Abstract:
A signal processing method is provided, which makes it possible to quickly find the cause of clipping or the like. A sound signal that is input is subjected to processing of adjusting at least one of sound volume and sound quality. It is determined whether the input sound signal satisfies a condition that the level of the sound signal exceeds a predetermined value at a plurality of metering points on a signal path along which the input sound signal is transmitted. An alarm is displayed on a screen when it is determined that the input sound signal satisfies the condition at at least one of the plurality of metering points.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a signal processing method, a program for implementing the method, and a signal processing apparatus that can be suitably used for mixing sound signals.  
           [0003]    2. Description of the Related Art  
           [0004]    Conventionally, there are known mixing apparatuses (signal processing apparatuses) which synthesize sound signals of multiple input channels. Many of these mixing apparatus have a clip lamp for warning of an excessive level (hereinafter referred to as ′clipping″) provided for the respective ones of input channels and mixing outputs. In recent years, a digital mixing apparatus has been developed which has AD converters provided for respective input channels and DA converters provided for respective output channels such that digital processing is performed at all parts other than inputs and output parts.  
           [0005]    In the digital mixing apparatus, however, sound signals are rapidly deteriorated by clipping. Accordingly, the digital mixing apparatus is required to quickly find the cause of clipping, etc. and take proper measures.  
         SUMMARY OF THE INVENTION  
         [0006]    It is therefore an object of the present invention to provide a signal processing method, a program for implementing the method, and a signal processing apparatus that make it possible to quickly find the cause of clipping or the like.  
           [0007]    To attain the above object, the present invention provides a signal processing method comprising an adjusting step of subjecting a sound signal that is input, to processing of adjusting at least one of sound volume and sound quality, a condition determining step of determining whether the input sound signal satisfies a condition that a level of the sound signal exceeds a predetermined value at a plurality of metering points on a signal path along which the input sound signal is transmitted, and an alarm display step of displaying an alarm when the condition determining step determines that the input sound signal satisfies the condition at at least one of the plurality of metering points.  
           [0008]    In a typical preferred form of the present invention, the signal processing method further comprises a mixing step of mixing the sound signal subjected to the adjusting processing and outputting the mixed sound signal.  
           [0009]    In a typical preferred form of the present invention, the sound signal comprises a plurality of sound signals input for a plurality of channels, respectively, and the plurality of metering points are provided on a signal path of each of the plurality of channels along which a corresponding one of the input sound signals is transmitted.  
           [0010]    In a typical preferred form of the present invention,the plurality of metering points on the signal path along which the input sound signal is transmitted include at least first and second metering points, the method further comprising a first display step of displaying a level of the sound signal at the first metering point on a first screen, and a second display step of displaying a level of the sound signal at the second metering point on a second screen, and the alarm is displayed on the first and second screen by the alarm display step.  
           [0011]    To attain the above object, the present invention also provides a program executed by a computer, comprising an adjusting module for subjecting a sound signal that is input, to processing of adjusting at least one of sound volume and sound quality, a condition determining module for determining whether the input sound signal satisfies a condition that a level of the sound signal exceeds a predetermined value at a plurality of metering points on a signal path along which the input sound signal is transmitted, and an alarm display module for displaying an alarm when the condition determining module determines that the input sound signal satisfies the condition at at least one of the plurality of metering points.  
           [0012]    To attain the above object, the present invention further provides a signal processing apparatus comprising an adjusting device that subjects a sound signal that is input, to processing of adjusting at least one of sound volume and sound quality, a condition determining device that determines whether the input sound signal satisfies a condition that a level of the sound signal exceeds a predetermined value at a plurality of metering points on a signal path along which the input sound signal is transmitted, and an alarm display device that displays an alarm when the condition determining device determines that the input sound signal satisfies the condition at at least one of the plurality of metering points.  
           [0013]    According to the above arrangement of the present invention, if the condition that the level of the sound signal exceeds the predetermined value is satisfied at any of the metering points, an alarm is indicated correspondingly to an channel to which the metering point belongs. This makes it possible to quickly find the cause of clipping or the like.  
