Abstract:
The present publication describes a calibration method and apparatus, in which an electrical calibration signal is formed, an audio signal is formed in the loudspeaker from the calibration signal, the response of the audio signal is measured and analyzed, and the system is adjusted on the basis of the measurement results. The calibration signal is formed in the loudspeaker in such a way that it is essentially a sinusoidal signal, the frequency of which scans at least substantially through the entire audio frequency range.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a calibration method in a sound-reproduction system, in which an electrical calibration signal is formed, an audio signal is formed in the loudspeaker from the calibration signal, the response of the audio signal is measured and analysed outside the loudspeaker, and the system is adjusted on the basis of the measurement results. 
     The invention also relates to a calibration apparatus. 
     2. Brief Discussion of the Related Art 
     According to the prior art, calibration methods are known, in which a test signal is fed to a loudspeaker. The response to the test signal is measured using a measuring system and the frequency response of the system is adjusted to be as even as possible using an equalizer. 
     A drawback of the state of the art is that the measuring arrangement is difficult and requires special equipment. The calibration arrangement cannot be generalized for different listening spaces and obtaining a reliable result always demands very precise planning and also the knowledge and skill to use the individual parts of the measuring system. 
     The invention is intended to eliminate the defects of the state of the art disclosed above and for this purpose create an entirely new type of method and apparatus for calibrating a sound-reproduction system. 
     SUMMARY OF THE INVENTION 
     The invention is based on the sound-reproduction equipment being connected, with the aid of a control network, to a calibration system built around a computer. 
     With the aid of a preferred first embodiment of the invention, the frequency response of the sound card of the computer can be calibrated using a generator external to the sound card, which is, however, controlled by the computer in which the sound card is. 
     According to a second preferred embodiment of the invention, the amplification of the sound card is calibrated using the voltage settings of the test signal. 
     According to a third preferred embodiment of the invention, the active loudspeaker is equipped with a signal generator, which can be used to create a logarithmically scanning sinusoidal test signal. 
     According to a fourth preferred embodiment of the invention, the level of the measuring signal is adjusted in such a way as to achieve the greatest possible signal-noise ratio. 
     According to a fifth preferred embodiment of the invention, the phase of the main loudspeaker and the subwoofer is set to be the same at the crossover frequency, with the aid of a sine generator built into the active subwoofer loudspeaker. 
     According to a sixth preferred embodiment of the invention, a logarithmic sine signal is used to equalize the frequency responses of the loudspeakers at the listening positioning (the location of the microphone), in order to eliminate differences in the mutual levels and time-of-flight delays of the loudspeakers in the loudspeaker system. 
     More specifically, the method according to the invention is characterized in that the calibration signal is formed in the loudspeaker in such a way that it is essentially a sinusoidal signal, the frequency of which scans at least substantially through the entire audio frequency range. 
     The apparatus according to the invention is, in turn, characterized in that the loudspeaker comprises means for forming an essentially sinusoidal electrical variable-frequency calibration signal, so that the calibration signal scans essentially through the entire audio-frequency range. 
     Considerable advantages are gained with the aid of the invention. 
     With the aid of the method according to the invention, any computer whatever, in which there is any sound card whatever, can be used to calibrate a sound-reproduction system, with the aid of an economical microphone. 
     The software implementing the invention can be installed in all the most common computer operating systems. 
     According to the first preferred embodiment of the invention, it is possible to envisage that the response of the sound card can be calculated using the FFT, e.g. H=FFT(y)/FFT(x), in which H is the frequency response, x a known generated signal, and y the acoustic response recorded by the sound card. However, this will not produce a result, unless the spectrum of the generated signal is continuous (energy at all the frequencies being examined), because otherwise the frequency response cannot be calculated (the signals x and y receive the value zero, in which case a quotient H does not exist at this frequency) at these frequencies, at which the energy content of the input signal is zero (or very small), thus this method cannot be used as a general solution. 
     Because the method according to the invention works with any sound card whatever of a computer, the frequency responses of the sound cards can differ from each other. 
     Measurement taking place using modelling according to the invention eliminates this problem. 
     A known method for eliminating the defects of the frequency response of a sound card is, for example, loopback measurement, in which the sound card generates a signal, which it records itself. In this method, the response of the output of the sound card cannot be distinguished from the response of the input. In the method according to the invention, only the output is measured, in which case the input by itself can be equalized. 
     The construction produced by the method is very simple to implement, because the pulse required for measurement is produced, for example, by the IO line of a micro-controller, the voltage produced by which is summed in the microphone signal. 
     This method can be built into the microphone amplifier, so that calibration can be performed transparently to the operator (without the operator knowing) and also at the same time as the acoustic measurement is recorded. 
