Abstract:
A circuit for processing broadcast signals that includes circuitry for receiving and processing broadcast signals which contain audio information and providing a first audio signal, and circuitry for controlling the amplitude of a received second audio signal in response to a first control signal, and providing a third audio signal wherein the circuit further comprises circuitry that receives the first audio signal and provides the second audio signal for automatically limiting the amplitude of the first audio signal in response to at least one reference signal.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the automatic limiting of signals. More particularly, the present invention relates to a circuit and method for automatically limiting the amplitudes of broadcast and/or transmitted audio signals. 
   2. Background Art 
   The following figures which illustrate the various embodiments of the background art and the present invention may incorporate the same or similar elements. Therefore, where the same or similar elements occur throughout the various figures, they will be designated in the same manner. 
     FIG. 1  illustrates a block diagram of a known circuit for varying the amplitude of audio signals. 
   This figure comprises a receiving circuitry  100  and output circuitry  110  and is applicable to an analogue and/or digital system. 
   The receiving circuitry  100  and output circuitry could for example be the respective audio front and back ends in a radio, television, video, satellite decoder etc. 
   The receiving circuitry  100  receives, via an input  120 , a transmitted or broadcast signal Si. The circuitry  100  then processes the signal Si and outputs, via its output  130 , a first audio, i.e. sound, signal A 1 . 
   The input  140  of the output circuitry  110  is connected to the output  130  of the receiving circuitry  100  and thus the output circuitry  110  receives the signal A 1 . The output circuitry  110  then processes the signal A 1  and outputs, via its output  150 , a second audio signal A 2 , which is then fed to a speaker system (not illustrated). 
   The output circuitry  110  also receives a control signal C 1  via another input  160 . This control signal C 1  is controlled by the user of the apparatus in which this circuitry of  FIG. 1  is utilised. This signal C 1  is used to control the peak, average and/or Root Mean Square (RMS) amplitude of the audio signal A 2 . Therefore, by increasing, for example, the RMS amplitude of the audio signal A 2 , the volume of the signal from the speakers is increased and vice-versa. The user can control the signal C 1  by, for example, a knob or button on said apparatus or by a remote control system that works in conjunction with said apparatus. 
   One problem associated with the arrangement of  FIG. 1  is that if there is a change in the RMS amplitude of the signal A 1 , then this will be proportionally reflected in the signal A 2  and hence the volume. The effect of this is that the user will have to readjust the control signal C 1  to return the volume emanating from the speakers, i.e. the RMS amplitude of the signal A 2 , to substantially the same level as before the change in the RMS amplitude of the signal A 1 . 
   An example of where changes in the RMS amplitude of the signal A 1  occur are in the commercial breaks of television, satellite and radio broadcasts. During these commercial breaks quite often the amplitude of the broadcast audio signal is increased, which results in an increased volume output. The purpose underlying this increase is to stimulate the listener and draw his attention to the commercial and hence the product, service etc. being advertised. However, the increase in the level of the volume can be as much as +6 dB for example, i.e. double the amplitude of the original signal, which results in the listener diving for the control knob/button or scrabbling for the remote control device in order to reduce the volume. Then when the commercial is over, the listener has to readjust the volume back to the acceptable level it was before the commercial. 
   Another example of where sudden changes in the amplitude of the signal A 1  occurs is during the tuning of a radio. The strengths, i.e. the amplitudes, of some signals are greater than others and it can be quite disturbing, and in some instances dangerous, when there is a sudden increase in the output volume: this is especially the case if one is tuning a car radio when driving for example. 
   OBJECTS &amp; SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to automatically compensate for variations, whether deliberate or otherwise, in the signals of audio systems and apparatus. 
   Another object of the present invention is to automatically maintain the amplitudes of signals within audio systems and apparatus to substantially a fixed level or substantially within a range of one or more levels. 
   In order to achieve these objects, the present invention proposes a circuit for processing broadcasted/transmitted signals that comprises: circuitry for receiving and processing the broadcast signals, which contain audio information, and providing a first audio signal; and circuitry for controlling, i.e. adjusting, the amplitude, i.e. the volume, of a received second audio signal in response to a first control signal and providing a third audio signal. The circuit further comprises circuitry, that receives the first audio signal and provides the second audio signal, for automatically limiting or adjusting the amplitude of the first audio signal in response to at least one reference signal. 
