Abstract:
A method of generating a test tone signal includes generating a fundamental tone signal, which is a sinusoidal signal having a predetermined frequency; generating a first group of harmonic tone signals having different frequencies that are integral multiples of the predetermined frequency; generating a second group of the harmonic tone signals having different frequencies that are integral multiples of the predetermined frequency, at least part of the second group of the harmonic tone signals having frequencies different from frequencies of the first group of the harmonic tone signals; adding the fundamental tone signal to the first group of the harmonic tone signals to generate a first test tone signal; adding the fundamental tone signal to the second group of the harmonic tone signals to generate a second test tone signal; and outputting the first and second test tone signals at predetermined intervals.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority from Japanese Patent Application No. JP 2005-121941 filed on Apr. 20, 2005, the disclosure of which is hereby incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to a method of generating a test tone signal and a test-tone-signal generating circuit. 
   In audio reproduction, audio reproduction systems have been evolving from 2-channel stereo systems into 5.1-channel audio, 7.1-channel audio, and more-than-7.1-channel audio systems as digital audio technologies and audio visual (AV) devices have been developed. However, in such multi-channel audio systems, it becomes difficult for a user to appropriately and manually set the sound balance between channels, frequency characteristics, and others. 
   In this situation, sound field correction devices that automatically set the sound balance, the frequency characteristics, and others have been supposed. The sound field correction devices supply a test tone signal to the speakers of multiple channels, pick up reproduced sounds from the speakers with microphones, and correct the characteristics of the channels so that the sound balance, the frequency characteristics, and others of the reproduced sounds are appropriately set. 
   However, in order to perform the sound field correction, it is necessary to first check connection of the speakers. This is because the user fails to obtain information used for the sound field correction in a state in which the speakers are not connected to the apparatus even if the test tone signal is output. 
   In addition, for example, a reproduction apparatus capable of reproducing 7.1-channel audio signals is possibly used as a reproduction apparatus for 5.1-channel audio signals because of the arrangement of the speakers or the like. Accordingly, it is necessary to check the presence of non-connected speakers (channels that are not used) in multi-channel reproduction apparatuses. 
   Related arts are disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2001-346299. 
   In the above setting or checking, a pink noise is generally used as the test tone signal. However, the pink noise is not ear-pleasing because the pink noise strikes the user&#39;s ear as noise burst. Furthermore, it is not acceptable that such a pink noise is output from a speaker each time a reproducing apparatus is used (is turned on). 
   It is desirable not to cause discomfort to a listener (user) in the checking of connection of speakers and to correctly check whether the speakers are connected. 
   SUMMARY OF THE INVENTION 
   According to an embodiment of the present invention, a method of generating a test tone signal includes the steps of generating a fundamental tone signal, which is a sinusoidal signal having a predetermined frequency; generating a first group of harmonic tone signals having different frequencies that are integral multiples of the predetermined frequency; generating a second group of the harmonic tone signals having different frequencies that are integral multiples of the predetermined frequency, at least part of the second group of the harmonic tone signals having frequencies different from those of the first group of the harmonic tone signals; adding the fundamental tone signal to the first group of the harmonic tone signals to generate a first test tone signal; adding the fundamental tone signal to the second group of the harmonic tone signals to generate a second test tone signal; outputting the first and second test tone signals at predetermined intervals. 
   According to the present invention, since the test tone composes a melody in the checking of whether the speakers are connected, the test tone does not make a listener uncomfortable. In addition, since the test tone includes the multiple harmonic tones, it is possible to correctly check whether the speakers are connected. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  are waveform diagrams illustrating embodiments of the present invention; 
       FIG. 2  is a table illustrating the embodiments of the present invention; 
       FIG. 3  is a table illustrating the embodiments of present invention; 
       FIG. 4  includes diagrams showing frequency spectra illustrating the embodiments of the present invention; 
       FIGS. 5A to 5D  are timing charts illustrating the embodiments of the present invention; 
       FIG. 6  is a table illustrating the embodiments of the present invention; 
       FIGS. 7A and 7B  are diagrams showing frequency spectra illustrating the embodiments of the present invention; 
       FIG. 8  is a block diagram showing a sound field correction device according to an embodiment of the present invention; 
       FIG. 9  is a block diagram showing part of the sound field correction device in  FIG. 8 ; 
       FIG. 10  is a flowchart showing a process in the sound field correction device in  FIG. 8 , according to an embodiment of the present invention; and 
       FIG. 11  is a flowchart showing another process in the sound field correction device in  FIG. 8 , according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Sinusoidal Signal 
   It is assumed that digital data DD that is to be converted into one cycle of a sinusoidal signal S 1 , shown in  FIG. 1A , by digital-to-analog conversion is stored in a memory. In this case, the digital data DD is given by sampling one cycle of the sinusoidal signal S 1  in N samples. Accordingly, N samples form one cycle. 
   It is also assumed that the following equation is satisfied:
 
N=2^n  (1)
 
where n denotes a natural number and ^ denotes power (“2^n” denotes two to the n-th power). In this example, for example, n=12 and, therefore, N=4096.
 
   It is further assumed that the samples of the digital data DD are written in the memory from “0” address to “4095” address in ascending order and that the digital data DD has a common format in digital audio. For example, the digital data DD has a quantifying bit number of 16 and is two&#39;s complement. 
   It is further assumed that “fS” denotes a clock frequency when the digital data DD is read out from the memory, “f1” denotes a frequency of the sinusoidal signal S 1  (f1=fs/N), and “TN” denotes one cycle period of the sinusoidal signal S 1  (TN=1/f1). 
   