           [0014]    The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a schematic block diagram showing the arrangement of a digital mixing apparatus as a signal processing apparatus according to an embodiment of the present invention;  
         [0016]    [0016]FIG. 2 is a schematic block diagram showing an algorithm that is executed according to the embodiment;  
         [0017]    [0017]FIG. 3 is a view showing an example of display in an input channel meter window with respect to a metering point MP 1 ;  
         [0018]    [0018]FIG. 4 is a view showing an example of display in an input channel meter window with respect to a metering point MP 2 ;  
         [0019]    [0019]FIG. 5 is a view showing an example of display in an input channel meter window with respect to a metering point MP 3 ;  
         [0020]    [0020]FIG. 6 is a view showing an example of display in an output channel meter window;  
         [0021]    [0021]FIG. 7 is a flow chart showing a window selecting routine;  
         [0022]    [0022]FIG. 8 is a flow chart showing an input metering point selecting routine;  
         [0023]    [0023]FIG. 9 is a flow chart showing a peak hold switching routine; and  
         [0024]    [0024]FIG. 10 is a flow chart showing a timer interruption routine. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    The present invention will be described in detail with reference to the drawings showing an embodiment thereof.  
         [0026]    Referring to FIG. 1, there is shown the construction of a digital mixing apparatus as a signal processing apparatus according to an embodiment of the present invention.  
         [0027]    1. Construction of Hardware  
         [0028]    First, there will now be described the construction of hardware of the digital mixing apparatus according to the present embodiment with reference to FIG. 1.  
         [0029]    In FIG. 1, an analog input unit  101  is comprised of a plurality of head amplifiers for amplifying microphone inputs from a plurality of channels, and a plurality of AD converters that convert output signals from the head amplifiers into digital signals while multiplexing them. An analog output unit  102  is comprised of a plurality of DA converters that convert the multiplexed digital signals of plural output channels into analog signals of the respective channels.  
         [0030]    The output signals from the analog output unit  102  are supplied mainly to a power amplifier that drives a speaker. A signal processing engine  110  carries out a mixing process, an effecting process, and the like on input signals supplied from the analog input unit  101 , and supplies the resulting signals to the analog output unit  102 . A console  120 , which is operated by a user such as a mixing engineer, controls the modes of the mixing process, the effecting process, etc. carried out by the signal processing engine  110 .  
         [0031]    A description will now be given of a suitable arrangement of the above-mentioned components in a concert hall. First, the analog input unit  101  is installed at a position close to performers, e.g. at the backstage in order to reduce the length of a microphone cable that transmits feeble analog signals. The console  120  is installed in a mixing booth located at the center of the seats or the like so as for the user to operate the console  120  while listening to sounds. Relatively loose limitations are imposed upon the installment locations of the analog output unit  102  and the signal processing engine  110 , since the analog output unit  102  handles relatively high-level analog signals and the signal processing engine  110  handles only digital signals. If these components are installed in the mixing booth, however, it is unavoidable to broaden the mixing booth and therefore necessitate reducing the number of seats. Therefore, it is preferable to install the components at the backstage or the like.  
         [0032]    In the signal processing engine  110 , an interface circuit  111  transmits and receives digital signals to and from the analog input unit  101  or the analog output unit  102  via a coaxial cable or the like. A DSP system  112  carries out a mixing process, an effecting process, and the like on input digital signals supplied from the analog input unit  101  via the interface circuit  111 , and supplies the resulting signals to the analog output unit  102  via the interface circuit  111 . A memory system  113  is used as a program memory and a data memory for the DSP system  112 .  
         [0033]    A CPU  116  receives commands from the console  120  via an interface circuit  114  according to a control program stored in a memory system  115 , and sets the contents of the memory system  113 , i.e. an algorithm and parameters executed by the DSP system  112 . The CPU  116  supplies information on the setting conditions of the algorithm in the DSP system  112  and monitor signals or the like for monitoring sound signals from the respective components to the console  120  via the interface circuit  114 .  
         [0034]    In the console  120 , a panel section  124  is comprised of an operating element group  125  composed of a fader, a switch, and the like, and a display group  126  that displays various kinds of information for the user. The operating element group  125  is provided with a keyboard and a mouse for use in inputting characters in order to enable window operations as is the case with ordinary personal computers. A CPU  123  transmits the contents of operations of the operating element group  125  to the signal processing engine  110  via an interface circuit  121 , and displays various kinds of data supplied from the signal processing engine  110  on the display group  126 . A memory system  122  is used as a program memory and a data memory for the CPU  123 .  
