Abstract:
The masking-effect of loud audio passages is utilized to reduce the audibility of the fan noise from a cooling fan in an audio system. A power monitor determines an output power of an audio amplifier and generates a power signal proportional to the output power. A switchable cooling fan is positioned to provide a cooling flow of air within the audio system when the cooling fan is turned on. A fan control is coupled to the power monitor and the switchable cooling fan. The fan control compares the power signal with a predetermined threshold and turns on the switchable cooling fan in response to the comparison indicating that reproduction of the amplified audio signals is at an output power level that will substantially mask audible noise created by the switchable cooling fan.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates in general to controlling a cooling fan in an audio system, and, more specifically, to running the cooling fan in a manner that masks audible noise generated by the cooling fan.  
           [0002]    Amplifiers used in audio systems cannot be designed with 100 percent efficiency and, therefore, always generate heat. Other components of the audio system, including any microcontrollers or other integrated circuits, media player mechanisms (e.g., CD or cassette tape), displays, transistors, or other discrete components, also generate heat during operation. Heat can especially be a problem in automotive audio systems in which all these elements are housed within a single module or box. Heat buildup can lead to improper circuit operation or failure, audio distortion, undesirable surface heating, or heating of the audio media (e.g., a CD ejected from the system may be hot to the touch). Consequently, heat dissipation or removal is an important consideration for any audio system.  
           [0003]    Heat exchange with ambient air is the primary method of removing heat from a typical audio system. In order to increase the heat exchange, a cooling fan may be provided for directing a flow of air through the audio system module so that fresh air is drawn from outside the module, over the heat-generating components, and back outside the module.  
           [0004]    A drawback of using a cooling fan in an audio system is the noise generated by the fan itself. Unless great care and expense are taken in designing and manufacturing the cooling fan, the level of noise generated can be considered objectionable by many users of the audio system. Even when cost is not a limitation, all noise cannot be eliminated. Furthermore, damage or structural or electrical changes over time may increase the noise level that is generated.  
           [0005]    Annoyance from fan noise can be minimized by running the fan intermittently, such as only when a certain amount of heat has built-up in the module. Many audio power amplifiers incorporate temperature sensing in their integrated circuits, but these typically only generate an output signal when the sensed temperature nears a level where a failure or improper operation may result (most audio systems use this signal to foldback or reduce the audio volume so that less heat is generated in the amplifier). Therefore, turning on the cooling fan in response to this signal is too late to prevent undesirable temperature levels. A separate temperature sensor can be used to sense the temperature within the module, but this adds an undesirable expense. Furthermore, intermittent operation of the cooling fan does not address the problem of audibility of the fan noise when the cooling fan is turned on.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention has the advantage of providing a cooling fan control strategy that operates the fan so that audibility of fan noise is minimized. The present invention uses the masking-effect of loud audio passages to reduce the audibility of the fan noise.  
           [0007]    In one aspect of the invention, an audio system for reproducing audio signals comprises an amplifier for amplifying the audio signals. A power monitor determines an output power of the amplifier and generates a power signal proportional to the output power. A switchable cooling fan is positioned to provide a cooling flow of air within the audio system when the cooling fan is turned on. A fan control is coupled to the power monitor and the switchable cooling fan. The fan control compares the power signal with a predetermined threshold and turns on the switchable cooling fan in response to the comparison indicating that reproduction of the amplified audio signals is at an output power level that will substantially mask audible noise created by the switchable cooling fan. Once the switchable cooling fan is turned on, the fan control keeps it turned on for a predetermined period. Even after the predetermined period, the switchable cooling fan is not turned off until the comparison indicates that reproduction of the amplified audio signals will no longer substantially mask audible noise created by the fan. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic, block diagram showing an audio system of the present invention.  
         [0009]    [0009]FIG. 2 is a flowchart showing a preferred fan control strategy of the present invention.  
