Patent Application
20130077796
Loudspeakers used in compact and portable devices require significant design compromises that may lead to suboptimal sound quality and loudness. A loudspeaker used in a compact device (e.g. a cellular phone, an electronic tablet, a laptop computer, a PDA (personal digital assistant), a media player etc.) is usually small. As a result, the sensitivity of the loudspeaker can be low and the diaphragm on the loudspeaker can have a limited range of motion. Often loudspeakers are overdriven in order to obtain the loudness needed to hear the audio signal coming from it.
Overdriving a loudspeaker can cause the voice coil in a loudspeaker to overheat because of the additional current needed to overdrive the speaker. Overheating the loudspeaker can cause permanent damage to a loudspeaker. For example, overheating can change the shape of the diaphragm of the loudspeaker. A loudspeaker where the diaphragm has changed shape from its original form distorts sound coming from the loudspeaker.
In addition to changing the shape of the diaphragm, overheating a voice coil can cause the glue holding the voice coil to the driver to melt. When the glue melts it can cause the voice coil to separate from the driver rending the loudspeaker inoperable. Overheating the voice coil can also cause the solder connecting an amplifier to the voice coil to melt, disconnecting the loudspeaker from the amplifier. The heat from the voice coil can also cause the insulation on the voice coil to melt. When the insulation melts, the metal in the voice coil can short to each other reducing the number of windings in the voice coil. Reducing the number of windings in the coil can limit the loudness of the loudspeaker and further heat the voice coil due to the lowered resistance.
Overdriving a loudspeaker can increase the mechanical stress on the loudspeaker causing it to fail. Loudspeakers used in compact devices are relatively cheap. However, damage to a loudspeaker in a compact device may cause a return of the entire device. In order to reduce the damage done to loudspeakers and improve the loudness and quality of the loudspeakers, the power applied to a loudspeaker needs to be controlled to reduce overheating of the voice coil.
The drawings and description, in general, disclose a method for protecting an electro dynamic transducer (e.g. loudspeaker) against thermal overload. As part of the method, an estimate of the temperature of the voice coil in the electro dynamic transducer is obtained while the power of an input audio signal is measured. After the estimate of the temperature is obtained and the power of the input audio signal is measured, the power of the audio input signal is reduced when a power limit is exceeded and/or a temperature limit is exceeded.
The voice coil 111 provides the motive to the cone 109 by the reaction of the magnetic field provided by the magnet 103 and the magnetic circuit 101 to the current flowing through the voice coil 111. By driving a current through the voice coil 111, a magnetic field is produced. This magnetic field causes the voice coil 111 to react to the magnetic field from the permanent magnet 103 fixed to the speaker's frame (not shown), thereby moving the cone 109 of the electro dynamic transducer 100. By applying an audio signal to the voice coil 111, the cone 109 will reproduce the sound pressure waves corresponding to the original audio signal.
The amount of power that a voice coil 111 may handle without damage is related to the thermal tolerance of the wire insulation, adhesive, and bobbin material, and may be influenced by the voice coil's position within the magnetic gap 105. In addition, the amount of power that a voice coil may use is limited by the amount of heat that can be removed from the voice coil 111.
An input audio signal 220 is applied to the controller 202 and the dynamic power limiter 204. The controller 202 measures the power of the input audio signal 220. When the power of the input audio signal 220 exceeds a predetermined power limit in the controller 202, the controller 202 instructs the dynamic power limiter 204 to attenuate the input audio signal 220. The predetermined power limit is obtained by measuring the heating of the voice coil 111 while varying the amount of power in the input audio signal 220. The dynamic power limiter 204, for example, can attenuate the input audio signal 220 by multiplying the input audio signal by factor of X where X ranges from 0 to 1.
The attenuated input audio signal 224 is then applied to the DAC 208. The DAC 208 converts the digital attenuated input audio signal 224 to an analog signal 226. The analog signal 226 then drives the power amplifier 210. The gain of the amplifier 210 in this example is G. Since the gain of the input 226 to the amplifier 210 can vary from 0 to 1, the gain of the output of the amplifier 210 can vary from 0 to G.
