Abstract:
A portable electronic device, such as a pager or a mobile station, has a housing and a vibration motor coupled to the housing for imparting vibration to it. Within the device, an electronic audio signal is preferably split into high and low frequency components by parallel filters. Pulses of the low frequency component are rectified, voltages of those rectified pulses are quantified and at least some are converted to pulse widths, and the train of pulse widths is amplified to drive the vibration motor, resonating the housing at in correspondence with the pulse amplitudes of at least some pulses of the original electronic audio signal. Various adaptive schemes are presented to control the number of modulated pulses that drive the vibration motor to enhance the user experience. The vibration motor may act as an audible sub-woofer and/or may provide a bass effect vibration within or beyond the audible range in correspondence to certain spaced pulses of the original audio signal.

Description:
FIELD OF THE INVENTION 
       [0001]    This disclosure relates to portable devices such as mobile radio terminals and pagers that have a vibration function for silently alerting a user to an incoming communication. It is particularly directed to a method and apparatus for driving a vibration mechanism with electronic audio signals to generate a vibration in correspondence with that audio signal. 
       BACKGROUND 
       [0002]    Portable electronic communication devices continue to integrate multiple functions beyond their core communications with great popularity, including for example still and/or video cameras, music storage and playing, GPS circuitry, and Internet operability. These desires of customers for multiple functionality lies in opposition to their desire for small size in the same communication device, so manufacturers increasingly seek to make components multi-functional and increase functionality via software to serve these competing customer desires while containing costs and size. One feature of enduring customer popularity is both an audible and a silent alert for incoming communications, common on devices such as mobile stations and pagers and selectable by a user. The silent alert is selected, for example, when the user desires to be made aware of an incoming call or page without interrupting all nearby participants in a meeting or social engagement. With few recent exceptions, the vibration mechanism has been dedicated solely to the silent alert function. 
         [0003]    The vibration mechanism typically includes a (relatively) massive rotor mounted to a shaft that is driven by an electric motor activated by an incoming communication when the user selects the silent mode. The rotor is purposefully not rotationally balanced about the shaft, so rotation of the shaft causes a noticeable vibration. The eccentricity of the rotor&#39;s mass about the shaft is selected to be substantial enough to cause the entire mobile station or pager to vibrate, gaining the attention of a user holding the device in his/her hand or pocket. Typically, the electric “vibration” motor is mounted directly to the housing of the device. 
         [0004]    Relevant teachings, by which the vibration mechanism may be used for the synthesis of low-frequency sound in addition to its traditional function of silent vibration alerts, is described in U.S. Pat. No. 6,809,635, issued on Oct. 26, 2004 and entitled “Mobile Terminal Using a Vibration Motor as a Loudspeaker and Method of Use Thereof”, which is hereby incorporated by reference. That incorporated patent also refers to U.S. Pat. Nos. 5,300,851; 5,373,207; 6,081,055; 5,783,899; 5,861,797 and 5,960,367, as well as European Patent Applications EP 0 688 125 A1; 1 001 249 A2 and 1 035 633 A1 as describing vibration motors and their implementations. These teachings are directed to increasing the functionality of such a vibration mechanism. 
       SUMMARY 
       [0005]    These teachings are directed to enhancing a user&#39;s sensory experience, such as when playing music. The invention may be implemented in devices such as mobile radio terminals that traditionally use a vibration mechanism for non-audio purposes, or in a portable music device such as an MP-3 player or broadcast radio receiver that traditionally does not employ a vibration mechanism. Alternatively, devices such as the above mobile radio terminals and portable music devices may serve as a source music device that provides the audio signal. In such alternative embodiments, devices of the present invention are coupled via a conductive wire or wireless (e.g., Bluetooth) connection to the otherwise independent source music device and function similarly as if they were physically integrated into the source device. Embodiments may provide an audible sub-woofer sound to the user, and/or a bass effect (‘kick’) that may or may not be within the audible frequency range that corresponds to certain notes sounded over a traditional speaker. 