     According to the second preferred embodiment of the invention, the unknown and varying delay caused by the operating system of the computer can be eliminated. The sensitivity of the output of the computer sound card (the size of the digital word in volts) can be calculated. 
     According to the third preferred embodiment of the invention, because the test signal is not fed to the loudspeaker from the computer, but arises in the loudspeaker, the test signal does not create other distortions or changes, in addition to the acoustic response. 
     Only the measuring microphone and the frequency response of the input of the computer sound card, in addition to the acoustic transfer path, affect the measuring signal. 
     Because the measuring signal is built in, it is always available. 
     Because the crest factor of the measuring signal is small, it produces a good signal-noise ratio. 
     According to the fourth embodiment of the invention, the following advantages are achieved. 
     As the distance of the microphone can vary greatly, the magnitude of the acoustic response produced by the measuring signal can vary within very wide limits. 
     Noise produced by the environment does not vary in the same way, but instead remains (in each room) relatively constant. 
     If the microphone is very close to the loudspeaker, the signal being recorded may be too large, in which case it will be peak-limited in the computer sound card. 
     If the microphone is very far away, the signal may be too small relative to ambient noise, in which case the signal-noise ratio will remain poor. 
     An advantageous signal-noise ratio can always be ensured with the aid of level setting. 
     Peak limiting of the measuring signal can be prevented by reducing the level of the signal. The signal-noise ratio can be improved by raising the level of the signal. 
     The setting of the level is known to the controlling computer all the time, and can be taken into account in calculations. 
     The following advantages are achieved with the aid of the fifth embodiment of the invention. 
     The correct phase settings are found, irrespective of where the loudspeaker is placed (the distance affects the sound level and the placing affects the phase). 
     The measurement corresponds to a real situation (in which the subwoofer and main loudspeaker operate simultaneously and repeat the same audio signal). 
     According to the sixth preferred embodiment of the invention, all the loudspeakers of the entire loudspeaker system are brought mutually to the correct level, to a virtual distance, and with an identical room response. 
     Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the present invention. 
         FIG. 1  shows a block diagram of one system suitable for the method according to the invention. 
         FIG. 2  shows a second calibration circuit according to the invention. 
         FIG. 3  shows graphically the signal according to the invention, which the computer sound card records. 
         FIG. 4  shows graphically a typical measured signal in the calibration arrangement according to the invention. 
         FIG. 5  shows graphically a test signal generated by the loudspeaker. 
     
    
    
     In the invention, the following terminology is used:
       1  loudspeaker     2  loudspeaker control unit     3  acoustic signal     4  microphone     5  preamplifier     6  analog summer     7  sound card     8  computer     9  measuring signal     10  test signal     11  USB link     12  control-network controller     13  control network     14  IO line     15  signal generator     16  loudspeaker element     18  interface device     50  calibration signal   

     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows the apparatus totality, in which loudspeakers  1  are connected to a computer  8  through a control network  13 , by means of an interface device  18 . 
     The interface device  18  contains a control-network controller  12  according to  FIG. 2 , a preamplifier  5  and an analog summer  6 , to which an IO line  15  coming from the control-network controller, through which IO line a test signal  10  is transmitted to the summer, is connected. 
       FIG. 2  contains the same functions as  FIG. 1 , but only one loudspeaker  1  is shown, for reasons of clarity. 
       FIG. 2  shows the apparatus totality of the invention, in which the loudspeaker  1  produces an acoustic signal  3 . For test purposes an acoustic signal  3  is created from an electrical calibration signal formed by the generator  15  of the control unit  2  of the loudspeaker itself. The control unit  2  typically contains an amplifier thus making the loudspeaker  1  an active loudspeaker. The test signal is preferably a sinusoidal scanning signal, such as is shown graphically among others in  FIG. 6 . The frequency of the calibration signal  50  ( FIG. 5 ) is scanned over the range of human hearing, preferably in such a way that this starts from the lowest frequencies and the frequency is increased at a logarithmic speed towards the higher frequencies. The generating  50  of the calibration signal is started by a signal brought to the control unit  2  of the loudspeaker  1  over the control bus  13 . The acoustic signal  3  is received by the microphone  4  and amplified by a preamplifier  5 . In the analog summer  6 , the signal coming from the preamplifier  5  is combined with the test signal  10 , which is typically a square wave. The analog summer  6  is typically a circuit implemented using an operation amplifier. The test signal  10  is obtained from the control unit  12  of the control network. In practice, the test signal can be obtained directly from the IO line  14  of the microprocessor of the control unit of the control network. 
     Thus, according to the invention the acoustic measuring signal  3  can be initiated by remote control through the control bus  13 . The microphone  4  receives the acoustic signal  3 , with which the test signal  10  is summed. The sound card  7  of the computer  8  receives a sound signal, in which there is initially the test signal and then after a specific time (the acoustic time-of-flight) the response  9  of the acoustic signal, according to  FIG. 2 . 
