Abstract:
In a howling detection circuit, an audio signal voltage generator generates an audio signal voltage based on a level of an input audio signal. A capacitor is chargeable and dischargeable by the audio signal voltage and develops a monitor voltage. An indication circuit is activated depending on the monitor voltage to provide an alert indicating that a howling noise is contained in the input audio signal. A discharging element discharges the capacitor according to a first time constant when the audio signal voltage is lower the monitor voltage, thereby lowering the monitor voltage. A charging element charges the capacitor according to a second time constant being set greater than the first time constant when the audio signal voltage is higher than the monitor voltage, thereby raising the monitor voltage so as to activate the indication circuit.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field of the Invention 
   The present invention relates to a howling detection circuit which is suitable for use in an audio mixer or the like. 
   2. Description of the Related Art 
   When howling occurs, a mixer or the like has to specify the source of the howling promptly and perform a level control, frequency characteristic control or the like in association with the specified source. Various techniques have been proposed with respect to a howling detection method. For example, disclosed in Japanese Patent Laid-open Publication No. Heisei 4-277977 is a technique that detects howling by measuring the levels of respective frequency components of an audio signal, comparing the measured level of each of the frequency components with those of the other frequency components and determining from the comparison whether a frequency component of an unnaturally high level is present in the frequency components of the audio signal. 
   However, the above-mentioned technique is disadvantageous in that complex processes, such as frequency component detection, etc., are required, resulting in complexity in configuration of a howling detection circuit and increase in cost when the howling detection circuit is installed in a mixer or the like. Particularly, because the mixer mixes audio signals of a plurality of input channels to generate an output signal, it must have the howling detection circuit for each of the input channels and, when howling occurs, promptly specify one of the input channels as the source of the howling. In this configuration, howling detection circuits of a number corresponding to that of the input channels are required, causing increase in cost. 
   Assume that an audio signal inputted from a microphone is amplified by an amplifier, outputted through a speaker and then fed back to the microphone. In this case, if a frequency component whose gain in one circulation of the input audio signal to the microphone exceeds “1” is present, howling occurs in that frequency component. Once the howling occurs, it continues ceaselessly as long as there is no change in the position relationship between the microphone and the speaker or the gain of the amplifier. Meanwhile, assume that a human voice or natural musical instrument&#39;s sound is inputted through the microphone. In this case, a “node” may be present in the voice or sound. This node may be regarded as a timing at which the variable level of the input audio signal falls during the utterance of the voice or sound. In this regard, whether or not the howling has occurred can be easily determined on the basis of the presence or not of the “node” in the microphone input. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above problems, and it is an object of the present invention to provide a howling detection circuit which is capable of detecting a howling with a simple configuration. 
   In order to achieve the above object, the present invention is characterized by a configuration described below. Here, elements in parentheses are references for illustrative purposes. 
   In accordance with the present invention, the above and other objects can be accomplished by the provision of a howling detection circuit comprising: an audio signal voltage generator ( 2 ,  4 ,  6  and  8 ) for generating an audio signal voltage (V 2 ) based on a level of an input audio signal; a capacitor ( 18 ); an indication circuit ( 22 ,  24 ,  26 ,  28  and  30 ) for providing an alert depending on a monitor voltage (V 3 ), the monitor voltage (V 3 ) being a voltage across the capacitor ( 18 ); a discharging element ( 12 ) for discharging the capacitor ( 18 ) according to a first time constant ((C10+C18)R12) when the monitor voltage (V 3 ) is higher than or equal to the audio signal voltage (V 2 ), namely when the audio signal voltage (V 2 ) is lower than or equal to the monitor voltage (V 3 ); and a charging element ( 16 ) for charging the capacitor ( 18 ) according to a second time constant (R16·C18) when the monitor voltage (V 3 ) is lower than the audio signal voltage (V 2 ), namely when the audio signal voltage (V 2 ) is higher than the monitor voltage (V 3 ), the second time constant (R16·C18) being larger than the first time constant ((C10+C18)R12). 
   Preferably the audio signal voltage generator ( 2 ,  4 ,  6  and  8 ) of the inventive howling detection circuit includes a limiter ( 6 ) for setting the audio signal voltage (V 2 ) substantially in proportion to the level of the input audio signal when the level of the input audio signal is lower than or equal to a predetermined value (Zener voltage Vz), and limiting increase in the audio signal voltage (V 2 ) against increase in the level of the input audio signal when the level of the input audio signal exceeds the predetermined value (Zener voltage Vz). 
