Patent Publication Number: US-7596238-B2

Title: Howling margin measuring device

Description:
The present application claims the benefit of priority of International Patent Application No. PCT/JP2004/009297 filed on Jun. 24, 2004, which application claims priority of Japanese Patent Application No. 2003-277975 filed Jul. 23, 2003. The entire text of the priority application is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a howling margin measuring device for measuring the howling margin of an acoustic system. 
     BACKGROUND ART 
     The howling margin of an acoustic system that includes microphones and speakers such as installed in a concert hall needs to be obtained in some cases. Such a howling margin has been heretofore measured by the following method. 
     Ordinary acoustic systems are equipped with a means for gain control. When an acoustic system is brought into a condition appropriate for use in a concert held in a concert hall for instance, the gain of the acoustic system is also set to a value appropriate for use in the concert. 
     A measurement of the howling margin of the acoustic system thus adjusted and set is generally made by a gain controlling means provided for the acoustic system. As the gain controlling means used for the measurement in this case, an output fader for a mixing console placed between a microphone and a speaker is commonly used. 
     Using, as a reference level, the condition of the gain controlling means of the acoustic system adjusted and set a concert, the operator controls the gain of the gain controlling means so as to gradually increase from the reference level. Until the acoustic system starts howling, the gain of the gain controlling means is increased by the operator. At the time when the acoustic system starts howling, the speaker makes sound waves having a specified frequency (howling frequency). The operator aurally checks this sound thereby knowing that generation of howling has started. Then, he obtains a howling margin from how much the gain of the gain controlling means when howling starts exceeds the reference level. For instance, if the reference level of the fader that serves as the gain controlling means is “−10 dB” and the level of the fader when the acoustic system starts howling is “−5 dB”, the howling margin is “5 dB”. 
     In the above method, however, a start of howling is determined depending on the sense of hearing of the operator. Therefore, a good operator&#39;s skill is required for making an accurate howling measurement. If an inexperienced operator measures a howling margin with the above method, the operator may fail in catching a howling sound while howling being generated and recklessly leave the howling to increase. In addition, the operator judges the level of the fader by reading the calibration mark provided beside the fader. Therefore, it is difficult for the operator to precisely read the calibration mark and therefore difficult to make an accurate measurement of a howling margin. 
     Further, such howling margin measurement dependent upon the skill of the operator is an obstacle to automatization of adjustment/setting of an acoustic system. 
     Although Japanese Unexamined Published Patent Application No. 09-247787 (Page 4, Paragraph No. 0024) makes mention of howling margin measurement, it does not concretely teach a system configuration nor method for making such a measurement. 
     DISCLOSURE OF THE INVENTION 
     A primary object of the invention is to provide a measuring device capable of making an objective and accurate measurement of a howling margin without depending upon the sense of hearing of the operator. 
     In accomplishing the above object, there has been provided, in accordance with the invention, a howling margin measuring device comprising processing means composed of gain controlling means and a compressor which are connected in series and controlling means, 
     wherein the gain. controlling means outputs an input sound signal after giving a gain thereto; 
     wherein if the level of a sound signal input to the compressor is equal to or higher than a threshold level, the compressor outputs the sound signal after compressing it with a specified ratio; 
     wherein the controlling means is capable of controlling the gain of the gain controlling means and reading the compression level of the compressor; and 
     wherein the controlling means reads the compression level of the compressor while gradually increasing the gain of the gain controlling means, determines whether or not howling has been generated based on whether the read compression level has a value equal to or higher than a specified value, and calculates a howling margin based on the gain of the gain controlling means when it is determined that howling has been generated. 
     According to the howling margin measuring device of the invention, a howling margin can be objectively and accurately measured without depending upon the sense of hearing of the operator. 
     The howling margin measuring device is preferably formed such that the determination on whether howling has been generated is done based on whether a condition in which the read compression level is equal to or higher than a specified level has continued for a specified period of time, which enables a correct judgment of howling generation. 
