Abstract:
The invention describes two alternative methods and corresponding devices for producing an energy signal y n  whose amplitude values represent the energy of an electrical signal s n . A first method and the corresponding device calculate the energy signal y n  according to the equation          y   n     =         tau   ·     s   n   2         2   ·     y     n   -   1           +       (     1   -     tau   2       )     ·     y     n   -   1                                   
     while a second method and the corresponding device are based on the equation,          y   n     =       y     n   -   1       +       tau     2        y     n   -   1                (       s   n   2     -     y     n   -   1     2       )                                 
     where tau is a specified parameter and n represents the clock pulse.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 09/412,921 filed Oct. 5, 1999, now U.S. Pat. No. 6,232,762. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the field of signal processing, and in particular to apparatus and method for determining the energy of a signal. 
     An ideal exponentially weighted root mean square (ERMS) detector for determining the energy of an analog signal s(u) is well known from the theory of electrical signal processing. The energy may be determined sequentially by executing three method steps: squaring the signal, integrating the squared signal, and extracting the root of the integrated signal. These method steps are also reflected in the following equation:          y        (       s        (   u   )       ,   t     )       =         1   T            ∫     -   ∞     t                s   2          (   u   )       ·          -       (     t   -   u     )     T                              u                                    
     which describes the functional principle of the ideal analog detector. The detector determines the energy y of the signal s(u) as its RMS value, weighted exponentially with a time constant T, as a function of time t. 
     The conventional digital ERMS detectors receive a digital signal s n  at their input in order to deliver a digital energy signal y n  from the output, with the amplitude values of the energy signal representing the energy of the signal s n . They are based on method steps known from theory. To convert these method steps, as a rule as shown in FIG. 4 a , they are formed as a series circuit consisting of a squaring element  1 , a low-pass filter  2 , and a root extractor  3 . 
     FIG. 4 b  shows one possible digital implementation for such a series circuit. Accordingly, squaring element  1  is formed from a first multiplying element  410  that multiplies the digital signal s n  by itself in order to provide the squared signal s 2 n and its output. The squared signal is then supplied as the input signal to the digital low-pass  2  weighted with a factor tau. 
     Within the low-pass  2 , the input signal is fed as a first summand to an adding element  420  which delivers at its output the desired energy signal but squared as y 2 n. As the second summand, the output signal y 2 n fed back through a state memory  430  weighted with the factor (1−tau) is supplied to adding element  420 . 
     Then the squared energy signal y 2 n is subjected to a root extractor  3 . The root extractor  3  comprises a second adding element  440  that receives the squared energy signal y 2 n and outputs the desired energy signal y n  at its output. To calculate the energy signal y n , adding element  440  adds the squared energy signal to two additional signals. These are firstly the energy signal y 2   n−1  fed back through a second state memory  450  from its own output and secondly a signal y 2   n−1  that is obtained by squaring and negating from the fed-back energy signal y n−1 . 
     The conventional calculation of the energy signal y n  shown here suffers from the following disadvantages: 
     By squaring the signal s n , its dynamic range is sharply increased. It is only possible to store the squared signal in a memory with a very large word width. 
     The square root routines used in conventional extraction of square roots converge slowly, often as a function of the magnitude of the amplitude of their input signal. They are therefore unsuitable for use in systems that require rapid convergence behavior of the detector, such as compander-expander systems for example. 
     SUMMARY OF THE INVENTION 
     An objective of the invention is to improve the methods and devices according to the species for determining the energy of a signal in such fashion that they exhibit faster convergence behavior and reduced computation cost, as well as less storage space. 
     According to a first embodiment of the method according to the invention, an energy signal y n  is calculated whose amplitude values represent the energy of signal s n , according to the equation                y   n     =       (       tau   *     s   n   2         2   *     y     n   -   1           )     +     (       (     1   -     tau   2       )     *     y     n   -   1         )               (   1   )                                
     with 
     tau: a specified parameter and 
     n: the clock pulse, whereby in Equation 1 the method steps of squaring, low-pass filtration, and root extraction are combined. 
     The parameter tau determines the time constant for the exponential weighting. It bears the following relationship approximately with the time constant T in the analog formula:        tau   =              (   0.5   )     *     (     1   -          (          T   *   fs       )         )         (     0.5   -          (          T   *   fs       )         )                                   
     where: 
     i=sqrt(−1) 
     fs=sampling frequency of the digital system. 
     Example 
     with fs 48 kHz and T=20 ms, tau is approximately 0.00104. 
