Patent Publication Number: US-2019182591-A1

Title: Device for calibrating microphones

Description:
The present patent application claims the priority benefit of French patent application FR16/57451 which is herein incorporated by reference. 
     BACKGROUND 
     The present application concerns a device and a method of calibration of microphones in an electronic system, particularly a sound recording system. 
     DISCUSSION OF THE RELATED ART 
     Certain electronic systems, particularly sound recording systems, may comprise a plurality of microphones, particularly to improve the quality of the recorded acoustic information and/or to extract information relative to the sound sources and/or to the surroundings. 
       FIG. 1  partially and schematically shows an example of a sound recording system  10  comprising a plurality of microphones  12  which are distributed on the site  14  where the sound recording is performed. Microphones  12  are coupled to an audio signal processing device  16 . Microphones  12  transmit to processing device  16  analog or digital electric signals originating from the conversion of the sound waves, and processing device  16  applies a processing to the digital audio signals based on the signals supplied by the microphones, and for example generates and stores digital audio files. 
     During the processing of the signals supplied by microphones  12 , processing device  16  generally operates by default as if the properties of microphones  12  were identical. An example of property is the delay between the time of reception of a sound wave by microphone  12  and the time at which processing device  16  starts performing a processing on the digital audio signal obtained by analog-to-digital conversion of the signal picked-up by microphone  12 . Such a delay is called transmission delay hereafter. Other examples of properties of the microphone are the phase shift and the gain at the conversion of the sound signal. 
     However, the properties of microphones  12  are generally not identical. For example, the transmission delays associated with the microphones are generally not identical and should be taken into account on generation of the audio files by processing device  16 , so that, when the audio files are listened to, a proper sound reproduction is obtained. Transmission delays particularly differ between analog microphones and digital microphones. An analog microphone transmits to the processing device an analog signal representative of the sound waves received by the microphone and the processing device performs the analog-to-digital conversion of the analog signal. An analog microphone is generally coupled to the processing device by a wire connection so that the duration of the transfer of the analog signal from the analog microphone to the processing device is negligible. A digital microphone comprises a digital-to-analog converter which converts the analog signal originating from the conversion of the sound signal into a digital signal transmitted to the processing device. Further, the transmission of the signals from the digital or analog microphone to processing device  16  may be a wireless transmission implementing electromagnetic waves. The transmission delay of the microphone then further comprises the delay for the transmission and the reception of the electromagnetic waves and also the delay for the possible coding and error correction processing carried out by the transmitter.  FIG. 1  schematically shows four microphones  12 , among which three microphones  12  coupled to processing device  16  by a wire connection  18  and one microphone  12  coupled to processing device  16  by a wireless connection  20 . 
     For certain applications, it may be necessary to provide a step of calibration of sound recording system  10  to determine the differences between the properties of microphones  12  and possibly determine means of compensation of such differences. As an example, the compensation of the differences between the transmission delays of microphones  12  may comprise the addition by an operator of variable delays, stored by processing device  16 , so that the times of beginning of audio file recording are identical. As an example, the compensation of the differences between the amplification ratios and the phase shifts of microphones  12  may comprise the addition of a filtering applied by processing device  16  to the signals supplied by microphones  12  so that the recorded audio files correspond to the audio files which would be obtained if the amplification ratios and the phase shifts of microphones  12  were identical. 
     An example of a method of calibrating sound recording system  10  comprises the emission by a sound generator  24 , for example, a loudspeaker, possibly controlled by processing device  16 , of a plurality of known sequences of sound signals, and the analysis of the audio files supplied by processing device  16  after the acquisition of the sound signal sequences by microphones  12 . 
     There exists a delay of propagation of each sound signal from loudspeaker  24  to each microphone  12 . This delay may vary from one microphone to the other according to the relative position of microphone  12  and loudspeaker  24 . A disadvantage is that it can then be difficult, based on the analysis of the audio files, to separate, for each microphone  12 , the transmission device associated with microphone  12  from the propagation delay. 
     It may then be difficult to automatically adapt the compensation means determined at the calibration to a new arrangement of the microphones, and an operator then generally has to perform a manual adaptation of the compensation means for each new arrangement of the microphones, which is a long and tedious operation. 
