Abstract:
A dynamic range compressor includes an input terminal for receiving input signal to be compressed, an amplifier unit for amplifying the signal to be compressed by an amplification factor, for deriving a compressed output signal, an output terminal for supplying the compressed output signal, a first envelope detector unit for deriving a first envelope signal from the input signal, and an amplifier control unit for generating an amplifier control signal in dependence of an envelope signal. Further including a second envelope detector unit for deriving a second envelope signal from the input signal, a first signal level prediction unit for generating a first prediction signal from the first envelope signal, a second signal level prediction unit for generating a second prediction signal from the second envelope signal, and a signal combination unit for combining the first and second prediction signals to generate a combined prediction signal.

Description:
The invention refers to a dynamic compressor in accordance with the Preamble of Claim  1 . A dynamic compressor of this type is known from a Master&#39;s thesis of the Queen Mary, University of London, entitled ‘A Design of a Digital, Parameter-automated, Dynamic Range Compressor’ by Dimitrios Giannoulis, dated 26 Aug. 2010. 
     DESCRIPTION OF THE PRIOR ART 
     In conventional dynamic compressors for audio signals, envelope signals are detected and a gain factor generator is controlled using these envelope signals. 
     There are techniques to lend the envelope signals an extended decay (“release”) because this allows signal distortions to be minimised. 
     Those conventional dynamic compressors, which work with forward control are specifically considered here because these have the advantage of little time delay; this advantage should be maintained. 
     Conventional dynamic compressors can be provided with a parameter regulator for the gain factor generator, which is provided to control the level value of the dynamically compressed signal. As a rule, this parameter control is based on a comparison with a target level value and a characteristic variable compression curve of the gain factor generator. 
     In cases, in which the application of the characteristic compression curve reduces the average level of the dynamically compressed signal in comparison with the input signal, there are techniques for equalising the amplification, known as “auto make-up gain”, which can work statically or signal-dependently. [Giannoulis, section 3.4, page 42] 
     DESCRIPTION OF THE INVENTION 
     When a level measurement is derived from the envelope with release properties in order to use this for comparison with the target level value, this has the disadvantage that an additional level error of the dynamically compressed signal may arise, which is in the order of magnitude of several dB and hence is potentially not tolerable. 
     The object of the invention is to reduce this level error. 
     The dynamic compressor, as defined in the preamble, is also identified in the characteristic part of the first Claim. Preferred embodiments of the invention are identified in Claims  2  to  7 . 
     The invention functions as follows. 
     In accordance with the invention, two different envelopes are derived from the input signal: the first envelope detector without release properties and the second envelope with release properties. This takes place using two associated envelope detectors, where the input signal is fed into the first envelope and the second envelope detector is coupled to the first envelope detector in series and is designed so that it generates the envelope signal with release properties from the envelope signal without release properties. It is also possible, however, for the input signal to be fed into both envelope detectors and the second envelope detector to be designed so that it generates the envelope signal with release properties directly from the input signal. The second envelope signal corresponds to the signal which is also fed into the gain factor generator of the dynamic compressor as the input signal. 
     A predicted envelope signal is derived from each of two envelope signals: the first predicted envelope signal from the envelope signal without release properties, the second predicted envelope signal from the envelope signal with release properties, and both from the current gain factor of the dynamic compressor. The respective predicted envelope signal comes about as a result of the multiplication of the respective envelope signal with the gain factor value. These predicted envelope signals provide estimated values of the corresponding, measurable envelopes of the compressed signal occurring due to the given gain factor value. In fact, however, envelopes of the compressed signal are not measured. A predicted level is derived from each of the two predicted envelope signals using the customary technique. A gain correction factor is derived by calculating the difference between both predicted levels. This provides information about the amount by which the levels of the compressed signal occurring due to the given gain factor value would vary if the gain factor value were derived from the envelope signal without release properties rather than from the envelope signal with release properties. 
     The level target value of the dynamic compressor is corrected by the gain correction factor. Thus, the above-mentioned level error is compensated. 