Because the analog signal 226 is attenuated, the output 216 of the power amplifier 210 supplies a smaller current to the voice coil 111 of the electro dynamic transducer 212 than would have been supplied if the input audio signal 220 would not have been attenuated. Because a smaller current is supplied, the heating of the voice coil 111 is reduced.
In the embodiment shown in
In an embodiment of the invention, a voltage 232 across the voice coil 111 of the electro dynamic transducer 212 and an estimate of the current 234 through the voice coil 111 are presented to the ADC 230. The outputs 214 and 216 of the ADC 230 are applied to the temperature estimator 206. Based on the voltage 216 and the current 214, the temperature estimator 206 outputs an estimate 218 of the temperature of the voice coil 111. The temperature estimate 218 may be calculated in several ways.
The temperature of the voice coil 111 may be estimated by measuring a the resistance of the voice coil 111. The resistance or dcr (direct current resistance) of voice coils typically range from 4 ohms to 16 ohms at room temperature, 73° F. However, as the temperature of a voice coil 111 increases due to current being conducted through it, the resistance of the voice coil 111 also increases. The resistance of a voice coil 111 in an electro dynamic transducer 212 at particular temperatures may be measured and cataloged. The results of these measurements may be included as part of a lookup table used in the temperature estimator 206. The temperature estimator 206 may also use an equation based on these measurements to give the temperature estimate 218.
The temperature estimate 218 is applied to the controller 202. In addition to the temperature estimate 218, an input audio signal 220 is applied to the controller 202. The controller 202 uses the temperature estimate 218 and the input audio signal 220 to dynamically control the gain of the dynamic power limiter 204. For example, when the power of the input audio signal 220 exceeds a predetermined power limit in the controller 202, the controller 202 instructs the dynamic power limiter 204 to attenuate the input audio signal 220. The dynamic power limiter 204, for example, can attenuate the input audio signal 220 by multiplying the input audio signal by factor of X where X ranges from 0 to 1.
The attenuated input audio signal 224 is then applied to the DAC 208. The DAC 208 converts the digital attenuated input audio signal 224 to an analog signal 226. The analog signal 226 then drives the power amplifier 210. The gain of the amplifier 210 is G. Since the gain of the input 226 to the amplifier 210 can vary from 0 to 1, the gain of the output of the amplifier 210 can vary from 0 to G.
Because the analog signal 226 is attenuated, the output 216 of the power amplifier 210 supplies a smaller current to the voice coil 111 of the electro dynamic transducer 212 than would have been supplied if the input audio signal 220 would not have been attenuated. Because a smaller current is supplied, the heating of the voice coil 111 is reduced.
In the previous example, the current supplied to the voice coil 111 was reduced. Because the current supplied to the voice coil 111 was reduced, the loudness of the electro dynamic transducer 212 would not be as loud as it would have been otherwise. However, because the current may only be limited for a short time, the perceived loudness of the electro dynamic transducer 212 does not change appreciably when compared to the case were the current is not reduced. The controller 202 dynamically changes in response to the power of the input audio signal 220 and the temperature estimate 218.
When neither a input signal power limit nor a temperature limit is exceeded, the controller 202 instructs the dynamic power limiter 204 to allow the input signal 220 to pass through the dynamic power limiter 204 without attenuation. As consequence, the loudness produced by this signal in the electro dynamic transducer 212 remains unchanged as well.
In the case where a temperature limit is exceeded and the input signal power limit is not exceeded, the controller 202 instructs the dynamic power limiter 204 to attenuate the input audio signal 220. The amount the input audio signal 220 is attenuated by the dynamic power limiter 204 when the temperature limit is exceeded and the input signal power limit is not exceeded is different from the amount the input audio signal 220 is attenuated when the temperature limit is exceeded and the input signal power limit is exceeded. The controller 202 adjusts the amount the input signal 220 is attenuated based on whether both the input signal power limit and the temperature limit are exceeded. In addition, the absolute value of the signal power limit and the absolute value of the temperature limit determine the amount of attenuation of the input audio signal 220.