       FIG. 3  shows the signal produced in the computer&#39;s sound card  7  by the method described above. The time t 1  is a randomly varying time caused by the operating system of the computer. The time t 2  to the start of the acoustic response  9  is mainly determined on the basis of the acoustic delay (time of travel), and random variation does not appear in it. The acoustic response  9  is the response of the loudspeaker-room system to the logarithmic sinusoidal scanning, the frequency of which is increasing. 
     In the first preferred embodiment of the invention, in which the frequency response of an unknown sound card is calibrated, the procedure is as follows. The pulse shape is generated by the controller  12  of the control network, which is connected to the computer&#39;s  8  sound card  7  and preferably to the computer&#39;s USB bus  11 . Under the control of a program run by the computer, the control-network controller produces the test signal  10 . The sound card  7  is used to record the received pulse shape, which arises as the response of the input of the computer  8  sound card  7  to the test signal. 
     A pulse wave  10  (in which there are two values: zero and a voltage corresponding to one) produced by the digital IO line  14  can be used as the input pulse. 
     The input pulse  10  can be summed (analogically) with the microphone signal. 
     The test signal  10  recorded in the sound card changes its shape due to the filtering caused by the sound card. It is known that the frequency response of the sound card is a bandpass frequency response, which includes a high-pass property (at low frequencies) and a low-pass property (at high frequencies). The original shape  10  of the test signal is known to the computer. A model, in which the original test signal travels through a filter depicting the filtering properties of the sound card, is applied to the recorded test signal  10 . In a preferred implementation, the parameters of the transfer function of the filter are selected with the aid of optimization using an adaptation method, in such a way that the filtered test signal  10  produced by this model corresponds in shape as accurately as possible to the real test signal recorded by the sound card. The frequency response H (b,a), in which b and a are the parameters of the frequency-response model, cause by filtering will then have been defined. 
     Using the frequency response thus defined, an equalizer is formed, by means of which the frequency response H can be equalized with the frequencies corresponding to the range of human hearing. The equalization thus defined is used later, when the acoustic responses are measured. When the measured acoustic response is corrected using this equalization, the filtering caused by the sound card is corrected at the frequencies in the range of human hearing. 
     The selection of the structure and degree of the transfer function being modelled can be used to affect the accuracy and the speed of the measurement. 
     According to the second preferred embodiment of the invention, the voltage of the test signal  15  produced by the IO line  14  is set to a specific value. 
     In this method, the generation of the known test signal  10  is combined to be part of the command that initiates the calibration signal  50  (log-sine scanning) produced by the loudspeaker. 
     The computer  8  records the signal, which consists of three parts. First is the test signal  10 , after it silence, the third to arrive at the microphone being the acoustic signal  3  produced by the loudspeaker, which is recorded as the response  9 . The following can be read from the recorded information:
         With the aid of the voltage of the test signal, the magnitude of the digital word recorded in the computer can be measured in volts. (Because the height of the pulse in volts can be known beforehand and the magnitude of the digital representation of the pulse can be examined from the stored signal.)   The time t 2  between the start of the test signal  10  and the start of the acoustic response  9  depicts the distance of the loudspeaker  1  from the measuring microphone  4 , and by using this information it is possible to calculate the distance of the loudspeakers  1  (reproducing the entire audio band) from the measuring point. Most advantageously this takes place by taking as the initial data for the FFT calculation a signal, which includes the signal recorded by the sound card  7  beginning from the start of the test signal (the start of the time t 2  in  FIG. 3 ) and setting the test signal  10  in it to zero before beginning the calculation.       

     The command to generate the test signal comes from the computer  8 . In practice however, it will be observed that the delay ( FIG. 3 , t 1 ) after which the command leaves, varies independently of the operating system (Windows, Mac OS X). This delay is random and cannot be predicted. Once the command has left, and because the command and test signal are linked to one and the same function, there is always a known and constant time from the generation of the test signal to the start of the generating of the measuring signal (i.e. the calibration signal). In addition to this, there is a time, which is affected only by the distance between the loudspeaker and the measuring microphone, to the start of the acoustically recorded measuring signal. 
     According to the third preferred embodiment of the invention, a generator  15 , which produces a calibration signal  50  that is precisely known beforehand, is built into the loudspeaker  1 . 
     The calibration signal produced by the generator  15  is sine-scanning, the speed of which frequency scanning increases in such a way that the logarithm of the frequency at the moment is proportional to the time, log(f)=k t, in which f is the momentary frequency of the signal, k is a constant defining speed, and t is time. The increase in frequency accelerates as time passes. 
     Because the test signal is precisely defined mathematically, it can be reproduced in the computer accurately, irrespective of the test signal produced by the loudspeaker  1 . 