         [0035]    2. Algorithm  
         [0036]    Referring next to FIG. 2, a description will be given of the algorithm employed in the present embodiment. This algorithm is implemented by the hardware shown in FIG. 1 and software. In FIG. 2, input channel processing sections  201 ,  202 , . . . ,  20   k  carry out an effect imparting process, a volume controlling process, a panning process (distribution of sound signals into right and left output channels), and the like with respect to the respective ones of the first, second, . . . , and the k th input channels. In the input channel processing section  201 , a head amplifier  211  and an AD converter section  212  are equivalent to the analog input unit  101  in FIG. 1.  
         [0037]    A tone control section  213  provides control of frequency characteristics, etc. of sound signals. The frequency characteristics, etc. are designated by an operating element of the operating element group  125  in the console  120 , and a filtering process or the like based on the operation of the operating element is carried out by the DSP system  112  in the signal processing engine  110 . A fader operating element  215  is included in the operating element group  125 . A multiplier section  214  multiplies a control input of the fader operating element  215  by an output signal from the tone control section  213 . The multiplication of the multiplier section  214  is implemented by calculation in the DSP system  112 .  
         [0038]    A panning processing section  216  controls the distribution ratio of sound signals in the right and left output channels. A stereo switch  217  switches the way of outputting sound signals between stereo outputting and monaural outputting. It should be noted that the monaural outputting means setting the distribution ratio of sound signals in the right and left output channels to 1:1 irrespective of the setting conditions of the panning processing section  216 . The setting of the distribution ratio of sound signals in the panning processing section  216  and the switching of the way of outputting in the stereo switch  217  are carried out by operating elements included in the operating element group  125  as is the case with the designation of the frequency characteristics, etc. by the tone control section  213 . The control of the setting of the distribution ratio of sound signals in the panning processing section  216  and the switching of the way of outputting in the stereo switch  217  is implemented by calculation in the DSP system  112 . It should be noted that the arrangements of the other input channel processing sections  202 , . . . ,  20   k  are identical with the arrangement of the input channel processing section  201  described above in detail.  
         [0039]    A left bus line  240  synthesizes left output signals from the input channel processing sections  201 ,  202 , . . . ,  20   k  by means of adder sections  241 ,  242 , . . . ,  24   k . Similarly, a right bus line  250  synthesizes right output signals from the input channel processing sections  201 ,  202 , . . . ,  20   k  by means of adder sections  251 ,  252 , . . . ,  25   k . The synthesis of the output signals by the respective bus lines  240 ,  250  is implemented by calculation in the DSP system  112 . A left output channel processing section  220  carries out an effect imparting process and a sound volume controlling process for a signal resulting from the synthesis by the left bus line  240 , and supplies the resulting signal to a DA converter section  260  for the left output channel. On the other hand, a right output channel processing section  230  carries out an effect imparting process and a sound volume controlling process for a signal resulting from the synthesis by the right bus line  250 , and supplies the resulting signal to a DA converter section  270  for the right output channel. The DA converter sections  260 ,  270  are equivalent to the analog output unit  102  in FIG. 1.  
         [0040]    In the left output channel processing section  220 , a tone control section  221  controls the frequency characteristics, etc. of the left output signal as is the case with the tone control section  213  in the input channel processing section  201 . The frequency characteristics are designated by an operating element included in the operating element group  125  in the console  120 , and a filtering process, etc. based on the operation of the operating element is carried out by the DSP system  112  in the signal processing engine  110 . A fader operating element  223  is included in the operating element group  125  as is the case with the above-mentioned fader operating element  215 . A multiplier section  222  multiplies a control input of the fader operating element  223  by an output signal from the tone control section  221 . The multiplication of the multiplier section  222  is implemented by calculation in the DSP system  112 . Similarly to the left output channel processing section  220 , a right output channel processing section  230  is comprised of a tone control section  231 , a multiplier section  232 , and a fader operating element  233 .  