     
       
         
             
           
             
               
                 
                   
                     
                       
                         
                           
                             
                               If 
                               ⁢ 
                               
                                 
                                     
                                 
                                 ⁢ 
                                 
                                     
                                 
                               
                               ⁢ 
                               fS 
                             
                             = 
                             
                               48 
                               ⁢ 
                               
                                   
                               
                               [ 
                               kHz 
                               ] 
                             
                           
                           , 
                         
                       
                     
                     
                       
                         
                           
                             f 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                           = 
                           
                             fS 
                             / 
                             N 
                           
                         
                       
                     
                     
                       
                         
                           = 
                           
                             48000 
                             / 
                             4096 
                           
                         
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                       
                   
                   ≈ 
                   
                     11.72 
                     ⁡ 
                     
                       [ 
                       Hz 
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
   
   Accordingly, when the digital data DD is read out from the memory at the clock frequency “fs”, sequentially reading out the samples one by one from the addresses of the memory gives one cycle of the sinusoidal signal S 1  having a frequency of 11.72 Hz (=f1) in the period TN, as shown in m=1 in  FIG. 1B . 
   When the digital data DD is read out from the memory, reading out the sample every two addresses and repeating the readout two times give two cycles of a sinusoidal signal S 2  having a frequency of 23.44 Hz (=2f1) in the period TN, as shown in m=2 in  FIG. 1B . 
   When the digital data DD is read out from the memory, reading out the sample every three addresses and repeating the readout three times give three cycles of a sinusoidal signal S 3  having a frequency of 35.16 Hz (=3f1) in the period TN, as shown in m=3 in  FIG. 1B . 
   The same applies to the subsequent cases. That is, when the digital data DD is read out from the memory, reading out the sample every m addresses and repeating the readout m times give m cycles of a sinusoidal signal Sm having a frequency (mf1) that is m times higher than the frequency f1 in the period TN. 
   Accordingly, the following equation is satisfied from Equation (2): 
                             fm   =     f   ⁢           ⁢     1   ⨯   m                   =     fs   /     N   ⨯   m                     (   3   )                 
where “fm” denotes a frequency of the sinusoidal signal Sm generated in the period TN.
 
   When m (m is a natural number) cycles of the sinusoidal signal Sm are fall within the period TN, as described above, frequency analysis of the sinusoidal signal Sm by fast Fourier transform (FFT) generates an amplitude only at the frequency positions of the sinusoidal signal Sm and generates no amplitude at other frequency positions. Consequently, it is not necessary to execute a window function in the frequency analysis to simplify the analysis. 
   Since the number of samples in the memory is given by Equation (1), it is unlikely to cause waste in the memory. Furthermore, it is possible to yield the digital data DD in one cycle by, for example, providing the first ¼ cycle of the digital data DD in the memory; reading out the digital data DD from the addresses of the memory in ascending order in the first ¼ cycle and reading out the digital data DD from the addresses of the memory in descending order in the second ¼ cycle; and reading out the digital data DD from the addresses of the memory in ascending order in the third ¼ cycle, reading out the digital data DD from the addresses of the memory in descending order in the fourth ¼ cycle, and inverting the sign (polarity) of the readout data. As a result, the memory area can be saved. 
   It is assumed that N=4096 and fs=48 kHz, as described above, when numerical values are shown in the following description. 
   Tonal Scale 
   Calculation of the frequency fm of the sinusoidal signal Sm according to Equation (3) when m=18 to 37 gives values shown in the second column in  FIG. 2 . These values of the frequency fm correspond to pitch names and frequencies of equal temperament shown in the third column in  FIG. 2 . The frequencies of equal temperament in  FIG. 2  are approximate values with respect to a frequency 445 Hz. 
   For example, when m=20, the frequency f20 of the sinusoidal signal S 20  is equal to 234.375 Hz. This frequency f20 corresponds to a sound having a pitch name A# (a sound of a pitch having a frequency of equal temperament of 235.896 Hz). Generally, it is said that the difference in pitch cannot be discriminated if the pitch is about three cents or less. 
   Accordingly, varying the value m gives the sounds having the pitch names shown in the third column in  FIG. 2 . This means that supplying the sinusoidal signal Sm to the speaker and varying the value m of the sinusoidal signal Sm allow a melody (music) to be played by using the sounds having the pitch names A, A#, B, C#, D#, F, F#, G, and G# shown in the third column in  FIG. 2 . As a result, supplying the sinusoidal signal Sm to the speaker allows the connection of the speaker to be checked, and sequentially varying the value m produces a melody formed of the test tones output from the speaker. 
   The values m may be made two raised to the power of values in  FIG. 2 , although not shown. In this case, it is possible to use sounds having frequencies an octave higher than the sounds having the pitch names in  FIG. 2 . 
   Harmonic Tone 
                             fmp   =     fm   ⨯   p                 =     fs   /     N   ⨯   m   ⨯   p                     (   4   )                 
where “Smp” denotes a harmonic tone signal of the p-th degree of the sinusoidal signal Sm and “fmp” denotes a frequency of the harmonic tone signal Smp. If p=1, fmp=fm and Smp=Sm.
 
