Abstract:
This invention intends to provide a tone signal detection circuit that accurately detects a tone signal regardless of a reception electric field strength. To accomplish this object, the tone signal detection circuit according to the invention is comprised of a first filter that extracts a signal of a specific frequency from an input signal, a second filter that excludes the signal of the specific frequency from the input signal and extracts signals of frequencies near the specific frequency, a subtractor that subtracts an output of the second filter from an output of the first filter, and a judgment circuit to judge whether the tone signal is present or not in the input signal on the basis of an output from the subtractor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a circuit that detects a tone signal. Specifically, the invention relates to a tone signal detection circuit used in an FM receiver or an FM portable telephone. 
     2. Description of the Related Art 
     A communication system using an FM receiver, FM portable telephone, or the like employs a tone signal in order to determine if a call is permissible. 
     The operation of a conventional tone signal detection means is as follows. First, a bandpass filter extracts a signal of a desired tone frequency band from an input signal. The extracted signal of the desired tone frequency band is then converted into a signal of a level corresponding to the amplitude of the extracted signal. When the level is more than a predetermined threshold, the input signal is judged to contain the tone signal. 
     However, the foregoing conventional detection means cannot discriminate noise from the tone signal, in case there are noise components near the passband of the bandpass filter, especially in FM radio communication systems in which, as the reception electric field strength decreases, the white noises of the signal increase and the signal decreases. From such circumstances, a tone signal detection circuit has been expected which can accurately detect a tone signal regardless of the reception electric field strength. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a tone signal detection circuit that accurately detects the tone signal regardless of the reception electric field strength. 
     In order to accomplish the foregoing object, the tone signal detection circuit according to the invention is comprised of: a first filter that extracts a signal of a specific frequency from an input signal, a second filter that excludes the signal of the specific frequency from the input signal and extracts signals of other frequencies near the specific frequency, a subtractor that subtracts an output of the second filter from an output of the first filter, and a judgment circuit that judges the presence of the tone signal in the input signal on the basis of an output from the subtractor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a block diagram to illustrate an example of the tone signal detection circuit according to the present invention; 
     FIG. 2 is a chart to explain the filter characteristic in the tone signal detection circuit according to the present invention; 
     FIG. 3 is a chart to compare the operation of the tone signal detection circuit according to the invention and that of the conventional detection circuit; 
     FIG. 4 is a block diagram to illustrate a second example of the tone signal detection circuit according to the present invention; 
     FIG. 5 is a block diagram to illustrate a third example of the tone signal detection circuit according to the present invention; 
     FIG. 6 is a block diagram to illustrate a fourth example of the tone signal detection circuit according to the present invention; 
     FIG. 7 is a block diagram to illustrate a fifth example of the tone signal detection circuit according to the present invention; 
     FIG. 8 is a block diagram of judgment circuit; 
     FIG. 9 is a chart to explain a first threshold and a second threshold in the judgment circuit of FIG. 8; and 
     FIG. 10 is a block diagram to illustrate a sixth example of the tone signal detection circuit according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Preferred embodiments of the present invention will hereafter be described in detail with reference to the accompanying drawings. 
     A first example of the tone signal detection circuit according to the present invention will be described with reference to FIG.  1 . 
     The concrete example 1 of the tone signal detection circuit consists of a first filter  1 , a second filter  2 , a first level converter  3 , a second level converter  4 , a judgment circuit  5 , and a subtractor  6 . 
     The first filter  1  extracts and outputs a signal including the frequency of a predetermined tone signal. The second filter  2  passes signals that include other frequencies near the frequency detected by the first filter  1  but exclude the detected frequency itself. The first level converter  3  measures a level of the output signal from the filter  1 . The second level converter  4  measures a level of the output signal from the filter  2 . 
     The subtractor  6  subtracts an output of the second level converter  4  from an output of the first level converter  3 . The judgment circuit  5  judges on the basis of an output from the subtractor  6 , for example, that the tone signal is present when the output from the subtractor  6  is more than a specific level, and that the tone signal is not present when the output from the subtract or  6  is less than the specific level. In the drawing, reference numeral  7  indicates an input terminal where the tone signal and signals including noises are input, and reference numeral  8  indicates an output terminal from where the judgment result of the tone signal is output. 
     The operation of the tone signal detection circuit of the first example will now be described. 
