Abstract:
Apparatus and method for transmitting and reproducing stereophonic audio signals are disclosed. The method comprises splitting first and second channels into two frequency bands, combining the lower frequency band signals of the two channels, and transmitting the combined signals of the two channels or signals representative thereof. The apparatus comprises: a splitter for splitting each of the two channels into two frequency bands, a combiner, for combining the lower frequency band signals of the two channels, and a transmitter for transmitting the combined signals, or signals representative thereof, and the higher frequency band signals of the two channels, or signals representative thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority to currently pending United Kingdom Patent Application number 0200499.2, filed on Jan. 10, 2002. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   N/A 
   BACKGROUND OF THE INVENTION 
   This invention relates to methods of and apparatus for transmitting stereophonic audio signals. This invention relates also to apparatus for reproducing stereophonic signals. 
   The Bluetooth standard allows for devices to communicate with each other in a wireless fusion with certain defined channels, including three 64 kb per second SCO channels. Each channel can carry sampled audio signals having a bandwidth of a 4 kHz with 8 bit samples. However, 4 kHz is regarded as not being suitable for use with portable hi-fi equipment, although it is regarded as being sufficient for carrying voice signals. 
   It is known to reduce the amount of digital data required to represent stereophonic sound signals using coders according to the MPEG-1 standard. Stereophonic signals reconstructed after MPEG-1 compression tend to be of a very high quality. However, MPEG coders and decoders are not cheap to produce, and they tend to have quite high power consumption. As a result, it is desirable to avoid their use in portable, battery-operated equipment. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   Objects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
   According to a first aspect of the invention, there is provided a method of transmitting stereophonic audio signals, the method comprising splitting each of the two channels into two frequency bands, combining the lower frequency band signals of the two channels, and transmitting the combined signals or signals representative thereof and the higher frequency band signals of the two channels or signals representative thereof. 
   According to a second aspect of the invention, there is provided a method of transmitting stereophonic audio signals, the method comprising: splitting each of the two channels into frequency bands, combining the lower frequency band signals of the two channels, shifting downwards in frequency the higher frequency band signals of the two channels, and transmitting the combined signal, or signals representative thereof, and the frequency shifted signals, or signals representative thereof. 
   According to a third aspect of the invention, there is provided apparatus for transmitting stereophonic audio signals, the apparatus comprising: a splitter for splitting each of the two channels into two frequency bands, a combiner for combining the lower frequency band signals of the two channels, and a transmitter for transmitting the combined signals, or signals representative thereof, and the higher frequency band signals of the two channels, or signals representative thereof. 
   According to a fourth aspect of the invention, there is provided apparatus for transmitting stereophonic audio signals, the apparatus comprising: a splitter for splitting each of the two channels into two frequency bands, a combiner for combining the lower frequency band signals of the two channels, a frequency shifter, for frequency shifting downwards the higher frequency band signals of the two channels, and a transmitter for transmitting the combined signal, or signals representative thereof, and the frequency shifted signals, or signals representative thereof. 
   According to a fifth aspect of the invention, there is provided apparatus for reproducing stereophonic audio signals, the apparatus comprising means for receiving signals transmitted over three audio channels, means for combining signals received over a first one of said channels with signals received over a second one of said channels, to provide a first channel output, and means for combining signals received over the first one of said channels with signals received over a third one of said channels, to provide a second channel output. 
   Additional objects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned through practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
   The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate at least one presently preferred embodiment of the invention as well as some alternative embodiments. These drawings, together with the description, serve to explain the principles of the invention but by no means are intended to be exhaustive of all the possible manifestations of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other aspects, features, and advantages of the present subject matter will be more apparent from the following more particular description of exemplary embodiments of the disclosed technology as set forth in the appended figures, in which: 
       FIG. 1  is a block schematic diagram illustrating the general concept of the invention; and 
       FIGS. 2 and 3  are circuit schematic diagrams of first and second exemplary implementations of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference now will be made in detail to the presently preferred embodiments of the invention. Each example is provided by way of explanation of the related technology, which is not restricted to the specifics of the examples. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present subject matter without departing from the scope or spirit of the subject matter. For instance, features illustrated or described as part of one embodiment, can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present subject matter cover such modifications and variations as come within the scope of the appended claims and their equivalents. 
   Referring to  FIG. 1 , apparatus  10  for transmitting stereophonic audio signals is shown schematically. The apparatus comprises left and right audio channel inputs  11 ,  12  of 8 kHz bandwidth inputs. The first splitter  13  passes the lowest 4 kHz of the signal received at the left channel input  11  to a summer  15 , and passes the highest 4 kHz to a downconverter  16 . The second signal splitter  14  similarly passes the lowest 4 kHz of the signals received at the right channel input to another input of the summer  15 , and passes the highest 4 kHz to a second downconverter  17 . The downconverters  16 ,  17  each downconverts the signals received at its input by 4 kHz, and supplies them to a respective one of first and second 64 kb per second wireless data channels  18 ,  19 . The summer  15  sums the signals it receives, and supplies them to a third 64 kb per second wireless data channel  20 . Each of the data channels  18 - 20  therefore carries a digitized audio signal occupying a bandwidth between 0 Hz and 4 kHz. 
   Stereophonic audio signals are reconstructed at a receiver end of the data channels  18 - 20 . First and second upconverters  21 ,  22  are connected one to each of the data channels  18 ,  19 . These upconverters each shift the frequency of signals received upwards by 4 kHz, which results in signals the same as those applied to the respective downconverters  16 ,  17 . Signals supplied by the first upconverter are added to signals received over the third data channel  20  in a second summer  23 , and the result supplied to a left channel output  24 . Similarly, a second summer  25  sums the signals supplied by the second upconverter  22  with signals received over the third data channel  20 , and supplies the result to a right channel output  26 . 