     Although it does not matter which of the gain controlling means and the compressor lies anterior or posterior to the other as far as they are connected in series, the compressor may be positioned in a stage posterior to the gain controlling means. 
     Preferably, the howling margin measuring device has displaying means for displaying a calculated howling margin. 
     Preferably, the processing means is capable of inputting a sound signal from a microphone and releasing an output signal to a speaker. 
     These objects as well as other objects, features and advantages of the description with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a howling margin measuring device according to one embodiment of the invention. 
         FIG. 2  is a block diagram showing a condition in which the howling margin measuring device shown in  FIG. 1  is incorporated into an acoustic system. 
         FIG. 3  is an input/output characteristic graph of a compressor. 
         FIG. 4  is a graph of the level a of an input signal input to the howling margin measuring device, the gain b of a gain controller and the compression level c of the compressor which are plotted on a common time axis. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring now to the accompanying drawings, a preferred embodiment of the invention will be described. 
       FIG. 1  is a block diagram of a howling margin measuring device  20  according to one embodiment of the invention. The howling margin measuring device  20  has a processing unit (processing means)  28 , a control unit (controlling means)  23  and a display unit  24 . 
     The processing unit  28  is comprised of a gain controller (gain controlling means)  21  located in an anterior stage and a compressor  22  located in a posterior stage, which are connected in series. 
     The gain controller  21  inputs an acoustic signal from an input terminal  25  serving as an input unit and outputs this signal after adding a gain to it. In this embodiment, the gain of the gain controller  21  can be arbitrarily set within the range of “−20 (dB)” to “+20 (dB)”. 
     The compressor  22  inputs an output signal of the gain controller  21  and outputs this signal to the output terminal  26  after processing it. The processing performed by the compressor  22  is compression of an input signal with a preset ratio, the input signal having a level higher than a preset threshold level. In this embodiment, the threshold level can be arbitrarily set within the range of “0 (dB)” to “10 (dB)”. The ratio can be arbitrarily set within the range of “1/1” to “∞/1”. The state where the ratio is “1/1” is such that even if the level of the input signal exceeds the threshold level, compression of the input signal will not be done at all. The state where the ratio is “∞/1” is such that if the level of the input signal exceeds the threshold level, the input signal is compressed until the threshold level is reached irrespective of the magnitude of the level (of the input signal). 
     The control unit  23  can control the gain controller  21  and the compressor  22 . More particularly, it can control the gain of the gain controller  21 , the threshold level for the compressor  22  and the ratio for the compressor  22 . The control unit  23  can read the compression level of the compressor  22 . In addition, the control unit  23  has a computing function and a timing function. Further, it can control driving of the display unit  24 . 
       FIG. 2  is a block diagram showing a condition in which the howling margin measuring device  20  is incorporated into an acoustic system  10 . 
     The acoustic system  10  is installed in a concert hall  40 . The acoustic system  10  includes a microphone  1 , a microphone amplifier  2 , a mixing console  3 , a power amplifier  4  and a speaker  5 . An output signal from the microphone  1  is input to the mixing console  3  through the microphone amplifier  2 . After adjusted by the mixing console  3 , the signal is amplified by the power amplifier  4  and made audible by the speaker  5 . 
     The howling margin measuring device  20  is interposed between the mixing console  3  and power amplifier  4  of the acoustic system  10 . 
     Next, reference is made to  FIGS. 1 and 2  to describe the operation of the howling margin device  20  when measuring the howling margin of the acoustic system  10  installed in the concert hall  40 . 
     The howling margin measuring device  20  starts operation provided that the acoustic system  10  installed in the concert hall  40  is adjusted and set so as to be in a usable condition. Specifically, this condition (i.e., the condition in which the acoustic system is adjusted and set so as to be usable in a concert) is utilized as a reference state and the howling margin of the acoustic system  10  in this reference state is measured by the howling margin measuring device  20 . 