     SUMMARY 
     An object of an embodiment is to provide a device of calibration of a sound recording system which overcomes all or part of the disadvantages of the previously-described devices. 
     Another object of an embodiment is for the calibration to be performed automatically. 
     Another object of an embodiment is for the calibration to be performed simply. 
     Another object of an embodiment is for the calibration to be performed rapidly. 
     Thus, an embodiment provides a calibration device comprising a sound recording system comprising microphones coupled to an audio signal processing device, the calibration device further comprising an enclosure at least partly containing the processing device, a sound generator coupled to the processing device, and a support of the microphones capable of maintaining the microphones at the same distance from the sound generator. 
     According to an embodiment, the device further comprises a battery of accumulators. 
     According to an embodiment, the sound generator is located between the support and the processing device. 
     According to an embodiment, the sound generator is in contact with the support. 
     According to an embodiment, the support is at least partly made of a resilient material. 
     According to an embodiment, the support comprises holes receiving the microphones, and at least one of the holes has a shape at least partly complementary to that of one of the microphones. 
     According to an embodiment, the minimum distance separating each microphone from the sound generator is shorter than 20 cm. 
     According to an embodiment, the number of microphones is greater than or equal to five. 
     An embodiment also provides the use of a calibration device such as previously defined for the calibration of the microphones of the sound recording system. 
     According to an embodiment, the use comprises the steps of: 
     emission of at least one sound signal by the sound generator; 
     picking-up of the sound signal by each microphone and, for each microphone, acquisition of a digital audio signal by the processing device; and 
     analysis of the digital audio signals to determine, for each microphone, at least one feature from among the transmission delay, the phase shift, and the conversion gain of the microphone. 
     According to an embodiment, the use further comprises the step of addition, for at least one of the microphones, of a delay by the processing device on subsequent acquisitions of digital audio signals representative of sound signals picked up by said microphone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, among which: 
         FIG. 1 , previously described, partially and schematically shows an example of a sound recording system; 
         FIGS. 2 and 3  are partial simplified cross-section views of an embodiment of a device of calibration of a sound recording system; 
         FIG. 4  is a partial simplified cross-section view, similar to  FIG. 3 , of another embodiment of a device of calibration of a sound recording system; 
         FIG. 5  is a block diagram of an embodiment of a method of calibration of a sound recording system; 
         FIG. 6  shows an example of envelope of an audio signal emitted by a loudspeaker and the digital audio signals acquired by a processing device of the calibration device of  FIG. 2  on implementation of the calibration method illustrated in  FIG. 5 ; and 
         FIG. 7  is a block diagram of a more detailed embodiment of a step of the calibration method illustrated in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     The same elements have been designated with the same reference numerals in the different drawings. For clarity, only those elements which are useful to the understanding of the described embodiments have been shown and are detailed. In particular, the structures of microphones and of a device for processing the sounds picked up by microphones are well known and will not be described in detail hereafter. 
     In the following description, when reference is made to terms qualifying absolute positions, such as terms “front”, “rear”, “top”, “bottom”, “left”, “right”, etc., or relative positions, such as terms “above”, “under”, “upper”, “lower”, etc., or to terms qualifying directions, such as terms “horizontal”, “vertical”, etc., it is referred to the orientation of the drawings or to a sound recording system in a normal position of use. The terms “approximately”, “substantially”, and “in the order of” are used herein to designate a tolerance of plus or minus 10%, preferably of plus or minus 5%, of the value in question. 
     According to an embodiment, for the calibration of microphones, it is provided to arrange the microphones close to a loudspeaker in a fixed and known configuration. The relative position between each microphone and the loudspeaker is then previously known. Preferably, the microphones are arranged so that the propagation device of the sound waves from the loudspeaker to each microphone is substantially constant. The determination of the transmission delays of the microphones is then eased. In the following description, the terms “sound signal” and “acoustic signal” are indifferently used. 
       FIGS. 2 and 3  are partial simplified cross-section views of an embodiment of a device  30  of calibration of a sound recording system  31 . 