         [0006]    Embodiments of the invention may be a method of transducing an audio signal in a portable electronic device. Transducing is used herein in its broad sense, converting an input signal of one form into an output signal of a different form. For example, different embodiments of the invention may convert an input electronic signal into an output vibration that may or may not be within normal human hearing frequencies. In the method, a series of driving pulses are generated by pulse-width modulating an electronic audio signal. A vibration motor is then driven with at least some of the series of driving pulses to transduce the electronic audio signal into a vibration, which may be heard by the user or merely felt if below the threshold for human hearing. The vibration motor is specifically configured to vibrate a housing of the portable electronic device. In one variation, generating the series of driving pulses includes, for each of the series of driving pulses, rectifying a sample of the electronic audio signal, determining an amplitude of the rectified sample, and then mapping the amplitude to a pulse width. 
         [0007]    Another embodiment of the invention is a portable electronic device, such as a pager or a mobile station. The device includes a housing and a vibration motor coupled to the housing for imparting vibration to the housing. Also included is a source for providing an electronic signal, such as an antenna for receiving a streaming audio signal or voice communications over a wireless link (e.g., Bluetooth or streaming download/podcasts) or a wired connection to a digital music player/mobile terminal, or a computer readable memory for storing an electronic file such as digital music or combined audio-visual files. A mapper is also in the device for converting a pulse amplitude to a corresponding pulse width. An amplifier is disposed between the mapper and the vibration motor for amplifying a signal from the mapper and further for driving the vibration motor with the amplified signal. In response the vibration motor produces a vibration. 
         [0008]    Another embodiment of the invention is a mobile station that includes means for providing an electronic audio signal, such as a stored media file or an antenna and receiver for receiving a media file or voice communications in real time. The mobile station further has means for modulating at least some pulse amplitudes of the electronic audio signal to corresponding pulse widths, and means such as an amplifier for driving a motor means with the corresponding pulse widths. The motor means may be a vibration motor with an imbalanced rotor. The vibration from the motor means may be within the audible range of human hearing, or it may be experienced by the user only as a touch input if beyond that frequency range. 
         [0009]    These and other aspects are set forth with particularity below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIGS. 1A-1D  are each schematic diagrams reproducing FIGS. 3-6 respectively of U.S. Pat. No. 6,809,635. 
           [0011]      FIG. 2  is a schematic block diagram showing music synthesized in both high and low frequency bands, the low band according to an embodiment of the present invention. 
           [0012]      FIG. 3  is a schematic block diagram showing further detail of the low-frequency band of  FIG. 2 . 
           [0013]      FIGS. 4A-B  are pulse diagrams showing adaptive control in converting audio signal pulses into excitation signals to a vibration motor. 
           [0014]      FIG. 5A-B  are similar to  FIGS. 4A-B  showing thresholds used to drive adaptive control. 
           [0015]      FIGS. 6A-B  are graphs with bass level (dB) versus pulse strength for direct mapping and adaptive mapping, respectively. 
           [0016]      FIG. 7  is a simplified block diagram showing automatic gain control and compression configured to adaptively map pulses to a vibration motor. 
           [0017]      FIG. 8  is a schematic block diagram of a mobile station in which the embodiment of  FIG. 2  may be disposed. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The invention is not limited to the following described embodiments, which provide readily adaptable implementations. One skilled in the art will see enhancements that keep within the scope described by the embodiments, and other devices in which the present invention may be disposed. As background, the teachings of U.S. Pat. No. 6,809,635 are briefly reviewed. 
         [0019]    U.S. Pat. No. 6,809,635 describes a mobile station that uses a conventional vibration motor to perform the additional function of reproducing audio communications received by the user of the mobile terminal. The use of the vibration motor either eliminates altogether the requirement for a separate loudspeaker or enhances the reproduction of lower frequency components of the received audio communications which permits the loudspeaker to function as a tweeter to reproduce only the higher frequency components of the received audio communications to produce an overall better reproduction of the received audio communication. 
         [0020]    In one embodiment reproduced here as  FIG. 1A , U.S. Pat. No. 6,809,635 describes applying a demodulated output signal  104 , in the form of audio communications, to an input of an audio amplifier  106  that amplifies the input audio communications to a sufficient signal level to drive the vibration motor  108  to reproduce an audible sound  110 . The gain level is chosen to provide the required output sound level, and the audio amplifier  106  may utilize amplification, which need not be a linear function of frequency so as to better reproduce the natural sound of the caller. 
