Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R3/00—Circuits for transducers, loudspeakers or microphones

Definitions

• the present invention relates generally to an interference reduction method in a multi-sensor reception system as well as a multi-sensor reception system implementing said interference reduction method.
• the invention more particularly finds application in the field of acoustic echo cancellation for sound recording and in the field of hands-free telephony.
• any device capable of transforming a physical quantity into a measurement signal for example an antenna or an acoustic transducer
• a reception system using a plurality of such sensors will be called a “multi-sensor reception system”.
• An antenna consisting of a plurality of such sensors, for example an array of sensors, will be called a “multi-sensor” antenna.
• a (t, f) represents a quantity a at time t and at the frequency / This makes it possible to describe a quantity in the frequency domain but which varies over time. As we know, the passage from the time domain to the frequency domain requires an observation of the quantity a in a time window. In this sense, we understand that A (t, f) is a known value at time t, but that its calculation may have required an observation of the quantity a for a certain duration. A (t, f) can be obtained from a by means of a spectral estimation in a time window.
• Ait, f) can be a signal, a spectral quantity, or the transfer function of a filter varying over time. Subsequently, the notations A t, f) and a will be used interchangeably to designate a quantity a whose spectrum varies over time.
• the notation a (t) is reserved for a quantity a at time t, varying in time.
• Fig. 1 schematically represents a multisensor reception system known from the state of the art.
• Such a system typically comprises a plurality of sensors 100 1? .., 100N transforming a physical quantity into received signals X x ⁇ t, f), .., X N (t, f), respectively.
• These signals are then used to perform beamforming using the filters 1 10 15 .., 110 N and the adder 120. More precisely, the signals V x (t, f), .., V N (t, f) at filter output
• HO. ⁇ l 10 N are summed in the summator 120 to provide a signal, called the antenna signal, noted Y (t, f).
• the filters in question introduce delays and / or phase shifts as well as a weighting of the signals received. It is clear that, in the general case, the transfer functions of these filters can depend on both time and frequency.
• the filters 110I, .., 1 10N simply perform a multiplication using complex weighting coefficients. These coefficients are determined according to the desired reception pattern, so that the main lobe of the reception pattern points in the direction of a useful signal source and has a given opening angle. The choice of coefficients is also dictated by the desired level of attenuation of the secondary lobes.
• these coefficients can be determined so as to introduce one or zeros in the reception diagram for directions in which there are interfering sources.
• the coefficients can be calculated adaptively in order to follow a mobile useful signal source or to reduce interference arriving at a variable angle of incidence.
• this interference reduction technique by means of the reception diagram does not make it possible to eliminate interference signals received by the main lobe of the diagram, which lobe can be large if the number of sensors is small.
• Fig. 15 illustrates a simple echo generation situation.
• a multi-media terminal 1500 comprising a monitor 1510, a pair of speakers 1520 and a multi-sensor acoustic antenna 1530 made up of microphones.
• the transmission channel is characterized by the direct propagation paths between the loudspeakers and the user (placed in front of the multi-media terminal).
• the arrows 1540 illustrate the phenomenon of echo generation, attributable here to the acoustic coupling between the loudspeakers and the acoustic antenna. The coupling is due to the propagation paths between the loudspeakers and the antenna as well as to the reflections of the signals emitted by the loudspeakers on the environment of the terminal (people, walls, objects etc.).
• Fig. 2 schematically reproduces a known system of echo cancellation reception, as described for example in the article by W. Kellermann entitled “Strategies for combining acoustic echo cancellation and adaptive beamforming microphone arrays” published in April 1997 in Proc.
• the multi-sensor system of FIG. 1 consisting of sensors 200 I , .., 200N, filters 210 13 .. 3 210N and summing device 220.
• the system further comprises an element 201 symbolizing a sensor or a socket making it possible to obtain a reference signal e of the signal interfere, also noted E (t, f) in frequency representation.
