiver according to the second embodiment of the invention. The bone-conducted sound pickup microphone 14, the directional microphone 15 and the receiver 17 are provided in such an ear-piece type acoustic transducing part 10 as depicted in FIG. 1. In this embodiment, the air-conducted sound signal from the directional microphone (the air-conducted sound pickup microphone 15) and the bone-conducted sound signal from the bone-conducted sound pickup microphone 14 are fed to an air-conducted sound dividing circuit 31A and a bone-conducted sound dividing circuit 31B via the amplifiers 21A and 21B of the transmitting-receiving circuit 20, respectively. As is the case with FIG. 2, the gains of the amplifiers 21A and 21B are preset so that input air-and bone-conducted sound signals of a vocal sound uttered in a no-noise environment have about the same level. The air-conducted sound dividing circuit 31A divides the air-conducted sound signal from the directional microphone 15 into first through n-th frequency bands and applies the divided signals to a comparison/control circuit 32 and signal select circuits 331 through 33n. The bone-conducted sound dividing circuit 31B divides the bone-conducted sound signal from the bone-conducted sound pickup microphone 14 into first through n-th frequency bands and applies the divided signals to the comparison/control circuit 32 and the signal select circuits 331 through 33n. In the present invention, the air- and bone-conducted sound signals need not always be divided (i.e. n=1), but when divided into frequency bands, they are divided, for example, every one or one-third octave, or into high and low bands, or high, intermediate and low bands.
A received signal dividing circuit 31R divides the received signal SR from an external line circuit via the input terminal 20R into first through n-th frequency bands and applies the divided signal to the comparison/control circuit 32. In this embodiment, the comparison/control circuit 32 is such one that converts each input signal into a digital signal by an A/D converter (not shown), and performs such comparison and control operations by a CPU (not shown) as described below. That is, the comparison/control circuit 32 calculates an estimated value of the ambient noise level for each frequency band on the basis of the air-conducted sound signals of the respective bands from the air-conducted sound dividing circuit 31A, the bone-conducted sound signals of the respective bands from the bone-conducted sound dividing circuit 31B and the received signals of the respective bands from the received signal dividing circuit 31R. The comparison/control circuit 32 compares the estimated values of the ambient noise levels with a predetermined threshold value (i.e. a reference value for selection) Nth and generates control signals C1 to Cn for the respective bands on the basis of the results of comparison. The control signals C1 to Cn thus produced are applied to the signal select circuits 331 to 33n, respectively. The signal select circuits 331 to 33n respond to the control signals C1 to Cn to select the air-conducted sound signals input from the air-conducted sound dividing circuit 31A or the bone-conducted sound signals from the bone-conducted sound signal dividing circuit 31B, which are provided to a signal combining circuit 34. The signal combining circuit 34 combines the input speech signals of the respective frequency bands, taking into account the balance between the respective frequency bands, and provides the combined signal to the speech transmitting output terminal 20T. The output terminal 20T is a terminal which is connected to an external line circuit.
FIG. 6 is a graph showing, by the solid lines 3A and 3B, a standard or normal relationship between the tone quality (evaluated in terms of the SN ratio or subjective evaluation) of the air-conducted sound signal picked up by the directional microphone 15 and the ambient noise level and a standard or normal relationship between the tone quality of the bone-conducted sound signal picked up by the bone-conducted sound pickup microphone and the ambient noise level. The ordinate represents the tone quality of the sound signals (the SN ratio in the circuit, for instance) and the abscissa the noise level. As indicated by the solid line 3A, the tone quality of the air-conducted sound signal picked up by the directional microphone 15 is greatly affected by the ambient noise level; the tone quality is seriously degraded when the ambient noise level is high. On the other hand, as indicated by the solid line 3B, the tone quality of the bone-conducted sound signal picked up by the bone-conducted sound pickup microphone 14 is relatively free from the influence of the ambient noise level; degradation of the tone quality by the high noise level is relatively small. Hence, the speech sending signal ST of good tone quality can be generated by setting the noise level at the intersection of the two solid lines 3A and 3B as the threshold value Nth and by selecting either one of the air-conducted sound signal picked up by the directional microphone 15 and the bone-conducted sound signal picked up by the bone-conducted sound pickup microphone, depending upon whether the ambient noise level is higher or lower than the threshold value Nth. It was experimentally found that the threshold value Nth is substantially in the range of 60 to 80 dBA. The characteristics indicated by the solid lines 3A and 3B in FIG. 6 are standard; the characteristics vary within the ranges defined by the broken lines 3A' and 3B' in dependence upon the characteristics of the microphones 14 and 15, the preset gains of the amplifiers 21A and 21B and the frequency characteristics of the input speech signals, but they remain in parallel to the solid lines 3A and 3B, respectively. The solid lines 3A and 3B are substantially straight.
