Abstract:
An audio apparatus comprising a modulator  12  for modulating a first ultrasonic signal  29  with an audio signal to provide a second ultrasonic signal  30 ; a transducer  15  for converting the second ultrasonic signal  30  into an ultrasonic pressure wave for transmission into a non-linear medium to allow demodulation of the ultrasonic pressure wave and thereby generate an audio pressure wave representative of the audio signal  29 , processing means  11  for modifying the audio signal to compensate for the demodulating properties of the non-linear medium; and means  8  for modifying the audio signal to compensate for the conversion characteristics of the transducer  15.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to audio apparatus, and in particular audio apparatus for radiating an ultrasonic pressure wave with a modulated audio signal. 
   Audio spotlighting, using a parametric audio system, provides a means for projecting highly directional beams of audible sound. This technology employs the non-linearity of a compressible material (such as air) to create audible by-products from inaudible ultrasound. This technique exploits an acoustic phenomenon called self-demodulation where low frequency audio beams of high directivity are generated from a high amplitude ultrasonic beam that has been modulated with an audio signal. Self-demodulation generates new frequencies in the received sound spectra based on the envelope frequency (i.e. the required audio signal) in a process akin to AM demodulation. Thus this technique provides a beam of audio sound with the focused directional properties of the original ultrasonic carrier beam, allowing distant targeting of specific listeners. This technique can produce predictable and controllable levels of audio sound and, despite the relatively weak effect of self-demodulation, is capable of generating substantial sound amplitudes due to the narrow spatial distribution of the acoustic energy. 
   The transmitted modulated ultrasonic wave can be considered as a collimated primary wave consisting of an amplitude modulated wave of pressure where the primary pressure wave is defined by 
   1) p 1 =P 1 E(t)sin(ω c t) 
   where P 1  is the amplitude of the primary beam pressure, ω c  is the carrier frequency and E(t) is the modulation envelope. For an amplitude modulated signal E(t) is equal to (1+mg(t)) where m is the modulation depth and g(t) is the audio signal. 
   As a result of p 1  interacting with the air the modulated audio signal demodulates creating an audible secondary pressure wave p 2  given by
 
                 p   2     ⁡     (   t   )       =         β   ⁢           ⁢     P   1   2     ⁢   A       16   ⁢   π   ⁢           ⁢     ρ   0     ⁢     c   0   4     ⁢   z   ⁢           ⁢   α       ⁢       ∂   2       ∂     t   2         ⁢       E   2     ⁡     (   τ   )                 2   )             
 
where β is the coefficient of nonlinearity (β air =1.2), ρ 0  is the ambient density of the medium, c 0  is the small signal wave propagation speed, A is the beam&#39;s cross-sectional area, z is the axial distance, α is the absorption coefficient of the medium. So, for example, where c=343 m/s, ρ 0 =1.2 kg/m 3 , α=0.6, and A=5×10 m −3 , a 140 dB ultrasound wave modulated with a 1 kHz signal would produce about 71 dB of audible sound at 1 m.
 
   The power of the resultant audio signal is proportional to the second derivative of the square of the modulation envelope. As a result significant coloration (i.e. a shift of signal power with respect to frequency) and distortion are introduced onto the demodulated audio signal as a result of the interaction of the ultrasonic wave with the non-linear medium. The coloration of the signal results in the low frequency audio components being suppressed by approximately 12 dB/octave; this is represented by the second derivative term of the modulation envelope. The distortion of the signal is represented by the square of the modulation envelope. 
   Processing the audio signal prior to modulation can minimize the effects of coloration and distortion that result from the interaction of the ultrasonic wave with the non-linear medium. The processing typically comprises a double integration filter to compensate for coloration of the audio signal and a square root operation to compensate for the distortion of the audio signal. 
   However, for the self-demodulation to occur high ultrasonic sound pressure levels are required. To generate these high pressure levels it is necessary to generate the ultrasonic pressure levels at or close to the resonant frequency of the transmitting transducer. Correspondingly the frequency response of the transducer can vary dramatically at this frequency. The variable transducer frequency response can significantly affect the quality of the demodulated audio pressure wave. 
     FIG. 1 , plot A shows the frequency spectrum of a white noise input signal constrained between 300 and 4000 Hz prior to modulation with an ultrasonic carrier signal. An example of the effects of self-demodulation and transducer conversion upon the input signal, using a typical transducer having a measured frequency response shown in  FIG. 2 , is shown in  FIG. 1 , plot B. 