         [0021]    In another embodiment reproduced here as  FIG. 1B , U.S. Pat. No. 6,809,635 describes disposing a capacitor  202  between the audio amplifier  106  and a speaker  204 . In this embodiment, the amplifier  106  drives both the speaker  204  and the vibration motor  108 , but the capacitor  202  acts as a high pass filter, substantially attenuating the lower frequencies so the speaker  204  acts as a tweeter that reproduces  212  only the higher frequency components of the audio signal  104 , leaving only the vibration motor  108  to reproduce  210  the lower frequency components. A third embodiment reproduced here as  FIG. 1C  uses a cross-over network (passive or active) to selectively split the signal output from the audio amplifier  106  into two frequency-distinguished channels  304 ,  306 . The lower-frequency channel  304  feeds an input of the vibration motor  108 , and the higher-frequency channel  306  feeds an input of the speaker  204 . 
         [0022]    A fourth embodiment of U.S. Pat. No. 6,809,635, reproduced here as  FIG. 1D , describes splitting the input signal among two branches, the signal on each branch then being frequency distinguished by high and low pass filters  310 ,  308  respectively. The output of these filters  310 ,  308  is then separately amplified by amplifiers  314 ,  312  of comparatively different gains. One amplifier  314  drives the speaker  204  and the other amplifier  308  drives the vibration motor  108 . The different gains reflect the difference in efficiency between the speaker/tweeter  204  and the vibration motor  108 . In each of the above embodiments of U.S. Pat. No. 6,809,635, the vibration motor  108  may be driven separately in its traditional silent-alert mode by a signal indicative of an incoming page/phone call rather than by an audio signal  104 . 
         [0023]    Now embodiments of this invention are described, wherein the audio signal is mapped to a driving signal for the vibration mechanism. Preferably only the filtered low frequency components thereof are mapped, the high frequency components being filtered previous to the mapping, such as by the low pass filter  308  ( FIG. 1D ) of the cross-over network ( FIG. 1C ). An embodiment adapting  FIG. 1D  is shown in  FIG. 2 . In  FIG. 2 , an audio signal  20  is split into two parallel paths  20 A,  20 B. The audio signal may derive from a locally stored digital music file, a demodulated voice signal or Internet-based streaming audio (e.g., a webcast or a “podcast”) received over a wireless link, or any of a variety of other audio signal sources wired or wireless. Preferably, only musical audio signals are mapped as described herein, where the device recognizes signals as musical audio signals by, for example, the compression format of those signals. 
         [0024]    Consider the audio signal  20  to be composed of both high and low frequency components, wherein frequency refers to frequency as transduced by a conventional speaker into an audible form between about 15-20,000 Hz, the normal range of human hearing for a young adult. Low frequency components may be those ranging up to about 2,000 Hz, 5,000 Hz, or 10,000 Hz, though any cutoff between low and high frequency components may be chosen based on specific response of hardware (e.g., the vibration mechanism) chosen for a particular embodiment. The high frequency components are then the remaining higher frequencies of the overall audible band. Of course, a mid-range speaker (not shown) may be used for fuller sound quality by transducing only those frequencies between the higher frequency components and the lower frequency components. For simplicity, the following description assumes no mid-range speaker. Processing of the signal  20  along the first path  20 A that transduces the high frequency components is as previously described: a high pass filter  22  attenuates low frequency components and allows only high frequency components to pass, a first amplifier  24  amplifies with a first gain and drives a tweeter  26  with its output. The first gain is matched to the audio response of the tweeter  26  as is well known in the acoustic arts. Generally, the first gain is relatively small as the actual transducing components (e.g., the former and cone) of the tweeter  26  are not massive and their vibration requires little power. 
         [0025]    Processing of the signal  20  along the second path  20 B that transduces the low frequency components is now described. The second path  20 B is of course parallel to the first  20 A, as both high and low components of the input signal  20  are re-combined in air once transduced. A low pass filter  28  attenuates high frequency components and allows only low frequency components to pass. A vibration mechanism, such as the electric vibration motor  30  previously described, is coupled to the output of the low pass filter  28  through a signal-converter block  32 . The signal-converter block  32  converts intensity of the low frequency component signal that is input into the block  32  into a driving signal length for the vibration motor  30  that is output from the block  32 . 
