Abstract:
An audio signal processor which modifies audio signal components outside the conventional audio frequency band. The processor includes a Delta Sigma Modulator (DSM) that receives a non-interpolated digital audio signal sampled at a frequency of at least 198 kHz.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation application of U.S. patent application Ser. No. 09/177,944, filed Oct. 23, 1998 now abandoned. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to audio signal processors. 
   2. Description of the Prior Art 
   Conventionally the audio band is deemed to be about DC to about 20 KHz and frequencies above that range are ignored as inaudible. In fact the audio band rolls off, rather than abruptly ending at 20 KHz, and some people with expert listening skills consider that frequencies above 20 KHz are audible or at least consider that they can hear the effects in the audible frequency band of frequency components outside that conventional band. In other words, frequencies above 20 kHz may affect elements of the audio response in the conventional audio band. 
   SUMMARY OF THE INVENTION 
   According to the present invention, there is provided an audio signal processor which modifies audio signal components not only in the conventional audio band but also in the range of frequencies from the conventional upper limit of the conventional audio band to frequencies greater than 24 kHz. 
   The conventional audio band is the range of frequencies from DC to 20 kHz or from about 20 Hz to 20 kHz. Whilst DC is inaudible, some audio signal processors have a frequency response down to DC. 
   The said range of frequencies above 20 kHz may extend to an upper limit of 30 khz or to 50 kHz or to 100 kHz. The processor of the invention is able to modify components in the whole of the band from DC or 20 Hz to the upper limit e.g. 100 kHz. The modification may be equalization such as gain control, frequency dependent gain control, frequency/phase characteristic control or any other form of modification conventional in the art. 
   It may be observed that some prior audio processors include transmission channels of bandwidth extending beyond 20 kHz but that prior audio processors do not modify audio signals outside the conventional audio band as far as is known to the present inventor. 
   In an embodiment of the present invention, the said audio components are sampled and digitized to produce digital audio components. 
   In a preferred embodiment of the invention, the audio signal components are sampled and digitized as 1-bit signals at a sampling rate of: e.g. 198 kHz or greater; or 1.4 MHz or greater; or preferably about 2.85 MHz e.g. 2.8224 MHz (64×44.1 kHz). 
   In the preferred embodiment, the processor includes a 1-bit Delta Sigma Modulator (DSM). The DSM may be a filter and/or a gain control and/or a signal adder or mixer. An example of a DSM is described hereinbelow. 
   The invention provides audio signal processing of very high quality. Although it appears unnecessary, according to conventional practice, to equalise over such wide frequency bands and at such high sampling rates as are used in the embodiments of the invention it is believed that so doing contributes to the fidelity of the processed audio signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings, in which: 
       FIG. 1  is a block diagram of an audio signal processor according to the present invention; 
       FIG. 2  is a schematic block diagram of an illustrative 1-bit audio signal mixer useful in the processor of  FIG. 1 ; and 
       FIG. 3  is a schematic block diagram of an integrator of the mixer of  FIG. 2 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , an audio signal processor has an input  2  for receiving audio signals from for example a stereo pair of microphones  4 . An anti-aliasing low-pass filter  6  passes signal components in the range of about DC to about 100 kHz. The audio signals are sampled and digitized in an analogue to digital converter ADC  8 . 
   The converter may be an n-bit converter where n is greater than one, e.g.  16  as is conventional in digital audio. The ADC  8  samples the signal at a suitable sample rate for n-bit digitization. The sample rate is set by a clock  12 . 
   Preferably the converter  8  is a 1-bit converter. It may have a sampling rate of 198 kHz or greater than 1.4 Mhz preferably 2.8224 MHz. 
   The digital signals are then modified in a processor such as an equalizer  10  which is able to modify over the whole frequency range not just in the conventional audio band. 
   The processor  10  may be for example: 
   an equalizer; 
   a digital signal mixer; 
   a processor which encodes audio signals for storage; 
   a processor which encodes audio signals for transmission; and/or 
   a processor which encodes audio signals for recording on a medium such as a CD. 
   The processor would be an n-bit processor if n-bit signals are produced by the converter  8 . 