     Such a measuring signal contains all the frequencies while the crest factor (the relation of the peak level to the RMS level) of the signal is very advantageous in that the peak level is very close to the RMS level, and thus the signal produces a very good signal-noise ratio in the measurement. 
     As the signal  50  ( FIG. 5 ) starts moving from the low frequencies and its frequency increases, the signal operates advantageously in rooms with a reverberation time that is usually longer at low frequencies than at high frequencies. 
     The generation of the calibration signal  50  can be initiated using a command given through remote control. 
     According to the fourth preferred embodiment of the invention, the magnitude of the calibration signal  50  produced in the loudspeaker can be altered through the control network  13 . 
     The calibration signal  50  is recorded. The magnitude of the acoustic response  9  of the calibration signal  50  relative to the calibration signal is measured. If the acoustic response  9  is too small, the level of its calibration signal  50  is increased. If the acoustic response  9  is peak limited, the level of the calibration signal  50  is reduced. 
     The measurement is repeated, until the optimal signal-noise ratio and level of the acoustic signal  9  have been found. 
     Level setting can be performed for each loudspeaker separately. 
     Because the extent to which the level has been altered is controlled by the computer  8  and thus known, this information can be taken into account when calculating the results, so that a reliable measurement result, which is scaled correctly relative to the level, will be obtained irrespective of the distance. 
     According to the fifth preferred embodiment of the invention, an internal sine generator is used in the subwoofer. The phase of the subwoofer is adjusted from the computer through the control network  13  and the acoustic signal is measured using the microphone. 
     Setting the subwoofer and the main loudspeaker to the same phase at the crossover frequency takes place in two stages.
         Stage 1: the levels of the subwoofer and the reference loudspeaker are set to be the same by measuring one or both levels separately and setting the level produced by each loudspeaker.   Stage 2: both loudspeakers repeat the same sine signal, which the subwoofer generates.   The common sound level is measured by the microphone.   The phase is adjusted and the phase setting at which the sound level is at a minimum is sought. The loudspeaker and subwoofer are then in an opposing phase.   The subwoofer is altered to a phase setting that is at 180 degrees to this, so that the loudspeaker and the subwoofer are in the same phase and thus the correct phase setting has been found.       

     According to the sixth preferred embodiment of the invention, the acoustic impulse response of all the loudspeakers  1  of the system is measured using the method described above. Such a calibration is shown in  FIG. 3 . 
     The frequency response is calculated from each impulse response. 
     The distance of the loudspeaker is calculated from each impulse response. 
     On the basis of the frequency response, settings of the equalizer filter that will achieve the desired frequency response in the room (even frequency response) are planned. 
     The (relative) sound level produced by the equalized response is calculated. 
     A delay is set for each loudspeaker, by means of which the measured response of all the loudspeakers contains the same amount of delay (the loudspeakers will appear to be equally distant). 
     A level is set for each loudspeaker, at which the loudspeakers appear to produce the same sound level at the measuring point. The level of each loudspeaker can be measured from the frequency response, either at a point frequency, or in a wider frequency range and the mean level in the wider frequency range can be calculated using the mean value, RMS value, or median. In addition, different weighting factors can be given to the sound level at different frequencies, before the calculation of the mean level. The frequency range and the weighting factors can be selected in such a way that the sound level calculated in this way from the different loudspeakers and subwoofers is subjectively as similar as possible. In a preferred implementation, the mean level is calculated from the frequency band 500 Hz-10 kHz, using the RMS value and in such a way that all the frequencies have the same weighting factor. 
     The subwoofer(s) phase is then adjusted as described above. 
     In the present application the term audio frequency range refers to the frequency range 10 Hz-20 kHz. 
     In one preferred embodiment of the invention, all the essential data of the system are recorded in a single file, or system setup file, which is based on information on the identity of the loudspeaker. Preferably each loudspeaker has an unequivocal identity, which is used for data management in the system setup file. This identity is preferably formed at the manufacturing stage of the loudspeaker  1 . The data system  8  updates the loudspeaker settings actively. By opening the file, the properties of the whole loudspeaker system are displayed and can also be updated through this file or the system setup file. 
     In a preferred implementation, the stages described above are performed in the following order:
         the acoustic responses of all the loudspeakers are recorded with the aid of the computer sound card,   the impulse response of the loudspeaker is calculated from each of the responses,   the time of travel of the sound is measured from each impulse response and the distance of the loudspeaker is calculated on its basis,   on the basis of the distance of each loudspeaker, the additional delay that makes the time of travel of the sound coming from the loudspeaker the same as that of the time of travel of the other loudspeakers is calculated,   the frequency response is calculated from each impulse response,   on the basis of the frequency responses, the levels of the loudspeakers are calculated,   a correction is calculated for each loudspeaker, which will make its level the same as that of the other loudspeakers.       

     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.