         [0026]    In some embodiments, the vibration motor may vibrate within the range of human hearing to act as a low frequency speaker or sub-woofer. In other embodiments, the motor does not vibrate in place of a woofer/sub-woofer but in conjunction with it to provide a bass effect or ‘kick’ stimulus. For example, the HPF  22  may removed so that the speaker  26  outputs both high and low human hearing frequencies, or another low-pass filtered output of the signal  20  may feed a traditional low-frequency speaker  27  through another amplifier  25  in addition to those components shown in  FIG. 2 . As shown in  FIG. 2 , a third parallel signal path  20 C passes the signal  20  through a separate low-pass filter whose pass frequency may differ from the first LPF  28  given the differences between speaker  27  and vibration motor  30 . Alternatively, the output of a single low pass filter  28  may feed both the low frequency speaker  27  and the vibration motor  30 , with the mapper  32  clipping the signal that it outputs to the vibration motor  30  so that it provides a ‘kick’ stimulus simultaneous with low frequency tones output from the low frequency speaker  27 . The kick stimulus is felt by a user through the touch sensation as vibrations of the device body, not necessarily as audible tones. 
         [0027]      FIG. 3  shows a further detail of an embodiment of the signal-converter block  32 , with representations of the signal present above the signal converter block  32  at various nodes between components of the block  32 . The low frequency component signal present at input node  40  has a sinusoidal waveform as illustrated above the input node  40 . A rectifier diode  42 , forward biased to allow current only from left to right as illustrated in  FIG. 3 , rectifies that sinusoidal waveform to produce at a first intermediate node  44  a rectified waveform as illustrated above that first intermediate node  44 . There are various forms a rectified waveform can take; that illustrated is merely a common one. A voltage block  46  then quantifies an intensity (voltage) of the rectified signal, which is output on a second intermediate node  48 . A mapper  50 , which may be for example a monostable multivibrator, a general purpose processor, or a digital signal processor, converts the signal intensity from the second intermediate node  48  to a pulse length, which is output to a third intermediate node  52  and illustrated above that third intermediate node  52  as a square pulse with a determinate length in milliseconds. A driver  54  then amplifies the signal on the third intermediate node  52  to drive the vibration motor  30  ( FIG. 2 ) with, a pulse length representative of the low frequency component (at node  40 ) of the original audible signal  20  ( FIG. 2 ). The driver  54  may be a part of the signal-converter block  32  or separate from it, but is preferably between the mapper  50  and the vibration motor  30 . 
         [0028]    Other implementations are also viable to embody the inventive concept. For example, rather than a rectifier diode and voltage block, the entire input signal at the input node  40  may be processed in a digital signal processor by a series of transistors to convert input signal amplitude (which may be determined in any number of manners, RMS, squared, etc.) to a pulse length used to set a driving frequency of the vibration motor  30 . Various different embodiments and implementations will be recognizable to those skilled in the art; the embodiment of  FIG. 3  is seen as advantageous for implementation in a mobile station due to its minimal use of processing power, relatively small size, and cost effectiveness. The various embodiments such as that of  FIG. 3  may also be imposed between the cross-over network and the vibration motor  108  of  FIG. 1C , or between a low-pass filter and the vibration motor of  FIG. 1B . 
         [0029]    The above description generally contemplates direct mapping, where each pulse of the input signal  20  that passes a low pass filter  28  is realized as an excitation of the vibration motor  30 . In some instances, this may result in excitation pulses to the motor that are too frequent, leaving a bass sound that is nearly continuous. In some instances, this will give a sub-woofer sound that is irritating, or at least which is not perceived by a user as enhancing the listening experience. Where end users experience an irritating sub-woofer for certain music, they may tend not to prefer the feature of a vibration motor  30  as sub-woofer at all. To this problem, embodiments of the present invention employ the concept of adaptive pulse control. 
         [0030]    Adaptive pulse control uses appropriate dynamic control of the filtered low frequency sound, in order to adapt the amount of vibration to be at a minimum but still perceivable in a wider range of music content, either as humanly audible or vibrations to be felt. In this arrangement, compression (limiter) and automatic gain control (AGC) algorithms are used. These enable an implementation, where basically short pulses of vibration are output and spaced from one another, instead of more continuous vibrations following the low frequency notes in some music. These are illustrated in  FIGS. 4A through 6 . 