   In this manner, in accordance with the present invention, the capacitor is rapidly discharged according to the first time constant when the monitor voltage is higher than or equal to the audio signal voltage, and it is slowly charged according to the second time constant when the monitor voltage is lower than the audio signal voltage. Therefore, if a “node” of voice or sound where the level of the input audio signal is lowered takes place during the utterance, the monitor voltage can be immediately reduced, thereby making it possible to detect a howling with a simple configuration. 

   
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram of a howling detection circuit according to an exemplary embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   1. Configuration of Embodiment 
   The configuration of a howling detection circuit according to an exemplary embodiment of the present invention will hereinafter be described with reference to  FIG. 1 . 
   The present howling detection circuit is provided for every input channel of a mixer to detect and indicate occurrence of howling in a corresponding input channel. In  FIG. 1 , the reference numeral  2  denotes an amplifier that amplifies an input audio signal S 1  of the corresponding input channel and outputs the amplified audio signal as an audio signal S 2 . The audio signal S 2  is applied across a Zener diode  6  through a resistor  4 . Here, a voltage V 1  appearing at a point P 1 , which is the cathode of the Zener diode  6 , is close to “0V” when the input audio signal S 1  has a negative polarity, and is in proportion to an instantaneous value of the input audio signal S 1  when the input audio signal S 1  has a positive polarity and the output level of the amplifier  2  is lower than or equal to a Zener voltage Vz of the Zener diode  6 . When the output level of the amplifier  2  exceeds the Zener voltage Vz, the voltage V 1  is close to the Zener voltage Vz. 
   Here, the Zener voltage Vz is set to a level that the amplitude of the audio signal S 2  is expected to reach when a howling occurs in the input audio signal S 1 . If the amplitude of the audio signal S 2  exceeds the Zener voltage Vz, the voltage V 1  has a substantially square waveform with an amplitude of the Zener voltage Vz. In this case, the waveform of the voltage V 1  is little influenced by variations in level of the input audio signal S 1 . The voltage V 1  is applied through a diode  8  to a capacitor  10  and resistor  12  connected in parallel. As a result, a voltage V 2  appearing at a point P 2 , which is a connection point of the diode  8  and resistor  12 , has a smoothed waveform of the voltage V 1 . In other words, in the case where a howling occurs in the input audio signal S 1  and the amplitude of the audio signal S 2  exceeds the Zener voltage Vz, the voltage V 2  is almost constant irrespective of the magnitude of the howling. Here, the capacitor  10  has a capacitance C10 of “2.2 μF” and the resistor  12  has a resistance R12 of “1 kΩ”. Therefore, if the level of the audio signal S 1  is lowered and the voltage V 1  is thus reduced, charge in the capacitor  10  is rapidly discharged through the resistor  12 , thereby causing the voltage V 2  to be rapidly reduced. 
   The signal voltage V 2  is then applied to a capacitor  18  through a parallel circuit of a diode  14  and resistor  16 . Here, a voltage appearing at a point P 3 , which is one end of the capacitor  18 , is monitor voltage V 3 . The diode  14  has an anode connected to the point P 3  and a cathode connected to the point P 2 . The resistor  16  has a relatively large resistance R16 of “68 kΩ”, and the capacitor  18  has a relatively large capacitance C18 of “22 μF”. As a result, in the case where the relationship of “V 2 &gt;V 3 ” is established with respect to the voltages V 2  and V 3 , the relatively large time constant of the resistor  16  and capacitor  18  is “R16·C18=1.496”, so the capacitor  18  is slowly charged, thereby causing the voltage V 3  to slowly approach the voltage V 2 . On the other hand, in the case where the relationship of “V 2 &lt;V 3 ” is established with respect to the voltages V 2  and V 3 , current flows from the capacitor  18  to the capacitor  10  through the diode  14 , thus maintaining a state of “V 2 ≈V 3 ”. That is, the value of “V 3 −V 2 ” is a forward voltage drop (about 0.6V) of the diode  14  at maximum. In this state, because charges in the capacitor  10  and capacitor  18  are discharged through the resistor  12 , the time constant for the discharging is “(C10+C18)R12=0.0242”, which is about “ 1/60” of that for the charging. 
   The monitor voltage V 3  is then applied to the base terminal of a transistor  26  through a current limiting resistor  20 . The emitter terminal of the transistor  26  is grounded through a diode  28  and the collector terminal thereof is applied with a predetermined direct current (DC) voltage from a power source  30  through a resistor  24  and a light emitting diode (LED)  22  for howling alert. Assume that the DC base voltage of the transistor  26  in an ON state thereof is VBE and the forward voltage drop of the diode  28  is VD. In this case, when the monitor voltage V 3  is higher than or equal to an ON voltage Von (=VBE+VD), the transistor  26  is turned on, thereby causing current to flow through the LED  22 , resistor  24 , transistor  26  and diode  28  and, thus, the LED  22  to be lit. 
