Patent Publication Number: US-2005123153-A1

Title: Signal compression/expansion device and mobile communication terminal

Description:
BACKGROUND OF THE INVENTION  
      1. Filed of the Invention  
      The present invention relates to a signal compression/expansion device, and more particularly to a signal compression/expansion device which controls an input signal with a gain in accordance with its level and a mobile communication terminal to which the signal compression/expansion device is applied.  
      2. Description of the Related Art  
      When voice signals and audio signals are amplified by an amplifier to be outputted to a speaker, a dynamic range is important. When a signal having large amplitude is inputted to the amplifier and the speaker, a waveform of the outputted signal is significantly distorted. When the sound with such distorted waveform is listened to, a listener feels it unpleasant. In order to solve such a problem, an automatic gain controller (AGC) has been generally used. In the automatic gain controller, when a signal reaches a predetermined level, a gain of an amplifier is decreased. As a result, an effective dynamic range of the amplifier is made wider. It should be noted that the “dynamic range” means a ratio of a level at which a signal/noise ratio (S/N ratio) has a predetermined value to a maximum level at which a signal saturates.  
      Particularly, a speaker built in a mobile communication terminal such as a cellular phone has an extremely smaller size than a speaker of a general an audio system, and its dynamic range and frequency characteristic are poor. Furthermore, a dynamic range of an amplifier for a speaker is narrow because of a limitation to a power supply. Accordingly, in a speaker built in a cellular phone, loud sound is distorted, and a low frequency area of the sound lacks sound pressure. Thus, a frequency characteristic has a peak in a high frequency range. Therefore, the automatic gain controller effectively widens a dynamic range. However, in a conventional automatic gain controller, an automatic gain control (that is, compression processing) has been performed for all ranges of a frequency band of an input signal, and a uniform dynamic range is formed.  
      Japanese Patent Laid-Open No. 2001-308730 discloses a digital receiver capable of widening a dynamic range of an A/D converter over a wide frequency band. In this receiver, a receiving signal over a wide band is divided by N band-pass filters. Furthermore, an automatic gain control amplifier amplifies the receiving signal so that the maximum signal level of an output of each band-pass filter becomes equal to 1/N of the maximum input level of the A/D converter, and adds the amplified receiving signals. As a result, signals having a very high level, which are included in the receiving signals, are suppressed, and signals having a very low level, which are included therein, are amplified: The digital receiver adjusts an amplification rate for each band of the input signal based on a signal level for each band, and forms a dynamic range. However, even if the foregoing technology is applied to a cellular phone showing poor dynamic range and frequency characteristic, it is difficult to obtain preferable sound from a speaker of the cellular phone.  
     SUMMARY OF THE INVENTION  
      A signal compression/expansion device of the present invention comprises a frequency band division circuit, a compression/expansion circuit and a band synthesis circuit. The frequency band division circuit divides an input signal into a plurality of frequency bands. The compression/expansion circuit receives the input signal of each frequency band, and controls a level of the input signal based on a predetermined gain. At least one compression/expansion circuit applies a different gain to the input signal in accordance with a level thereof. The band synthesis circuit adds output signals from the respective compression/expansion circuit. The band synthesis circuit adds outputted signals from the respective compression/expansion circuits.  
      A mobile communication terminal of the present invention comprises a sound circuit, the signal compression/expansion device having the foregoing constitution and a speaker for converting a signal, which is outputted from a variable gain amplification circuit, to a sound.  