         [0041]    In the input channel processing section  201 , the level of a sound signal is sequentially metered at an input end of the tone control section  213 , an input end of the multiplier section  214 , and an output end of the multiplier section  214 . These points of metering will be called metering points MP 1 , MP 2 , MP 3 . In the left output channel section  220 , the level of a sound signal is sequentially metered at an input end of the tone control section  221 , an input end of the multiplier section  222 , and an output end of the multiplier section  222 . These points of metering will be called metering points L 1 , L 2 , L 3 . Likewise, in the right output channel processing section  230 , the level of a sound signal is sequentially metered at an input end of the tone control section  231 , an input end of the multiplier section  232 , and an output end of the multiplier section  232 . These points of metering will be called metering points R 1 , R 2 , R 3 .  
         [0042]    3. Operation  
         [0043]    3.1 Outline of Displaying Process  
         [0044]    A description will now be given of the operation of the present embodiment.  
         [0045]    First, when the digital mixing apparatus is activated and the user performs a predetermined operation by means of the operating element group  125 , a meter window  300  as shown in FIG. 3 is displayed on the display group  126 . In FIG. 3, the meter window  300  is comprised of an input channel meter window  302  and an output channel meter window  304  with two tabs. Tabs  302   a ,  304   a  are provided at the top of the windows  302 ,  304 , respectively. In the illustrated state, however, the window  304  is not displayed on the display section  126  except for the tab  304   a.    
         [0046]    The input channel meter window  302  is intended to monitor metering points of the input channel processing sections  201 ,  202 , . . . ,  20   k,  and a plurality of level meters  310  corresponding to the respective ones of the first, second, . . . , k th channels are displayed in the input channel meter window  302 . These level meters  310  are intended to indicate the level at the metering point MP 1 , MP 2 , or MP 3  in the form of a histogram. Reference numerals  312 ,  314 ,  316  denote metering point setting switches provided correspondingly to the metering points MP 1 , MP 2 , MP 3 , respectively. The metering point setting switches  312 ,  314 ,  316  are intended to alternatively select one metering point to be monitored in each input channel.  
         [0047]    A peak hold switch  318  is provided to set an on-off state representing whether the respective level meters  310  provide a peak hold display or not. The peak hold display means displaying the level of a peak value in each level meter  310  continuously (the display of the peak value may be continued only over a predetermined period of time, or may be continued until any canceling operation such as switching-off of the peak hold switch  318  is carried out). In a normal operating state, the peak hold display is preferably ON. The top of each level meter  310  is especially called a clip display section  308 . A Σ display section  306  is provided at the upper side of the clip display section  308 .  
         [0048]    A detailed description will now be given of the clip display section  308  and the Σ display section  306 . If the level of the sound signal at any one metering point selected as the metering point reaches the maximum value, the clip display section  308  of the corresponding level meter  310  is lighted. On this occasion, if the peak hold display is ON, the clip display section  308  is continuously lighted even if the level of the sound signal at the metering point is subsequently lowered. This enables the user to see the metering point at which clipping occurs.  
         [0049]    TheΣ display section  306  is lighted when clipping occurs at any one metering point of the corresponding input channel. If the peak hold display is ON, the Σ display section  306  is continuously lighted even if the level of the sound signal at the metering point is subsequently lowered. For example, assuming that clipping occurs at the metering point MP 3  of the second input channel while the metering point MP 1  in each input channel is monitored in the input channel meter window  302 , the Σdisplay section  306  of the second input channel is lighted even if clipping does not occur in any of the level motors  310 .  
         [0050]    [0050]FIG. 3 is based on the above assumption. In FIG. 3, among the metering point setting switches  312 ,  314 ,  316  and the peak hold switch  318 , the lighted (ON) switches are indicated in white. That is, the metering point MP 1  is selected as the metering point by the user, and the peak hold display is ON. In the clip display section  308  and the Σ display section  306  as well, lighted section and areas are indicated in white. In the case of the second input channel (CH 2 ) for example, the clip display section  308  is unlighted. This means that clipping has not occurred at the metering point MP 1  of the second input channel after the peak hold display was turned on on the last occasion.  
         [0051]    On the other hand, the Σ display section  306  of the second input channel is lighted. This means that clipping has occurred at the metering point MP 2  or MP 3 .  
         [0052]    [0052]FIG. 4 shows the input channel meter window  302  in a case where the metering point MP 3  is selected as the metering point by the user. In FIG. 4 as well, the clip display section  308  of the second input channel is unlighted.  