         [0031]      FIG. 4A  illustrates a graph of low frequency signal intensity over time. The solid bass-intensity line  51  represents an analog version of the lower frequency portions of the input audio signal  20 . Consider this an analog version of a signal output from the low pass filter  28  that is input into the signal converter block  32 . The vibration pulses  53  result from pulse-width mapping the intensity of the bass-intensity line  51  in the signal converter block  32 . Direct mapping is shown in  FIG. 4A , where each pulse-width modulated pulse mapped from the input signal (represented as line  51 ) is an excitation pulse to the vibration motor  30 . Direct mapping a first bass movement a yields six excitation pulses  53  that are output to the vibration motor  30 . Similarly, a second bass movement b yields two excitation pulses  53  to the vibration motor  30 , and a third bass movement c yields four excitation pulses  53 . As illustrated in  FIG. 4A , each set of pulses  53  associated with a single bass movement are closely spaced in time. Since the pulse length derives from the bass intensity, these excitation pulses can  53  blend together when experienced by a user, especially considering the mechanical/inertial constraints of the physical vibration motor and its unbalanced rotor.  FIG. 4B  illustrates an adaptive pulse control solution, where each bass movement a, b, c, results in only a single pulse  53  output to the vibration motor  30 . This may be considered a ‘kick’ effect, wherein each bass movement begins with a sub-woofer imitating pulse to the vibration motor  30 . Using the ‘kick’ effect, the adaptive pulse control may be used in addition to a traditional woofer or other speaker, rather than in place of a woofer/low-frequency response speaker. 
         [0032]      FIG. 5A-B  illustrate one implementation to achieve the result of  FIG. 4B .  FIG. 5A  is identical to  FIG. 4A , and  FIG. 5B  is identical to  FIG. 4B  except for the addition of an adaptive threshold  55  and an adaptive time delay  57  in  FIG. 5B . The adaptive threshold  55  is a threshold intensity at which the signal converter block  32  is allowed to provide an output that drives the vibration motor  30 . Any input signal with intensity below that threshold  55  yields no excitation pulse  53  to the vibration motor  30 . Whether the pulses corresponding to signal intensity below the threshold  55  are generated at the signal converter block  32  and inhibited from driving the motor  30 , or not generated in the signal converter block  32  at all, are variances in implementation. Upon the bass intensity line  51  positively crossing the threshold level  55 , shown in  FIG. 5B  by reference numbers  59 , the signal converter block  32  becomes active and maps the input signal to an output pulse  53  (or any inhibiting of generated pulses is suspended in an alternate implementation). 
         [0033]    In one embodiment, an adaptive time delay  57  is imposed from the start of the output pulse  53 , or from the instant  59  at which the bass intensity positively crossed the adaptive threshold  55 . During that adaptive time delay  57 , the signal converter block  52  is prevented from providing an output excitation signal to the vibration motor  30 , resulting in a ‘dead time’ for which the actual bass intensity line  51  is irrelevant to stimulation of the vibration motor  30 , by inhibiting generation of additional pulses or blocking all pulses except the first pulse during that ‘dead time’. The inventors have concluded that a span of about 100-200 milliseconds is an appropriate adaptive time delay  57  for enhancing the user sub-woofer/vibration experience, though other time periods may be used in keeping with this invention. 
         [0034]    In another embodiment, the signal converter block  32  is prevented from providing an output (or the output is blocked from the vibration motor  30 ) until after the bass intensity line  55  crosses the threshold  55  in the negative direction, shown as reference numbers  59 ′ in  FIG. 5B . Of course, the first excitation pulse  53  sent after the bass intensity line  51  first crossed the threshold  55  in the positive direction is not inhibited from being sent to the motor  30 . In order to avoid short-spaced excitation pulses  53  when the bass intensity line  51  varies frequently across the threshold  55 , one threshold may be used for the positive crossing  59  to generate one pulse and a different, preferably lower threshold may be used for the negative crossing  59 ′ to enable the signal converter block  32  to again provide an output to the motor  30 . Only one threshold  55  is needed for the adaptive time delay  57  embodiment. 