     These and further objects are achieved by means of a dynamic compressor as described in the attached claims, which are considered an integral part of the present description. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention is further explained in the following figure description by reference to examples embodiment of the dynamic compressor according to the invention. Here, 
         FIG. 1  depicts an example embodiment of the dynamic compressor, 
         FIG. 2  depicts an example embodiment of a level prediction unit in the dynamic compressor in  FIG. 1 , 
         FIG. 3  depicts an example embodiment of the amplifier control unit in the dynamic compressor in  FIG. 1 , 
         FIG. 4  depicts the behaviour of various signals in the dynamic compressor in  FIG. 1 , 
         FIG. 5  depicts a second example embodiment of the dynamic compressor in accordance with the invention and 
         FIG. 6  depicts an example embodiment of the amplifier control unit of the dynamic compressor in accordance with  FIG. 5 . 
     
    
    
     DESCRIPTION OF THE FIGURES 
       FIG. 1  depicts a first example embodiment of the dynamic compressor. 
     The dynamic compressor  100  in  FIG. 1  is provided with an input terminal  102  for receiving an input signal to be compressed. The input terminal  102  is coupled to an input of an amplifier device  104 , which is equipped for amplifying the input signal to be compressed with a gain factor, to obtain a compressed output signal. An output of the amplifier device  104  is coupled to an output terminal  106  for generating the compressed output signal. 
     A first envelope detector unit  108  is provided for deriving a first envelope signal from the input signal. For this purpose, an input of the first envelope detector unit  108  is coupled to the input terminal  102  of the dynamic compressor. A second envelope detector unit is provided to derive a second envelope signal from the input signal. In this example embodiment, the second envelope detector unit is formed by a series of blocks  108 , that is, the first envelope detector unit, and  110 . In addition, the dynamic compressor  100  incorporates an amplifier control unit  112  for generating an amplifier control signal as a function of an envelope signal. For this, an input  113  of the amplifier control unit  112  is coupled to an output of the second envelope detector unit  108 ,  110 . An output  115  of the amplifier control unit  112  is coupled to a control signal input  117  of the amplifier device  104 . 
     The dynamic compressor is also provided with a first level prediction unit  114  for generating a first prediction signal from the first envelope signal. For this, an input of the first level prediction unit  114  is coupled to an output of the first envelope detector unit  108 . 
     A second level prediction unit  116  is provided for generating a second prediction signal from the second envelope signal. For this, an input of the second level prediction unit  116  is coupled to the output of the second envelope detector unit  108 ,  110 . 
     Outputs of the first and second prediction units  114  and  116  are coupled to corresponding inputs of a signal combining unit  118 , which is equipped to combine the first and second prediction signals for obtaining a combined prediction signal. In accordance with the invention, the amplifier control unit  112  is also equipped for generating the amplifier control signal as a function of the combined prediction signal. For this, an output of the signal combining unit  118  is coupled to a second input  119  of the amplifier control unit  112 . 
     In the example embodiment in accordance with  FIG. 1 , an input of the second envelope detector unit  110  is coupled to an output of the first envelope detector unit  108 . It would also be possible for the input of the second envelope detector unit  110  to be coupled directly to the input terminal  102  of the dynamic compressor. In this case, the signal processing in the second envelope detector unit is equivalent to the sequential signal processing in the detector units  108  and  110 , as they are switched in  FIG. 1 . 
     The first prediction unit  114  is provided with a multiplication unit  120  for multiplying the first envelope signal with a first multiplication factor as a function of the amplifier control signal. For this, an input of the multiplication unit  120  is coupled to the output of the first envelope detector unit  108  and a control signal input  121  of the multiplication unit  120  is coupled to the output  115  of the amplifier control unit  112 . In addition, a level measuring device  122  is provided for deriving the first prediction signal from the first envelope signal multiplied in the multiplying unit  120 . 
     The second prediction unit  116  is provided with a multiplication unit  124  for multiplying the second envelope signal with a second multiplication factor as a function of the amplifier control signal. For this, an input of the multiplication unit  124  is coupled to the output of the second envelope detector unit  108 ,  110  and a control signal input  125  of the multiplication unit  124  is coupled to the output  115  of the amplifier control unit  112 . In addition, a level measuring device  126  is provided for deriving the second prediction signal from the second envelope signal multiplied in the multiplication unit  124 . 
