Abstract:
An automatic gain control circuit for use with a motorcycle radio and headset features an adjustable control for setting the maximum signal allowed to reach the headset, while allowing normal signal levels below that point to be controlled at the motorcycle radio. The circuit may be easily incorporated for use with a previously installed radio as well as an enhancement for new radio designs. Isolation transformers on both the input and output of the circuit eliminate the necessity to ground the circuit. Bass boost and internal gain circuits compensate for internal losses and headphone loading characteristics to provide near-transparent signal transfer below the set maximum signal level.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to the field of radio automatic gain control, and more specifically to automatic gain control of motorcycle radio signals received through a headphone.  
           [0002]    Headphones offer many obvious advantages when used as radio receivers, particularly in a high-noise environment. A potential disadvantage of headphone use, however, results if a dangerously or painfully loud signal is emitted through the headphone and the user is unable to quickly or conveniently pull the headphone away from the ears or lower the signal level with a manual control.  
           [0003]    Motorcycle officers commonly wear headphones that are connected to their police radios, but hands-free operation is particularly critical for a motorcycle officer because of the need to give full attention to the control of the motorcycle. Constant manual changing of the volume control in this environment is both distracting and dangerous, yet the potential problem of the user receiving an unacceptably loud signal is particularly high. It is likely, for example, that a police officer will set the volume level for “normal” operation relatively high to compensate for the noisy background level that accompanies the sounds of the motorcycle engine, sirens, and highway noise. While the volume level in radios found on police motorcycles is normally independent of the received radio frequency signal level, is it highly dependent on the talker voice level. Officers who set their listening level for “normal” may have a need to adjust the volume control to protect their ears from excessively loud talkers.  
           [0004]    Excessively loud talking levels are likely if the talker is excited or feels he must speak loudly to overcome local ambient noise. As a result, a listening officer may be placed in a hazardous situation while the vehicle is in motion if he is required to manually move his hands from the handlebars to manually adjust the radio volume. The problem is compounded if either the high volume level or the fact that he has reduced the volume reduces his ability to hear a subsequent speaker talking at or below a more “normal” volume level.  
         BRIEF DESCRIPTION OF THE PRIOR ART  
         [0005]    Headset control circuits are known that attempt to control peak volume level in various ways. For example, high volume signals may be eliminated by circuits that suppress any signal voltage above a predetermined level. Disadvantages of such circuits include the fact that they are not readily adjustable for use in different environmental conditions, and also that they commonly introduce unpleasant distortions into the received signal.  
           [0006]    Alternative devices for noise suppression generally are also capable of attenuating the peak volume level of a received signal. U.S. Pat. No. 6,118,878 to Jones describes an active noise cancellation system employing a complex arrangement of mechanical and electrical devices including microphones and sound generators. The additional cost, complexity, and power requirements of such systems are not readily compatible with the environment in which police motorcycles operate.  
           [0007]    Automatic gain control (AGC) circuits are also known for limiting the maximum volume level of a received signal. U.S. Pat. No. 5,369,711 to Williamson, III, describes an AGC circuit for limiting the peak volume level in a headset by controlling the signal across a capacitor. The principal disadvantages of this approach for the application identified above are complexity of the feed-forward control circuit used and difficulties in setting of the “nominal” output level within the AGC device.  
           [0008]    It is desired to have an automatic gain control circuit that efficiently limits the maximum volume level in a headphone while drawing very little power. The automatic gain control circuit can be added to or detached from existing headset circuits without modification to the existing cable harness. Ideally, the automatic gain control circuit will control only the maximum level of the signal reaching the headphone, with the nominal signal controlled at the signal source.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention is a headphone automatic gain control circuit intended primarily as an accessory attachment to radio systems currently installed on police motorcycles. It overcomes deficiencies in prior art devices with a much-simplified analog feedback circuit, with proper attack and decay time achieved by inherent low pass filter and hysteresis characteristics of a photosensitive resistor-based optical isolator. The present invention allows setting peak acceptable sound level presented to a headset, without restricting control of nominal level. “Nominal” gain control remains at the radio receiver, where it can be adjusted to compensate for varying background noise levels to which a police motorcycle is subjected in normal operation. Bass boost and internal gain compensate for internal losses and headphone loading characteristics to provide near-transparent signal transfer below a set maximum signal level. Variable gain control minimizes signal distortion over prior art “clipping” techniques. A gain control loop reacts quickly to excessive signal levels to reduce the signal to the headphones, then recovers within approximately the same time as the radio squelch circuit, so that the level for the next talker is unaffected by the reaction to an overly loud talker.  