   The harmonic tone signal Smp of the p-th degree is also a harmonic tone signal on the basis of a fundamental tone that is generated from the sinusoidal signal Sm. That is, the signal Sm is the fundamental tone signal and the signal Smp is the harmonic tone signal for the fundamental tone signal. 
   When the fundamental tone signal Sm is mixed with the harmonic tone signals Smp to reproduce sounds, the reproduced sounds have the same pitch but have different tones if the fundamental tone signal Sm has the constant frequency fm even though the harmonic tone signals Smp have different frequencies fmp. 
   Accordingly, supplying the mixed signals generated by mixing the fundamental tone signal Sm with multiple harmonic tone signals Smp having different degrees p to the speaker allows various frequency components to be supplied to the speaker. Even if the frequency characteristic of the speaker has a dip or a standing wave exists in the room, it is possible to correctly check whether the speaker is connected. 
   According to the embodiments of the present invention, the fundamental tone signal Sm is mixed with the multiple harmonic tone signals Smp to generate a test tone signal STT. 
   Frequency Components of Test Tone Signal STT 
     FIG. 3  is a table showing examples of the harmonic tone signals Smp included in the test tone signal STT. In the examples in  FIG. 3 , one fundamental tone signal Sm is mixed with five harmonic tone signals Smp. The first and second columns in  FIG. 3  show the pitch names and their values m of the sounds provided by the fundamental tone signal Sm of the test tone signal STT. The pitch names and their values m in the first and second columns in  FIG. 3  correspond to those in the third and first columns in  FIG. 2 . 
   Variables k in the third column show combination numbers of the fundamental tone signal Sm and the five harmonic tone signals Smp. Variables p in the fourth column show degrees of the harmonic tone signals Smp mixed with the fundamental tone signal Sm. For example, the pitch name A# has three values 1 to 3 for the variable k. As also shown in  FIG. 4 , if k=1, the fundamental tone signal S 20  (m=20) is mixed with the harmonic tone signals S 2002 , S 2004 , S 2011 , S 2020 , and S 2033  (p=2, 4, 11, 20, and 33) to generate the test tone signal STT. 
   If k=2, the fundamental tone signal S 20  (m=20) is mixed with the harmonic tone signals S 2002 , S 2005 , S 2010 , S 2017 , and S 2034  (p=2, 5, 10, 17, and 34) to generate the test tone signal STT. If k=3, the fundamental tone signal S 20  (m=20) is mixed with the harmonic tone signals S 2002 , S 2007 , S 2008 , S 2019 , and S 2032  (p=2, 7, 8, 19, and 32) to generate the test tone signal STT. 
   Referring to  FIG. 3 , when the combination k is varied for the same pitch name, the harmonic tone signal Smp in p=2 is fixed but only the four harmonic tone signals Smp (the harmonic tone signals Smp having the values other than two (p≠2)) having higher frequencies are varied in value p in order not to damage the image of the sounds that have the same pitch name but have different combinations of the harmonic tones (different variables k). 
   The frequency f2033 of the harmonic tone signal S 2033  in p=33 in k=1 of the pitch name A# (m=20) is calculated according to Equation (4) as follows: 
   
     
       
         
           
             f 
             ⁢ 
             
                 
             
             ⁢ 
             2033 
           
           = 
           
             
               48000 
               / 
               
                 4096 
                 ⨯ 
                 20 
                 ⨯ 
                 33 
               
             
             ⁢ 
             
               
 
             
             ⁢ 
             
                 
             
             ≈ 
             
               7734.4 
               ⁢ 
               
                   
               
               [ 
               Hz 
               ] 
             
           
         
       
     
   
   Referring to  FIG. 3 , the harmonic tone signal Smp having the highest frequency is the harmonic tone signal S 3634  in p=34 in k=2 of the pitch name G# (m=36) . The frequency f3634 of the harmonic tone signal S 3634  is calculated according to Equation (4) as follows: 
             f   ⁢           ⁢   3634     =       48000   /     4096   ⨯   36   ⨯   34       ⁢     
     ⁢           ≈     14343.8   ⁢           [   Hz   ]             
This means that the test tone signal STT includes the frequency components over a wide range in an audio frequency band.
 
   Since the sounds having the pitch names A and B, in  FIG. 3 , are not used, the degree p of the corresponding harmonic tone signal S 19   p  and S 21   p  is blank. For example, the pitch name C# does not have the combination in k=3 for the same reason. Conversely, the number of combinations, or variables k may be increased if more combinations are necessary for the sound having the pitch name A#. 
   Output Format of Test Tone Signal STT 
     FIG. 5A  shows a format (timing chart) when the test tone signal STT is output. The test tone signal STT is generated during a test period TT, which includes a preparation period TR, a check period TC, and a rendering period TE. 
   During the preparation period TR, the volume of a test tone that is to be output from the speaker during the subsequent check period TC is set to an appropriate value. During the check period TC, connection of the speaker of each channel is actually checked. The rendering period TE is used for rendering termination of the test tone and is not used for checking the connection of the speaker. 
   The preparation period TR, the check period TC, and the rendering period TE each include four unit periods TU. Each unit period TU has a length corresponding to the two cycles TN in  FIG. 1A , as shown in  FIG. 5B . The frequency component of the test tone signal STT is varied every unit period TU. 
   The test tone signal STT is generated by mixing the fundamental tone signal Sm with the harmonic tone signal Smp, and the number of cycles of the fundamental tone signal Sm and the harmonic tone signal Smp in the period TN is an integer. Accordingly, the phase of the test tone signal STT is smoothly varied even in a boundary between the periods TN in the unit period TU and in a boundary between the unit period TU and the subsequent unit period TU. 
   With the above numeric values, 
   
     
       
         
             
           
             
               
                 
                   TU 
                   = 
                   
                     TN 
                     ⨯ 
                     2 
                   
                 
               
             
             
               
                 
                   = 
                   
                     4096 
                     / 
                     
                       48000 
                       ⨯ 
                       2 
                     
                   
                 
               
             
             
               
                 
                   = 
                   
                     171 
                     ⁢ 
                     
                         
                     
                     [ 
                     
                       m 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       sec 
                     
                     ] 
                   
                 
               
             
           
         
       
     
   
   The test period TT is calculated by the following equation: 
   
     
       
         
             
           
             
               
                 
                   TT 
                   = 
                   
                     TR 
                     + 
                     TC 
                     + 
                     TE 
                   
                 
               
             
             
               
                 