   According to another embodiment of the present invention, the circuitry for automatically limiting or adjusting the amplitude of the first audio signal comprises: circuitry, that receives the second audio signal, for providing an output signal in response to the amplitude of the second signal; circuitry for comparing the output signal and said at least one reference signal and providing a second control signal in response to the output signal and said at least one reference signal; and circuitry, that receives the first audio signal and that is controlled in response to the second control signal, for providing the second audio signal. 
   According to another embodiment of he present invention, the circuitry for providing: the output signal; the second control signal; and for providing the second audio signal are implemented by analogue and/or digital means. 
   According to another embodiment of the present invention, the circuitry for providing: the output signal; the second control signal; and for providing the second audio signal are implemented by hardware digital circuitry. 
   According to another embodiment of the present invention, the digital means can be represented by one or more digital signal processing algorithms and/or by one or more software routines. 
   According to another embodiment of the present invention, the digital means is implemented by any combination of hardware digital circuitry, one or more digital signal processing algorithms, and one or more software routines. 
   According to another embodiment of the present invention, the circuitry for providing: the output signal is a Root-Mean Square extractor circuitry; the second control signal is an integrating comparator; and the circuitry for providing the second audio signal is an attenuator. 
   According to another embodiment of the present invention, the Root-Mean Square extractor circuitry comprises a series connected rectifier and low pass filter. 
   According to another embodiment of the present invention, the circuitry for providing the second control signal comprises a current sourcing/sinking comparator having a capacitor connected between its output terminal and a reference voltage, i.e. ground potential for example. 
   According to another embodiment of the present invention, the circuitry for providing the second audio signal is a multiplying digital-to-analogue converter. 
   According to another embodiment of the present invention, the circuit is included in an apparatus that receives broadcast and/or transmitted signals. 
   According to other embodiments of the present invention, the circuit is included in circuitry and/or an apparatus that receives television, satellite, and/or radio signals. 
   The present invention also proposes a method for processing broadcast or transmitted signals that comprises the steps of: receiving and processing the broadcast or transmitted signals, which contain audio information, and providing a first audio signal; and controlling the amplitude of a received second audio signal in response to a first control signal and providing a third audio signal. The method further comprises the step of automatically limiting, i.e. adjusting, the amplitude of the first audio signal in response to at least one reference signal and providing a second audio signal. 
   According to another embodiment of the present invention, the step of automatically limiting the amplitude of the first audio signal comprises: providing an output signal in response to the amplitude of the second signal; comparing the output signal and said at least one reference signal and providing a second control signal in response to the output signal and said at least one reference signal; and receiving the first audio signal and controlling said first audio signal in response to the second control signal, for providing the second audio signal. 
   According to another embodiment of the present invention, the method is practiced in circuitry and/or an apparatus that receives television, satellite and/or radio signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, as well as other advantages and features, of the present invention will become apparent in light of the following detailed description and accompanying drawings among which: 
       FIG. 1  has already been depicted as exposing the state of art and the problem to overcome; 
       FIG. 2  illustrates a block diagram of a circuit for automatically limiting the amplitude of audio signals according to the present invention; 
       FIG. 3  illustrates an example of a circuit diagram for automatically limiting the amplitude of an audio signal according to the present invention; and 
       FIGS. 4   a – 4   f  illustrate waveforms associated with the circuitry of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  illustrates a block diagram of a circuit for automatically limiting the amplitude of audio signals according to the present invention. 
   In addition to the circuitry  100  and  110  of  FIG. 1 ,  FIG. 2  also includes circuitry  200  for automatically limiting or adjusting the amplitude of the audio signal A 1  in response to at least one reference signal Ref 1 . This reference signal Ref 1  may be pre-defined during the design phase and/or may or may not be adjustable afterwards. 
   Since the circuitry  100  and  110  of  FIG. 1  has already been described, Its operation will hereafter be omitted for the purposes of brevity. 
   According to the present invention, the circuitry  200  comprises: an attenuator  210 , signal processing circuitry  220 , and an integrating comparator  230 . 
   A first input  250  of the attenuator  210 , which according to the present invention is preferably variable, is connected to the output  130  of the circuitry  100  and receives the signal A 1 . The output  260  of the attenuator  210 , which carries a second audio signal A 2 ′, is connected to the respective inputs  140  and  270  of the respective circuitry  110  and  220 . The output  275  of the circuitry  220 , which carries a feedback signal FB that is derived from the audio signal A 2 ′, is connected to a first input  280  of the integrating comparator  230 . The second input  285  of the integrating comparator  230  receives the reference signal Ref 1 . The output  290  of the integrating comparator  230 , which carries a second control signal C 2 , is connected to a second input  295  of the attenuator  210 . 