           [0010]    Accordingly, it is an object of the present invention to provide gain control that is not dependent on the real time output voltage of the feedback amplifier.  
           [0011]    It is also an object to provide a gain control circuit that is immune from feedback bias such as is generated by field effect transistors (FETs).  
           [0012]    It is another object to provide a gain control circuit utilizing an optical feedback device.  
           [0013]    Is another object to provide a gain control circuit employing only a single adjustable gain amplifier stage.  
           [0014]    It is still another object to provide a gain control circuit that utilizes variable gain rather than “clipping” to minimize signal distortion.  
           [0015]    It is a further object to reduce the number of required parts, potentially reducing device cost and power drain.  
           [0016]    An automatic gain control circuit having these and other advantages includes a first inverting variable gain amplifier having an input for receiving audio signals of varying levels and an output. A second inverting variable gain amplifier has an input connected to the output of the first amplifier and an output, and a light emitting diode has an input connected to the output of the second amplifier, the diode providing an optical signal in response to a control signal. A photosensitive resistor responds to the optical signal and has an output connected to the input of the first amplifier, the first amplifier being responsive to the output of the photosensitive resistor to vary the gain of the first amplifier. A variable gain feedback circuit is connected between the output of the first amplifier and the input of the diode. The feedback circuit is adjustable to limit only the maximum level of the signal at the output of the first amplifier by controlling the optical signal from the diode. Normal signal levels below that point are unaffected by the feedback circuit, which permits them to be controlled at the motorcycle radio.  
           [0017]    A motorcycle communications system employing the invention includes a radio, a headset, and an automatic gain control circuit. The gain control circuit includes a first inverting variable gain amplifier having an input connected to the radio for receiving audio signals of varying levels from the radio and an output connected to the headset for providing signals to the headset. A second inverting variable gain amplifier has an output and an input connected to the output of the first amplifier. A light emitting diode has an input connected to the output of the second amplifier, the diode providing an optical signal in response to a control signal. A photosensitive resistor responds to the optical signal and has an output connected to the input of the first amplifier, which responds to the output of the photosensitive resistor to vary the gain of the first amplifier. A variable gain feedback circuit connects the output of the first amplifier to the input of the diode and is adjustable to limit the maximum level of the signal at the output of the first amplifier by controlling the optical signal from the diode, while allowing normal signal levels below that point to be controlled at the motorcycle radio. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    Other objects and advantages of the invention will become apparent from a study of the following specification when viewed in light of the accompanying drawing, in which:  
         [0019]    [0019]FIG. 1 is a block diagram of one embodiment of the invention;  
         [0020]    [0020]FIG. 2 is a block diagram of a power supply circuit for use with the embodiment of FIG. 1;  
         [0021]    [0021]FIG. 3 is a more detailed circuit diagram of the embodiment of FIG. 1;  
         [0022]    [0022]FIG. 4 is a circuit diagram of the power supply circuit of FIG. 2;  
         [0023]    [0023]FIG. 5 is a plot illustrating the peak signal transfer characteristic of the embodiment of FIG. 1; and  
         [0024]    [0024]FIG. 6 illustrates the embodiment of FIG. 1 packaged as an accessory for use with existing motorcycle radios.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    [0025]FIG. 1 illustrates a circuit  20 , in which an isolation transformer  24  receives a signal from an input  22  and provides a signal to an input loss compensation circuit  26 . A compression stage circuit  28  receives a signal from the input loss compensation circuit  26 . The compression stage circuit includes a variable amplifier  30  and a photo-electronic gain control circuit  32 . A signal from the compression stage circuit  28  is provided to an output loss pre-compensation circuit  34  and to a compression point adjustment circuit  36 . The compression point adjustment circuit  36  provides a signal to the photo-electronic gain control circuit  32 , which, in turn, provides gain control to amplifier  30 . An output transformer  38  receives a signal from the output loss pre-compensation circuit  34  and provides a signal to an output  40 .  
         [0026]    [0026]FIG. 2 illustrates a power supply circuit  44  for use with the circuit  20  of FIG. 1. A battery voltage input  46  is connected to a diode  48 . A positive voltage regulator  50  converts the switched battery voltage, which would normally be +12 volts, to a voltage suitable for operating circuit  20 . An output  54  permits connection of the regulated positive voltage to circuit  20 . A negative voltage converter  52  receives the signal from the positive voltage regulator  50  and provides a negative voltage as needed to output  56  for operation of circuit  20 . The diode  48  connected between input  46  and positive voltage regulator  50  provides reverse polarity protection for the circuit  44 .  