                   = 
                   
                     TU 
                     ⨯ 
                     4 
                     ⨯ 
                     3 
                   
                 
               
             
             
               
                 
                   = 
                   
                     2.048 
                     ⁢ 
                     
                         
                     
                     [ 
                     sec 
                     ] 
                   
                 
               
             
           
         
       
     
   
   After the test tone signal STT is supplied to a speaker under test, the sound having the frequency component corresponding to the test tone signal STT is output from the speaker under test. After the sound output from the speaker under test is picked up with a microphone, the test tone signal STT is output from the microphone, as shown in  FIG. 5C  (the test tone signal STT output from the microphone is hereinafter referred to as a “reply signal STT”). In this case, the reply signal STT is delayed by a time Td corresponding to the distance between the speaker under test and the microphone with respect to the test tone signal STT (in  FIG. 5B ) supplied to the speaker. 
   Hence, as shown in  FIG. 5D , the frequency analysis of the reply signal STT over a predetermined analysis period TA for every unit period TU of the reply signal STT output from the microphone can check whether the speaker under test is connected and can also check the frequency characteristic etc. of the corresponding channel. 
   Since the same content is repeated twice during the two periods TN in the unit period TU of the reply signal STT output from the microphone, as shown in  FIG. 5C , there is room for the time position of the analysis period TA. Accordingly, for example, when the reply signal STT is output from the microphone, the frequency analysis of the reply signal STT may be started upon rising of the output reply signal STT. In this case, it is not necessary to strictly consider the delay time Td of the picked-up reply signal STT. 
   Since the test tone signal STT is generated by mixing the fundamental tone signal Sm with the harmonic tone signals Smp, making the analysis period TA equal to the period TN causes the number of cycles of the reply signal STT during the analysis period TA to be an integer. Hence, it is not necessary to execute the window function in the frequency analysis, thus simplifying the analysis. 
   Content of Test Tone Signal STT 
     FIG. 6  illustrates the relationship between audio channels and the pitch names of the sounds included in the test tone signal STT.  FIG. 6  illustrates 7.1-channel reproduction. The vertical axis represents the following channels: 
                                               C: center channel               L: left front channel   R: right front channel           LS: left surround channel   RS: right surround channel           LB: left rear channel   RB: right rear channel                        
The horizontal axis represents the test period TT including the preparation period TR, the check period TC, and the rendering period TE, each of which includes the four unit periods TU. The pitch name of the sound used for checking the speaker is shown in each cell in  FIG. 6 .
 
