Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03G—CONTROL OF AMPLIFICATION
  • H03G3/00—Gain control in amplifiers or frequency changers without distortion of the input signal
  • H03G3/20—Automatic control
  • H03G3/30—Automatic control in amplifiers having semiconductor devices
  • H03G3/32—Automatic control in amplifiers having semiconductor devices the control being dependent upon ambient noise level or sound level
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/38—Transceivers, i.e. devices in which transmitter and receiver form a structural unit and in which at least one part is used for functions of transmitting and receiving
  • H04B1/40—Circuits
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03G—CONTROL OF AMPLIFICATION
  • H03G7/00—Volume compression or expansion in amplifiers
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03G—CONTROL OF AMPLIFICATION
  • H03G7/00—Volume compression or expansion in amplifiers
  • H03G7/06—Volume compression or expansion in amplifiers having semiconductor devices
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04M—TELEPHONIC COMMUNICATION
  • H04M1/00—Substation equipment, e.g. for use by subscribers
  • H04M1/60—Substation equipment, e.g. for use by subscribers including speech amplifiers
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04M—TELEPHONIC COMMUNICATION
  • H04M1/00—Substation equipment, e.g. for use by subscribers
  • H04M1/60—Substation equipment, e.g. for use by subscribers including speech amplifiers
  • H04M1/6016—Substation equipment, e.g. for use by subscribers including speech amplifiers in the receiver circuit