     Calibration device  30  comprises an enclosure  32  having the components of sound recording system  31  arranged therein, particularly a sound signal processing device  34 . Enclosure  32  further contains a sound generator  36 , for example, a loudspeaker, preferably coupled to processing device  34  and controlled by processing device  34 . Loudspeaker  36  may be coupled to processing device  34  by a wire connection or be controlled by processing device  34  over a wireless connection, particularly implementing electromagnetic waves. Loudspeaker  36  is preferably located above processing device  34 . Processing device  34  for example corresponds to the product commercialized by Aaton-Digital under trade name Cantar-X3. Processing device  34  may comprise a dedicated electronic circuit and/or a processor, for example, a microcontroller, capable of executing the instructions of a computer program stored in the memory. Loudspeaker  36  may be a wideband loudspeaker. A battery of electric accumulators  38  for the power supply of processing device  34  and/or of loudspeaker  36  may be arranged in enclosure  32 . Battery  38  may be located between processing device  34  and loudspeaker  36 . As a variation, processing device  34  may be located between loudspeaker  36  and battery  38 . 
     According to an embodiment, loudspeaker  36  and/or battery  38  of electric accumulators may be totally or partly integrated to processing device  34 . 
     Sound recording system  31  further comprises microphones  40  coupled to processing device  34 . The number of microphones  40  may be in the range from 2 to 50, preferably from 2 to 20. As an example, in  FIGS. 2 and 3 , a processing device  34  coupled to five microphones  40  has been shown. Each microphone  40  may be coupled to processing device  34  by a wire connection or by a wireless connection implementing the transmission of electromagnetic waves. As an example, in  FIG. 2 , five microphones  40  have been shown, among which two microphones, each coupled to processing device  34  by a cable  42 , and one microphone transmits signals to processing device  34  over a wireless connection, not shown. 
     Each microphone  40  comprises a transducer capable of receiving an acoustic signal S(t) and of converting it into an analog electric signal S e (t), also called analog audio signal, where t indicates a time variable. Each microphone  40  has a transfer function H which, in the frequency range, is provided by the following relation (1): 
         H (ω)= A (ω)exp( i Φ(ω))  (1)
 
     where ω is the pulse of the acoustic signal, A is the conversion gain, which may depend on frequency, and Φ is the phase shift, which may depend on frequency. 
     Processing device  34  is capable of determining, for each microphone  40 , a digital audio signal S n  based on analog audio signal S e (t). Microphone  40  may transmit analog audio signal S e (t) to processing device  34 , which then converts the analog audio signal to obtain digital audio signal S n . As a variation, microphone  40  may perform the analog-to-digital conversion of analog audio signal S e (t) and directly supply digital audio signal S n  to processing device  34 . For each microphone  40 , processing device  34  is capable of performing a processing on digital audio signal S n  to supply a digital audio file. The processing may comprising conditioning digital audio signal S n , for example, applying a filter to the digital audio signal, mixing the digital audio signal with another digital audio signal, and/or recording the digital audio signal comprising digital audio signal S n  and possibly additional data. The transmission delay of microphone  40  corresponds to the delay between the time when the microphone starts receiving sound signal S(t) and the time when processing device  34  starts the processing applied to the digital audio signal, for example, the time when processing device  34  starts conditioning digital audio signal S n , the time when processing device  34  starts mixing digital audio signal S n  with another digital signal, or the time when processing device  34  starts storing the digital audio file representative of sound signal S(t). 
     Calibration device  30  further comprises a support  44  at least partially arranged in enclosure  32  and comprising holes  46  having microphones  40  at least partly arranged therein. Preferably, support  40  is in contact with loudspeaker  36 . According to an embodiment, support  44  comprises a resilient material at least at the level of each hole  46  so that support  44  may slightly deform on introduction of microphone  40  into hole  46 . Support  44  is for example at least partly made of foam. According to an embodiment, each hole  46  has a shape complementary to a portion of a microphone  40  so that, when a microphone  40  is arranged in a hole  46 , microphone  40  remains substantially stationary with respect to enclosure  32 , for example, by the friction exerted by support  44  on microphone  40 . The holes  46  present in support  44  may be identical. As a variation, holes  46  may have different shapes in the case where microphones having different shapes are used. In  FIG. 3 , holes  46  are shown as being aligned. 