   The frequency response of a transducer can be flattened at the resonant frequency. However this requires considerable damping to be added to the transducer, and a corresponding drop in ultrasonic pressure level. This in turn would require a transducer with a large radiating surface area, which is not suitable for small devices, for example a mobile communication device and in particular a radiotelephone. 
   SUMMARY OF THE INVENTION 
   In accordance with an aspect of the present invention there is provided audio apparatus comprising a modulator for modulating a first ultrasonic signal with an audio signal to provide a second ultrasonic signal; a transducer for converting the second ultrasonic signal into an ultrasonic pressure wave for transmission into a non-linear medium to allow demodulation of the ultrasonic pressure wave and thereby generate an audio pressure wave representative of the audio signal; processing means for modifying the audio signal to compensate for the demodulating properties of the non-linear medium; and means for modifying the audio signal to compensate for the conversion characteristics of the transducer. 
   This provides the advantage of enabling the effects of the demodulation process and the transducer conversion characteristics on the audio signal to be minimized. This can allow the size of the transducer to be reduced while retaining the performance of the transducer. 
   Typically the means for modifying the audio signal to compensate for the conversion characteristics of a transducer is a transducer response filter. 
   Most preferably the processing means comprises a double integration filter and a square root operator. As the operator processes the audio signal non-linearly, the characteristics of the second filter are preferably derived empirically by tone adjustment for the required frequency range of the audio signal. 
   The invention will now be described, by way of one example only, with reference to the accompanying drawings, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1 , plot A shows the frequency spectrum of a test audio input signal to an audio apparatus according to one embodiment of the present invention; 
       FIG. 1 , plot B shows the frequency spectrum of  FIG. 1 , plot A after self-demodulation without correction; 
       FIG. 2  shows a typical measured frequency response of a suitable transducer for use in audio apparatus according to an embodiment of the present invention; 
       FIG. 3  shows a radiotelephone having audio apparatus according to an embodiment of the present invention; 
       FIG. 4  shows audio apparatus according to an embodiment of the present invention; 
       FIG. 5 , plot A shows the frequency spectrum of the test audio signal output after self-demodulation with correction for self-demodulation; 
       FIG. 5 , plot B shows the frequency spectrum of the test audio signal output from audio apparatus according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 3  shows a radiotelephone  1  with audio apparatus  2 , a speech decoder  3 , a channel decoder  4 , a receiver  5  and an antenna  6 . In operation the receiver  5  receives a speech encoded digital signal  20  from a base station (not shown) via antenna  6 . The receiver  5  demodulates the received digital signal  20  and passes the demodulated signal  21  to channel decoder  4  which corrects for errors that may have occurred during the transmission process by using error protection bits encoded within the received signal. The receiver  5  typically samples the received signal  20  at 8 kHz. The decoded digital signal  22  is provided to speech decoder  3  which decodes the speech and passes the digital decoded signal  23  to audio apparatus  2  to generate an acoustic representation of the received speech signal as described in detail below. The audio apparatus  2  may be mounted within the radiotelephone  1 . Typically however, to obtain the required audio power levels and to support hands free use of the radiotelephone  1 , the audio apparatus  2  is mounted separately to the radiotelephone  1 , for example, in conjunction with a hands free kit or a car kit for hands free use of a radiotelephone in a car. 
     FIG. 4  shows the audio apparatus  2  which comprises a double integration filter  7 , a transducer response filter  8 , a DC up-shifter  9 , an up-sampler  10 , a square root operator  11 , a modulator  12 , an ultrasonic signal source  13 , a digital to analog converter  14  and an ultrasonic transducer  15 . 
   The audio apparatus  2  is a parametric device that radiates an inaudible ultrasonic pressure wave with an audio signal modulated onto the ultrasonic pressure wave. The transmitted ultrasonic wave interacts with air (i.e. a compressible non-linear medium) to cause the ultrasonic wave to self-demodulate, thereby causing the modulated audio signal to become audible. 
   Digital signal  23  is provided to double integration filter  7 , which boosts the low frequencies by 12 dB/octave akin to integrating the signal twice. Double integration filter  7  compensates for the effects of coloration that occur during the self-demodulation process, and is linear in nature. Typically, the double integration filter  7  is a simple recursive filter. The double integration filter  7  provides the double integrated digital signal  24  to transducer response filter  8 . 