In an embodiment of the invention, the controller 202 may be a PID (proportional integral derivative) controller. A PID controller is a generic control loop feedback mechanism widely used in industrial control systems. A PID controller calculates an “error” value as the difference between a measured process variable (e.g. temperature or power) and a desired set point for the variable. The controller attempts to minimize the error by adjusting the process control inputs.
The PID controller calculation involves three separate constant parameters, and is accordingly sometimes called three term control: the proportional, the integral and the derivative values. These values can be interpreted in terms of time: P depends on the present error, I on the accumulation of past errors, and D is a prediction of future errors, based on current rate of change. The weighted sum of these three actions is used to adjust the process via a control element such as the temperature of a voice coil.
During step 304, the power of an input audio signal 220 is measured. During step 306, it is determined whether the measured power of the input audio signal 220 exceeds a predetermined power limit. When the measured power of the input audio signal 220 exceeds the predetermined power limit, the input audio signal 220 is attenuated as shown in step 310. When the measured power of the input audio signal 220 does not exceed the predetermined power limit, it is determined during step 308 if the temperature of the voice coil 111 exceeds a predetermined temperature limit. When the temperature of the voice coil 111 exceeds the predetermined temperature limit, the input audio signal is attenuated as shown in step 310.
When the temperature of the voice coil 111 does not exceed the predetermined temperature limit, the input audio signal is not attenuated and is passed directly to an amplifier to be amplified as shown in step 312. The amplifier, as shown in step 314, then amplifies the input audio signal. Next the amplifier drives the voice coil 111. The attenuated signal from step 310 is also amplified in step 314 when a power limit or a temperature limit is exceeded.
The process shown in
The instantaneous temperature of the voice coil 111 may be estimated by applying the instantaneous resistance of the voice coil 111 to a lookup table or an equation contained in the temperature estimator 206. The values used in the lookup table or in the equation are obtained by measuring the resistance of the voice coil at different temperatures prior to using the temperature estimator.
In another embodiment of the invention, the instantaneous current conducted through the voice coil 111 may be determined using a current sensor. A current sensor is a device that detects electrical current (AC or DC) in a wire, and generates a signal proportional to it. The generated signal can be an analog voltage, an analog current or a digital output. The generated signal along with the voltage across the voice coil can then be used to calculate the instantaneous resistance of the voice coil. The instantaneous temperature of the voice coil 111 may be estimated by applying the instantaneous resistance of the voice coil 111 to a lookup table or an equation.
In another embodiment of the invention, the current sensing is part of (built into) the amplifier 210. In this embodiment, the current drawn through the coil 111 may be calculated by measuring the voltage drop across the output transistors of the amplifier 210. The amplifier 210 in this example is a class D amplifier. However, other types of amplifiers such as class A or class AB could be used.
In the embodiment of the invention shown in
Because low frequency signals require more power to be reproduced, the current needed to drive the voice coil 111 is reduced when low frequency signals are removed from the input audio signal 504. Reducing the amount of current needed to drive the voice coil 111 also reduces the heating of the coil. Therefore, removing low frequency signals from the input audio signal helps protect the electro dynamic transducer 212 from overheating.
The foregoing description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiments were chosen and described in order to best explain the applicable principles and their practical application to thereby enable others skilled in the art to best utilize various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments except insofar as limited by the prior art.
This application is related to Ser. No. ______/______ (TI-71350) entitled “Thermal Control of Voice Coils in Loudspeakers”, and to Ser. No. ______/______ (TI-70802) entitled “Over-excursion Protection for Loudspeakers”, filed on even date herewith and are hereby incorporated by reference for all that is disclosed therein.