   According to the present invention, a purpose of the attenuator  210 , the signal processing circuitry  220 , and integrating comparator  230  is to limit the amplitude of the audio signal A 2 ′, in response to the reference signal Ref 1 , to a desired threshold, by automatically compensating for variations beyond said threshold of the amplitude of the signal A 1 . According to the present invention it is preferable to limit the RMS amplitude, however the present invention can be used to limit the average or peak amplitude of the signal A 1 . 
   By way of an example of the operation of the circuit according to the present invention consider the following, which is applicable to television, satellite and radio transmitted broadcasts. 
   The attenuator  210 , which could be a network or transducer, is controlled by control signal C 2 , which is in turn controlled by the signals Ref 1  and FB. Assume that, during ‘normal’ broadcast, i.e. when the RMS amplitude of the broadcast audio signal is not deliberately increased, the signals Ref 1  and FB are at values such that the attenuator  210  provides substantially 0 dB&#39;s of attenuation: therefore the RMS value of the audio signal A 2  substantially equals that of A 1 . The user controls the output volume from the speakers, i.e. the RMS amplitude of the audio signal A 3  that appears on the output  150  of the circuitry  110 , to its desired level by altering the control signal C 1 , which has the effect of either amplifying or attenuating the signal A 2 ′ within the circuitry  110 . Assume now that the broadcast is not ‘normal’, i.e. the RMS amplitude of the broadcast audio signal has been deliberately increased by +6 dB for example. According to the present invention, an object of the processing circuitry  220  and the integrating comparator  230  is to stimulate the attenuator  210  such that it attenuates the signal A 1 , preferably by the same amount that it was amplified by. Initially, since the attenuator provides an attenuation of 0 dB, the signal A 2 ′ has substantially the same RMS amplitude as A 1 . The increase in the amplitude of signal A 2 ′ is detected by the processing circuitry  220 , whose output signal FB changes as a result of this increase. Assume for example that the FB is a voltage signal that increases from a value V 1  to a value V 2 . Also assume that the reference signal Ref 1  supplied to the integrating comparator  230  is a voltage signal that has a value V 3 , which is slightly greater than V 1 , so as to allow for minor increases in the signal FB, but less than V 2 . As the signal FB increases from V 1  to V 2  it triggers the comparator  230  so that the control signal C 2  stimulates the attenuator such that its attenuation of the signal A 1  changes rom 0 dB to −6 dB, thereby restoring the level of the signal A 2 ′ to substantially its previous value. It should be noted that the response time for the circuitry  200  to attenuate the amplified signal A 1  will be such that the user will not notice any appreciable change in the output volume of the apparatus. 
   In a system where the increase in the signal A 1  during a not ‘normal’ broadcast is fixed: not ‘normal’ in the sense that the broadcast audio signal in purposely increased; it is sufficient enough to fix the amount of attenuation provided by the attenuator  210  so that the attenuator  210  can just be switched on when attenuation is needed and vice-versa. 
   However, according to the present invention, it is preferable to be able to have variable attenuation that is controllable so as to be able to attenuate the signal A 1  by the same amount that it was amplified. As an example, assume A 1  is increased by only +3 dB, as opposed to +6 dB, then it is preferable that the attenuator  210  attenuates it by −3 db and so forth. 
   It should be noted that, according to the present invention, it is preferable that the user should have the option to be able to control whether or not attenuation during advertising actually takes place or not. The user can therefore control, using control signal C 3 , whether or not the circuitry  200 , or any part of it, operates such that it provides the necessary attenuation, or not as the case may be, during the advertising breaks. The user may control this attenuation function with a switch or button or the like, or alternatively, via a remote control apparatus with, in the case of a television or monitor, an On Screen Display (OSD) facility for example. 
   According to the present invention, it is preferable to design a multi-standard system especially for television, video, satellite applications etc. This is easily achieved with the aid of a microcontroller or microprocessor and some memory (not illustrated). The memory can De pre-programmed with various information about the peak, average and/or RMS values of the broadcast signals and even standards throughout the various countries of the world. The correct settings can easily be called up during the assembly and testing phases of the apparatus. The attenuator can then be controlled directly by, or in conjunction with, the microcontroller/microprocessor so that it provides the correct attenuation as and when it is required. 