   For example, during the first unit period TU in the preparation period TR, the test tone signal STT includes the fundamental tone signal Sm having the pitch name G# and is supplied to the speaker of the center channel C. Accordingly, the sound of the pitch name G# is output from the speaker of the center channel C during the first unit period TU. 
   During the second unit period TU in the preparation period TR, the test tone signal STT includes the fundamental tone signals Sm having the pitch name F and pitch name G#. The test tone signal STT including the fundamental tone signal Sm having the pitch name F is supplied to the speaker of the left front channel L and the test tone signal STT including the fundamental tone signal Sm having the pitch name G# is supplied to the right front channel R. Accordingly, the sound of the pitch name F is output from the speaker of the left front channel L and the sound of the pitch name G# is output from the speaker of the right front channel R during the second unit period TU. 
   During the third unit period TU in the preparation period TR, the test tone signal STT includes the fundamental tone signals Sm having the pitch name C# and pitch name F. The test tone signal STT including the fundamental tone signal Sm having the pitch name C# is supplied to the speaker of the left surround channel LS and the test tone signal STT including the fundamental tone signal Sm having the pitch name F is supplied to the right surround channel RS. Accordingly, the sound of the pitch name C# is output from the speaker of the left surround channel LS and the sound of the pitch name F is output from the speaker of the right surround channel RS during the third unit period TU. 
   The test tone signal STT including the fundamental tone signals Sm having the corresponding pitch names is supplied to each channel in the same manner as described above. Hence, the sounds of the pitch names are output from the speakers of the channels in a pattern shown in  FIG. 6 . Referring to  FIG. 6 , the unit period TU in a blank cell has no signal (is mute). During a period TM having a length TU immediately before the test period TT, all the channels have no signal for a reason described below and all the channels are mute. 
   The frequencies of the fundamental tone signals Sm included in the test tone signal STT are varied so as to output the sounds having the pitch names shown in  FIG. 6  when the test tones are output from the speakers. In contrast, the variables k showing the combination numbers of the fundamental tone signal Sm and the harmonic tone signals Smp are varied in accordance with the numeric values shown in parentheses in  FIG. 6 . 
   Specifically, during the first unit period TU in the preparation period TR, the test tone signal STT having the pitch name G# is supplied to the center channel C, and the test tone signal STT during the first unit period TU is generated by mixing the fundamental tone signal Sm with the harmonic tone signals Smp in k=1. 
   During the second unit period TU in the preparation period TR, the test tone signal STT having the pitch name G# is supplied to the right front channel R, and the test tone signal STT during the second unit period TU is generated by mixing the fundamental tone signal Sm with the harmonic tone signals Smp in k=2. In addition, during the second unit period TU in the preparation period TR, the test tone signal STT having the pitch name F is supplied to the left front channel L, and the test tone signal STT during the second unit period TU is generated by mixing the fundamental tone signal Sm with the harmonic tone signals Smp in k=1. 
   Similarly, when the same pitch name is used, particularly when the sound having the same pitch name is used during the continuous two unit periods TU, as in the first and second unit periods TU in the preparation period TR, the variables k showing the combination numbers of the fundamental tone signal Sm and the harmonic tone signals Smp are varied in accordance with the numeric values shown in parentheses in  FIG. 6 . Accordingly, for example, although the sounds of the same pitch name G# are output during the first unit period TU and the second unit period TU in the preparation period TR, the signals output during the first and second unit periods TU have different frequency components and different tones. 
   Even when the sounds of the same pitch name are used during the continuous two unit periods TU, varying the variables k showing the combination numbers of the harmonic tone signals Smp allows the check to be more correctly performed. In other words, since a room where the audio reproduction is performed usually contains a certain amount of acoustic reverberation, the acoustic reverberation during one unit period TU sometimes remains until the analysis period TA ( FIG. 5D ) in the subsequent unit period TU. 
   However, varying the variables k showing the combination numbers every unit period TU, as described above, allows the acoustic reverberation during the previous unit period TU to be filtered in the analysis, so that it is possible to check whether the speaker is connected without being affected by the reverberation and, therefore, the connection can be correctly checked. 
   In order to vary the components of the test tone signal STT as shown in  FIG. 6 , a “tone frequency list” and a “tone sequence list” should be provided. The tone frequency list includes the correspondence between the pitch names and the variables m, p, and k, as shown in  FIG. 3 . The tone sequence list includes the correspondence between the channels, the pitch names, and the variables k for every unit period TU, as shown in  FIG. 6 . 
   Referring to the tone frequency list and the tone sequence list in the generation of the test tone signal STT to vary the variables m, p, and k for every channel and for every unit period TU allows the test tone to be output in the pattern in  FIG. 6 . 
   Background Noise and Method of Determining Speaker Connection 
   As shown in  FIG. 6 , all the channels has no signal and are mute during the period TM having a length of unit period TU immediately before the test period TT. This mute period TM is provided in order to avoid an effect of background noise on the checking of the connection of the speaker. 
   When the test tone output from the speaker is picked up and the reply signal STT yielded from the pickup of the test tone is analyzed to measure the level of each frequency component of the test tone, the analytical result (frequency components) contains the frequency component of the background noise. Accordingly, it is necessary to consider the frequency component of the background noise in the determination of the connection of the speaker from the analytical result of the test tone. An exemplary method of determining the connection of the speaker in consideration of the background noise will now be described. 
   First, the background noise during the mute period TM is picked up to perform the frequency analysis, the level of each frequency component is calculated, as shown in  FIG. 7B , and the calculated level is temporarily stored. Here, it is sufficient to store the levels of only the components of the frequencies equal to those of the fundamental tone signal Sm and the harmonic tone signals Smp included in the test tone signal STT and it is not necessary to store the levels of the components of other frequencies. The frequencies to be stored can be determined from the tone frequency list. 
   Next, during the preparation period TR, the test tone signal STT is supplied to the speaker under test and the test tone output from the speaker under test is picked up. The reply signal STT yielded from the pickup of the test tone is subjected to the frequency analysis and the level of each frequency component is calculated, as shown in  FIG. 7A . Referring to  FIG. 7A , signals Sx 1  to SX 6  show the frequency components of the fundamental tone signal Sm and the five harmonic tone signals Smp and the remaining frequency components are of the background noise. The signals Sx 1  to Sx 6  generally have different levels depending on the frequency characteristic of the speaker and include the frequency components of the background noise. 