• the reference signal the interfering signal, a picked up or picked up signal, allowing the signal to be reconstructed. interfere.
• the reference signal could be the signal transmitted to a loudspeaker and the element 201 could be a socket on the loudspeaker control circuit.
• the reference signal e is filtered by an echo canceller filter 205 of transfer function H e (t, f).
• This filter actually models the echo propagation channel.
• the filter output is subtracted from the antenna signal 230 to provide a signal Z (t, f), ideally rid of the echo.
• this echo cancellation technique is also called adaptive acoustic echo cancellation.
• the coefficients of the echo canceller filter 205 are calculated adaptively in the calculation module 206, so as to minimize the mean square value of the error signal Z (t, f).
• the performance of acoustic echo cancellers is limited by the size of the room in which the sound recording system is used. Indeed, in the case of large rooms, the echo canceller must identify an acoustic impulse response (transfer function H e (t, f)) which can reach several seconds, which leads to a considerable increase in digital complexity of the adaptation algorithm.
• the duration of the impulse response to be identified has a direct influence on the performance of the algorithm: the convergence time (i.e. the speed of adaptation of the filter) as well as the mismatch (i.e. i.e. the difference between the effective reduction of the echo level and the theoretical maximum reduction) increases with the duration of the impulse response.
• the convergence time i.e. the speed of adaptation of the filter
• mismatch i.e. i.e. the difference between the effective reduction of the echo level and the theoretical maximum reduction
• the general objective of the present invention is to propose a method of eliminating interfering signals for a multi-sensor reception system, which does not have the drawbacks of the state of the art.
• one of the goals of the present invention is to provide a method of eliminating interfering signals, in particular of an echo, having a higher adaptation speed than in the prior art.
• a secondary object of the present invention is to provide a simple method of eliminating interfering signals capable of taking into account a plurality of interfering sources, including when the reception system is multi-channel.
• the problem is solved by the invention defined by an interference reduction method for a reception system using a multisensor antenna and at least one channel trainer supplying an antenna signal from the signals received by the various sensors of said receiver. antenna.
• the transfer function of a first filter called the first post-filter, is estimated from a reference signal making it possible to regenerate said interference, and said antenna signal is filtered by said first postfilter.
• said transfer function is obtained from a short-term estimate and a long-term estimate of the spectral density of said reference signal, for example from a ratio between these two estimates.
• the short-term estimation and the long-term estimation of the spectral density are preferably obtained by low-pass filtering of a spectrum of the reference signal.
• the short-term estimate ⁇ ⁇ (t, f) of the spectral density is obtained by recursive filtering of the type: where E (t, f) is a spectral component of the reference signal at frequency / and at time t, a is a coefficient between 0 and 1, ⁇ t is the delay in the loop of the recursive filtering and. denotes the conjugation operation and the long-term estimate ⁇ (t, f) of the spectral density is obtained by recursive filtering of the type:
• the transfer function of a second filter is estimated from said received signals, before or after filtering by said channel filters, and the signal is filtered. antenna by said second post-filter.
• the transfer function of said second post-filter is estimated from an average of the power spectral densities and an average of the interspectral power densities of said received signals, after filtering by said channel filters.
• the transfer function W s (t, j) of said second post-filter is estimated by:
• ⁇ vv (/, /) and ⁇ ., v (t, f) are respectively estimates of the spectral densities and interspectral power densities of the signals received after channel filtering
• b t (f) are the transfer functions of the different channel filters free of phase-shifting terms
• N is a number of antenna sensors
• ⁇ (.) denotes the actual value or the module.
• the signals received by the various sensors being filtered by at least one set of channel filters before being summed to provide said antenna signal, the transfer function of an second filter, said second post-filter, from said signals received after filtering by said channel filters, as well as from the antenna signal, and the antenna signal is filtered by said second post-filter.