         [0010]    [0010]FIG. 3 is a schematic, block diagram showing a preferred method and apparatus for determining output power being consumed by the audio amplifiers. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0011]    Referring to FIG. 1, an audio system  10  provides amplified audio reproduction signals to output transducers (i.e., loudspeakers)  11 ,  12 ,  13 , and  14 . In a preferred embodiment, audio system  10  is an automotive audio system and speakers  11 - 14  comprise right-front (RF), left-front (LF), right-rear (RR), and left-rear (LR) channels. Audio system  10  has an enclosure  15  which retains all the main heat-generating components of the audio system other than speakers  11 - 14 . Nevertheless, the present invention is also applicable to enclosures containing only some of the heat-generating components (e.g., a separate power amplifier module).  
         [0012]    Audio system  10  includes at least one audio source  16 , such as an AM/FM tuner, a cassette tape mechanism, or a CD mechanism, for providing left and right stereo audio signals. In the preferred embodiment, digital signal processing is employed in audio system  10  for achieving superior performance at low cost and high packaging efficiency. For an analog audio source (e.g., a tuner or cassette tape), analog-to-digital converters  17  and  18  convert analog audio signals into digital audio signals for processing by a digital signal processor (DSP)  20 . For a digital audio source, a direct connection may be provided to DSP  20  from audio source  16 .  
         [0013]    DSP  20  performs audio processing functions which are known in the art, including volume control, tone control, balance control, fade control, and special effects as determined by commands from a microprocessor  27 , which in turn responds to user controls (e.g., input switches or push buttons, not shown). The processed digital audio signals from DSP  20  are coupled to digital-to-analog converters (DAC)  21 - 24 , and the resulting analog signals are provided to power amplifiers  25  and  26  for driving speakers  11 - 14 . Amplifiers  25  and  26  are two- channel (i.e., stereo) amplifiers for handling front and rear channels, respectively, and each may be comprised of a TDA8566 car radio power amplifier sold by Philips Semiconductors, for example.  
         [0014]    A cooling fan assembly  30  includes an integral motor and a motor driver. The motor driver may simply be a switch for energizing a dc motor or may also include regulating circuitry or a power converter depending upon the type of motor being used. Microcontroller  27  provides a control signal for switching cooling fan  30  on and off. When turned on, cooling fan  30  creates an airflow as shown which draws outside air through cooling fan  30  into enclosure  15 . Airflow escapes enclosure  15  through a plurality of outlets  31 . Cooling fan  30  may be comprised of a 30×30 mm 2  fan AFB0312LA manufactured by Delta Electronics, Inc., for example.  
         [0015]    The present invention uses the ability of loud sounds to mask a sound that is of a lower volume and is substantially unchanging (e.g., has a constant sound level and tone). In the preferred embodiment, cooling fan  30  is turned on when either of the conditions exists that 1) loud enough audio signals are being reproduced that audible fan noise will be substantially masked, or 2) a high temperature has been reached in the power amplifiers. Microcontroller  27  is connected to a power monitor  32  which indirectly infers the output power being expended in the power amplifiers based on the level of the DSP output signals to DACs  21 - 24 . Since power amplifiers  25  and  26  typically provide a fixed amount of gain, the level of the DSP output signals is directly proportional to the power consumed by the power amplifiers. Alternatively, the actual outputs of the power amplifiers could be measured. Still other embodiments for determining output power of the amplifiers will occur to those skilled in the art. Since the greatest amount of heat is generated at times of loud audio output, by running cooling fan  30  during all times of loud audio output, the preferred embodiment achieves sufficient heat exchange so that elevated temperatures within the audio module are avoided under most conditions.  
         [0016]    Power amplifiers  25  and  26  include thermal warning circuits  33  and  34 , respectively. These provide a temperature warning signal in the event that an internal temperature of the power amplifiers reaches a threshold temperature (e.g., 150° 0  C.). The temperature warning signals are provided to microprocessor  27  which turns on cooling fan  30  if the temperature warning signals occur, regardless of the loudness of the audio signals at that time.  
         [0017]    The overall fan control strategy of the preferred embodiment is shown in FIG. 2. In initialization step  40 , a counter for counting a variable “Loud_Count” is reset and a timer is reset. The timer measures a time window, or equivalently a sample-count window, used to ensure that a loud and sustained section of audio material is present rather than just a short burst of loud sound during an overall quiet passage. A time window of about 1 minute is preferably employed, during which a plurality of output power samples are taken (preferably at regular intervals). The time window can be established by using a timer or by counting out a predetermined number of output power samples.  