     According to an embodiment, enclosure  32  may be a monoblock part or comprise a plurality of parts coupled to one another. According to an embodiment, enclosure  32  may comprise a frame having an inner wall having a shape complementary to that of the different elements housed in enclosure  32 . As a variation, wedges may further be arranged in enclosure  32 , between enclosure  32  and processing device  34 , battery  38 , loudspeaker  36 , and/or support  44 , to ease the holding in position of these elements in enclosure  32 . As an example, enclosure  32  is made of resilient material. 
       FIG. 4  is a view similar to  FIG. 3  of another embodiment of support  44  where holes  46  are arranged at the corners of a regular polygon. As a variation, holes  46  may be distributed in rows and in columns. 
     According to another embodiment, support  44  may comprise a plurality of microphone clamps, preferably coupled by a rigid frame to one another, each microphone  40  being held by one of the clamps on use of calibration device  30 . 
     When microphones  40  are arranged on support  44 , the capsules of microphones  40  are preferably placed relative to loudspeaker  36  so that the sound waves emitted by the loudspeaker are substantially planar when they reach microphones  40  and reach at the same time the capsules of microphones  40 . According to an embodiment, the capsules of microphones  40  are arranged at an equal distance from loudspeaker  36 . 
     According to an embodiment, the distance between the capsule of each microphone  40  and loudspeaker  36  is in the range from 2 cm to 20 cm, preferably from 2 cm to 10 cm. 
       FIG. 5  shows an embodiment of a method of calibration of sound recording system  31 . 
     Step  50  corresponds to the assembly of calibration device  30 , which comprises stacking, in enclosure  32 , processing device  34 , battery  38 , loudspeaker  36 , and support  44 . Enclosure  32  holds processing device  34 , battery  38 , loudspeaker  36 , and support  44  in position. Enclosure  32  ensures that microphones  40  remain stationary relative to loudspeaker  36  during the calibration operation. The method carries on at step  52 . 
     At step  52 , sound signals are emitted by loudspeaker  36 . The sound signals are picked up by microphones  40 . Each sound signal may correspond to a pure sound emitted for a determined emission time period, that is, to a sound signal at a single frequency. The frequency of the pure sound may be constant during the emission time period or may vary during the emission time period. As an example the frequency of the pure sound may increase or decrease with a constant variation rate during the emission time period, which corresponds to a frequency ramp. The method carries on at step  54 . 
     At step  54 , for each microphone  40 , a digital audio signal S n  is acquired by processing device  34  from the sound signal picked up by the microphone at step  52 , where digital audio signal S n  may be received or determined by processing device  34 . As previously described, the processing device may perform various processings on digital audio signals S n , and particularly supply and store audio files. 
     Steps  52  and  54  are repeated from each sound signal supplied by loudspeaker  36 . The method carries on at step  56 . 
       FIG. 6  schematically shows an example of envelope S H  of the control signal of loudspeaker  36  for the emission of a sound signal and digital audio signals S n1  and S n2  acquired by processing device  34  from the sound signals which are picked up by two microphones  40  on emission of the sound signal by loudspeaker  36 . Envelope S H  for example successively comprises a rising edge Attack, a steady level area Sustain, and a falling edge Release. However, other shapes of envelope S H  may be used. Time t 1  corresponds to the time of detection of the rising edge of signal S n1  and time t 2  corresponds to the time of detection of the rising edge of signal S n2 . Call Δt the delay of time t 2  with respect to time t 1 . 
     Referring again to  FIG. 5 , at step  56 , features of microphones  40  are determined by processing device  34  based on the analysis of the digital audio signals S n  supplied at step  54 . The features may be the transmission delay, the phase shift, and/or the amplification ratio of each microphone  40 . Advantageously, since the shape of support  44  is previously known, the relative positions between the microphones  40  placed in holes  46  and loudspeaker  36  are previously known. The propagation delay of each sound signal from loudspeaker  36  to each microphone  40  is thus previously known. Further, preferably, the configuration of microphones  40  is defined so that the propagation delays of each sound signal emitted by loudspeaker  36  to microphones  40  are substantially identical. 