• the transfer function of said second post-filter is estimated from an average of the interspectral power densities of said received signals, after filtering by said channel filters, and from an estimation of the spectral density of the antenna signal.
• ⁇ V V ⁇ (t, f) and ⁇ w (t, /) are respectively the spectral and interspectral power densities of the signals received after channel filtering
• b, (/) are the transfer functions of the different channel filters removed in terms of re-phasing
• Nest a number of antenna sensors
• ⁇ (.) denotes the actual value or the module.
• the filtering of the antenna signal by the first post-filter and that of the second post-filter are applied in a combined manner, by filtering the antenna signal by means of a post-filter, called the first combined post-filter. having the transfer function a combination of the transfer functions of said first and second post-filters.
• a statistical analysis is carried out of spectral components of the transfer function of the second post-filter and / or of the transfer function of the first combined post-filter and that an indication of presence is deduced therefrom. or lack of a useful signal.
• the statistical analysis is also carried out on spectral components of the transfer function of the first post-filter.
• said statistical analysis uses a criterion of spectral occupancy rate and / or variance of said spectral components.
• a switching signal is generated from said indication of presence or absence of a useful signal and the antenna signal is filtered by means of the first combined post-filter when the switching signal is in a first state and is filtered by means of a second combined post filter when the switching signal is in a second state, the transfer function of the second combined post filter being a combination of the transfer function of the first post -filter and a predetermined attenuation.
• the received signals being filtered by a plurality of sets of channel filters to form a plurality of channel signals
• a statistical analysis is carried out of spectral components of the transfer functions of the second post-filters associated with the different channel filter sets and that the channel with the highest probability of the presence of a useful signal is deduced therefrom.
• the statistical analysis is also carried out on spectral components of the transfer function of the first post-filter.
• said statistical analysis uses a criterion of spectral occupancy rate and / or variance of said spectral components.
• the antenna signal is then obtained from the channel signals relating to the channel having the highest probability of presence of useful signal.
• the invention is also defined by a reception system comprising a multisensor antenna, at least one channel trainer and interference reduction means, said interference reduction means being adapted to implement said method of reduction of interference.
• Fig. 1 schematically represents a multi-sensor reception system known from the state of the art
• Fig. 2 schematically represents a multi-sensor reception system with echo cancellation, as known from the state of the art
• Fig. 3A schematically represents a multi-sensor reception system according to a first embodiment of the invention
• Fig. 3B schematically represents a multi-sensor reception system according to a variant of the first embodiment of the invention
• Fig. 4 schematically represents a multi-sensor reception system according to a second embodiment of the invention.
• Fig. 5 schematically represents a multi-sensor reception system according to a variant of the second embodiment of the invention
• Fig. 6 schematically represents a multi-sensor reception system with useful signal detection
• Fig. 7 schematically represents a multi-sensor reception system according to a third embodiment of the invention
• Fig. 8 schematically represents a module of the multisensor reception system shown in FIG. 7 according to a first alternative embodiment
• Fig. 10 schematically represents another module of the multi-sensor reception system shown in FIG. 7;
• Fig. 12 shows an example of a multi-sensor reception system according to the third embodiment of the invention
• Fig. 13 schematically represents a module of the multi-sensor reception system shown in FIG. 12;
• FIG. 3A illustrates a first embodiment of the invention.
• the reception system comprising a plurality of sensors 300J, .., 300N, filters 310 15 .., 310N and a summator 320 performing the channel formation.
• An element 301, sensor or socket provides a reference signal e of the interfering signal.
• An estimation module 351 which will be detailed below, calculates from said reference signal the transfer function W e (t, f) of a filter 350 at the antenna output, for this reason called "post-filter"".