To switch between the air- and bone-conducted sound signals in accordance with the ambient noise level, it is necessary to calculate an estimated value of the ambient noise level. FIG. 7 is a graph showing, by the solid line 4BA, a standard relationship of the ambient noise level (on the abscissa) to the level ratio (on the ordinate) between an ambient noise signal picked up by the directional microphone 15 and an ambient noise signal picked-up by the bone-conducted sound pickup microphone 14 in the listening or speech receiving or silent states. FIG. 8 is a graph showing, by the solid line 5BA, a standard relationship of the ambient noise level to the level ratio between a signal (the air-conducted sound signal plus the ambient noise signal) picked up by the directional microphone 15 and a signal (the bone-conducted sound signal plus the ambient noise signal) picked-up by the bone-conducted sound pickup microphone 15 in the talking or double-talking state. As shown in FIGS. 7 and 8, the characteristic in the listening or silent state and the characteristic in the talking or double-talking state differ from each other. Hence, the level VA of the air-conducted sound signal from the directional microphone 15, the level VB of the bone-conducted sound signal from the bone-conducted sound pickup microphone 15 and the level VR of the received signal from the amplifier 27 are compared with the reference level values VRA, VRB and VRR, respectively, to determine if the transmitter-receiver is in the listening (or silent) state or in the talking (or double-talking) state. Next, the level ratio VB /VA between the bone-conducted sound signal and the air-conducted sound signals picked up by the microphones 14 and 15 in the listening or silent state is calculated, and the noise level at that time is estimated from the level ratio through utilization of the straight line 4BA in FIG. 7. Depending upon whether the estimated noise level is higher or lower than the threshold value Nth in FIG. 6, the signal select circuits 331 to 33n each select the bone-conducted sound signal or air-conducted sound signal. Similarly, the level ratio VB /VA between the bone-conducted sound signal and the air-conducted sound signal in the talking or double-talking state is calculated, then the noise level at that time is estimated from the straight line 5BA in FIG. 8, and the bone-conducted sound signal or air-conducted sound signal is similarly selected depending upon whether the estimated noise level is above or below the threshold value Nth.
Next, the operation of the transmitter-receiver will be described. Incidentally, let is be assumed that there are prestored in a memory 32M of the comparison/control circuit 32 the reference level values VRA, VRB and VRR, the threshold value Nth and the level ratio vs. noise level relationships shown in FIGS. 7 and 8. Since the speech signals and the received signals divided into the first through n-th frequency bands are subjected to exactly the same processing until they are input into the signal combining circuit 34, the processing in only one frequency band will be described using reference numerals with no suffixes indicating the band.