         [0027]    Referring now to FIG. 3, the isolation transformer  24  may be a 300/600 ohm audio transformer such as the model 42TL023 audio transformer distributed by Mouser Electronics. Output transformer  38  may be a 500/16 ohm audio transformer such as the Mouser Electronics model 42TL026 audio transformer. The function of the transformers is to prevent grounding of either side of the headset circuit, which is an important safety consideration for a vehicle-mounted radio. The input loss compensation circuit  26 , output loss pre-compensation circuit  34 , and compression point adjustment circuit  36  may all be implemented with inverting operational amplifiers. A single quad JFET-input general purpose operational amplifier module such as Mouser Electronics model TL084IN may be used to supply all of the operational amplifiers required for circuit  20 . The function and design of input loss compensation circuit  26  is well known in the prior art and is not further described here. Bass boost and internal gain provided by circuits  26  and  34  compensate for internal losses and headphone loading characteristics to provide near-transparent signal transfer below the set maximum signal level. Input losses result from normal internal loss of the isolation transformer  24 , from use of a termination resistor (not shown) to reflect near-normal headphone impedance back to the radio output, and a high frequency noise shunting capacitor  23  across the input load resistor  25 . Almost all of the loss, however, results from the isolation transformer  24 .  
         [0028]    The circuit  34  pre-compensates for coupling loss in the output transformer  38 . The negative input of an amplifier  29  is connected to the output of the amplifier  30  through a resistor  31  having a value R 9 . The negative input of a resistor  39 , having a value R 10 , resistor  40 , having a value R 11 , and capacitor  42  produce a bass-boost “shelving filter.” At DC (direct current), capacitor  42  opens that path including resistor  40 , so gain is set by the ratio R 10 /R 9 . As the frequency increases, the impedance of capacitor  42  drops, and that impedance plus R 11  starts to shunt resistor  39 , reducing the gain. Past the 3 dB point of capacitor  42  and resistor  40 , the gain quickly approaches [(R 10 *R 11 )/(R 10 +R 11 )/R 9 . Preferably, this “shelf” will be set for about 6 dB less gain at 1000 Hz than at 300 Hz. Capacitor  41  introduces a high frequency roll-off. Preferably, the 3 dB point will be set for about 10 kHz, which will eliminate high frequency noise resulting from excessive AGC bandwidth. Also, this helps reduce the 20 kHz whistle from the negative voltage converter, which may be audible to operators having particularly sensitive hearing.  
         [0029]    Compression stage circuit  28  and compression point adjustment circuit  36  constitute an optical feedback loop that provides gain control. The negative input of amplifier  30  is connected to the output of amplifier  26  through a resistor  27 . A photosensitive resistor  35  is connected in parallel with a feedback resistor  29  of amplifier  30 . Photosensitive resistor  35  is illuminated by a light emitting diode (LED)  33 , the intensity of which is determined by the current flowing through it. The varying resistance of photosensitive resistor  35  varies the gain of amplifier  30 . A manually controllable variable resistor  37 , connected as a negative feedback resistor across the operational amplifier  36 , provides the compression point (maximum volume) adjustment by controlling the current flow through the LED  33 . The photosensitive resistor  35  acts as a variable resistor as current flow through the LED  33  varies, thereby providing the negative feedback resistance to effect gain control via operational amplifier  30 .  
         [0030]    The input resistor  27  and feedback resistor  29  of amplifier  30  are selected for a desired nominal amplifier gain. The photosensitive resistor  35  is selected for a “dark” resistance value that is much greater than that of feedback resistor  29  of amplifier  30 . The photosensitive resistor  35  is also selected for an “illuminated” resistance that is much less than that of feedback resistor  29  of amplifier  30 . As a result, the gain of the operational amplifier  30  is unaffected until the feedback level, i.e., the output of operational amplifier  44 , causes LED  33  to sufficiently illuminate photosensitive resistor  35 . At that point, the gain of operational amplifier  30  is forced well below unity. Accordingly, by setting the gain of operational amplifier  44 , the feedback loop provides sharp output limiting.  
         [0031]    Photosensitive resistor  35  will have a response time to a rising signal level that is selected to provide sufficiently rapid response to minimize the amount of excess signal power reaching the headset without producing significant signal distortion. A response time of about 2-5 milliseconds has been found to work well in operational tests. The response time of photosensitive resistor  35  to a falling signal level is selected to provide gain recovery to the nominal level more slowly than a normal word-to-word time interval, but before the next transmission would normally be received. That is, the reaction time should be sufficiently long to avoid causing noticeable fluctuations in the speech signal passed to the headphone, while sufficiently short to allow recovery of operational amplifier  30  to nominal gain before a signal from the next speaker is received. A response time of about 500 milliseconds has been found to work well in operational tests.  