• the transfer function W e (t, f) of the post-filter is determined so that in the absence of an interfering signal, it is worth 1 (the filter
• the estimation module 351 advantageously calculates the transfer function W e (t, f) of the post-filter in the following manner:
• ⁇ is a constant, called short-term time constant, between 0 and 1 and where a ⁇ and ⁇ 2 are constants such that 0 ⁇ o: 2 ⁇ ! ⁇ l, a ⁇ representing a time constant at long term and ⁇ 2 a long term time constant.
• the calculations of expressions (2) to (4) are carried out simply by means of a first order recursive filtering, ⁇ t being the delay present in the filter loop.
• the constants is chosen very close to 1 so as to take into account the past over a long period while the values of and ⁇ 2 are chosen lower so as to react more quickly to variations in spectral density of the reference signal.
• Fig. 3B illustrates a variant of the first embodiment of the invention.
• Fig. 4 illustrates a second embodiment of the invention.
• the reference signal e of an interference signal is received or sampled.
• the estimation module 451 determines, as before, the transfer function W e (t, f) post-filter 450.
• This embodiment differs from the previous one in that there is provision for additional post-filtering by means of a post-filter 440 placed upstream or downstream of the post-filter 450.
• the post-filter 440 has a transfer function Wft, f) provided by an estimation module
• the post-filtering technique in order to improve the signal-to-noise ratio at the output of a multi-sensor antenna was notably described in the article by C. Marro et al. entitled "Analysis of noise reduction and deverberation techniques based on microphone arrays with postfiltering" published in May 1998 in IEEE Trans. on Speech and Audio Processing, Vol. 6, No. 3, pages 240-259. The main results are recalled below.
• the signal x, at the level of the sensor i is modeled by the sum of the useful signal received after propagation to the sensor and of the noise n,; • for each sensor, the noise n t and the useful signal received are decorrelated;
• the optimal post-filter W s is that which minimizes the mean square error between the desired signal s and the signal at the output of the filter. As shown in the article by KU Simmer et al. in the article “Time delay compensation for adaptive multichannel speech enhancement Systems” published in Proc. ISSE-92 in September 1992, the expression of this optimal filter can be written from the useful signal s and the average noise n at the antenna output:
• ⁇ jyt, f) and ⁇ m (t, f) are the spectral power densities of the wanted signal and of the noise at the output of the channel formation.
• Fig. 5 illustrates a variant of the second embodiment.
• the elements 500 15 .., 500N, 510 I , .., 510N, 520, 501, 550, 551 are respectively identical to the elements 400 1 , .., 400 N , 410 ⁇ , .., 410 N , 420, 401, 450, 451 of FIG. 4.
• the estimate (13) or (14) may relate only to a subset of the signals x, or v researcher, the mean of the spectral and interspectral densities then being taken only on this subset. Said estimation can be carried out in the frequency domain or in the time domain.
• the module 541 of FIG. 5 performs the estimation of W s (t, f) according to expression (11). For this, it uses on the one hand the antenna signal y (Y (t, f) in frequency representation) and, on the other hand, either the signals x, (vector X (t,)) after having delivered them in phase, ie directly the signals v, (vector N (t, /)).
• the multi-sensor reception systems shown in Figs. 4 and 5 allow, as we have seen, to significantly improve the signal to noise ratio at the antenna output.
• the post-filters 440 and 450 or 540 and 550 tend to distort the noise, which can prove to be annoying, in particular in sound recording applications.
• Fig. 6 illustrates a multi-sensor reception system with switchable post-filter as set out in the above-mentioned application.
• the latter performs a set of operations which will be detailed more far and which aim to identify by means of analysis of variance and / or spectral occupancy rate an index of presence of useful signal in the spectrum of W s (t, f).
• the module 661 provides a binary indicator for the presence / absence of a useful signal denoted P_A (t) which is temporally smoothed in the low-pass filtering module 662 to give a gain value G (t).
• Fig. 7 illustrates a multi-sensor reception system according to a third embodiment of the invention.
• This embodiment uses post-filtering and switching of presence / absence of useful signal in FIG. 6.
• the elements 700 1 , .., 700 N , 710 1 , .., 710 N , 720, 730, 740, 741, 760, 762, 763 are respectively identical to the elements 600 1; .., 600 N , 610 l5 .., 610 N , 620, 630, 640, 641, 660, 662, 663 of the latter.