   Preferably the converter  8  is a 1-bit converter and the processor  10  is a 1-bit processor. An example of a processor is a signal mixer. In the case of a signal mixer the mixer has a plurality of inputs each of which receives an audio signal having components in the range DC to greater than 24 kHz as described above. 
   An example of a 1-bit audio signal mixer is shown in  FIG. 2  and is described in more detail in co-pending UK patent application 9624671.5 (I-96-24, S96P5063 GB00, P/1509) incorporated herein by reference. 
   Referring to  FIG. 2 , the signal combiner comprises an nth order Delta-Sigma Modulator (DSM) where  n  is 1 or more. The example shown in a third order DSM (n=3) but  n  may be greater than 3. 
   The order of the DSM is defined by the number of integrator sections. In the DSM of  FIG. 2 , there are two inputs  4 A and  4 B for receiving first and second 1-it input signals. The DSM has: n integrator stages comprising a first stage and n−1 intermediate stages; and a final stage. The first stage comprises a three input adder  61 , a first 1-bit multiplier a 1  connected to the first input  4 A of the DSM a second 1-bit multiplier b 1  connected to the second input  4 B of the DSM, a third 1-bit multiplier connected to the output of the DSM, and an integrator  71 . The first, second and third multipliers a 1 , b 1 , c 1 , multiply a 1-bit signal by coefficients A 1 , B 1  and C 1  respectively. Each intermediate stage comprises: an adder  62 ,  63  having four inputs; an integrator  72 ,  73 ; a first coefficient multiplier a 2 , a 3  connected to the first input of the DSM for multiplying the first 1-bit signal by a coefficient A 1 , A 2 , A 3 ; a second coefficient multiplier b 2 ,b 3  connected to a second input of the DSM for multiplying the second 1-bit signal by a coefficient B 1 , B 2 , B 3 ; and a third coefficient multiplier c 2 , c 3  connected to a the output of the DSM for multiplying the 1-bit output signal of the DSM by a third coefficient C 2 , C 3 . The adder of each stage adds the output of the integrator of the preceding stage to the output of each 1-bit multiplier connected thereto. 
   The final stage of the DSM comprises an adder  64  having three inputs; a first coefficient multiplier a 4  for multiplying the first signal by a first coefficient A 4 ; a second coefficient multiplier b4 for multiplying the second signal by a second coefficient B 4 . The adder  64  adds the output of the integrator  73  of the preceding stage to the outputs of the multipliers a4 and b4. The adder  64  has an output connected to a quantizer Q. 
   The multipliers a 1  to a 1 , b 1  to b 4  and c 1  to c 4  are all 1-bit multipliers, which multiply each bit of the 1-bit signals applied to them by  p  bit coefficients to produce  p  bit multiplicands. 
   The adders  61  to  64  and the integrators  71  to  73  operate on the  p  bit signals. 
   The  p  bit signals are represented in twos complement form for example whereby positive and negative numbers are represented. 
   The quantizer Q is a comparator having a threshold level of zero. Negative inputs to the quantizer are encoded as −1 (logic 0) and positive inputs as +1 (logical 1), to produce the 1-bit output at output  5 . 
   The first and second 1-bit signals are applied to inputs  4 A and  4 B. A synchronisation circuit  40  is provided to synchronise the first and second signals to a local clock provided by a clock circuit  41 . The synchronisation circuit may separately synchronize the two input signals to the local clock. Clock circuit  41  also controls the clocking of the DSM. 
   The coefficients A 1  to A 4 , B 1  to B 4  and C 1  to C 3  may be chosen using the method described in Annex A to provide 
   a) circuit stability; and 
   b) noise shaping. 
   The coefficients C 1  to C 3  have fixed values to provide the noise shaping. 
   The coefficient A 1  to A 6  and B 1  to B 4  define zeros of the transfer function of the input signals and thus control the gain applied to the signals. 
   In accordance with one embodiment of the present invention, the coefficients A 1  to A 4  and B 1  to B 4  are chosen to sum the first and signals in fixed proportions defined by the coefficients. Thus coefficients A 1  to A 4  may be different from B 1  to B 4 . The coefficients A 1  to A 4  may equal corresponding coefficients B 1  to B 4 . 