Definitions

• Telephone with means for enhancing the subjective signal impression in the presence of noise means for enhancing the subjective signal impression in the presence of noise.
• the invention relates to an audio restoration device comprising means for measuring noise and means for compressing the dynamic of an audio signal according to a compression law selected from various possible laws.
• the invention also relates to an audio restoration method comprising a step for measuring noise and a step for compressing the dynamic of an audio signal according to a compression law selected from various possible laws.
• the invention finally relates to a telephone including such a device, or implementing such a method.
• the invention finds important applications, notably for mobile telephones that are used in particularly noisy environments. When the ambient sound level becomes too high, the audio signal is immersed in the noise that renders the use of the telephone highly uncomfortable.
• European patent application EP 0 661 858 A2 describes an audio restoration device that comprises means for modifying the dynamic of the received audio signal (that is to say, the ratio between the highest amplitude and the lowest amplitude of the signal) as a function of the ambient background noise.
• This restoration device brings good results when the received audio signal doesn't contain too much noise, that is to say when the noise included into the received audio signal doesn't have too high amplitudes.
• a first object of the invention is to propose a device which brings better results when the audio signal contains noise. This is achieve with the audio restoration devices as claimed in claim 1 of the present application.
• Another object of the invention is to propose an particularly efficient way of adapting the compression law as a function of the measured noise, notably the remote noise. This object is achieved with the audio restoration devices as claimed in claim 2 of the present application.
• the compression ratio, the reference level and the transition threshold are functions of the measured noise, notably the remote noise.
• a further object of the invention is to propose a type of compression law which is particularly efficient when the remote noise is high.
• an expansion phase is added for expanding the dynamic of the audio signal for the amplitudes that are lower than an expansion threshold (this expansion threshold is lower than the transition threshold) so as to reduce the remote noise.
• this expansion threshold is lower than the transition threshold
• the value of this expansion threshold is a function of the measured noise.
• - Fig. 1 represents an example of a telephone including an audio restoration device
• - Fig. 4 is a block diagram summarizing the various steps used for the selection of a compression law, thereafter for the calculation of the gain to be applied to the audio signal in accordance with the selected compression law, and
• FIG. 5 is a diagram summarizing the steps of an audio restoration method according to the invention.
• Fig. 1 is represented an example of a telephone 1 including an audio restoration device 2.
• This telephone notably includes a microphone 10 connected to an analog/digital converter 20 itself connected to a speech coder 30.
• This speech coder 30 is connected, on the one hand, to a channel coder 40 and, on the other hand, to means 50 for measuring background noise N ⁇ .
• the output of the channel coder 40 is connected to a conventional radio transceiver circuit 60.
• This radio transceiver circuit 60 is also connected to a channel decoder 70 that processes the signals received by the telephone.
• This channel decoder 70 is connected to a speech decoder 80 that produces an audio signal Uin.
• This speech decoder 80 is connected, on the one hand, to means 90 for measuring remote noise N r contained in the audio signal Uin and, on the other hand, to means 100 for compressing the dynamic of the audio signal Uin.
• the measurements of local noise Ni and remote nose N r performed by the noise measuring means 50 and 90 are applied to the input of the compression means 100. These measurements are used by the compression means 100 for determining the compression law to be applied to the audio signal Uin.
• the compression means 100 deliver an audio signal Uout that is applied to the input of a digital/analog converter 110, itself connected to an earphone 120.
• Said noise measuring means comprise: - conventional means for distinguishing an only-noise signal from a signal containing speech and noise (they may be, for example, speech detection means), - means for measuring the power of the only-noise signals.
• the noise can be considered to be stationary over a period of the order of 2s (whereas speech is only stationary over a period of the order of 20ms), it is sufficient to renew the noise measurement with each only-noise signal received.
• the compression means 100 have for their object to compress the dynamic of c the audio signal as a function of the local noise and, in a preferred embodiment, as a function of the measured remote noise.
• the compression of the dynamic of a signal in fact corresponds to a multiplication of each sample of this signal by a gain that depends on the amplitude of said sample.
• Fig. 2 are represented three compression laws of a first family of laws. This family of laws corresponds to a first type of evolution of the gain as a function of the amplitude.
• references of the type X JB are used for designating the value in dB of a variable X, whereas the references of the type X (without an index) are used for designating the linear value of a variable X.
• X JB log(X).
• the gain GdB is a decreasing linear function of the amplitude Uin B -
• Uinl and Uin2 be two amplitudes of the audio signal taken to the input of the compression means, and Uoutl and Uout2 the two corresponding amplitudes obtained on the output of the compression means. From the equation (1) may be derived that the following relation links the two amplitudes Uoutl and Uout2:
• Fig. 3 are represented three other compression laws of a second family of laws.
• This second family of laws corresponds to a second type of evolution of the gain plotted against amplitude.
• These laws are identical to those of Fig. 2 for the amplitudes Uin d B that are higher than a transition threshold T2 dB that is lower than or equal to C dB - Below the transition threshold T2 dB , the gain GdB has a constant value Gmax d ⁇ whatever the amplitude UindB considered.
• a limitation of the maximum gain applicable to the samples of the audio signal has been introduced. This embodiment permits the limitation of the amplification of the remote noise contained in the audio signal.