     FIG. 3  illustrates an example of a circuit diagram for automatically limiting the amplitude of an audio signal according to the present invention. 
   The circuitry  200  of  FIG. 2  is illustrated in further detail in this present figure. The attenuator  210  comprises a multiplying digital-to-analogue (D/A) converter; the processing circuitry  220  a rectifier  300  and low pass filter  305 ; and the integrating comparator comprises a comparator COMP and an integrator INT. 
   The multiplying D/A comprises two voltage followers  310 ,  315  and a switched resistive attenuation control circuit  320 . The signal A 1  is applied to the control circuit  320  after having been buffered by the voltage follower  310 . The control circuit  320  receives the control signal C 2  from the comparator  230 . The control circuit  320  can be controlled by a microprocessor or microcontroller (not illustrated), as indicated by the dashed input  325 . This control input  325  can be used to actively adjust the amount of attenuation provided by the control circuitry  320  in response to predetermined references for example. The signal A 2  is derived from the output  330  of the circuit  320  via the voltage follower  315 . 
   The rectifier  300  recifies the signal A 2 ′ before it is passed through the low pass filter  305 . The resulting output signal FB from the filter  305  is the Root-Mean Square (RMS) value of the signal A 2 ′. 
   The signal FB is compared with the reference signal Ref 1 , so that for example., when FB is greater than Ref 1 , the output signal C 2  from the integrating comparator  230  is a positive ramp, which is used to control the circuit  320 . 
     FIGS. 4   a – 4   f  illustrate waveforms associated with the circuitry of  FIG. 3 . 
   It should be noted that the following figures are not to scale and are representations of the underlying principles and that the waveform of  FIG. 4   a  only illustrates the positive envelope of a broadcast signal. 
   When the signal A 1  increases, at time t 0 , from its ‘normal’ value to its increased value, as illustrated in  FIG. 4   a , this causes the value of the signal FB to start to increase, as illustrated in  FIG. 4   b . When the voltage value of the signal FB becomes greater than approximately that of the reference voltage Ref 1 , the comparator&#39;s output  335 , in this particular example, changes to a high state, as illustrated in  FIG. 4   c . The resulting output signal C 2  from the integrator, which can be implemented by a capacitor (not illustrated) for example, is a positively increasing ramp. The signal C 2  acts upon the attenuation control circuit  320  such that it attenuates the signal A 1 , as can be seen by the resultant attenuated signal A 2 ′ illustrated in  FIG. 4   f . Naturally, the attenuation of the signal A 1  to produce the attenuated signal A 2 ′ results in an attenuation of the signal FB 
   When the signal A 1  has beer attenuated by the attenuation control circuit  320  such that the value of FB equals that of Ref 1 , then the output of the comparator COMP reduces to its low state, as illustrated at time t 1  in  FIG. 4   b.    
   As a result of the comparator returning to its low state, the output signal C 2  starts to reduce, which reduces the attenuation of the signal A 1 , which in turn increases the RMS value of the signal A 2 ′. Therefore, the signal FB increases such that it becomes greater than approximately Ref 1 , as illustrated at time t 2  in  FIG. 4   b . This in turn causes the output from the comparator COMP to change to a high state, that in turn causes the signal C 2  to increase, which in turn causes the attenuator  210  to increase the attenuation of the signal A 1  and so on. 
   Therefore, he attenuation oscillates about its mean value, of say −6 dB for example. However, the circuit can be designed such that the oscillations or adjustments made to the attenuation are not prone to detection by the listener. One way of achieving this is to have relatively long time constants associated with the charging and discharging of the integrator INT. However, these time constants are such that the listener would neither detect any substantial difference in the output volume when the there is an increase/decrease in the signal A 1  or the oscillations in the signal A 2 ′. 
     FIG. 3  is just one example of how to automatically compensate for variations in the amplitude of an audio signal. The same can be achieved whether the system is analogue and/or digital. The implementation of the basic block diagram of  FIG. 2  can be either: analogue; and/or digital, either hardware and/or by means of one or more Digital Signal Processing (DSP) algorithms and/or one or more software routines. These types of solutions will be known to those skilled in the art. 
   Although this invention has been described in connection with certain preferred embodiments, it should be understood that the present disclosure is to be considered as an exemplification of the principles of the invention and that there is no intention of limiting the invention to the disclosed embodiments. On the contrary, it is intended that all alternatives, modifications and equivalent arrangements as may be included within the spirit and scope of the appended claims be covered as part of this invention.