• the system of FIG. 7 differs from that of FIG. 6 in that it implements an elimination of interfering signals according to the principle of the invention.
• 701 denotes an element making it possible to receive or take a reference signal from an interfering signal
• 751 denotes an estimation module identical to the module 351 of FIG. 3A.
• the estimation modules 741 and 751 respectively estimate the transfer function W s (t, f) of a first post-filter and the transfer function W e (t, f) of a second post-filter, the first allowing to increase the signal to noise ratio subject to the aforementioned hypotheses and the second allowing to eliminate interference (or interference as in Fig. 3B) for which there is a reference.
• the transfer functions W s (t, f) and W e (t, f) (or more precisely their estimates) are provided, as well as their product W (t, f) obtained by the multiplier 725, to a statistical analysis module 761 similar to module 661 and the function of which will be detailed below.
• This module provides a binary indicator of presence / absence of useful signal P_A (t) which is smoothed temporally to provide the gain G (t).
• the output W ⁇ t, f) W s (t, f) .W e (t, f) of the multiplier is transmitted to the filter 740.
• the estimation module 751 transmits the indication W e (t) to the filter 750, which allows it to carry out the transfer function G S A- W e (tj).
• the attenuation G S A depends on the frequency while remaining constant over time, which can in particular make it possible to systematically reject certain parts of the spectrum, independently of the interference.
• the antenna output is filtered by the combination of the two post-filters as in FIG. 5 whereas, when the useful signal is absent, it is simply subject to filtering by W e (t, f) (or W e v ⁇ t, f)) and attenuation.
• the functional diagram of the statistical analysis module 761 is shown in FIG. 8.
• W s (t, f) We extract from W s (t, f), by extraction means 811, a set of frequencies F ocp set by the user.
• the spectrum thus obtained possibly undergoes a non-linear transformation (not shown) to give more relevant information.
• a logarithmic transformation in decibels
• the spectral components reduced to the set of frequencies F ocp are then compared to a threshold SOC s in the comparator 812.
• a threshold SOC s in the comparator 812.
• the occupancy rate ⁇ r O 0 'c * p p (t) is then compared with a predetermined threshold value TOC "*.
• This comparison provides a binary signal p_a'.
• the binary signal p_a ' indicates a useful signal is present when the occupancy rate ⁇ '(t) is greater than the TOC * threshold.
• the processing chain consisting of the extraction module 821 (resp. 831), of the comparator 822 (resp. 832), the occupancy rate calculation module 823 (resp. 833) and the comparator 824 (resp. 834) provides a binary signal p_a w (resp. p_a ').
• the binary signal p_a * gives a indication of the presence of the useful signal without taking the interference signal into account, on the other hand, the binary signal p_a 'gives an indication of the presence of the interference signal.
• the binary signal p_a w gives an indication of the presence of useful signal from global information taking into account both the useful signal and the inter signal.
• the combination function used may in particular depend on the gain value G (t). We may indeed wish to favor more or less the indicator p_ a w depending on whether or not we are already in the presence of a useful signal.
• the binary signal p_ ⁇ "indicates that a useful signal is present if the variance va 'is less than the threshold value VAR s .
• the processing chain consisting of the extraction module 921 (resp. 931), comparator 922 (resp. 932), variance calculation module 923 (resp. 933) and comparator 924 (resp. 934) provide a binary signal p_a w (resp. p_a e ).
• the three binary signals are combined in 950 to provide the binary indicator P_A (t)
• the remarks made for the first example in Fig. 8 also apply here mutatis mutandis.
• the low-pass filter 762 is shown schematically in FIG. 10. We will denote P the state of presence and A the state of absence of the useful signal.
• the function of the filter 762 is to continuously decrease the gain G (t) to a value S mm when passing from state P to state A and to increase the gain G (t) to value S max during the pass in reverse.
• the input signal is filtered by the low-pass filter 1030 of time constant TA, which conditions the descent time of G (t).
• the outputs of the two low-pass filters are connected to the inputs of a second switch 1040 which selects, using the binary indicator P_A (t), the output of the low-pass filter with time constant T A if the useful signal is absent and the output of the low-pass filter with time constant ⁇ if the useful signal is present.