      The signal compression/expansion device of the present invention controls a level of each signal having being subjected to a band division with a predetermined gain, in accordance with a dynamic range of an amplifier and an audio system. Therefore, it is possible to suppress occurrence of distortion, and to output a preferable sound from a speaker.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and advantages of the present invention will become apparent from the following detailed description when taken with the accompanying drawings in which:  
       FIG. 1  is a block diagram of a signal compression/expansion device of an embodiment of the present invention;  
       FIG. 2  is a graph showing a frequency characteristic of a frequency band division circuit;  
       FIGS. 3A and 3B  are block diagrams of an embodiment of a compression/expansion circuit;  
       FIGS. 4A, 4B ,  4 C and  4 D are graphs showing examples of an I/O characteristic of the compression/expansion circuit;  
       FIG. 5  is a block diagram showing an embodiment of a mobile communication terminal; and  
       FIG. 6  is a block diagram of the signal compression/expansion device of another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Preferred embodiments of a signal compression/expansion device of the present invention are described below. Referring to  FIG. 1 , a frequency band division circuit  10  receives a signal through an input terminal  40 , and divides the signal into N frequency bands. Then, the frequency band division circuit  10  outputs signals  41 ,  42 ,  43 , . . . , and  4 N corresponding to the respective frequency bands. A compression/expansion circuits  21 ,  22 ,  23 , . . . , and  2 N respectively receive the signals  41 ,  42 ,  43 , . . . , and  4 N. The respective compression/expansion circuits adjust levels of the input signals in accordance with their levels, and output the signals. The compression/expansion circuits are capable of possessing intrinsic I/O characteristics respectively, and output the signals after compressing/expanding levels of the input signals based on the respective I/O characteristics. The compression can mean that the input signal is output in a state where its level is suppressed. The expansion can mean that the input signal is output in a state where its level is elevated. A frequency band synthesis circuit  30  adds the signals  51 ,  52 ,  53 , . . . and  5 N, which are outputted from the respective compression/expansion circuits  21 ,  22 ,  23 , . . . and  2 N, and outputs an addition result to an output terminal  50 . The output terminal  50  is connected to a speaker or an amplifier for driving the speaker, which are not illustrated in  FIG. 1 .  
      Herein, in at least one circuit among the compression/expansion circuits  21 ,  22 ,  23 , . . . and  2 N, a gain applied to an input signal smaller than a predetermined level (threshold value) differs from that applied to another input signal higher than the predetermined level. A compression/expansion circuit which compresses/expands the input signal with a constant gain can be also arranged. Note that a “level” does not mean an instantaneous value of a signal but means an effective value or a value equivalent to a short-time average such as a power or an envelope signal. The compression/expansion circuit variously compresses/expands levels of signals in respective frequency bands in accordance with a dynamic range of a speaker or an amplifier for driving the speaker. As a result, occurrence of distortion of an acoustic signal outputted from the speaker is suppressed.  
       FIG. 2  is a graph illustrating an example of a frequency characteristic of the frequency band division circuit  10 . The circuit  10  divides the input signal to N bands including the frequency bands F 1 , F 2 , . . . , FN−1, and FN, and output the signals. The circuit  10  has N band pass filters, for example.  
       FIG. 3A  and  FIG. 3B  show examples of the compression/expansion circuit. The compression/expansion circuit has a variable amplification circuit  60  and a level detection circuit  61 . Referring to  FIG. 3A , the compression/expansion circuit has a feed-forward type of configuration. A signal  4 K is inputted to the variable amplification circuit  60  and the level detection circuit  61 . The level detection circuit  61  monitors a signal level of the input signal  4 K, and compares the signal level with a predetermined threshold value. Thereafter, the level detection circuit  61  outputs a comparison result to the variable amplification circuit  60 . The variable amplification circuit  60  changes its amplification rate (that is, gain) in accordance with the signal received from the level detection circuit  61 . An increase in the rate corresponds to an expansion of the input signal, and a decrease in the rate corresponds to a compression of the input signal. Alternatively, when the rate is smaller than one, the signal can be compressed. When the rate is larger than one, the signal can be expanded. The variable amplification circuit  60  outputs a signal  5 K to the frequency synthesis circuit  30 . Referring to  FIG. 3B , the compression/expansion circuit has a feedback type of configuration. The input signal  4 K is inputted to the variable amplification circuit  60 . The level detection circuit  61  monitors a signal level of the signal  5 K outputted from the variable amplification circuit  60 , and outputs a monitor result to the variable amplification circuit  60 . The variable amplification circuit  60  changes its amplification rate (that is, gain) in accordance with the monitor result which is outputted from the level detection circuit  61 . The level detection circuit  61  is capable of comparing the level of the output signal of the variable amplification circuit  60  with a predetermined threshold value, and of outputting a comparison result to the variable amplification circuit. Alternatively, the level detection circuit  61  is capable of outputting the detected level of the output signal of the variable amplification circuit  60  to the variable amplification circuit  60 .  