The comparison/control circuit 32 compares, at regular time intervals (of one second, for example), the levels VA, VB and VR of the air-conducted sound signal, the bone-conducted sound signal and the received signal input from the air-conducted sound dividing circuit 31A, the bone-conducted sound dividing circuit 31B and the received signal dividing circuit 31R with the predetermined reference level values VRA, VRB and VRR, respectively. When the level VR of the received signal SR is higher than the predetermined value VRR and the level VA of the air-conducted sound signal picked up by the directional microphone 15 and the level VB of the bone-conducted sound signal picked up by the bone-conducted sound pickup microphone 14 are smaller than the predetermined values VRA and VRB, respectively, the comparison/control circuit 32 determines that this state is the listening state shown in the table of FIG. 9. When the level VR of the received signal level VR is smaller than the predetermined value VRR and the levels VA and VB of the air-conducted sound signal and the bone-conducted sound signal are both smaller than the predetermined values VRA and VRB, the circuit 32 determines that this state is the silent state. In these two states the comparison/control circuit 32 calculates the level ratio VB /VA between the air-conducted sound signal from the air-conducted sound dividing circuit 31A and the bone-conducted sound signal from the bone-conducted sound dividing circuit 31B. Based on the value of this level ratio, the comparison/control circuit 32 refers to the relationship of FIG. 7 stored in the memory 32M to obtain an estimated value of the corresponding ambient noise level. When the estimated value of the ambient noise level is smaller than the threshold value Nth shown in FIG. 6, the comparison/control circuit 32 supplies the signal select circuit 33 with a control signal C instructing it to select and output the air-conducted sound signal input from the air-conducted sound dividing circuit 31A. When the estimated value of the ambient noise level is greater than the threshold value Nth, the comparison/control circuit 32 applied the control signal C to the signal select circuit 33 to instruct it to select and output the bone-conducted sound signal input from the bone-conducted sound dividing circuit 31B.
On the other hand, when the received signal level VR is smaller than the reference level value VRR and the levels VA and VB of the air-conducted sound signal by the directional microphone 15 and the bone-conducted sound signal by the bone-conducted sound pickup microphone 14 are larger then the predetermined reference level values VRA and VRB, the comparison/control circuit 32 determines that this state is the talking state shown in the table of FIG. 9. When the received signal level VR is larger than the reference level value VRR and the levels VA and VB of the air-conducted sound signal and the bone-conducted sound signal are larger than the predetermined reference level values VRA and VRB, the comparison/control circuit 32 determines that this state is the double-talking state. In these two states the comparison/control circuit 32 calculates the level ratio VB /VA between the bone-conducted sound signal and the air-conducted sound signal and estimates the ambient noise level N through utilization of the relationship of FIG. 8 stored in the memory 32M.
When the thus estimated value of the ambient noise level N is smaller than the threshold value Nth shown in FIG. 6, the comparison/control circuit 32 applies the control signal C to the signal select circuit 33 to cause it to select and output the air-conducted sound signal input from the air-conducted sound dividing circuit 31A. When the estimated value N of the ambient noise level is greater than the threshold value Nth, the circuit 32 applies the control signal C to the signal select circuit 33 to cause it to select and output the bone-conducted sound signal input from the bone-conducted sound dividing circuit 31B.
The comparison/control circuit 32 has, in the memory 32M for each of the first through n-th frequency bands, the predetermined threshold value Nth shown in FIG. 6 and the level ratio vs. noise level relationships representing the straight characteristic lines 4BA and 5BA shown in FIGS. 7 and 8. The comparison/control circuit 32 performs the same processing as mentioned above and applies the resulting control signals C1 to Cn to the signal select circuits 331 to 33n. The signal combining circuit 34 combines the speech signals from the signal select circuits 331 to 33n, taking into account the balance between the respective frequency bands.
While in the above the embodiments have been described to estimate and compare the noise level with the threshold value and control the signal select circuits 331 to 33n accordingly in any state described in the table of FIG. 9, it is also possible to employ a scheme that estimates the noise level only in the silent or listening state and uses the thus estimated noise level to effect control in the talking state and the double-taking state. In such an instance, the characteristic data of FIG. 8 need not be stored in the memory 32M. In contrast to this, the estimation of the noise level may be made only in the talking or double-talking state, in which case the estimated noise level is used for control in the talking or double-talking state. In this instance, the characteristic data of FIG. 7 is not needed.