• the remote noise that is present in the audio signal generally corresponds to amplitudes that are lower than the transition threshold T2.
• the risk of amplifying the remote noise exists all the more as the compression is large, that is to say, that the compression rate is low.
• the value at which a choice is made to limit the maximum gain applicable to the audio signal is thus all the lower as the compression is stronger (in Fig. 3 there is Gmax 3 dB > Gmax 2 B > Gmaxid ⁇ )-
• 3C 3T2 do ⁇ ⁇ 0; ⁇ > 0; — - > 0 (3)
• the compression is very effective when the local noise Ni is considerable.
• the compression then permits to rebalance the amplitudes of the audio signal by increasing the low amplitudes and by diminishing the high amplitudes relative to a reference amplitude C.
• the perception may also be enhanced by increasing the average level of the signal.
• at least one of the following measures is to be taken: increase the reference amplitude C, diminish the transition threshold T2 to start the compression earlier, diminish the compression rate ⁇ .
• Fig. 4 are represented in the form of blocks the various steps of an example of a gain calculation method to be applied to a sample based on noise measurements N r and Nj.
• the invention has been explained so far by using an evolution law of the gain in dependence on the amplitude of the audio signal (equation (1)).
• the use of the energy E permits to smooth the evolution of the signal Uout and thus to avoid distortions in the speech signal.
• G(k) [C/E(k)] (1" ⁇ )
• E(k) is the energy of the audio signal for the k th sample of the audio signal.
• the energy E is obtained by filtering the amplitude • Uin* : the z-transform of the transfer function of the filter is thus written as ot/[l-(l- ).z " '].
• Fig. 4 are represented three blocks 200, 210 and 220 for calculating parameters T2, C and ⁇ which characterize the compression law to be used. These blocks receive on the input the measurements N r and Ni of the remote noise and of the local noise respectively, and derive therefrom the values of the parameters T2, C and ⁇ by applying the functions fi, f 2 and f 3 .
• the transition threshold T2 obtained at the output of the block 200 is supplied to a calculation block 230 that calculates the maximum MAX between this threshold T2 and the energy E(k) calculated for the sample k.
• the block 240 receives on the input the value of the parameters C, ⁇ and MAX and it derives therefrom the value G of the gain to be applied to the sample k.
• Fig. 5 are summarized the various steps of an audio restoration process according to the invention.
• step 300 the audio signal Uin and the measurements of noise N r and Ni are applied to the compression means 100.
• step 310 permits to make a decision to activate or deactivate the compression.
• the compression is deactivated when the remote noise N r is high, whatever the level of the local noise , and when the remote noise N r is low or moderate, and when the local noise Ni is low;
• the compression is activated when the remote noise N r is low or moderate and when the local noise is high or moderate.
• the output audio signal Uout is equal to the input audio signal Uin (arrow 311). If the compression is activated (arrow 312) the next step 320 is changed to. At step 320, the amplitude • Uin* of the audio signal is calculated. Then, in step 330, this amplitude is filtered to obtain the energy E of the audio signal (the z-transform of the transfer function of the filter is written as ⁇ /[l-(l- ⁇ ).z " ']). The next step 340 is the calculation step of the gain G to be applied to the input signal. This step is described in detail with reference to Fig. 4. In step 350, the audio signal Uin is multiplied by the gain G that has been calculated, so that the output signal Uout is equal to (G.Uin).
• and N r were measurements made directly on the local and remote signals received by the telephone. These measurements may also be measurements of residual noise, that is, measurements of the local noise and/or remote noise after the signals received by the telephone have passed through conventional noise reduction devices. Such an embodiment permits to diminish the constraints linked with the level of the measurements Ni and N r in the choice of the compression law. - It is also possible to calculate the parameters that characterize a family of laws by using discontinuous functions of the local noise and of the remote noise.
• the gain evolution itself may be a discontinuous function of the amplitude or energy of the audio signal. And in that case a table is advantageously used for storing the values to be assigned to the gain as a function of the compression parameters that will have been calculated.
• the step 310 which permits to deactivate the compression in certain cases, is optional.
• Fig. 6 is shown a compression law that corresponds to a third type of gain evolution as a function of the amplitude. This law is identical with those represented in Fig. 3 for the amplitudes UindB that are higher than an expansion threshold Tld ⁇ T2 B- Below this expansion threshold TldB, the gain G d ⁇ is a growing linear function of the amplitude Uin d B-
• Tld ⁇ T2 B- Below this expansion threshold TldB
• the gain G d ⁇ is a growing linear function of the amplitude Uin d B-
• this example of embodiment is introduced an expansion of the dynamic of the audio signal for the amplitudes lower than Tl B - This expansion enables to reduce the remote noise present in the audio signal, and thus to enhance the listening comfort for the user.
• this expansion threshold Tl is identical with that of the transition threshold T2.
• Fig. 7 is represented a compression law that corresponds to a fourth type of gain evolution as a function of the amplitude.
• This law is of the same type as that of Fig. 6, but the transitions between the three zones defined by the thresholds Tl dB and T2 d ⁇ are progressive, so that this law is described by a curve and no longer by a succession of segments of straight lines.

Applications Claiming Priority (3)

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• 1998
  • 1998-09-29 FR FR9812159A patent/FR2783991A1/fr active Pending
• 1999
  • 1999-09-14 KR KR1020007005764A patent/KR20010032522A/ko not_active Application Discontinuation
  • 1999-09-14 CN CN99802540A patent/CN1289500A/zh active Pending
  • 1999-09-14 EP EP99952451A patent/EP1044549A1/de not_active Withdrawn
  • 1999-09-14 WO PCT/EP1999/006787 patent/WO2000019686A1/en not_active Application Discontinuation
  • 1999-09-14 JP JP2000573062A patent/JP2002526983A/ja active Pending
  • 1999-10-29 TW TW088118780A patent/TW444488B/zh not_active IP Right Cessation

Non-Patent Citations (1)

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