   A signal to noise (S/N) ratio of the signal Sx 1  to a noise component N 1  having a frequency equal to that of the signal Sx 1 , among the noise components whose levels are stored ( FIG. 7B ), is calculated and the calculated S/N ratio is set as a value V 1 . Similarly, S/N ratios of the signals Sx 2  to Sx 6  to noise components having frequencies equal to those of the signals Sx 2  to Sx 6  are respectively calculated and the calculated S/N ratios are set as values V 2  to V 6 . If the signals Sx 1  to Sx 6  includes a signal Sx 1  (signal Sx 4  in FIG.  7 A) having a level less than a predetermined value VTH, the above S/N ratio is not calculated and the corresponding value Vi is set to zero. 
   Among the values V 1  to V 6 , a value Vj (j is any of one to six) having the highest S/N ratio is selected and the value Vj is compared with a predetermined value VREF. It is determined that the checked speaker is connected if Vj&gt;VREF and that the checked speaker is not connected if Vj≦VREF. 
   In the above method, the value of the highest S/N ratio, among the S/N ratios of the sinusoidal signal Sm and the harmonic tone signals Smp included in the reply signal STT to the noise components, is compared with the predetermined value VREF to determine whether the corresponding speaker is connected. Accordingly, it is possible to correctly determine whether the speaker is connected without being affected by the frequency characteristic of the speaker or the standing wave in the room. 
   When the acoustic reverberation is continued, it is preferable that the maximum values among the values V 3  to V 6 , instead of the maximum value among the values V 1  to V 6 , be compared with the predetermined value VREF, in consideration of the long decay time in lower frequencies. The comparison of the maximum value, among the values V 3  to V 6 , with the predetermined value VREF reduces the effect of the acoustic reverberation, thereby preventing erroneous determination to improve the accuracy of the determination. 
   Audio-Visual Reproducing Apparatus 
     FIG. 8  is a block diagram showing a sound field correction device  20  according to an embodiment of the present invention. In the example in  FIG. 8 , the sound field correction device  20  is included in an existing AV reproducing apparatus as an adaptor. 
   Example of Reproducing System 
   Referring to  FIG. 8 , the AV reproducing apparatus includes a signal source  11  of an AV signal, a display  12 , a digital amplifier  13 , and speakers  14 C to  14 RB. The signal source  11  is, for example, a digital versatile disk (DVD) player or a satellite tuner. In the example in  FIG. 8 , an output from the signal source  11  has a digital visual interface (DVI) format. A digital video signal DV and digital audio signals for the seven channels, which are encoded into one serial signal DA, are output from the signal source  11 . 
   The display  12  receives an input in the DVI format and normally receives the digital video signal DV output from the signal source  11 . The digital amplifier  13  is a class D amplifier. Specifically, the digital amplifier  13  also normally receives the digital audio signal DA output from the signal source  11 . The digital amplifier  13  separates the digital audio signal DA into signals for the respective channels and performs the class D amplification for the signals for the respective channels to output analog audio signals for the respective channels. 
   The audio signals output from the digital amplifier  13  are supplied to the speakers  14 C to  14 RB for the respective channels. The speakers  14 C to  14 RB are arranged at the center, the left front side, the right front side, the left side, the right side, the left rear side, and the right rear side, respectively. 
   Sound Field Correction Device 
   Exemplary Structure of Sound Field Correction Device 
   Referring to  FIG. 8 , the sound field correction device  20  according to the embodiment of the present invention is connected to a signal line between the signal source  11  and the display  12  and digital amplifier  13 . The digital video signal DV output from the signal source  11  is supplied to the display  12  through a delay circuit  21 . The delay circuit  21  is used for lip synchronization, which delays the digital video signal DV by a time corresponding to a delay time of the digital audio signal DA for the sound field correction to synchronize an image with the corresponding reproduced sound. The delay circuit  21  is, for example, a field memory. 
   In addition, in the sound field correction device  20 , the digital audio signal DA output from the signal source  11  is supplied to a decoder circuit  22 , where the digital audio signal DA is separated into digital audio signals DC to DRB for the respective channels. Among the digital audio signals resulting from the separation, the digital audio signal DC for the center channel is supplied to a correction circuit  23 C for the center channel. The correction circuit  23 C includes an equalizer circuit  231  and a switch circuit  232 . The digital audio signal DC supplied from the decoder circuit  22  is supplied to the switch circuit  232  through the equalizer circuit  231 . 
   In this case, the equalizer circuit  231  is, for example, a digital signal processor (DSP). The equalizer circuit  231  controls the delay, frequency, and phase characteristics and the level of the received digital audio signal DC to perform the sound field correction for the digital audio signal DC. The switch circuit  232  is connected in a manner shown in  FIG. 8  during normal watching and listening, and is connected in a state reverse to the state in  FIG. 8  when the connection of the speakers  14 C to  14 RB is checked. Accordingly, during the normal watching and listening, the audio signal DC subjected to the sound field correction, supplied from the equalizer circuit  231 , is output from the switch circuit  232 . The audio signal DC is supplied to an encoder circuit  24 . 
   Furthermore, the audio signals DL to DRB for the remaining channels, separated by the decoder circuit  22 , are supplied to the encoder circuit  24  through correction circuits  23 L to  23 RB. The correction circuits  23 L to  23 RB each have a structure similar to that of the correction circuit  23 C. Hence, during the normal watching and listening, the audio signals DL to DRB subjected to the sound field correction are output from the correction circuits  23 L to  23 RB. 
   In the encoder circuit  24 , the audio signals DC to DRB for the respective channels, supplied to the encoder circuit  24 , are mixed into one serial signal DS and the serial signal DS is supplied to the digital amplifier  13 . Hence, during the normal watching and listening, the digital audio signal DA supplied from the signal source  11  is subjected to the sound field correction in the correction circuits  23 C to  23 RB and is supplied to the speakers  14 C to  14 RB. As a result, a reproduced sound whose sound field is corrected to a state appropriate for the environment in which the speakers are arranged is output from the speakers  14 C to  14 RB. 
   In order to realize the sound field correction and the checking of whether the speakers  14 C to  14 RB are connected, a signal generating circuit  31  and a control circuit  32  are provided in the sound field correction device  20 . The signal generating circuit  31  is a DSP and generates the test tone signal STT during the test period TT, as described above. The control circuit  32  is a microcomputer. When the signal generating circuit  31  generates the test tone signal STT, the control circuit  32  refers to the tone frequency list and the tone sequence list to control generation of the test tone signal STT and determines whether the speakers are connected on the basis of the analytical result during the analysis period TA. 