In the case where the estimated noise level N is compared with the threshold value Nth for each frequency band and the air-conducted sound signal picked up by the directional microphone 15 is switched to the bone-conducted sound signal by the bone-conducted sound pickup microphone 14 on the basis of the result of comparison as described previously with reference to the FIG. 5 embodiment, the timbre of the speech being sent may sometimes undergo an abrupt change, making the speech unnatural. To solve this problem, an area NW of a fixed width as indicated by N- and N+ is provided about the threshold value Nth of the ambient noise level shown in FIG. 6; when the estimated noise level N is within the area NW, the air-conducted sound signal from the directional microphone 15 and the bone-conducted sound signal from the bone-conducted sound pickup microphone 14 are mixed in a ratio corresponding to the noise level, and when the estimated noise level N is larger than the area NW, the bone-conducted sound signal is selected, and when the estimated noise level is smaller than the area NW, the air-conducted sound signal is selected. By this, it is possible to reduce the abrupt change in the timbre prior to or subsequent to the switching operation.
The modification of the FIG. 5 embodiment for such signal processing can be effected by using, for example, a signal mixer circuit 33 depicted in FIG. 10A in place of each of the signal select circuits 331 to 33n. In this example, the corresponding air-conducted sound signal and bone-conducted sound signal of each frequency band are applied to variable loss circuits 33A and 33B, respectively, wherein they are given losses LA and LB set by control signals CA and CB from the comparison/control circuit 32. The two signals are mixed in a mixer 33C and the mixed signal is applied to the signal combining circuit 34 in FIG. 5.
The losses LA and LB for the air-conducted sound signal and the bone-conducted sound signal in the area NW need only be determined as shown in FIG. 10B, for instance. For brevity's sake, setting Nth =(N+ +N-)/2, the area width to D=N+ -N-, the minimum values LA0 and LB0 of the losses LA and LB to 0 dB, respectively, and their maximum values LAMAX and LBMAX to the same LMAX dB, the loss LA in the area NW can be expressed, for example, by the following equation: ##EQU1## Similarly, the loss LB can be expressed by the following equation: ##EQU2## The value of the maximum loss LMAX is selected in the range of between 20 and 40 dB, and the width D of the area NW is set to about 20 dB, for instance. When the estimated noise level N is larger than the area NW, the bone-conducted sound signal is not given any loss (LB =0) and is applied intact to the mixer 33C. On the other hand, the air-conducted sound signal is not given the loss LMAX but instead the variable loss circuit 33A is opened to cut off the signal. Similarly, when the estimated noise level N is smaller than the area NW, the air-conducted sound signal is not given any loss (LA =0) and is fed intact to the mixer 33C, whereas the bone-conducted sound signal is cut off by opening the variable loss circuit 33B. The comparison/control circuit 32 determines the losses LA and LB for each band as described and sets the losses in the variable loss circuits 33A and 33B by the control signals CA and CB.
With such signal processing as described above, it is possible to provide smooth timbre variations of the speech being sent when the air-conducted sound signal is switched to the bone-conducted sound signal or vice versa. Moreover, if the levels of the air-conducted sound signal and the bone-conducted sound signal input into the variable loss circuits 33A and 33B are nearly equal to each other, the output level of the mixer 33C is held substantially constant before and after the switching between the air- and bone-conducted sound signals and the output level in the area NW is also held substantially constant, ensuring smooth signal switching. Incidentally, the signal select processing by the signal select circuits 331 to 33n in FIG. 5 corresponds to the case where the width D of the area NW is set to zero in the processing in the modified embodiment depicted in FIGS. 10A and 10B. Hence, it can be said, in a broad sense, that the signal select circuits 331 to 33n also contribute to the mixing of signals on the basis of the estimated noise level.