     Envelope detectors, such as the first envelope detector unit  108 , are generally known and typically incorporate a rectifier and a low-pass filter. The block  110  incorporates a release circuit, also generally known, which typically incorporates a storage element, such as a capacitor, which is charged quickly, but discharges slowly. The second envelope detector unit  108 ,  110  is thus an envelope detector with release properties. 
     In the example embodiment in  FIG. 1 , the level measuring devices  122  and  126  may be built from a series circuit of an integrator circuit  201  and a logarithmising circuit  202 , as shown in  FIG. 2 . In this way, the prediction signals are generated as prediction signals converted in the logarithmised region. In this case, the signal combining unit  118  is a subtracting circuit. If the prediction signals were generated in the non-logarithmised region, the signal combining unit would be a dividing circuit. 
     Where the combined prediction signal  119  has been generated in the logarithmised region, the amplifier control unit  112  may appear as shown in  FIG. 3 . The amplifier control unit  112  in  FIG. 3  incorporates a signal combining unit  302 , a first converter  304  and a second converter  306 . The first converter  304  is configured for receiving the envelope signal from the second envelope detector unit  108 ,  110  and for converting the envelope signal into the logarithmised region. For this, the output of the converter  304  is coupled to an input  308  of the signal combining unit  302 . The input  119  of the amplifier control unit  112  is coupled to a second input  310  of the signal combining unit  302 . An output  312  of the signal combining unit  302  is coupled to an input of the converter  306 . If the input signals of the signal combining unit  302  are both in the logarithmised region, then the signal combining unit  302  is configured as an adding circuit. The second converter  306  is configured for receiving the output signal of the signal combining unit  312  and for converting this output signal in the non-logarithmised region. 
     Where the combined prediction signal has been generated in the non-logarithmised region, the signal combining unit  302  is configured as a multiplying circuit and the converters  304  and  306  can be omitted. 
     In addition, there may be another scaling circuit  314  in the amplifier control unit  112 , which, if the signals in the amplifier control unit  112  are in the logarithmised region, is realised as a multiplying circuit, for multiplying the output signal of the converter  304  with a value G. If the signals are in the amplifier control unit  112  in the non-logarithmised region, then the scaling circuit  314  is an exponentiating circuit, whereby the output signal of the converter  304  is exponentiated with the value G. This scaling circuit  314  enables the level of compression of the dynamic compressor to be set as required by selecting the value G. Here, G is typically in the [−1,0] region, although not necessarily restricted to this. If G=0, then no dynamic compression occurs. If, for example, G=−1, then an intense dynamic compression occurs. 
     Another signal combining unit  316  may possibly be interposed between the converter  304  and the signal combining unit  314 . In the logarithmised region, this signal combining unit  316  is a subtracting circuit; otherwise, it is a dividing circuit. This signal combining unit  316  allows the level operating point of the dynamic compressor to be set as required. The level operating point corresponds to the value T, whereby in the logarithmised region T is a level value based on the level value of the input signal, or else a factor based on the amplitude of the input signal. 
     The behaviour of certain signals in the dynamic compressor in  FIG. 1  is represented in  FIG. 4 . The input signal at the input terminal  102  is depicted in  FIG. 4 a    as a function of time. As can be seen in  FIG. 4 a   , low-amplitude signal components are alternated with high-amplitude signal components, whereas the pauses, i.e. the low-amplitude segments on the left-hand side make up a large fraction and make up a small fraction on the right-hand side. The continuous line b 2  in  FIG. 4 b    depicts the output signal of the first envelope detector unit  108 . It can be seen in  FIG. 4 b    that the output signal of the envelope detector unit  108  rapidly follows the decreasing amplitudes of the input signal a. The continuous line b 1  in  FIG. 4 b    depicts the output signal of the second envelope detector unit  108 , 110 . It can be seen in  FIG. 4 b    that the output signal of the envelope detector unit  108 , 110  only very slowly follows the decreasing amplitudes of the input signal a. 