         [0053]    This means that clipping has not occurred at the metering point MP 3  in the second input channel after the peak hold display is turned on on the last occasion.  
         [0054]    [0054]FIG. 5 shows the input channel meter window  302  in a case where the metering point MP 2  is selected as the metering point by the user. In FIG. 5, the clip display section  308  in the second input channel is lighted. It will be learned that the Σ display section  306  in the second input channel is lighted due to clipping at the metering point MP 2 .  
         [0055]    [0055]FIG. 6 shows a state in which the output channel meter window  304  is displayed in the meter window. In FIG. 6, the level meter  310 , the clip display section  308  and the Σ display section  306  are displayed with respect to each of the metering points L 1 , R 1 , L 2 , R 2 , L 3 , R 3  of the output channels, and the respective levels at the metering points are indicated as is the case with the input channel meter window  302 . In the example shown in FIG. 6, clipping has occurred at the metering point L 1 , and the Σ display sections  306  at all the metering points L 1 , L 2 , L 3  of the left output channel to which the metering point L 1  belongs are lighted.  
         [0056]    3.2 Window Selecting Routine (FIG. 7)  
         [0057]    A description will now be given of a concrete procedure for carrying out the above described displaying process.  
         [0058]    First, in a default state when the digital mixing apparatus has just been activated, the meter window is displayed such that the input channel meter window  302  is displayed at the forefront on the screen as shown in FIG. 3. On this occasion, if either one of the tabs  302   a ,  302   b  is clicked using the mouse included in the operating element group  125  of the console  120 , a window selecting routine in FIG. 7 is started. If the program proceeds to a step SP 2  in FIG. 7, it is determined whether the input channel has been selected for display or not (i.e. whether the tab  302   a  has been clicked or not).  
         [0059]    If the determination result is positive (YES) in the step SP 2 , the program proceeds to a step SP 4  wherein the input channel meter window  302  is displayed on the display group  126 . On the other hand, the determination result is negative (NO) in the step SP 2 , the output channel meter window  304  is displayed on the display group  126 . If either one of the windows  302 ,  304  is thus displayed, the routine is terminated.  
         [0060]    3.3 Input Metering Point Selecting Routine (FIG. 8)  
         [0061]    If any one of the metering point setting switches  312 ,  314 ,  316  is clicked using the mouse while the input channel meter window  302  is displayed, an input metering point selecting routine in FIG. 8 is started. If the program proceeds to a step SP 12  in FIG. 8, it is determined which point has been selected among the metering points MP 1 , MP 2 , MP 3 , and the program proceeds to different steps according to the results of the determination.  
         [0062]    First, if the metering point setting switch  312  is clicked using the mouse, it is determined that the metering point MP 1  has been selected and the program proceeds to a step SP 14 . In the step SP 14 , the metering point setting switch  312  is lighted, and the contents of the level meter  310  and the clip display section  308  are set according to the result of level metering at the metering point MP 1  in each input channel. If the metering point setting switch  314  or  316  is clicked using the mouse, the program proceeds to a step SP 16  or SP 18  wherein the contents of the level meter  319  and the clip display section  308  are set according to the result of level metering at the metering point MP 2  or MP 3  (see FIGS. 4 and 5). The routine is then terminated.  
         [0063]    3.4. Peak Hold Switching Routine (FIG. 9)  
         [0064]    If the peak hold switch  318  is clicked using the mouse while either one of the windows  302 ,  304  is displayed, a peak hold switching routine in FIG. 9 is started. If the program proceeds to a step SP 8  in FIG. 9, the on-off state of the peak hold display is inverted to terminate the routine. If the peak hold display is turned on as a result of the inversion, the peak hold switch  318  is set lighted, and if the peak hold is turned off, the peak hold switch  318  is set unlighted.  
         [0065]    3.5 Timer interruption routine (FIG. 10)  
         [0066]    If either one of the windows  302 ,  304  is displayed, a timer interruption occurs in the CPU  123  at predetermined time intervals to start a timer interruption routine in FIG. 10. If the program proceeds to a step SP 22  in FIG. 10, it is determined whether the input channel meter window  302  is displayed at the forefront on the screen or not. If the determination result is positive (YES), the program proceeds to a step SP 26  wherein a numeral “1” is assigned to a variable (channel number) j. If the program then proceeds to a step SP 28 , it is determined whether or not clipping has been detected at any one of the metering points MP 1 , MP 2 , MP 3  in the j th input channel.  