• the common output of switch 1040 provides the smoothed gain signal Gif).
• the transfer functions are analyzed on the basis of a variance or spectral occupancy rate criterion by a first statistical analysis module 1145 and the channel ko having the highest probability of receiving a useful signal is selected.
• the index ko generated by the module 1145 selects by means of the multiplexer 1115 the signals V ! 0 , .., ⁇ 0 which provides them to the summator 1120 to form the channel in question.
• the transfer function of the post-filter associated with channel kb is transmitted to a second statistical analysis module 1161 identical to module 761 in FIG. 7.
• the first statistical analysis module 1145 also receives the transfer function W e t, f) from the estimation module 1151 and takes it into account for the selection of the channel kb. The first statistical analysis module 1145 then performs all of the statistical analysis operations and directly supplies the binary indicator P_A (f).
• the elements 1130, 1140, 1150, 1160, 1162 and 1163 are in both cases identical to the elements 730, 740, 750, 760, 762 and 763 in FIG. 7.
• Fig. 12 gives an example of the third embodiment of the invention in an application to taking hands-free sound for interactive communication contexts (teleconferencing, communicating personal computers, etc.).
• the acoustic echo that is to say the signal of the distant speaker emitted by the loudspeaker constitutes the interfering signal.
• the disturbance signal e is taken directly from the loudspeaker.
• the temporal samples are indexed by n representing the index temprel at discrete time.
• the representation in the frequency domain is ensured by a discrete Fourier transform inside a sliding time window.
• the signals x, (") received by the microphones 1200 l ., 1200 N are subjected to a weighting in the time window by means of the filters 1205 l ., 1205 N then to a discrete Fourier transform (TFD) in the short term in 1207 ⁇ , .., 1207 N -
• TFD discrete Fourier transform
• the reference signal e taken from the loudspeaker is subjected like the microphone signals to a short-term Fourier transform in 1208 after weighting in 1206.
• the representation of e in the frequency domain, as obtained at the output of 1208 is written with the previous notations:
• a ⁇ is more much closer to 1 than ⁇ .
• a ⁇ - 0.9999 and a% 0.9.
• the detection of the useful signal is carried out on the basis of a statistical analysis of the frequency values of the filter W (p, ⁇ g ) alone.
• the functional diagram of the statistical analysis module 1261 is given in FIG. 13.
• the occupancy rate ⁇ o w cp (jA) is then compared to an occupancy threshold STOC w in the comparator 1324.
• the comparator delivers a binary indicator p_ ⁇ W (p) of presence of useful signal which here is none other that the global indicator P_A (p) since here only the transfer function W (p, ⁇ q ) is used for the detection of useful signal.
• the gain smoothing filter is digital.
• the gain G (p) is smoothed by a recursive filtering conditioned by the state of P_A (p):
• the quantities ⁇ p and ⁇ _ are time constants which respectively determine the rates of ascent and descent of G (p). More precisely, the predetermined gains S min and S max are switched by the binary indicator P_A (p) to the common output of the switch 1410. This output is connected on the one hand to the input of the recursive filter 1420 of time constant ⁇ j. and at the input of the recursive filter 1430 of time constant ⁇ . The outputs of the two filters are switched by switch 1440, controlled by the binary indicator P_A (p). Returning to FIG. 12, the antenna output switching is provided by the switch 1230 controlled by the signal K (p).
• the switching signal K (p) is obtained by comparison of the smoothed gain G (p ) at a predetermined threshold ST, that is:
• the invention is not limited to the elimination of acoustic echo in a multi-sensor sound pickup system, but generally applies to any multi-sensor reception system having a reference of one or more interference signals.

Landscapes

• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• Acoustics & Sound (AREA)
• Signal Processing (AREA)
• Noise Elimination (AREA)
• Mobile Radio Communication Systems (AREA)

Applications Claiming Priority (3)

Publications (1)

Family

ID=8868810

Family Applications (1)

Country Status (4)

Families Citing this family (35)

Family Cites Families (11)

• 2001
  • 2001-10-25 FR FR0113933A patent/FR2831717A1/fr active Pending
• 2002
  • 2002-10-23 WO PCT/FR2002/003637 patent/WO2003037033A1/fr not_active Application Discontinuation
  • 2002-10-23 EP EP02790538A patent/EP1438870A1/de not_active Withdrawn
  • 2002-10-23 US US10/493,580 patent/US20040264610A1/en not_active Abandoned

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events