   The transducer response filter  8  corrects for characteristics of the ultrasonic transducer  15 , as described in detail below. The transducer response filter  8  provides the corrected signal  25  to DC up-shifter  9 . 
   The DC up-shifter  9  re-scales the data and shifts the voltage of the corrected digital signal  25  so that all signal voltages are positive, thus ensuring the square root operation only has to work on positive values, thereby avoiding complex filtering. 
   The DC up-shifted signal  26  is provided to up-sampler  10 . The up-sampler  10  re-samples the 8 kHz signal at typically 120 kHz. The purpose of up-sampler  10  is to increase the frequency range of the signal in preparation for the square rooting of the signal. A consequence of square rooting the received signal is the creation of an infinite series of harmonics. For distortion to be eliminated all these harmonics must be reproduced. Therefore, to ensure harmonics above 4 kHz are reproduced, the signal is re-sampled at a higher frequency. Sample rates other than 120 kHz may be used dependent on the operating frequencies of the ultrasonic transducer. The re-sampled signal  27  is provided to the square root operator  11 . 
   The square rooting operator  11  compensates for the effects of distortion that occur during the self-demodulation process, and is non-linear in nature. 
   The square root operator  11  is typically performed by means of a look up table, as is well known to a person skilled in the art. 
   The square rooted signal  28  is provided to modulator  12  for modulation with an ultrasonic signal  29  from ultrasonic signal source  13 . To minimize the risk of harm to humans or animals, the ultrasonic frequency should be higher than approximately 40 kHz. Due to increased signal absorption by the air at higher frequencies the upper highest feasible frequency limit is typically of the order of 200 kHz. 
   The digital modulated ultrasonic signal  30  is provided to digital to analog converter  14  for converting the digital signal  30  to a representative analog signal. The analog modulated ultrasonic signal  31  is provided to ultrasonic transducer  15 . Transducer  15  radiates the modulated ultrasonic signal as an inaudible ultrasonic pressure wave. 
   To obtain the required ultrasonic pressure levels required for self-demodulation to occur, the transducer  15  will typically be chosen to have its resonance frequency at the frequency of the ultrasonic carrier signal  29 . 
   For a 40 kHz ultrasonic signal a suitable transducer is the MuRata MA4OB8S. This transducer has a frequency response as shown in  FIG. 2 , which has a narrow resonance band at 40 kHz. To obtain the required power levels a plurality of transducers will typically be required, for example 19 transducers will provide 55 dB&#39;s of audio speech signal. 
   As the square root operator  11  is non-linear, it is not possible for the 15 transducer response filter  8  to have a single optimum filtering characteristic for all frequencies. Therefore, the transducer response filter  8  is determined empirically for a required frequency range. For example, if the resultant spectra of the self-demodulated signal shows a gradual power drop from 300 Hz to 4 kHz after correction for self-demodulation, the transducer response filter  8  is selected to boost the signal over this frequency range. The characteristics of the transducer response filter are therefore, typically, the inverse response of the resultant spectra of the self-demodulated signal. The transducer response filter is designed typically using a recursive filter design package, for example the Yule-Walk package, which models the transducer response filter characteristics using the inverse of the self-demodulated signal. 
   Typically the transducer response filter characteristics are determined during the manufacture of the audio device. However, it is possible for the characteristics of the transducer response filter to be determined dynamically, for example while during use of the audio device. 
     FIG. 5 , plot A shows the transducer conversion effect upon the input signal shown in  FIG. 1 , plot A.  FIG. 5 , plot B shows the corrected spectrum using a transducer response filter  8  empirically derived using  FIG. 5 , plot A for determining the effect of the transducer conversion. For different frequency and modulation depths the square root non-linear operator  11  will vary the effects of the transducer conversion and accordingly different frequency and modulation depths will require the transducer response filter  8  to be modified empirically as described above. 
   The transducer response filter  8  is typically a simple recursive filter. 
   The present invention may include any novel feature or combination of features disclosed herein either explicitly or implicitly or any generalization thereof irrespective of whether or not it relates to the presently claimed invention or mitigates any or all of the problems addressed. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention. The applicant hereby gives notice that new claims may be formulated to such features during prosecution of this application or of any such further application derived therefrom. For example, it will be appreciated that an analog audio signal can be processed and modulated with an ultrasonic carrier signal, also that the pre-processing filters and/or operator can be used to process the audio signal after modulation of the audio signal.