         [0032]    A combination of the light emitting diode  33  and the photosensitive resistor  35  suitable for use in this embodiment is available from SILONEX Corporation as Optocoupler Model NSL-32.  
         [0033]    The single adjustable gain amplifier stage of circuit  20  responds to root-mean-square (RMS) signal levels and is inherently a low pass device. As a result, the attack and decay characteristics of the photosensitive resistor provide response and recovery characteristics similar to that of the squelch circuit of the supported radio, without the requirement for separate timing elements as is found in many prior art devices. As a result, the volume level for a next, quieter, talker is unaffected by the reaction to an overly loud talker.  
         [0034]    The power supply circuit illustrated in FIG. 4 includes a voltage regulator  50  that can be implemented with a 3-terminal positive voltage regulator circuit such as is manufactured as model 7808 by U.S. Microwaves Corporation. In a preferred embodiment, voltage regulator  50  reduces the +12 volt battery voltage to +8 volts to drive the operational amplifiers in circuit  20 . Voltage converter  52  converts the +8 volts from voltage regulator  50  to −8 volts, which is also needed to drive the operational amplifiers in circuit  20 . Voltage converter  52  may be implemented with an Intersil Corporation model ICL7660S voltage converter.  
         [0035]    [0035]FIG. 5 illustrates a representative peak signal transfer characteristic of the circuit of FIG. 1, with the x-axis representing the maximum input signal level (volume) provided at terminal  22  and the y-axis representing the maximum output level provided at terminal  40 . As control loop gain is increased, the input signal level at which gain compression occurs is pushed lower. The curve illustrates the relationship between maximum input level and maximum output level as control loop gain is increased. As shown by line segment  62 , that relationship is linear until the compression point is reached. Past the compression point, higher maximum levels force a near real-time reduction in gain, restraining the maximum output signal level to the preset value. While line segments  66  and  68  are illustrated as perfectly flat, it will be readily understood by those skilled in the art that an exactly correct curve will be only close to flat. This results from the fact that increasingly higher maximum input levels do push maximum outputs slightly higher. Also, the loop delay required to minimize signal distortion allows sharply increasing signals to exceed the set limit for a few milliseconds before gain is suppressed. It is believed that this is more desirable than the signal distortion that would result from a more rapid loop response. The curve defined by line segments  62  and  64  describes the input/output relationship when rheostat  37  is set for minimum loop gain. As rheostat  37  is adjusted to increase the loop gain, the maximum output level decreases, as represented by arrow  68 , to a point where the input/output relationship is defined by line segments  62  and  66 . Adjustment of rheostat  37  to further increase the loop gain will continue to drop the maximum output level as illustrated.  
         [0036]    The embodiment of FIG. 1 may be incorporated into a radio design or it may be implemented as an accessory for use with existing motorcycle radios. FIG. 6 illustrates an embodiment of the invention packaged as an accessory  70  for existing motorcycle radios. A weatherized, shock and vibration resistant box  72  contains the electronic circuits described above, which are accessed through a multi-pin connector  82 . A rheostat having a knob  84  permits manual adjustment of variable resistor  37 . Alternatively, knob  84  may be replaced with a screwdriver-adjusted locking control as is well known in the prior art. Four connectors  74 ,  76 ,  78  and  80  are provided for connecting the invention into the wiring harness of the existing motorcycle electrical and radio systems. In particular, a first connector  74  will route the headset connection (SPKR HI and SPKR LO, the two sides of a balanced input signal) through circuit  20  while providing a normal-through for the microphone connections. A second connector  78  delivers the output of circuit  20  to the radio headset input. Connectors  76  and  80  provide power to circuit  20  and the emergency plug that is a conventional part of most prior art police motorcycle radios. Preferably a switched battery source will be tapped by connector  76  so that circuit  20  will be powered only when the radio is on.  
         [0037]    Adjustment of the maximum volume level of the circuit is made by the operator when the system is installed. It would not normally be necessary to adjust the circuit again by that operator. To perform the initial adjustment, the single control  84  is used to set the maximum acceptable signal level to the headphone. Using control  84 , the operator initially adjusts rheostat  37  to minimum rotary loop gain. While wearing the headphone, the operator sets the radio volume control at a point that is “too loud,” and then adjusts rheostat  37  until an acceptable signal level is obtained. The radio volume control is then returned to what the operator considers to be a normal listening level and the radio is ready for use.  
         [0038]    While the preferred forms and embodiments of the invention have been illustrated and described, it will be apparent to those of ordinary skill in the art that various changes and modification may be made without deviating from the inventive concepts set forth above.