   A microphone  33  is provided for picking up test tones output from the speakers  14 C to  14 RB. The reply signal STT output from the microphone  33  is supplied to an analog-to-digital (A/D) converter circuit  35  through a microphone amplifier  34 . The reply signal STT is converted into a digital signal in the A/D converter circuit  35 . 
   The digital signal is supplied to an analysis circuit  36 . The analysis circuit  36  is, for example, a DSP and performs frequency analysis for the test tone output from the speakers  14 C to  14 RB during the analysis period TA. The analytical result is supplied to the control circuit  32 . Control signals are supplied from the control circuit  32  to equalizer circuit  231 C to  231 RB and switch circuits  232 C to  232 RB in the correction circuits  23 C to  23 RB. In addition, various operation switches  37  are connected to the control circuit  32  and a display device, for example, a liquid crystal display (LCD) panel  38  in which the check results are displayed is also connected to the control circuit  32 . 
   Operation of Sound Field Correction Device  20   
   After a check switch among the operation switches  37  is operated, the mute period TM is started. During the mute period TM, the control circuit  32  causes the switch circuits  232 C to  232 RB in the correction circuits  23 C to  23 RB to be connected in the state reverse to the state in  FIG. 8 . The control circuit  32  controls the signal generating circuit  31  so that the test tone signal STT becomes a mute signal. Hence, no sound is output from the speakers  14 C to  14 RB. 
   The background noise during the mute period TM is picked up by the microphone  33 . At the same time, the signal of the background noise that has been picked up is subjected to the frequency analysis in the analysis circuit  36 , and the analytical result is supplied to the control circuit  32  and is stored therein. 
   Next, the sound field correction device  20  enters the test period TT. During the test period TT, the control circuit  32  causes the switch circuits  232 C to  232 RB in the correction circuits  23 C to  23 RB to be connected in the state reverse to the state in  FIG. 8 . The control circuit  32  controls the signal generating circuit  31  so that the test tone signal STT is generated, and the generated test tone signal STT is supplied to the switch circuits  232 C to  232 RB. The fundamental tone signal Sm and the harmonic tone signals Smp of the test tone signal STT are varied in the manner shown in  FIG. 3  because the variables m, p, and k are varied for every unit period TU in the manner shown in  FIG. 6 , and the combination of the fundamental tone signal Sm and the harmonic tone signals Smp is also varied. 
   The test tone signal STT is supplied to the encoder circuit  24  through the switch circuits  232 C to  232 RB. The test tone signal STT is mixed into one serial signal DS in the encoder circuit  24 , and the serial signal DS is supplied to the digital amplifier  13 . As a result, the test tone is output from the speakers  14 C to  14 RB in the sequence shown in  FIG. 6  during the preparation period TR, the check period TC, and the rendering period TE in the test period TT. 
   The test tone is picked up by the microphone  33 . The picked-up reply signal STT is subjected to the frequency analysis every analysis period TA in the analysis circuit  36  and the analytical result is supplied to the control circuit  32 . 
   Since the test tone signal STT during the preparation period TR is used for setting the output level from the speakers  14 C to  14 RB during the subsequent check period TC to an apparatus value, the level of the test tone signal STT is relatively low. The level of the test tone signal STT at this time can be determined in consideration of the analytical result of the background noise during the proximate mute period TM. 
   During the check period TC, it is determined whether the speaker of each channel is connected from the analytical result in the analysis circuit  36 . The determination result is supplied to the LCD panel  38  in which the connection states of the speakers  14 C to  14 RB are displayed. 
   During the rendering period TE, the control circuit  32  controls the equalizer circuits  231 C to  231 RB in the correction circuits  23 C to  23 RB based on the analytical result during the check period TC so that the sounds output from the speakers  14 C to  14 RB have, for example, flat frequency characteristics. 
   After the test period TT is terminated, the control circuit  32  causes the switch circuits  232 C to  232 RB in the correction circuits  23 C to  23 RB to be connected in the state shown in  FIG. 8 . The control circuit  32  also controls the signal generating circuit  31  so that the test tone signal STT becomes mute. Hence, it is possible to reproduce the video signal DV and the audio signal DA from the signal source  11 . 
   Example of Signal Generating Circuit  30   
     FIG. 9  shows an example in which the signal generating circuit  31  is structured as a separate circuit. In the example in  FIG. 9 , the digital data DD to be converted into one cycle of the sinusoidal signal S 1  shown in  FIG. 1A  is stored in a random access memory (ROM)  41 . The digital data DD is read out at a ratio of one address per m addresses of the ROM  41  during the period TN. This readout is repeated m times to extract the sinusoidal signal Sm that is stored in a memory  421 . 
   The sinusoidal signal Sm in the memory  421  is read out at a ratio of one address per p addresses of the memory  421 . This readout is repeated p times to extract the harmonic tone signals Smp. The extraction of the harmonic tone signals Smp is performed five times with the degree p being varied in the manner shown in  FIG. 3 . Specifically, since p=2, 4, 11, 20, or 33 if the pitch name is A# and k=1, the harmonic tone signal Smp is extracted with p being equal to two in the first extraction, the harmonic tone signal Smp is extracted with p being equal to four in the second extraction, . . . , and the harmonic tone signal Smp is extracted with p being equal to 33 in the fifth extraction. 
   The harmonic tone signal Smp in the first extraction is stored in a memory  422 , the harmonic tone signal Smp in the second extraction is stored in a memory  423 , . . . and the harmonic tone signal Smp in the fifth extraction is stored in a memory  426 . Accordingly, the sinusoidal signal Sm and the five harmonic tone signals Smp are concurrently stored in the memories  421  to  246 . 
   The sinusoidal signal Sm and the harmonic tone signals Smp in the memories  421  to  426  are concurrently read out every period TN, and the readout sinusoidal signal Sm and harmonic tone signals Smp are subjected to level adjustment in level adjustment circuits  431  to  436  and are supplied to an adder circuit  44 . The sinusoidal signal Sm and harmonic tone signals Smp are added in the adder circuit  44  and the added signal is extracted through a terminal  45 . The signal extracted through the terminal  45  is distributed to the corresponding channel by a distribution circuit (not shown) and is output as the test tone signal STT. 
   The signal extracted through the terminal  45  corresponds to one channel of the test tone signal STT. In the examples in  FIGS. 3 and 6 , the test tone signals STT for three channels are concurrently processed. Hence, the signal generating circuits  31  in  FIG. 9  for further two channels are provided and a signal resulting from mixing the added signals for the three channels is used as the test tone signal STT. When the signal generating circuit  31  is a DSP or a central processing unit (CPU), the processing in the memory  421  and the components downstream thereof should be performed for the digital data DD in the ROM  41 . 