         [0018]    In step  41 , a check is made to determine whether an active temperature warning signal is being received from any power amplifier. If such a warning signal is being received, then the cooling fan is turned on in step  42 . Otherwise, a power level is determined in step  43  for the current time interval or sample interval within the overall 1-minute time window. Each sample or time interval may be about 1 second, for example. In step  44 , the power level is compared to a power threshold which is chosen to represent a loudness which, if sustained for a long enough time, will audibly mask the fan noise. The power threshold takes into account the level of noise produced by the cooling fan, for example. If the instantaneous power level is greater than the power threshold, then Loud_Count is incremented by 1 in step  45 , otherwise step  45  is skipped.  
         [0019]    In step  46 , a check is made to determine whether the 1-minute time window has expired (e.g., whether 60 power level samples have been taken at 1 second intervals). If not, then a return is made to step  41 . Otherwise, Loud_Count is compared to a count threshold in step  47 . The count threshold defines the extent to which loud power level measurements must dominate an audio passage from moment to moment (i.e., second to second) in order to achieve a desired masking effect. A count threshold in the range of about 50% (i.e., 30 samples out of 60 in the present example) may be employed. If Loud_Count exceeds the count threshold, then the cooling fan is activated in step  42 . Otherwise, the cooling fan is deactivated in step  48  (unless it was not then active, in which case it is left in a deactivated state in step  48 ).  
         [0020]    After the cooling fan is activated in step  42 , it is left on for a predetermined delay period (e.g., 3 minutes) in step  49 . This ensures a substantial amount of heat exchange can take place each time the cooling fan is started-up and avoids excessive cycling of the cooling fan which would be undesirable because fan noise would be most noticeable during its starting and stopping transients.  
         [0021]    After the 3 minute delay of step  49  or the fan deactivation of step  48 , a return is made to step  40  for re-initialization. If the cooling fan is running at the time of the return to step  40 , it continues to run during the next series of power level measurements. Thus, the cooling fan will actually run for a minimum time of 4 minutes in the present example. Specifically, the cooling fan continues to run until a 1-minute time window is encountered which does not satisfy the count threshold (i.e., is not loud enough to audibly mask the fan noise). Consequently, the cooling fan is operated at substantially all times during which its audible noise will be masked, plus the short amount of time it takes to verify that sufficient loudness of the audio signal to achieve masking is no longer present.  
         [0022]    [0022]FIG. 3 shows a preferred embodiment for determining the power level within an audio system using digital signal processing. Since power is proportional to voltage squared, each of the separate audio signal channels to be input to the power amplifiers (i.e., signals LF, LR, RF, and RR) is squared in a respective squaring block  51 - 54 . The squared values from blocks  51 - 54  are added together in a summing block  55 . The sum is integrated over a predetermined interval in an integrator  56 . The predetermined interval preferably corresponds to the 1-second sampling interval of each power level sample. The power signal from integrator  56  may be provided from DSP  20  to microcontroller  27  for comparison to the power threshold as described above.  
         [0023]    [0023]FIG. 4 shows an alternative embodiment wherein a microcontroller is not required to take action during each individual power level measurement. Beginning at step  60 , this alternate fan control method waits for about one minute (i.e., the microcontroller executes the fan control strategy once per minute). In step  61 , the thermal warning register from the amplifier IC is read. During the 1-minute waiting period, power is measured within the DSP so that a measurement is available from the DSP that quantifies the average power during the previous minute. The microcontroller reads the average power level in step  62 .  
         [0024]    In step  63 , a check is made to determine whether the fan is already on. If the fan is not activated, then a check is made in step  64  whether the average power level is greater than a power threshold. If greater than the threshold, then the fan is activated in step  65  and a return is made to step  60 . Otherwise, a check is made in step  66  whether the thermal warning is active. If the thermal warning is active then the fan is activated in step  65 , otherwise a return is made to step  60 .  
         [0025]    If step  63  determines that the fan is already activated, then a check is made in step  67  whether the average power level is less than the power threshold. If it is not, then a return is made to step  60 . If the average power level is below the power threshold, then a check is made in step  68  to determine whether the thermal warning is active. If it is, then a return is made to step  60 . If the thermal warning is not active, then the fan is de-activated in step  69  and a return is made to step  60 .