     An example of a method of analyzing the digital audio signals is described in patent application FR 2764088. The analysis method may comprise comparing the digital audio signals with one another. As an example, for each sound signal emitted by loudspeaker  36  and for each pair of microphones comprising a first microphone and a second microphone, the analysis method may comprise determining delay Δt between time t 1  of beginning of first digital audio signal S n1  relative to time t 2  of beginning of the second digital audio signal S n2  such as acquired by processing device  34 . As an example, the time of beginning of a digital audio signal may correspond to the time at which the digital audio signal exceeds a threshold. According to another example, the digital audio signal is compared with a template by displacement in time of the template with respect to the digital audio signal until a criterion is fulfilled, for example, the maximum coverage of the digital audio signal by the template. The time of beginning of the digital audio signal is then obtained from the determined position of the template. 
     As a variation, rather than a processing applied to the two digital audio signals associated with a pair of microphones  40 , a simultaneous processing of more than two digital audio signals associated with more than two microphones  40  may be performed. 
     At step  58 , processing device  34  may modify some of its operating parameters according to the results obtained at step  56 . According to an embodiment, processing device  34  is capable of modifying, for each microphone  40 , the delay between the real beginning of the digital audio signal acquired by processing device  34  and corresponding to an audio signal picked up by microphone  40  and the beginning of the processing applied by processing device  34  to the digital audio signal. Such a delay is called waiting delay hereafter. As a variation, the processing device may shift the time of beginning of the digital audio signal by a time period equal to the waiting delay. 
     According to an embodiment, processing device  34  automatically modifies the waiting delays associated with the microphones so that the processings of the digital audio signals by the processing device start simultaneously, as if the times of beginning of the digital audio signals were identical. According to an embodiment, at step  56 , processing device  34  determines for which microphone  40  delay Δt is the longest and, at step  58 , the waiting delays associated with the other microphones are then modified so that the time of beginning of each digital audio signal corresponds to the time of beginning of the digital audio signal having the longest delay. 
     Independently from what has been previously described, the processing performed by processing device  34  may comprise introducing an additional delay for certain digital audio signals, particularly by delaying the beginning of the recording of digital audio files, for example, to obtain a desired sound effect (the creation of an echo, the obtaining of a distance perception, etc.). 
     At step  60 , processing device  34  determines whether the digital audio signals fulfill certain criteria. If the digital audio signals do not fulfill the criteria, the method returns to step  52  and a new calibration operation is implemented. As an example, the digital audio signals may be compared with templates. If the digital audio signals fulfill the criteria, the method carries on at step  62 . 
     At step  62 , processing device  34  determines the phase shift between digital audio signals acquired by processing device  34  from the sound signals picked up by microphones  40 . Step  62  may be omitted. 
     At step  64 , processing device  34  may indicate to an operator that the calibration operation is over. As an example, it may be provided to display information on a display screen indicating the delay associated with each microphone  40 . 
       FIG. 7  shows a more detailed embodiment of a method of determination of the phases of the audio signals at previously-described step  62 . 
     At step  70 , sound signals are emitted by loudspeaker  36 . The sound signals are picked up by microphones  40 . Each sound signal corresponds to a pure sound emitted for a determined emission time, that is, to a sound signal at a single frequency. The frequency of the pure sound may be constant during the emission time period or may vary during the emission time period. As an example, the frequency of the pure sound may increase or decrease with a constant variation rate during the emission time period, which corresponds to a frequency ramp. 
     At step  72 , for each sound signal and for each pair of microphones comprising a first microphone and a second microphone, processing device  34  may determine the sum of the first digital audio signal associated with the first microphone and of the second digital audio signal associated with the second microphone to determine the phase shift between the first and second digital audio signals. 
     At step  74 , processing device  34  may modify digital audio signals S n  to compensate for the phase shifts determined at step  72 . According to an embodiment, at step  74 , processing device  34  determines the digital audio signal having a correct phase and the digital audio signals which do not have the right phase are modified. As an example, the correct phase corresponds to the phase common to the greatest number of digital audio signals among all the signals acquired by processing device  34 . 
     Steps  52  to  64  of the calibration of microphones  40  of sound recording system  31  may advantageously be carried out rapidly and automatically by recording device  31 . 
     Specific embodiments have been described. Various alterations, modifications, and improvements will readily occur to those skilled in the art.