     The tone signal having noises superposed thereon is input to the first filter  1  and the second filter  2  through the input terminal  7 . 
     FIG. 2 is a chart to illustrate the characteristics of the first filter  1  and the second filter  2 , when the tone signal is set at 6000 Hz. The first filter  1  possesses a frequency passband of which the center frequency is 6000 Hz. The second filter  2  possesses a passband of frequencies near 6000 Hz excluding 6000 Hz. Accordingly, the first filter  1  outputs the “tone signal + noises”, and the second filter  2  outputs the “noises”. 
     The outputs from the first filter  1  are converted into signals of levels corresponding to its amplitudes by the first level converter  3 . The outputs from the second filter  2  are converted into signals of levels corresponding to its amplitudes by the second level converter  4 . The subtractor  6  subtracts the output level of the second filter  2  from the output level of the first filter  1 . That is, the subtractor  6  subtracts the level of the “noises” from the level of the “tone signal + noises”, and outputs the level of the tone signal. The judgment circuit  5  judges the level of the tone signal to thereby detect the tone signal. 
     FIG. 3 is a chart that compares the operation of the tone signal detection circuit according to the invention with that of the conventional detection circuit. 
     The curve shown by the dotted line illustrates an output of the conventional bandpass filter, and the curve shown by the solid line illustrates an output of the subtractor  6  according to the first example of the present invention. As shown in the drawing, in the conventional filter, the level of the filter output in the noise area is almost the same as the output level obtained in the strong electric field. That is, in the conventional filter, it is difficult to detect the signal in the noise area. In the first example of the present invention on the other hand, the filter output level in the noise area is substantially different from the output level in the strong electric field. Accordingly, the first example of the present invention discriminates the noise area from the strong electric field. Further, when this example is applied to an FM receiver and even if there are dispersions in the noise levels by each unit, the noise components can be removed, and accordingly the tone signal can be stably detected. 
     The second example of the present invention will be described below. 
     FIG. 4 is a block diagram illustrating a second example of the tone signal detection circuit according to the invention. 
     The present example has a lowpass filter  9  added on the prior stage of the judgment circuit  5 , in contrast to the configuration of the first example shown in FIG.  1 . The lowpass filter  9  smoothes the output from the subtractor  6 . The smoothing time constant of the lowpass filter  9  is appropriately determined according to the condition of use, or the like. The other configuration is the same as that of the first example of FIG. 1, and the description will be omitted. 
     The operation of the tone signal detection circuit of the second example will be described below. 
     The operation of the second example from when a signal is input to the input terminal  7  until when the signal is output from the subtractor  6  is the same as that of the first example of FIG. 1, and the description will be omitted. The level of a tone signal calculated by the subtractor  6  is smoothed by the lowpass filter  9 , and the output therefrom is input to the judgment circuit  5 . The judgment circuit  5  judges the level of the tone signal to thereby detect the tone signal. The interpolation of the lowpass filter  9  will prevent the judgment circuit  5  from outputting the judgment result, “tone signal absent”, in the case that the tone signal is not momentarily input due to the fading phenomenon or the like. 
     The third example of the present invention will be described below. 
     FIG. 5 is a block diagram illustrating third example of the tone signal detection circuit according to the present invention. 
     The third example has a first lowpass filter  10  added on the output of the first level converter  3  and a second lowpass filter  11  added on the output of the second level converter  4 , in contrast to the configuration of the first example of FIG.  1 . 
     The first lowpass filter  10  and the second lowpass filter  11  smooth the output from the first level converter  3  and the output from the second level converter  4 , respectively. The other configuration is the same as that of the example of FIG. 1, and the description will be omitted. 
     The operation of the tone signal detection circuit of the third example will be described below. 
     The operation of the concrete example 3 from when a signal is input from the input terminal  7  until when the first filter  1  outputs the “tone signal + noises” and the second filter  2  outputs the “noises” is the same as that of the first example of FIG. 1, and the description will be omitted. The first lowpass filter  10  calculates the average level of the “tone signal + noises” output from the first level converter  3 . The second lowpass filter  11  calculates the average level of the “noises” output from the second level converter  4 . The average levels calculated by the first and second lowpass filters  10 ,  11  are output to the subtractor  6 . The subtractor  6  subtracts the output level of the second level converter  4  from the output level of the first level converter  3 . That is, the subtractor  6  subtracts the average level of the “noises” from the average level of the “tone signal + noises”, and outputs the average level of the tone signal. The judgment circuit  5  judges the average level of the tone signal to thereby detect the tone signal. 