     The explanations of the parameters tau and n likewise apply to all the following equations in the specification. 
     In the method performed according to Equation 1, the integrated method step of extracting the root exhibits a quadratic convergence behavior which has a very advantageous effect on the dynamic behavior of the entire process. 
     In addition, the method step of root extraction in the step of low-pass filtration is integrated, so that the calculation expense is reduced. 
     To work the method, it is no longer necessary to store the squared signal s 2 n with its large dynamic range; instead, it is sufficient to store the amplitude values found for the energy signal y n , which, because of the fact that the root has been extracted, exhibit a value range considerably reduced by comparison with the dynamic range of the squared signal s 2 n. For this reason, in the method according to the invention, a memory with a relatively small word width can be used. 
     According to one advantageous improvement on the method, the calculation of the first summand in Equation 1 includes the step “multiply signal s n  by an auxiliary signal tau/2y n−1  to obtain a product signal” and “multiply the product signal by the signal s n ” or alternatively the steps “square the signal s n ” and “multiply the squared signal s 2 n by the auxiliary signal tau/2y n−1 ”. Depending on the selected implementation of the method, one alternative or the other can contribute to reducing the computation and storage cost. 
     The advantages given for the first embodiment of the method according to the invention apply similarly to a corresponding device. 
     It is advantageous for the corresponding device according to the invention to have an inverter to receive the energy signal y n  and to output an inverted signal b n =tau/2y n , designed so that it, by the equation                b   n     =       b     n   -   1       *       2      k     tau     *     (       (         (     1   +   k     )     *   tau       2      k       )     -     (       b     n   -   1       *     y   n       )       )               (   2   )                                
     with 
     k: a constant preferably between 0.5 and 1, it provides the specified link between the inverted signal b n  and the energy signal y n . The convergence behavior of the inverter can be influenced by the choice of the constant k; in this manner, the quadratic convergence behavior of the entire device can be optimized as well. 
     The explanation for the constant k likewise applies to all the following equations in the specification. 
     According to a second embodiment of the method according to the invention, the goal is achieved especially by virtue of the fact that the energy signal y n  is calculated according to the equation                y   n     =       (       tau   *     s   n   2         2   *     y     n   -   1           )     +     (       (     1   -     tau   2       )     *     y     n   -   1         )               (   3   )                                
     with the method steps of squaring, low-pass filtering, and root extraction being combined in Equation 3. 
     In the method performed according to Equation 3, the method step of root extraction exhibits a quadratic convergence behavior, which has a highly advantageous effect on the dynamic behavior of the entire method. 
     In addition, the method step of low-pass filtration is integrated in the step of root extraction, so that calculation cost is reduced. 
     To work the method, it is no longer necessary to store the squared signal s 2 n with its wide dynamic range; instead, it is sufficient to store the currently determined value for the energy signal y n , which, because of the root extraction that has been performed, has a value range that is considerably reduced by comparison with the dynamic range of the squared signal s 2 n. For this reason, in the method according to the invention, a memory with a much reduced word width can be used. 
     The listed advantages of the second embodiment of the method according to the invention also apply to a corresponding device. 
     For the device according to the second embodiment, it is likewise advantageous for the reasons given for it to have the inverter described above. 
    
    
     These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 a  implementation of a detector according to a first embodiment; 
     FIG. 1 b  is an alternative embodiment of the detector according to FIG. 1 a;    
     FIG. 2 is an implementation of a detector according to a second embodiment; 
     FIG. 3 is an implementation of an inverter according to the invention; 
     FIG. 4 a  is a block diagram of a prior art detector; and 
     FIG. 4 b  is an implementation of the prior art detector from FIG. 4 a.    
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A detailed description of the two preferred embodiments of the invention follow, with reference to FIGS. 1 a ,  1   b ,  2 , and  3 . 
     The first embodiment of a detector according to the invention, shown in FIG. 1 a , calculates an energy signal y n  whose amplitude values represent the energy of the signal s n  according to the following equation:                y   n     =       (       tau   *     s   n   2         2   *     y     n   -   1           )     +       (       (     1   -     tau   2       )     *     y     n   -   1         )     .               (   1   )                                
     For hardware or software implementation of Equation 1, the detector has a first multiplying element  110  and a second multiplying element  120 , an adding element  130 , and a state memory  140 . 