         [0067]    If the determination result is positive (YES), the program proceeds to a step SP 30  wherein the Σ display section  306  of the j th input channel in the input channel meter window  302  is set lighted. Further, in the step SP 30 , the clip display section  308  of the j th input channel at the metering point where clipping has been detected is set lighted. If the program then proceeds to a step SP 32 , it is determined whether the channel number j is equal to the maximum channel number k or not. If the determination result is negative (NO), the program proceeds to a step SP 34  wherein the channel number j is incremented by “1” and the program returns to the step SP 28 .  
         [0068]    On the other hand, if clipping has not been detected at any of the metering points MP 1 , MP 2 , MP 3  in the j th input channel, the determination result is negative (NO) in the step SP 28  and the program then proceeds to a step SP 36 . In the step SP 36 , it is determined whether the peak hold display is ON or not. If the determination result is negative (NO), the program proceeds to a step SP 38  wherein the Σ display section  306  and the clip display section  308  of the j th input channel are set unlighted and the program then proceeds to the step SP 32 .  
         [0069]    On the other hand, if the determination result is positive (YES) in the step SP 36 , the program proceeds to the step SP 32  while skipping the step SP 38 . Therefore, if the peak hold display is ON and clipping has been detected at any of the metering points MP 1 , MP 2 , MP 3 , the Σ display section  306  of the j th input channel and the corresponding clip display section  308  are continuously set lighted. Therefore, the user can find a metering point where clipping has occurred according to the state of the Σ display section  306 . The channel number j of the input channel to be processed is sequentially incremented in the step SP 34 , and the steps SP 28  to SP 38  are repeatedly executed with respect to the j th input channel. If the steps SP 28  to SP 38  have been repeated with respect to all the input channels, the routine is terminated.  
         [0070]    Although the above description is based on the case where the input channel meter window  302  is displayed, a description will now be given of a case where the output channel meter window  304  is displayed. If the window  304  is displayed, the determination result is negative (NO) in the step SP 22  and the program proceeds to a step SP 24 . In the step SP 24 , the same process as in the steps SP 28  to SP 38  is carried out with respect to the right and left output channels.  
         [0071]    The example in FIG. 6 assumes that clipping has occurred at the metering point L 1 . In this case, the same process as in the step SP 30  is carried out to set the display section  306  lighted at the metering points L 2 , L 3  as well as the metering point L 1  as shown in FIG. 6.  
         [0072]    4. Variations  
         [0073]    It should be understood that there is no intention to limit the invention to the above described embodiment, but on the contrary, the invention is to cover all modifications, alternate constructions and equivalents falling within the spirit and scope of the invention as described below.  
         [0074]    1) Although in the above described embodiment, the present invention is applied to the digital mixing apparatus, it goes without saying that the present invention may be applied to an analog mixing apparatus. The analog mixing apparatus is implemented by removing the AD converter section  212 , etc. and the DA converter sections  260 ,  270  in the block diagram of FIG. 2 and constructing or replacing the other respective components by analog circuits. In such an analog mixing apparatus, the level of a sound signal is monitored at the respective metering points to detect clipping state at the metering points MP 1 , MP 2 , MP 3  of each input channel. If clipping is detected at any metering point, a lamp (Σ display section) corresponding to the input channel to which the metering point belongs is lighted to achieve the same effects as in the above described embodiment.  
         [0075]    2) Although the above described embodiment assumes that the control program is executed by the CPU  123  in the console  120 , the console  120  may be replaced by a universal personal computer or the like. In this case, the control program may be stored in a storage medium such as a floppy disk and a CD-ROM so that the control program can be distributed as an application program for general-purpose personal computers.  
         [0076]    3) In the above described embodiment, the Σ display section  306  only capable of coping with clipping in the input channels is lighted while the input channel meter window  302  is displayed, and the_display section  306  only capable of coping with clipping in the output channels is lighted while the output channel meter window  304  is displayed. The invention may be modified such that if clipping occurs in an output channel while the input channel meter window  302  is displayed, or if clipping occurs in an input channel while the output channel meter window  304  is displayed, it is possible to indicate some alarm to that effect so that the user can see it.