     The interpolation of the first lowpass filter  10  and the second lowpass filter  11  will prevent the judgment circuit  5  from outputting the judgment result, “tone signal absent”, when the tone signal is not momentarily input due to the fading phenomenon or the like. Further, the filtering by each lowpass filter is individually processed to each of the outputs from the first filter  1  and the second filter  2  and accordingly, when the variation of the tone signal is different from that of noises found in nature, an optimized lowpass filter can be designed to each of them. Consequently, this circuit configuration makes it possible to achieve a circuit having a higher detection accuracy. 
     The fourth example of the present invention will be described below. 
     FIG. 6 is a block diagram to illustrate a fourth example of the tone signal detection circuit according to the invention. 
     The concrete example 4 has a third lowpass filter  9   a  added on the prior stage of the judgment circuit  5 , in contrast to the configuration of the third example of FIG.  5 . The lowpass filter  9   a  smoothes the output from the subtractor  6 . The smoothing time constant of the lowpass filter  9   a  is appropriately determined according to the condition of use, or the like. The other configuration is the same as that of the foregoing third example, and the description will be omitted. 
     The operation of the tone signal detection circuit of the fourth example will be described. 
     The operation of the fourth example from when a signal is input to the input terminal  7  until when the signal is output from the subtractor  6  is the same as that of the concrete example  3 , and the description will be omitted. The level of a tone signal calculated by the subtractor  6  is smoothed by the lowpass filter  9   a , and the output therefrom is input to the judgment circuit  5 . The judgment circuit  5  judges the level of the tone signal to thereby detect the tone signal. The tuning of the time constant of the lowpass filter  9   a  makes it possible to set the time for detecting the tone signal. Accordingly, this circuit configuration makes it possible to achieve a circuit having a higher detection accuracy. 
     The fifth example of the present invention will be described below. 
     FIG. 7 is a block diagram illustrating a fifth example of the tone signal detection circuit according to the present invention. 
     The fifth example has a filter  12  added on after the input terminal  7 , in contrast to the configuration of the first example of FIG.  1 . This filter  12  limits signals within, for example, 300 Hz to 3400 Hz being the frequency band of human voices. The other configuration is the same as that of the first example of FIG. 1, and the description will be omitted. 
     The operation of the tone signal detection circuit of the fifth example will be described below. 
     The tone signal having noises superposed thereon is input to the first filter  12  through the input terminal  7 . The filter  12  limits the frequency band of the signals input thereto, and outputs the signal processed by the filter  12  to the first level converter  3  and the second level converter  4 . The operation after this stage is the same as that of the first example of FIG. 1, and the description will be omitted. The interpolation of the filter  12  will remove the influence of the signals within the frequency band that the filter  12  limits on the tone signal. That is, setting the frequency band of the voice signal as a frequency band that the filter  12  limits removes the influence of the voice signal on the tone signal, when the tone signal and the voice signal overlap each other and input. 
     The judgment circuit  5  used in the examples of FIGS.  1  and  4 - 7  through  5  will be described below. 
     FIG. 8 is a block diagram of the judgment circuit  5 . 
     The judgment circuit  5  consists of a switch  51 , a first comparator  52 , a first counter  53 , a second comparator  54 , and a second counter  55 . 
     The first comparator  52  compares a level of an output signal from the subtractor  6  with a predetermined first level, and outputs an ON-signal when the level of the output signal from the subtractor  6  is more than the first level. The first counter  53  counts a frequency during which the ON-signal output from the first comparator  52  continues, and outputs a judgment result of “tone signal present” when the continued frequency is more than a predetermined first threshold. 
     The second comparator  54  compares the level of the output signal from the subtractor  6  with a second level that is predetermined at a lower value than the first level, and outputs an OFF-signal when the level of the output signal from the subtractor  6  is less than the second level. The second counter  55  counts a frequency during which the OFF-signal output from the second comparator  54  continues, and outputs a judgment result of “tone signal absent” when the continued frequency is more than a predetermined second threshold. 
