Abstract:
To improve the aural sensation from an input signal of an audio reproduction system a bandwidth extension device can be used. The bandwidth extension device includes a first and a second signal path and an analyzer for selecting the created signal which fits most for the current input signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a bandwidth extension device comprising an input for receiving an input signal, filter means for filtering the input signal, creating means for creating an adapted signal with a lower frequency part than the input signal. 
     The invention further relates to an audio reproduction system comprising such a bandwidth extension device. 
     2. Description of the Related Art 
     Such a device is known from European Patent Application No. EP-A-0240286, corresponding to U.S. Pat. No. 4,790,014. To improve the aural sensation in low-pitched (that is: signals in the very low frequency band) sound reproduction by an audio reproduction system or the like, a sub-harmonics generator is used to create this low-pitched signal. In this way, a lower pitch signal is created than is present in the incoming signal. 
     A disadvantage of the known sub-harmonics generator and the known bandwidth extension device is that the amplitude of the sub-harmonics generator is not linear with the input voltage. Known sub-harmonics generators usually clip the input signal and then divide the signal by two, for example, with a flip-flop, also, for example, a phase-locked loop is used to obtain the sub-harmonics. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to improve such a bandwidth extension device and to obtain such a bandwidth extension device, which is simpler. To this end a first aspect of the invention provides a bandwidth extension system as described above, characterized in that the bandwidth extension device comprises at least a first and a second signal path each having filter means and creating means for creating a first and a second adapted signal, analyze means for comparing the first and second adapted signal with the input signal, and summing and weighting means for selecting the adapted signal which corresponds most with the input signal. 
     In this way, a bandwidth extension system is obtained which supplies a low-pitched signal part with an amplitude proportional with the original signal. 
     Further, the created sub-harmonics have no discontinuities as opposed to the sub-harmonics of the prior art bandwidth extension device which have discontinuities. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention and additional features, which may optionally be used to implement the invention to advantage, will be apparent from and elucidated with reference to the examples described below and hereinafter and shown in the figures, in which: 
         FIG. 1  shows an example of a bandwidth extension device according to the invention; 
         FIG. 2  shows first diagrams of the harmonics streams of a signal in the frequency domain; 
         FIG. 3  shows second diagrams of harmonics streams of a signal in the frequency domain; 
         FIG. 4  shows a second example of a bandwidth extension device according to the invention; 
         FIG. 5  shows a third example of a bandwidth extension device according to the invention; and 
         FIG. 6  shows an example of an audio reproduction system according to the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows an example of a bandwidth extension device BD according to the invention. On an input I, the bandwidth extension device receives an input signal, for example, an audio signal. The input I is coupled to a first and a second band-pass filter BPF 11  and BPF 12 , respectively. These band-pass filter form two band-limited parts from the (narrow-band) input signal, for example, one of 300-600 Hz (BPF 11 ) and one of 300-450 Hz (BPF 12 ). 
     The output of the band-pass filter BPF 11  is coupled to a harmonics generator HG 1  and the output of the band-pass filter BPF 12  is coupled to a harmonics generator HG 2 . 
     The two harmonics generators HG 1  and HG 2  respectively produce two separate streams of harmonics of the selected frequency bands. Frequency components in the first band will be extracted as if the lowest harmonic in the narrow-band signal is the second harmonic, frequency components in the second band will be extracted as if the lowest harmonic in the narrow-band signal is the third harmonic, in this example. 
     The output of the harmonics generator HG 1  is coupled to an input of switching means SM and the output of the harmonics generator HG 2  is coupled to the other input of the switching means. The output of the switching means is coupled via a band-pass filter BPF 2  to an output O of the bandwidth extension device BD. 
     It is not necessary that the filtering (BPF 2 ) is equal for both branches, therefore instead of the band-pass filter BPF 2  after the switching means, it is also possible to have two separate band-pass filters placed in both branches just before the switch means SM. 
     Analyze means AM is used to decide which of the two harmonics streams in the frequency domain is the most appropriate one. The analyze means supplies, as decision signal, a signal ss to the switching means SM. The analyze means receives the output signals from both harmonics generators, and further the analyze means receives the input signal from the input I and make a decision on the basis hereof. 
     Of course, this decision is not valid for the whole excerpt, so this “best fit decision” should be made every so many samples. Two examples of harmonics streams in the frequency domain are given in  FIGS. 2 and 3 . 
     It appeared that it is not necessary to choose which of the harmonics generator gives the best results, when dealing with unvoiced speech. For ease in this example, unvoiced speech is treated the same as voiced speech. 
     To save computing time, FFT can be used, which means that the number of samples within the input signal should be a power of 2. 
     A possible way to decide whether two signals contain similar information is to take their cross-correlation function. Two signals with similar information are maximally correlated with each other. 