     The input signal s n  is supplied to two inputs of the first multiplying element  110  so that the squared input signal s 2 n is available at its output. The squared signal is then fed to the second multiplying element  120  that multiplies it by an auxiliary signal tau/2y n−1 . A product signal calculated by the second multiplying element  120  is supplied to a first input of adding element  130 , which provides the energy signal y n  to be determined at its output. To calculate the energy signal y n , the latter is fed back through state memory  140 , weighted with a factor 1-tau/2, to a second input of adding element  130 . 
     With the first embodiment of the detector according to the invention wired in this fashion, a low-pass filter is obtained in which a root extractor is integrated. 
     An alternative design, shown in FIG. 1 b , of the first embodiment of the detector differs from the design according to FIG. 1 a  only in the wiring of the two multipliers  110  and  120 . According to the alternative design according to FIG. 1 b , the multiplying element  110  multiplies the signal s n  initially by the auxiliary signal tau/2y n−1 . The product signal at the output of the first multiplying element is then supplied to multiplying element  120 , which multiplies the product signal times the signal s n . 
     In the alternative designs of the detector according to FIGS. 1 a  and  1   b , the product signals at the outputs of the second multiplier  120  are identical. 
     According to the second embodiment of the detector shown in FIG. 2, an energy signal y n  whose amplitude values represent the energy of a signal, s n  are calculated according to the following equation:                y   n     =       y     n   -   1       +       tau     2        y     n   -   1                  (       s   n   2     -     y     n   -   1     2       )     .                 (   3   )                                
     For hardware or software implementation of Equation 3, the detector according to the second embodiment has a first multiplying element  210  and a second multiplying element  230 , a first adding element  220  and a second adding element  240 , a state memory  260 , and a squaring element  250 . 
     The signal s n  is supplied to the two inputs of the first multiplying element  210 , so that the squared signal s 2   n  is provided at the output of the first multiplying element. The squared signal is delivered to a first input of the first adding element  220  whose output signal is supplied to the second multiplying element  230 . The second multiplying element  230  multiplies the output signal of the adding element  220  times the auxiliary signal tau/2y n−1  and delivers the resultant product signal to a first input of the second adding element  240 , which delivers the energy signal y n  to be produced at its output. The output signal of adding element  240  is fed back through the state memory  260  to a second input of the second adding element, 240 . In addition, the output signal y n−1  of the state memory  260  is fed back following negation through the squaring element  250  connected downstream to the second input of the first adding element  220 . 
     With the second embodiment of the detector according to the invention wired in this fashion, a root extractor is formed into which a low-pass filter is integrated. 
     FIG. 3 shows the implementation of an inverter for converting the energy signal y n  into a signal b n =tau/2y n . The inversion takes place according to the invention by the following equation:                b   n     =       b     n   -   1       *       2      k     tau     *       (       (         (     1   +   k     )     *   tau       2      k       )     -     (       b     n   -   1       *     y   n       )       )     .               (   2   )                                
     For a hardware or software implementation of Equation 2, the inverter has a first multiplying element  310  and a second multiplying element  330 , an adding element  320 , and a state memory  340 . 
     The first multiplying element  310  multiplies the received energy signal y n  by the output signal b n  from the inverter fed back via the state memory  340 . The output signal of the first multiplying element  310  is negated and supplied to adding element  320 , which adds it to the summand (1+k)*(tau/2k). The sum signal at the output of adding element  320  is weighted with the factor 2k/tau and supplied to the second multiplying element  330 , that multiplies it by the output signal b n−1  of the state memory  340 . At the output of the second multiplying element  330 , the output signal b n  from the inverter is provided, and supplied simultaneously to the state memory  340  as an input signal. 
     The following is true of all versions of the ERMS detector: 
     The method supplies inaccurate results if y n  suddenly rises sharply, so that: y n &gt;(c*y n−1 ). Here c is a constant which can be chosen depending on the desired accuracy. A good accuracy is obtained with c&lt;1.5. 
     In order to improve the accuracy in this case, the following iterations can be performed as well. This is necessary only with an abrupt change in signal energy and strict requirements for accuracy. This results in the following modification:          y     n   ,   m       =       tau   ·       s   n   2       2        y     n   ,     m   -   1               +         (     1   -     tau   2       )     ·     y     n   -   1                       and                 y     n   ,   m       =       y     n   -   1       +       tau     2        y     n   ,     m   -   1                  (       s   n   2     -     y     n   -   1     2       )                                
     where: 
     m=additional iteration index. 
     The iterations are continued until          1   c     &lt;            y     n   ,   m         y     n   ,     m   -   1                &lt;   c                          
     As a rule, even with sharp jumps in the input signals, two to three iterations will suffice. 
     Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.