         [0035]      FIGS. 6A-B  illustrate the advantages of adaptive pulse control as compared to the direct mapping. In the direct mapping shown in  FIG. 6A , lower intensity of bass notes yield minimal pulse strength/duration, which the user experiences as minimal or no vibration at all from the vibration motor  30 . Conversely, at high bass note intensity, direct mapping yields very intense pulse strengths (very long pulses), which to a user can overwhelm other sounds of the audio signal  20  produced by conventional speakers  26 , or be experienced as continuous vibration without clear relation to the underlying music. With adaptive pulse mapping as illustrated in  FIG. 6B , the vibration pulse intensity (duration) is limited by the gain control and signal compression that are typically already applied to the signal  20 . As was discussed above, gain control applied to different speakers (e.g., tweeter and woofer) varies based on the signal, as well known in the art. Also known in the art is to vary compression of the signal  20  based on the frequency response, yielding a compression curve. This is inherent in the compression format used to digitize the musical audio signal, and common compression formats for music include MP3 and AAC. The portable device already uses gain control and compression in order to process the audio signal. Gain applied to a portion of the signal rises with signal intensity in order to drive the traditional speaker properly. That same gain control curve, and the corresponding compression curve, are used to adaptively control the vibration pulse output from a bass signal input. Both those curves depend in part from signal intensity, especially AGC. In  FIG. 6B , the slope of the adaptive curve (in solid lines) results from adapting to signal compression, while the position of the solid line curve varies along the horizontal axis based on gain control. 
         [0036]    Implementation of  FIG. 6B  is shown in simplified block diagram form at  FIG. 7 , which is imposed between the low pass filter  28  and the signal converter block  32  of  FIG. 2 . The output from the low pass filter  28  ( FIG. 2 ) is input into an automatic gain control AGC block  61  which applies an AGC algorithm. Many such algorithms are known in the digital audio signal processing arts for driving a speaker. A compression block  63  then applies a compression algorithm to the signal, shifting the slope of the intensity/pulse length curve from the dotted line (direct mapping) to the solid line (adaptive mapping) as shown in  FIG. 5B . The output of the compression block  63  then becomes the input at node  40  of  FIG. 3  for the previously described signal converter block  32 . 
         [0037]    For implementation within a mobile station, the voltage of the vibration pulse may be pulse-width modulated PWM at 100% for optimum fidelity where the vibration motor is used as an audible subwoofer, though in some embodiments this may be changed to some value greater than or less than 100% so that the pulse width (node  52 ) is proportionally scaled to voltage amplitude (node  48 ). It is also preferable that the pulse length (node  52 ) is constrained to be less than about 50 milliseconds. The inventors have determined that pulse lengths above about 50 milliseconds tend to exceed the nominal RPM, and the subwoofer starts to feel too intense, leading to an uncomfortable user experience. In addition, if played longer than about 50 milliseconds, the experience to the user is no longer a ‘kick’ but more or less a continuous stimulation leading to an uncomfortable user experience. In general, the ideal operating range for embodiments of the present invention, when used strictly as an audio device rather than providing a sensory ‘kick’ accompanying traditional speaker output, is seen to be between about 70 Hz and about 200 Hz. Commonly available vibration motors  30  tend not to be sufficiently responsive to reproduce sound below 70 Hz, and above 200 Hz the result is seen as overstimulating and uncomfortable. Note that pulse width modulation, pulse duration modulation, and pulse length modulation refer to the same general concept of modulating an output pulse to an instantaneous sample of an input wave by varying a leading, trailing, or both edges of that pulse to achieve a particular spacing between those edges. 
         [0038]    Given the mass of certain vibration motor embodiments, some implementations may need to actively synchronize outputs from the speaker(s)  26 ,  27  and the vibration motor  30 . While each may receive their input simultaneously or nearly so, the signal response of the speaker  26 ,  27  far exceeds that of more massive vibration motors  30 . One embodiment to ensure that the output, rather than just the input, of both transducers are synchronized is to impose a delay buffer  29  along the first path  20 A and third path  20 C of  FIG. 2 . The delay buffer  29  delays the signal feed to the speakers by a fixed time interval that corresponds to the response differential between the speakers  26 ,  27  and the vibration motor  30 . As the response times of a traditional woofer  27  and tweeter  26  typically vary only a negligible amount, especially in portable devices of the handheld size that include two speakers  26 ,  27 , a single delay buffer  29  may be used from which the first  20 A and third  20 C signal paths diverge. In this variation, the second signal path  20 C that feeds the vibration motor  30  branches from the source  20  prior to the delay buffer  29 . 