     The above can be used to select the harmonics stream in the frequency domain, which has the highest correlation with the narrow-band input signal. The harmonics stream consists of a fundamental and its higher harmonics. If the higher harmonics of the narrow-band signal are (roughly) the same as those of one of the harmonics streams, then that stream will most likely contain the right sub-harmonics too. This is in principle only valid for voiced speech. 
       FIG. 2  shows an example of the harmonics streams in the frequency domain. A wide-band ( 1 ) signal is filtered to obtain a narrow-band signal ( 2 ). Of this signal, the first frequency component ( 3 ) is taken to generate two streams of harmonics, one as if this component was the second harmonic of the wide-band signal ( 4   a ) and one as if it was the third ( 4   b ). Comparison of the plot ( 2 ) with plots ( 5   a ) and ( 5   b ) define which stream is most correlated and will most probably contain the right sub-harmonics ( 6   a ) and ( 6   b ). In this case ( 6   b ) fits best. 
     The whole spectra are used for this decision. All signals in consideration are, therefore, band-limited from 300 to 1200 Hz. 
     Since the energy of the harmonics streams is not equal, it may be necessary to have some kind of normalization. This can be done by dividing both cross-correlation functions by the square root of the harmonics streams&#39; auto-correlation function of so-called lag zero. 
     All processing, such as filtering and generating harmonics, introduces delay and phase distortion. Hence, the cross-correlation function does not have to be symmetric around the origin. As a decision value, use is made of the maximum of the cross-correlation function. 
       FIG. 3  shows another example of the harmonics streams in the frequency domain. Also, in this example, the wide-band ( 1 ) signal is filtered to a narrow-band signal ( 2 ). Also here, of this signal, the first frequency component ( 3 ) is taken to generated two streams of harmonics, one as if this component was the second harmonic of the wide-band signal ( 4   a ) and one as if it was the third ( 4   b ). In this example, stream  6   a  fits best. 
       FIG. 4  shows a second example of a bandwidth extension system BD 4 . At the input I 4 , the bandwidth extension device receives an input signal, for example, an audio signal. This input I 4  is coupled to n band-pass filters BPF 411  . . . BPF 41   n . The band-pass filters have, in this example, the following frequency ranges: 
     BPF 411 : 300-400 Hz 
     BPF 412 : 350-450 Hz . . . 
     BPF 41   n: . . . .    
     The output of each band-pass filter is coupled to an input of switching means SM 4 . The output of the switching means is coupled to the harmonics generator HG 4 . The output of the harmonics generator is coupled, via a band-pass filter BPF 42 , to the output  04  of the bandwidth extension device BD 4 . 
     The input I 4  is also coupled to analyze means AM 4  for analyzing the input signal and supplying a switching signal ss 4  to the switching means SM 4  and a control signal hs 4  to the harmonics generator HG 4 . 
     In this way, it is possible to create more opportunities for choosing the part of lower part of the frequency band for which the creation of sub-harmonics fits best by this input signal. 
       FIG. 5  describes a third example of a bandwidth extension device BD 5  according to the invention. At the input I 5 , the bandwidth extension device receives an signal. Also, in this example, two signal paths are available. The first one comprising a band-path filter BPF 511  and a mixer MIX 51  and a second path comprising a band-pass filter BPF 512  and a mixer MIX 512 . 
     In this example, the analyze means AM 5  also receives the input signal and the signals after the band-pass filters. Here, the analyze means supplies a first control signal w 1  and a second control signal w 2 . These control signals are supplied to the respective mixers MIX 51  and MIX 52 . The outputs of the mixers are coupled to a summing device SUMS for summing the two weighted signals and supplying, via the band-pass filter BPF 52 , the summed weighted signals to the output O 5  of the bandwidth extension device BD 5 . 
       FIG. 6  shows an example of an audio reproduction system AS 6  comprising a bandwidth extension device BD 6  according to the invention. The audio reproduction system further comprises an input I 6  for receiving an input signal. The input is coupled to a time delay device TD 6  for compensating for the delay of the bandwidth extension device, the bandwidth extension device supplies, in this example, a control signal d 6  to the time delay device. The output of the time delay device and the output of the bandwidth extension device are coupled to a summing device SUM 6  for summing the (delayed) original signal and the bandwidth extended low frequency signal, at the output ASO 6  of the audio reproduction system AS 6 . 
     It should be noted that above the invention has been described on the basis of some examples. 
     The man skilled in the art is well aware of alternatives within the scope of the invention. 
     For example, it is also possible to send information about the optimal filtering together with the signal to the “receiver”. This is known in the art “non-blind” versus “blind” as described before. In that case, the analyze means only has to decode the information about which filer is optimal for this signal and supply, for example, the control signals w 1  and w 2  (see  FIG. 5 ) to the mixers. 
     Further, instead of using the time domain, the invention can also be used in the frequency domain using the inverse Fourier Transform (H H*9ejw)). 
     Above are examples described (see  FIGS. 2 and 3 ) with divide factors  2  and  3 . Of course, also higher factors can be used.