   Software for Checking Speaker Connection 
     FIG. 10  shows a routine  100  executed by the control circuit  32  in the above determination of whether the speaker is connected. The routine  100  includes the frequency analysis performed in the analysis circuit  36  (hence, the analysis circuit  36  is not connected). 
   When a check switch, among the operation switches  37 , is operated, in Step S 101 , the routine  100  in the control circuit  32  is started (start of the mute period TM). In Step S 102 , it is presumed that no speaker is connected for all the channels that can be processed by the sound field correction device  20 . 
   In Step S 103 , the background noise signal output from the A/D converter circuit  35  is supplied to the control circuit  32 . In Step S 104 , the supplied background noise signal is subjected to the frequency analysis to measure the level of the background noise for every frequency component. In Step S 105 , the level of the background noise for every frequency component, measured in Step S 104 , is compared with a predetermined noise level VNL. This comparison should be performed for the frequency components having the frequencies equal to those of the sinusoidal signal Sm and harmonic tone signals Smp included in the test tone signal STT by referring to the tone frequency list. 
   In Step S 106 , the control circuit  32  determines whether the comparison result is less than the predetermined noise level VNL. If the noise level of any of the frequency components is less than the predetermined noise level VNL, the routine  100  proceeds from Step S 106  to Step S 111 . In Step S 111 , the noise level for every frequency component, measured in Step S 104 , is stored in a memory in the control circuit  32  (termination of the mute period TM). 
   In Step S 112 , the signal generating circuit  31  is controlled in accordance with the tone sequence list and the tone frequency list to generate the test tone signal STT over the period from the preparation period TR to the rendering period TE, and the generated test tone signal STT is supplied to the digital amplifier  13 . In Step S 113 , the routine  100  is terminated (termination of the rendering period TE). 
   If the control circuit  32  determines in Step S 106  that the noise levels of all the frequency components exceed the predetermined noise level VNL, the routine  100  proceeds from Step S 106  to Step S 107 . In Step S 107 , the control circuit  32  determines whether the number of times the background noise level is measured (measurement for every mute period TM) reaches a predetermined value. If the number of times the background noise level is measured does not reach the predetermined value, the routine  100  goes back from Step S 107  to S 102  to repeat the measurement of the background noise level for every frequency component. 
   If the control circuit  32  determines in Step S 107  that the number of times the background noise level is measured reaches the predetermined value, the routine  100  proceeds from Step S 107  to S 108 . In Step S 108 , for example, the control circuit  32  displays the necessity to improve the environment to reduce the background noise in the LCD panel  38 . Then, in Step S 113 , the routine  100  is terminated. 
   A routine  120  shown in  FIG. 11  is executed at timings shown in  FIG. 5  in parallel with the generation of the test tone signal STT in Step S 112 . Referring to  FIG. 11 , in Step S 121 , the routine  120  is started. In Step S 122 , the reply signal STT output from the A/D converter circuit  35  is supplied to the control circuit  32  and is subjected to the frequency analysis during the analysis period TA. In Step S 123 , the frequency components subjected to the frequency analysis in Step S 122  is subjected to frequency separation for every speaker (channel). This frequency separation is performed by referring to the tone frequency list and the tone sequence list. 
   In Step S 124 , the level of each frequency component, separated in Step S 123 , is compared with the predetermined value VTH ( FIG. 7A ) for every speaker. If the level of the frequency component is higher than the predetermined value VTH, the routine  120  proceeds from Step S 124  to Step S 125 . If the level of the frequency component is lower than the predetermined value VTH, the routine  120  proceeds from Step S 124  to Step S 126 . 
   In Step S 125 , the level of the frequency component, separated in Step S 123 , is compared with the level of the frequency component of the background noise, stored in Step S 111 , and the S/N ratios (the values V 1  to V 6 : the values V 3  to V 6  for a higher accuracy) are calculated for every frequency component of the test tone signal STT. In Step S 126 , the test tone signal STT having the highest S/N ratio is extracted from the S/N ratios calculated in Step S 125 . In Step S 127 , the highest S/N ratio (value Vj), extracted in Step S 126 , is compared with the predetermined value VREF. 
   As described above, the comparison shows that the speaker under test is connected if Vj&gt;VREF and that the speaker under test is not connected if Vj≦VREF. In Step S 128 , the determination result is supplied to the LCD panel  38  and the connection states of the speakers  14 C to  14 RB are displayed in the LCD panel  38 . In Step S 129 , the routine  120  is terminated. 
   It is possible to determine whether the speaker of each channel is connected in the routines  100  and  120 . 
   Summary 
   Since the test tone formed of the test tone signal STT composes a melody in the sound field correction device  20  described above, the test tone does not make a listener uncomfortable, unlike the pink noise. In addition, since the test tone signal STT is composed of the sinusoidal signal Sm and the harmonic tone signals Smp, the test tone signal STT includes various frequency components. As a result, it is possible to correctly check whether the speakers  14 C to  14 RB are connected even if the frequency characteristics of the speakers  14 C to  14 RB have dips or the standing wave exists in the room. 
   Because of the test tone signal STT including various frequency components, the analytical result can be used to check the frequency characteristics of sounds output from the speakers  14 C to  14 RB or correct the frequency characteristics. In addition, since the combination k of the sinusoidal signal Sm and the harmonic tone signals Smp included in the test tone signal STT is varied every unit period TU, the connection of the speakers  14 C to  14 RB can be checked in the analysis without being affected by the reverberation in the previous unit period TU, thus realizing the correct checking. 
   Since the unit period TU of the test tone signal STT corresponds to m cycles of the sinusoidal signal Sm, the test period TT can be set to around two seconds. Accordingly, stress is not applied to the listener not only when the checking of the connection is instructed with the operation switches  37  but also when the connection of the speakers  14 C to  14 RB is checked each time the AV apparatus or the sound field correction device  20  is turned on. On the contrary, the test tone composing a melody can be used as an opening sound indicating the startup of the apparatus. 
   Others 
   The sound field correction device  20  shown in  FIG. 8  may be integrated with the signal source  11 , the digital amplifier  13 , or an AV amplifier (not shown). The digital audio signals DC to DRB output from the correction circuits  23 C to  23 RB may be supplied to a downstream amplifier directly or after being subjected to digital-to-analog (D/A) conversion. 
   The processing in the signal generating circuit  31  and the analysis circuit  36  may be realized by a microcomputer serving as the control circuit  32 . 
   It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.