   In accordance with another embodiment of the present invention, the coefficients A 1  to A 4  and B 1  to B 4  are variable to allow the first and second signals to be mixed in variable proportions. The variable coefficients A 1  to A 4 , B 1  to B 4  are generated by a coefficient generator  42 . Generator  42  may be a coefficient store, storing sets of coefficients which are addressed by a variable addressing arrangement responsive to a control signal CS. 
   Alternatively the coefficients generator  42  maybe a micro computer which generates the coefficients in response to a control signal. 
     FIG. 3  shows, schematically, an example of an integrator  71 , 72 , 73 . It comprises an adder  30  and a 1-bit delay  31  with a feedback path from the output of the delay to an input of the adder. Thus the output of the delay is added to the signal input to the adder. The adder  30  may be separate from, or implemented by, the adder  61 , 62 , 63 . 
   In a modification of the DSM of  FIG. 3 , the second input  4 B and the coefficient multipliers B 1 ,B 2 , B 3 , B 4  are omitted. 
   The coefficients A 1  to A 4  (and B 1  to B 4  if provided) may be chosen to apply a predetermined filter characteristic to the signal in addition to noise shaping. 
   The microphone  4  of  FIG. 1  has a bandwidth which is at least DC to greater than 24 kHz. 
   The microphone  4  may be replaced by another audio signal source able to produce signal components in the bandwidth of at least DC to greater than 24 kHz. The source may be an audio recorder/reproducer. 
   Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims. 
   Calculating Coefficients 
   This annex outlines a procedure for analysing a fifth order DSM and for calculating coefficients of a desired filter characteristic. 
   A fifth order DSM is shown in FIG. A having coefficients a to f and A to E, adders  6  and integrators  7 . Integrators  7  each provide a unit delay. The outputs of the integrators are denoted from left to right s to w. The input to the DSM is a signal x[n] where [n] denotes a sample in a clocked sequence of samples. The input to the quantizer Q is denoted y[n] which is also the output signal of the DSM. The analysis is based on a model of operation which assumes quantizer Q is simply an adder which adds random noise to the processed signal. The quantizer is therefore ignored in this analysis. 
   The signal y[n]=fx[n]+w[n] i.e. output signal y[n] at sample [n] is the input signal x[n] multiplied by coefficient f plus the output w[n] of the preceding integrator  7 . 
   Applying the same principles to each output signal of the integrators  7  results in Equations set 1. 
         y   ⁡     [   n   ]       =       fx   ⁡     [   n   ]       +     w   ⁡     [   n   ]             
         w   ⁡     [   n   ]       =       w   ⁡     [     n   -   1     ]       +     ex   ⁡     [     n   -   1     ]       +     Ey   ⁡     [     n   -   1     ]       +     v   ⁡     [     n   -   1     ]             
         v   ⁡     [   n   ]       =       v   ⁡     [     n   -   1     ]       +     dx   ⁡     [     n   -   1     ]       +     Dy   ⁡     [     n   -   1     ]       +     u   ⁡     [     n   -   1     ]             
         u   ⁡     [   n   ]       =       u   ⁡     [     n   -   1     ]       +     cx   ⁡     [     n   -   1     ]       +     Cy   ⁡     [     n   -   1     ]       +     t   ⁡     [     n   -   1     ]             
         t   ⁡     [   n   ]       =       t   ⁡     [     n   -   1     ]       +     bx   ⁡     [     n   -   1     ]       +     By   ⁡     [     n   -   1     ]       +     s   ⁡     [     n   -   1     ]             
         s   ⁡     [   n   ]       =       s   ⁡     [     n   -   1     ]       +     ax   ⁡     [     n   -   1     ]       +     Ay   ⁡     [     n   -   1     ]             
 
   These equations are transformed into z-transform equations as well known in the art resulting in equations set 2. 