   2. Operation of Embodiment 
   In the above configuration, when a howling occurs in the input audio signal S 1  and the amplitude of the audio signal S 2  exceeds the Zener voltage Vz of the Zener diode  6 , the voltage V 1  has a substantially square waveform with an amplitude of the Zener voltage Vz and the signal voltage V 2  immediately rises to a smoothed level of the voltage V 1 . At this time, if the relationship of “V 2 &gt;V 3 ” is established with respect to the voltages V 2  and V 3 , the capacitor  18  is slowly charged through the resistor  16 . In the case where the howling occurs, the monitor voltage V 3  becomes higher than or equal to the ON voltage Von after this state continues for several seconds. As a result, the transistor  26  is turned on to light the LED  22 . Because the present howling detection circuit is installed for each input channel of the mixer as stated above, the LED  22  for howling alert is also provided for each input channel of the mixer and disposed on an operating panel of the mixer. Accordingly, if the LED  22  is lit, the user can immediately recognize that the possibility of howling generation is high in the associated input channel and thus readily take necessary measures, including reducing a fader of the associated input channel. 
   On the other hand, the amplitude of the audio signal S 2  may incidentally exceed the Zener voltage Vz even in a no howling state. However, in the case where the input audio signal S 1  corresponds to, for example, a human voice or general musical instrument&#39;s sound inputted through the microphone, a “node” of the voice or sound, namely, a timing at which the level falls, is present in the audio signal S 2 . At the moment that the level of the audio signal S 2  falls, the voltage V 2  is immediately reduced so as to become equal to the voltage V 3 . Then, the charges stored in the capacitor  10  and capacitor  18  are rapidly discharged through the resistor  12 , so that the voltages V 2  and V 3  become adequately low. In this manner, in the case where the input audio signal S 1  purely contains a human voice or general musical instrument&#39;s sound, the charge stored in the capacitor  18  is discharged before the voltage V 3  reaches the ON voltage Von, so the LED  22  is not lit. 
   As described above, the audio signal voltage generator generates the audio signal voltage V 2 , which has a variable voltage level when the input audio signal V 1  is free of the howling noise, and which has a monotonous voltage level when the input audio signal V 1  contains the howling noise, such that the variable voltage level of the audio signal voltage V 2  enables the discharging element  12  to avoid raising of the monitor voltage V 3 , while the monotonous voltage level of the audio signal voltage V 2  enables the charging element  16  to enhance raising of the monitor voltage V 3 . 
   As described above, according to the present embodiment, it is possible to detect a howling appearing in the input audio signal S 1  with a simple circuit. In particular, when the level of the audio signal S 1  rises slowly from “0”, the voltage V 2  increases slowly in proportion to the amplitude of the audio signal S 1 . However, after the amplitude of the audio signal S 2  reaches the Zener voltage Vz of the Zener diode  6 , the voltage V 2  is limited so as to be almost constant. Therefore, when a howling occurs, it is possible to detect the howling occurrence on the basis of substantially the same reference based on the “duration” of the howling almost irrespective of the “level” of the howling. 
   3. Modified Embodiments 
   Although the preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, as in the following examples. 
   (1) Although the charging time constant of the capacitor  18  has been disclosed in the above embodiment to be “about 60 times” as large as the discharging time constant of the capacitor  18 , the time constant ratio is not limited thereto. The effect of the invention can be obtained as long as the charging time constant is greater than the discharging time constant. For example, the charging time constant is preferably “10 times” or more as large as the discharging time constant, more preferably “30 times” or more, and most preferably “50 times” or more. 
   As described above, the charging time constant is set greater than 10 times of the discharging time constant, more preferably “30 times” or more, and most preferably “50 times” or more, so that the discharging element  12  rapidly discharges the capacitor  18  as compared to the charging of the capacitor  18  and the charging element  16  gradually charges the capacitor  18  as compared to the discharging of the capacitor  18 . Typically, the discharging time is set less than 0.1 second while the charging time is set more than 1 second. 
   (2) Although the howling detection circuit disclosed in the above embodiment is applied to the audio mixer, the present invention is not limited thereto, but is applicable to howling detection in various audio devices. For example, the circuit of the above embodiment may be built in an individual microphone. In this case, provided that the LED  22  is lit, the user of the associated microphone will be able to recognize occurrence of a howling in the associated microphone and thus take necessary measures, including changing the direction of the microphone.