     The adjustment and setting of the acoustic system  10  to bring it into the reference state can be done by the operator or by an automatic adjustment system (not shown). 
     During the adjustment of the acoustic system  10 , It is necessary to prevent the howling margin measuring device  20  from affecting the acoustic system  10 . To this end, there may be provided, for instance, a bypass path (not shown) for bypassing the howling margin measuring device  20 . Alternatively, the gain controller  21  of the howling margin measuring device  20  may be set to 0 dB and the ratio may be set to “1/1” so that the compression level of the compressor  22  is constantly kept to be 0 dB. 
     After completion of the adjustment and setting to bring the acoustic system  10  into its reference condition, the howling margin measuring device  20  starts up. 
     In the howling margin measuring device  20 , the gain controller  21  and the compressor  22  are brought into their respective initial conditions. The initial condition of the gain controller  21  is a condition in which gain is set to a value that is small enough not to cause howling in the acoustic system  10 . In this embodiment, the gain of the gain controller  21  is set to “0 db”. The setting of the gain of the gain controller  21  is done by the control unit  23 . 
     The initial condition of the compressor  22  is a condition in which the threshold level for the compressor  22  is set to a specified value and the ratio is set to a specified value. In this embodiment, the threshold level is set to “6 dB” and the ratio is set to a value exceeding “1/1” such as “2/1”. 
       FIG. 3  is an input/output characteristic graph of the compressor  22 . Solid line in  FIG. 3  is a characteristic line of the compressor  22  in the initial condition described above (that is, the threshold level is 6 dB and the ratio is “2/1”). Broken line in  FIG. 3  is a characteristic line of the compressor  22  when no compression is done at all. The difference between the broken line and the sold line when the input level is equal to or higher than the threshold level (6 dB) is the compression level. 
     After the gain compressor  21  and the compressor  22  are set to their respective initial conditions, the gain of the gain controller  21  is gradually increased in the howling margin measuring device  20 . The gain is increased by a specified level per unit time. In this embodiment, the gain increases at a speed of “1 dB/sec”. The gain control of the gain controller  21  is also performed by the control unit  23 . 
     Meanwhile, the control unit  23  monitors the compression level of the compressor  22 . Since the control unit  23  can read the compression level of the compressor  22 , the compression level can be obtained at any time. Although the control unit  23  reads the compression level at certain time intervals, it looks like monitoring the compression level in a substantially continuous manner since the time interval of reading is short. 
     In this way, the control unit  23  gradually increases the gain of the gain controller  21  while constantly monitoring the compression level of the compressor  22 . 
     As far as the gain of the gain controller  21  is sufficiently small, howling does not occur in the acoustic system  10 . Therefore, the level of the output signal of the microphone  1  is small and the level of the signal input to the compressor  22  does not exceed the threshold level. 
     If the gain of the gain controller  21  becomes great to a certain degree, howling will occur at a specified frequency. When howling occurs, the output compressor  22  rapidly increase. As a result, the level of the input signal of the compressor  22  exceeds the threshold level, causing the compressor  22  to start signal compression. 
     The control unit  23  always monitors the compression level of the compressor  22  as discussed earlier and, at the same time, constantly makes a judgment to check if the compression level has a value equal to or higher than a specified value. In this embodiment, the specified value is “1 dB”. If the compression level is judged to be 1 dB or more, timing starts, being triggered by this judgment. The monitoring of the compression level continues after the start of timing. 
     If the compression level does not become less than 1 dB after an elapse of a specified time (which is “5 seconds” in this embodiment) after the start of timing, it is then determined that howling has been being generated in the acoustic system  10  and a howling margin is decided based on the gain of the gain controller  21  at the time of this determination. 