FIG. 11 illustrates in block diagram a modified form of the FIG. 5 embodiment, in which as is the case with the first embodiment of FIGS. 1 and 2, the omnidirectional microphone 16, the amplifier 21U and the noise suppressing circuit 23 are provided in association with the direction microphone 15 and the output from the noise suppressing circuit 23 is fed as an air-conducted sound signal to the air-conducted sound dividing circuit 31A. This embodiment is identical in construction with the FIG. 5 embodiment except for the above. In this embodiment, when the transmitter-receiver is in the silent or listening state, a switch 35 is opened and only the air-conducted sound signal provided via the amplifier 21U from the omnidirectional microphone 16 is applied to the noise suppressing circuit 23, from which it is fed intact to the air-conducted sound dividing circuit 31A, and the air-conducted sound signals divided into respective frequency bands are applied to the comparison/control circuit 32. As in the FIG. 5 embodiment, the comparison/control circuit 32 estimates the ambient noise levels through utilization of the relationships shown in FIG. 7 and, based on the estimated levels, generate the control signals C1 to Cn for signal selection (or mixing use in the case of using the FIG. 10A circuit configuration), which are applied to the signal select circuits 331 to 33n (or the signal mixing circuit 36). After this, the switch 35 is turned ON to pass therethrough the air-conducted sound signal from the directional microphone 15 to the noise suppressing circuit 23, in which its noise components are suppressed, and then the air-conducted sound signal is fed to the air-conducted sound dividing circuit 31A. This is followed by the speech sending signal processing by the same signal selection or mixing as described previously with respect to FIG. 5.
Although in the embodiments of FIGS. 5 and 11 the comparison/control circuit 32 has been described to convert the signals input thereto to digital signals and generate the control signals C1 to Cn on the basis of the level ratio-noise level relationships stored in the memory 32M, the comparison/control circuit 32 may also be formed as an analog circuit, for example, as depicted in FIG. 12. In FIG. 12 there is shown in block form only a circuit portion corresponding to one of the divided subbands. A pair of corresponding subband signals from the air-conducted sound signal dividing circuit 31A and the bone-conducted sound signal dividing circuit 31B are both applied to a level ratio circuit 32A and a comparison/logic state circuit 32E. The level ratio circuit 32A calculates the level ratio LB /LA between the bone- and air-conducted sound signals in an analog fashion and supplies level converter circuits 32B and 32C with a signal of a level corresponding to the calculated level ratio.
The level converter circuit 32B performs a level conversion based on the relationship shown in FIG. 7. That is, when supplied with the level ratio VB /VA, the level converter circuit 32B outputs an estimated noise level N corresponding thereto and provides it to a select circuit 32D. Similarly, the level converter circuit 32C performs a level conversion based on the relationship shown in FIG. 8. That is, when supplied with the level ratio VB /VA, the level converter circuit 32C outputs an estimated noise level corresponding thereto and provides it to the select circuit 32D. On the other hand, the comparison/state logic circuit 32E compares the levels of the corresponding air- and bone-conducted sound signals of the same subband and the level of the received speech signal with the reference levels VRA, VRB and VRR, respectively, to make a check to see if these signals are present. Based on the results of these checks, the comparison/state logic circuit 32E applies a select control signal to the select circuit 32D to cause it to select the output from the level converter circuit 32B in the case of State 1 or 2 shown in the table of FIG. 9 and the output from the level converter circuit 32C in the case of State 3 or 4.
The select circuit 32D supplies a comparator circuit 32F with the estimated noise level N selected in response to the select control signal. The comparator circuit 32F compares the estimated noise level N with the threshold level Nth and provides the result of the comparison, as a control signal C for the subband concerned, to the corresponding one of the signal select circuits 311 to 31n in FIG. 5 or 11. In this instance, it is also possible to make a check to determine if the estimated noise level N is within the area NW or higher or lower than it as described previously with respect to FIG. 10B, instead of comparing the estimated noise level N with the threshold value Nth ; if the estimated noise level N is within the area NW, the control signals CA and CB corresponding to the difference between the estimated noise level N and the threshold level Nth, as is the case with Eqs. (5) and (6), are applied to the signal mixing circuit of the FIG. 10A configuration to cause it to mix the air-conducted sound signal and the bone-conducted sound signal; when the estimated noise level N is higher than the area NW, the bone-conducted sound signal is selected and when the estimated noise level N is lower than the area NW, the air-conducted sound signal is selected.
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