      An example of an I/O characteristic of the compression/expansion circuit is shown below. In  FIG. 4A , when a level of an input signal exceeds a threshold value th 0 , an amplification rate (i.e. gain) decreases. When the level of the input signal exceeds the threshold value th 0 , the level of the input signal can be compressed. In  FIG. 4B , the amplification rate does not change, and a level of an output signal changes linearly with respect to the level of the input signal. There is a chance that the input signal may not be compressed/expanded. In  FIG. 4C , when the level of the input signal exceeds the threshold value th 1 , the amplification rate increases. Alternatively, when the level of the input signal is equal to the threshold value th 1  or less, there is a chance that the level of the output signal may be compressed. When the level of the input signal is equal to the threshold value th 1  or more, there is a chance that the level of the output signal may not be compressed/expanded. In  FIG. 4D , when the level of the input signal exceeds a threshold value th 2 , the amplification rate decreases. When the level of the input signal is equal to the threshold value th 2  or less, there is a chance that the level of the input signal may be expanded. When the level of the input signal exceeds the threshold value th 2 , there is a chance that the level of the input signal may be compressed. The compression/expansion circuit can have a plurality of threshold values.  
       FIG. 5  shows an example of a mobile communication terminal loading the foregoing signal compression/expansion device. The mobile communication terminal  5  comprises a sound source circuit  2 , a signal compression/expansion device  1 , an amplification circuit  3  and a speaker  4 . The sound source circuit  2  generates a sound source signal such as incoming call sound (e.g. incoming call melody). An output signal from the sound source circuit  2  is inputted to the signal compression/expansion device  1 , and compressed/expanded depending on respective frequency bands. The amplification circuit  3  amplifies the signal compressed/expanded to drive the speaker  4 . Note that if the level of the output signal of the signal compression/expansion device  1  is high enough to drive the speaker  4 , the amplification circuit  3  needs not to be provided.  
       FIG. 6  shows another example of the signal compression/expansion device of the present invention. The frequency band division circuit  10  divides the input signal into three frequency bands. The compression/expansion circuit  21  corresponding to the low frequency band F 1  has an I/O characteristic shown in  FIG. 4D . This compression/expansion circuit  21  expands the input signal with a high amplification rate in order to compensate sound pressure of the speaker at a low frequency band. Another reason why this operation is performed is because it is difficult for a human to listen to a low frequency signal of a small sound volume. The compression/expansion circuit  22  corresponding to the intermediate frequency band F 2  has an I/O characteristic shown in  FIG. 4A . In this I/O characteristic, a threshold value th 4  is set to a region of a large input level (that is, th 4 &gt;th 3 ). Since distortion occurs in a region where a level of the input signal is high, a compression characteristic is desirable. The compression/expansion circuit  23  corresponding to the high frequency band F 3  has an I/O characteristic shown in  FIG. 4A . However, since a high frequency band sometimes has a peak, a threshold value th 5  is set to a region of a low input level. When the level of the input signal becomes high, sound in a high frequency is emphasized. To prevent this phenomenon, the compression characteristic is preferable. In addition, an amplification rate of the compression/expansion circuit can be set to a low value so that the output level equal to the threshold value th 5  or less is made to be low.  
      The signal compression/expansion device can be configured as a device for processing an analog signal. Furthermore, the signal compression/expansion device can be realized by a digital signal processing device. For example, the frequency band division circuit  10  is realized by a Fourier transform circuit, and the band synthesis circuit  30  is realized by an inverse Fourier transform circuit. Thus, it is possible to execute operations of the compression/expansion circuits  21 ,  22 ,  23 , . . . and  2 N by the digital signal processing.  
      While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by the present invention is not limited to those specific embodiments. On the contrary, it is intended to include all alternatives, modifications, and equivalents as can be included within the spirit and scope of the following claims.