     The switch  51  is connected to the terminal  57  when the first counter  53  outputs the judgment result “tone signal present”, so that the output signal from the subtractor  6  is supplied to the second comparator  54 . And, the switch  51  is connected to the terminal  56  when the second counter  55  outputs the judgment result “tone signal absent”, so that the output signal from the subtractor  6  is supplied to the first comparator  52 . 
     The first level and the second level will be described with reference to FIG.  9 . As shown in the drawing, the first level is sufficiently higher than the level in the noise area, and the second level is equivalent approximately to the upper limit of the level in the noise area. 
     The operation of the judgment circuit  5  will be described. 
     When the tone signal is not yet detected, the switch  51  is connected to the terminal  56 . The input signal to the judgment circuit  5  is input to the first comparator  52  through the switch  51 . The first comparator  52  compares the input signal level with the first level at every specified time. The first comparator  52  outputs the ON-signal when the input signal level is more than the first level, and outputs the OFF-signal When the input signal level is less than the first level. The first counter  53  counts a frequency during which the ON-signal output from the first comparator  52  continues, outputs the “tone signal present” as the judgment result when the continued frequency is more than the first threshold, and switches the connection of the switch  51  into the terminal  57 . 
     When the switch  51  is connected to the terminal  57 , the input signal to the judgment circuit  5  is input to the second comparator  54  through the switch  51 . The second comparator  54  compares the input signal level with the second level at every specified time. The second comparator  54  outputs the ON-signal when the input signal level is more than the second level, and outputs the OFF-signal when the input signal level is less than the second level. The second counter  55  counts a frequency during which the OFF-signal output from the second comparator  54  continues, outputs the “tone signal absent” as the judgment result when the continued frequency is more than the second threshold, and switches the connection of the switch  51  into the terminal  56 . 
     Carrying out such an operation, the judgment circuit  5  outputs the judgment result of “tone signal present” when the reception field strength is sufficiently strong and this strength is stable. When the tone signal is not input continuously for a certain period, the judgment circuit  5 , outputs the judgment result of “tone signal absent”. The application of the judgment circuit  5  to the examples of FIGS.  1  and  4 - 7  makes it possible to detect the tone signal fluctuating near the detection frequency reliably with a least detection time. 
     A sixth example of the present invention will be described below. 
     FIG. 10 is a block diagram illustrating a sixth example of the tone signal detection circuit according to the present invention. 
     The sixth example has an electric field strength detector  13  added on, in contrast to the configuration of the foregoing example of FIG.  6 . This field strength detector  13  detects an electric field strength of a reception signal, and on the basis of the detected electric field strength controls the characteristic of the first filter  1 , the characteristic of the second filter  2 , the characteristic of the first lowpass filter  10 , the characteristic of the second lowpass filter  11 , the characteristic of the third lowpass filter  9   a , and the first and the second level and the first and the second threshold of the judgment circuit  5 . The other configuration on the drawing is the same as that of the example of FIG. 6, and the description will be omitted. 
     The operation of the tone signal detection circuit of the sixth example will be described below. 
     The electric field strength detector  13  detects an electric field strength of a reception signal, and the detector  13  carries out the following controls on the basis of the detected value. As to the first filter  1  and the second filter  2 , the detector  13  sharpens the filter characteristic (value of Q) when the field strength is weak, and broadens the filter characteristic when the field strength is strong. As to the first lowpass filter  10 , the second lowpass filter  11 , and the third lowpass filter  9   a , the detector  13  controls the time constant so as to decrease the smoothness when the field strength is strong, and to increase the smoothness when the field strength is weak. As to the first level and the second level of the judgment circuit  5 , the detector  13  sets the first level to a slightly lower level and sets the second level to a slightly higher level when the field strength is strong and sets the first level to a slightly higher level and sets the second level to a slightly lower level when the field strength is weak. As to the first threshold and the second threshold of the judgment circuit  5 , the detector  13  sets to a slightly lower value when the field strength is strong, and sets to a slightly higher value when the field strength is weak. As a result, the judgment circuit  5  sets the value such that the judgment result is obtained in a shorter time when the field strength is strong, and sets the value such that the judgment accuracy is improved when the field strength is weak. 
     The foregoing examples have been described as the tone detection circuit of the present invention. Since the tone detection circuit of this invention is able to detect the level of “tone signal + noises” and the level of “noises”, it is also able to calculate the S/N ratio being the ratio of the two levels. That is, the tone detection circuit can be applied as an S/N detection circuit. 
     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.