         [0039]    A method according to an exemplary embodiment is then rectifying an audio signal or at least its low frequency component, measuring intensity of the rectified signal, mapping that intensity to a pulse length, and driving a vibration mechanism with the pulse length. Preferably, the high frequency components of the audio signal are filtered out prior to rectifying, though they may be filtered between rectifying and measuring intensity with some minimal loss of efficiency. 
         [0040]      FIG. 8  is a block diagram of an exemplary mobile station  60  in which the present invention may be disposed. These blocks are functional and the functions described below may or may not be performed by a single physical entity as described with reference to  FIG. 8 . A display user interface  62 , such as a circuit board for driving a visual display screen, and an input user interface  64 , such as a unit for receiving inputs from an array of user actuated buttons, are provided for interfacing with a user. The user may select between the traditional silent alert mode at the input user interface  64 , so that the vibration mechanism is actuated, silently, upon receipt of a page or incoming call. The MS  60  further includes a power source  66  such as a self-contained battery that provides electrical power to a motherboard  68  that controls functions within the MS  60 . The motherboard represents one or more circuit boards on which a general processor and/or a digital signal processor are disposed, as well as the diodes, amplifiers, and mappers described with reference to  FIGS. 2-3 . Within the processor (general or DSP) of the motherboard  68  are functions such as digital sampling, decimation, interpolation, encoding and decoding, modulating and demodulating, encrypting and decrypting, spreading and despreading (for a CDMA compatible MS  60 ), and additional signal processing functions known in the art. 
         [0041]    Voice or other aural inputs are received at a microphone  70  that may be coupled to the processor of the motherboard  68  through a buffer memory  72 . Computer programs such as drivers for the display  62 , algorithms to modulate, encode and decode, data arrays such as look-up tables, and the like are stored in a main memory storage media  74  which may be an electronic, optical, or magnetic memory storage media as is known in the art for storing computer readable instructions and programs and data. The main memory  74  is typically partitioned into volatile and non-volatile portions, and is commonly dispersed among different storage units, some of which may be removable. The memory  74  may also store music or other audio files that may serve as an audio signal source for the invention as detailed above. The MS  60  communicates over a network link such as a mobile telephony link via one or more antennas  76  that may be selectively coupled via a T/R switch  78  or diplex filter, to a transmitter  80  and a receiver  82 . The MS  60  may additionally have secondary transmitters and receivers for communicating over additional networks, such as a WLAN, WIFI, Bluetooth®, or to receive digital video broadcasts. Any of these links may serve as a source for the said audio signal processed by the present invention. Known antenna types include monopole, di-pole, planar inverted folded antenna PIFA, and others. The various antennas may be mounted primarily externally (e.g., whip) or completely internally of the MS  20  housing. Audible output from the MS  60  is transduced at a speaker  84  and at a vibration mechanism  88 , as detailed above. 
         [0042]    While described above as incorporated within a mobile station, the invention is not so limited. For example, the invention may be embodied as a portable electronic device that is physically separated from a mobile station, radio, MP3 player, or other external source device. That external source device need not be portable itself. The portable electronic device of such an embodiment has disposed in it the vibration motor for vibrating its own housing, and a plug-in input port and/or an antenna/receiver arrangement as the source for receiving an electronic signal from the external source device. A wired connection between the portable electronic device and the external source device couples at the input port. A wireless connection, such as over a Bluetooth or other personal area network, a low power FM band (or other broadcast radio), or other wireless protocol is established with the antenna/receiver, which serves as the source of the signal in this embodiment. 
         [0043]    Various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings. As but some examples, the use of other similar or equivalent components or combinations of components may be attempted by those skilled in the art. However, all such and similar modifications of the teachings of this invention will still fall within the scope of the non-limiting embodiments of this invention. Furthermore, some of the features of the various non-limiting embodiments of this invention may be used to advantage without the corresponding use of other features. For example, advantages of the present invention may be gained without employing the tweeter to transduce high frequency audio components. As such, the foregoing description should be considered as merely illustrative of the principles, teachings and exemplary embodiments of this invention, and not in limitation thereof.