   The result is the transmission of 16 kHz of audio signals over channels having a combined bandwidth of 12 kHz. This is achieved without any reduction in signal quality of the higher frequencies, but at the expense of inaccurate reproduction of lower frequency signals. However, this is not considered to be a problem in many circumstances since, with most recorded music, it is uncommon to find a significant difference between the low frequency components of the left and right channels. Also, the human ear is much less able to discern the direction of origin of low frequency sound than that of high frequency sound, so a human listener is unlikely to be able to detect a difference between the apparatus  10  being used and not being used. This applies whether sound is reproduced using speakers or using head phones. 
   An analog implementation of the apparatus  10  is shown at  30  in  FIG. 2 , in which reference numerals are re-used for like elements. Referring to  FIG. 2 , the apparatus  30  includes, as the first signal splitter  13 , a high-pass filter  31  and a low-pass filter  32 . Each of the filters  31 ,  32  has a cut-off frequency of 4 kHz. The second signal splitter  14  similarly comprises a second high-pass filter  33  and a second low-pass filter  34 , also having cut-off frequencies of a 4 kHz. The downconverters  16 ,  17  are formed from first and second mixers  35 ,  36 , which are commonly connected to a 4 kHz square wave oscillator  37 . The outputs of the mixers  35 ,  36  are filtered by respective low-pass filters  38 ,  39 , each having a 4 kHz cut-off frequency, to remove the unwanted sum frequencies. The summer  15  is constituted by an amplifier  40  having a feedback resistor  41 . 
   On the receiver side, the upconverters  21 ,  22  are formed by respective mixers  42 ,  43 , which are commonly fed by a 4 kHz square wave oscillator  44 . The mixers  42 ,  43  of the upconverters  21 ,  22  are succeeded by respective high-pass filters  45 ,  46 , which each have a cut-off frequency of 4 kHz. The summers  23 ,  25  are constituted by respective amplifiers  47 ,  48  having a respective feedback resistor  49 ,  50 . In one embodiment, the data cannels  18 - 20  are Bluetooth audio channels. To this end, the apparatus  10  includes analog-to-digital converters (ADCs), a modulator, a radio transmitter, a radio receiver, a demodulator and digital-to-analog converters (DACs), which are not shown. The signals transmitted over the data channels  18 - 20  are not, therefore, the signals provided by the filters  38 ,  39  and the summer  15 . Rather, the transmitted signals are representative of the signals provided by the filters  38 ,  39  and the summer  15 . The representative signals are processed at the receiver side to reconstruct the signals provided by the filters  38 ,  39  and the summer  15 . 
   A digital implementation of the apparatus  10  is shown at  60  in  FIG. 3 . Reference numerals are reused from  FIG. 1  for like elements. Referring to  FIG. 3 , the apparatus  60  comprises left and right ADCs  61 ,  62 , each of which samples signals received at its respective input  11 ,  12  and provides 16 k samples thereof per second at its output. A first digital signal processor (DSP)  63  is arranged to receive the sampled left channel signals, to perform high-pass filtering to eliminate signals having a frequency less than 4 kHz, to downconvert the result by 4 kHz and to low-pass filter the downconverted signal to eliminate signals having a frequency above 4 kHz. Signals provided by the first DSP  63  have a sampling rate of 8 k bits per second. A second DSP  64  performs the same functions in respect of signals provided by the right channel ADC  62 . The samples provided by the left and right ADCs  61 ,  62  are also high-pass filtered, to remove signals having frequencies over 4 kHz, by respective third and fourth DSPs  65 ,  66 . Signals emanating from the third and fourth DSPs are added together by a digital summer  15 , which provides output samples at 8 k bits per second. The signals provided by the first and second DSPs  63 ,  64  and by the adder  15  are prepared for transmission over respective Bluetooth 64 k bits per second voice channels  18 - 20  by apparatus which is not shown. 
   At the receiver end, apparatus which is not shown demodulates the Bluetooth transmitted data, and returns it to three separate 8 k bits per second digital channels. The signals received over the third data channel  20  are processed by a fifth DSP  67 , which doubles their sampling rate. Signals received over the second channel  18  are processed by a sixth DSP  68 , which is arranged to upconvert the signals by 4 kHz and then to high-pass filter the result to remove components having frequencies less than 4 kHz. The result is samples at 16 k bits per second, which is provided to second summer  23 . A seventh DSP  69  performs the same functions on signals received over the second channel  19 , and provides 16 k bits per second samples to the third summer  25 . The second and third summers add the signals received from their respective DSP  68 ,  69  to signals provided by the fifth DSP  67 , resulting in 16 k bit per second samples at their respective output. The summers  23 ,  25  are connected to their respective output  24 ,  26  by respective DACs  70 ,  71 . 
   Various alternative embodiments exist. In one embodiment, analog signals are processed at the transmitter side, as in the  FIG. 2  embodiment, and processed digitally at the receiver side, as in the  FIG. 3  embodiment. In another embodiment (not shown), signals are processed digitally at the transmitter side, and analog signals are processed at the receiver side. 
   The use of Bluetooth SCO channels is not essential to the invention. Any suitable channels could be used, depending on the bandwidth requirements. Also, the split of the higher frequency components from the lower frequency components is alterable. Where three 6 kHz channels are available, for example, stereo signals having a bandwidth of 12 kHz may be transmitted by removing the lower 6 kHz of each channel, summing these signals and transmitting the sum over a third channel. In this case, the left and right channels each carry signals having a bandwidth of 6 kHz, and the third channel carries signals having a 6 kHz bandwidth. 
   While at least one presently preferred embodiment of the invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.