         Y   ⁡     (   z   )       =       fX   ⁡     (   z   )       +     W   ⁡     (   z   )             
           W   ⁡     (   z   )       ⁢     (     1   -     z     -   1         )       =       z     -   1       ⁡     (       eX   ⁡     (   z   )       +     EY   ⁡     (   z   )       +     V   ⁡     (   z   )         )           
           V   ⁡     (   z   )       ⁢     (     1   -     z     -   1         )       =       z     -   1       ⁡     (       dX   ⁡     (   z   )       +     DY   ⁡     (   z   )       +     U   ⁡     (   z   )         )           
           U   ⁡     (   z   )       ⁢     (     1   -     z     -   1         )       =       z     -   1       ⁡     (       cX   ⁡     (   z   )       +     CY   ⁡     (   z   )       +     T   ⁡     (   z   )         )           
           T   ⁡     (   z   )       ⁢     (     1   -     z     -   1         )       =       z     -   1       ⁡     (       bX   ⁡     (   z   )       +     BY   ⁡     (   z   )       +     S   ⁡     (   z   )         )           
           S   ⁡     (   z   )       ⁢     (     1   -     z     -   1         )       =       z     -   1       ⁡     (       aX   ⁡     (   z   )       +     AY   ⁡     (   z   )         )           
 
   The z transform equations can be solved to derive Y(z) as a single function of X(z) (Equation 3) 
         Y   ⁡     (   z   )       =       fX   ⁡     (   z   )       +         z     -   1         (     1   -     z   1       )       ⁢           ⁢     (       eX   ⁡     (   z   )       +     EY   ⁡     (   z   )       +         z     -   1         1   -     z     -   1           ⁢     (       dX   ⁡     (   z   )       +     DY   ⁡     (   z   )       +         z     -   1         1   -     z     -   1           ⁢     (       cX   ⁡     (   z   )       +     CY   ⁡     (   z   )       +         z     -   1         1   -     z     -   1           ⁢     (       bX   ⁡     (   z   )       +     BY   ⁡     (   z   )       +         z     -   1         1   -     z     -   1           ⁢     (       aX   ⁡     (   z   )       +     AY   ⁡     (   z   )         )         )         )         )         )             
 
   This may be reexpressed as shown in the right hand side of the following equation, Equation 4. A desired transfer function of the DSM can be expressed in series form 
         Y   ⁡     (   z   )         X   ⁡     (   z   )           
 
given in left hand side of the following equation and equated with the tight hand side in Equation 4. 
           Y   ⁡     (   z   )         X   ⁡     (   z   )         =           α   0     +       α   1     ⁢     z       -   1     ⁢                 +       α   2     ⁢     z     -   2         +       α   3     ⁢     z     -   3         +       α   4     ⁢     z     -   4         +       α   5     ⁢     z     -   5               β   0     +       β   1     ⁢     z       -   1     ⁢                 +       β   2     ⁢     z     -   2         +       β   3     ⁢     z     -   3         +       β   4     ⁢     z     -   4         +       β   5     ⁢     z     -   5             =     
     ⁢                 f   ⁡     (     1   -     z     -   1         )       5     +       z     -   1       ⁢       e   ⁡     (     1   -     z     -   1         )       4       +       z     -   2       ⁢       d   ⁡     (     1   -     z     -   1         )       3       +                     z     -   3       ⁢       c   ⁡     (     1   -     z     -   1         )       2       +       z     -   4       ⁢     b   ⁡     (     1   -     z     -   1         )         +       z     -   5       ⁢   a                                     (     1   -     z     -   1         )     5     -       z     -   1       ⁢       E   ⁡     (     1   -     z     -   1         )       4       -       z     -   2       ⁢       D   ⁡     (     1   -     x     -   1         )       3       -                   z     -   3       ⁢       C   ⁡     (     1   -     z     -   1         )       2       -       z     -   4       ⁢     B   ⁡     (     1   -     z     -   1         )         -       Z     -   5       ⁢   A                       
 
   Equation 4 can be solved to derive the coefficients f to a from the coefficients α 0  to α 5  as and coefficients E to A from the coefficients β 0  to β 5  as follows noting that the coefficients α n  and β n  are chosen in known manner to provide a desired transfer function. 
   f is the only z 0  term in the numerator. Therefore f=α 0 . 
   The term α 0 (1−z −1 ) 5  is then subtracted from the left hand numerator resulting in α 0 +α 1 z −1  . . . + . . . α 5  z −5 −α 0 (1−z −1 ) 5  which is recalculated. 
   Similarly f(1−z −1 ) 5  is subtracted from the right hand numerator. Then e is the only z −1  term and can be equated with the corresponding α 1  in the recalculated left hand numerator. 
   The process is repeated for all the terms in the numerator. 
   The process is repeated for all the terms in the denominator.