     Various methods may be taken for determining a howling margin based on the gain of the gain controller  21 . For instance, the value, which is obtained from subtraction of a specified minute gain from the gain of the gain controller  21  when howling is judged to be generated, may be determined to be a howling margin. For instance, if the gain of the gain control  21  when howling is generated is “9 dB” and the minute gain is “1 dB”, the howling margin is determined to be “8 dB”. This minute gain may be the minimum unit (i.e., the amount of gain increased by one step) of change in the gain that can be given to the gain controller  21  by the control unit  23 . 
     The control unit  23  controls the display unit  24  so as to display the howling margin thus determined (calculated). 
     It has been described earlier that the control unit  23  starts timing, being triggered by the judgment in which the compression level of the compressor  22  is determined to be 1 dB or more. However, if the compression level becomes less than 1 dB within a specified period of time (5 sec.) after the start of timing, it is not determined that howling has occurred and therefore the timing operation is reset. The reason for this is as follows. 
     In the concert hall  40 , there is a likelihood that an accidental big sound due to other factors than howling may be generated. For instance, when the door for the entrance of the concert hall  40  is opened, a high-level sound is momentarily input to the microphone  1 . Like this case, a loud sound caused by other factors than howling does not last in many cases. Therefore, even if the control unit  23  determines through detection that the compression level of the compressor  22  is 1 dB or more, generation of howling is not admitted unless this condition last long (i.e., generation of howling is not admitted if the condition continues only for less than 5 sec.). 
       FIG. 4  is a graph of the level a of an input signal input to the howling margin measuring device  20 , the gain b of the gain controller  21  and the compression level c of the compressor  22  which are plotted on a common time axis.  FIG. 4  shows the state after the gain b of the gain controller  21  starts to rise until the compressor  22  starts compression. 
     As seen from  FIG. 4 , the gain b of the gain controller  21  gradually rises from 0 dB. At the input signal level a, the waveform at the level of −10 dB or less is due to the background noise of the concert hall  40 . An abrupt rise from the background noise level is seen at the input signal level a. This is due to generation of howling. Compression by the compressor  22  starts substantially simultaneously with the generation of howling. 
     Thus, the howling margin measuring device  20  detects howling from the compressing condition of the compressor  22  without depending on the sense of hearing of the operator. And, it measures a howling margin based on the gain of the gain controller  21  and the compression level of the compressor  22 . As a result, a howling margin can be objectively, accurately measured. In addition, the threshold level for the compressor  22  can be set low, which makes it possible to reliably control howling (feedback). Further, since the output level at the time of feedback generation can be limited to a low level by the compressor  22 , damage to the acoustic system  10  can be avoided, and an acoustic impact will not be given to the measurer. 
     One embodiment of the howling margin measuring device  20  has been described hereinabove with reference to  FIGS. 1 to 4 . 
     In the above embodiment, a howling margin is calculated, determining that howling has been generated, in cases where the condition in which the compression level of the compressor  22  is equal to or higher than a specified value (e.g., “1 db”) has continued for a specified period of time (e.g., 5 sec.). Alternatively, generation of howling may be determined immediately after detecting the condition in which the compression level of the compressor is equal to or higher than a specified level. In other words, generation of howling may be determined without confirming that the above condition has continued for the specified period of time. And a howling margin may be calculated from the gain of the gain controlling means at that time. 
     In the above embodiment, the processing unit  28  is formed such that the gain controller  21  is placed in an anterior stage while the compressor  22  being in a posterior stage. An alternative form of the processing unit  28  may be such that the compressor  22  is placed in an anterior stage with the gain controller  21  placed in a posterior stage and these components are connected in series. In this case, the processing unit can operate similarly to the processing unit  28  of the above embodiment. 
     Numerous modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, the description is to be construed as illustrative only, and is provided for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details of the structure and/or function maybe varied substantially without departing from the spirit of the invention and all modifications which come within the scope of the appended claims are reserved. 
     INDUSTRIAL APPLICABILITY 
     The howling margin measuring device of the invention is useful for the technical field of acoustic systems, because it can objectively accurately measure a howling margin for an acoustic system without depending on the skill of the operator.