     The continuous line c 2  in  FIG. 4 c    depicts the output signal of the first multiplication unit  120  and the continuous line c 1  depicts the output signal of the second multiplying unit  124 . Here, the effect of the multiplication by the amplifier control signal f, which will also be described below, becomes evident. The ripple of the output signals of the multiplication units  120  and  124  is smaller, compared with the output signals of both envelope detectors  108  or  108 ,  110 . Therefrom, it is discernible that the output signal of the multiplication unit  124  behaves as if it had an imagined envelope of the compressed output signal, and that the output signal of the multiplication unit  120  behaves as if it had another imagined envelope of the compressed output signal in the hypothetical case where the amplifier control signal had been derived from the envelope signal without release properties b 2 , rather than from the envelope signal with release properties b 1 . 
     The continuous line d 2  in  FIG. 4 d    depicts the output signal of the first predictor unit  114  and the continuous line d 1  depicts the output signal of the second prediction unit  116 . Due to the release properties in the block  110 , the output signal d 1  of the prediction unit  114  is greater than the output signal d 2  of the prediction unit  116 . Both signals, d 1  and d 2 , display an increasing level as soon as the pause fraction of the input signal a becomes smaller. The increase, however, is smaller for the signal, which was derived from the envelope with release properties, namely d 1   
       FIG. 4 e    depicts the output signal e of the signal combining unit  118 . As can be seen in  FIG. 4 d   , both signals d 1  and d 2  converge during the interval in which the pause fraction of the input signal a is smaller. This means that the output signal e of the signal combining unit  118  becomes smaller over this interval. 
       FIG. 4 f    depicts the amplifier control signal f of the amplifier control unit  112 , and  FIG. 4 g    then depicts the dynamically compressed output signal g of the dynamic compressor  100 . The dynamic compression can be recognised insofar as, compared with the input signal a, the differences between low amplitude and high amplitude have become smaller for the signal g. 
       FIG. 5  depicts a second example embodiment of the dynamic compressor in accordance with the invention. The dynamic compressor in accordance with  FIG. 5  depicts great similarities with the dynamic compressor in accordance with  FIG. 1 . Similar elements of the dynamic compressors are indicated with the same reference numbers in both Figures. The differences consist in another design of the second envelope detector unit, indicated in  FIG. 5  by  510 , and of the amplifier control unit, indicated in  FIG. 5  by  512 . The envelope detector unit  510  is constructed from a series circuit of the blocks  108  and  110  in  FIG. 1  and works in the same way. It can also be seen that the output of the envelope detector unit  108  is also coupled to a third input  517  of the amplifier control unit  512 . 
     On the other hand, the amplifier control unit  512  may, where the combined prediction signal  119  was generated in the logarithmised region, appear like the unit depicted in  FIG. 6 . The amplifier control unit  512  in  FIG. 6  is a development of the amplifier control unit  112  in  FIG. 3 . Elements with the same reference numbers in both amplifier control units  112  in  FIGS. 3 and 512  in  FIG. 6  are identical. The amplifier control unit  512  is also provided with a signal block  524 , a signal combining unit  520  and a signal combining unit  522 . The input  526  of the signal block  524  is coupled to the input  517  of the amplifier control unit  512 . One output  528  of the signal block  524  is coupled to a respective one input of both signal combining units  316  and  520 . An output of the signal combining unit  520  is coupled to an input of the signal combining unit  522 . The output of the scaling circuit  314  is coupled to a second input of the signal combining unit  522 . An output of the signal combining unit  522  is coupled to the input  308  of the signal combining circuit  302 . 
     The signal block  524  may look like the signal block in  FIG. 2 , already shown for the level measuring units  122  and  124 . The signal block can thus also be provided with a series circuit of an integrator circuit and of a logarithmising circuit. 
     The signal combining unit  520  in the logarithmised region is a subtracting circuit and the signal combining unit  522  is an adding circuit. Similarly to in the amplifier control unit  112 , the signal block  524  and the signal combining units  316 ,  520  and  522  allow the level operating point of the dynamic compressor to be set as required. On the other hand, the level operating point corresponds to the value T. In contrast to the circuit in  FIG. 1  using the amplifier control unit  112  in accordance with  FIG. 3 , however, the circuit in  FIG. 5  using the amplifier control unit  512  in accordance with  FIG. 6  also allows the difference between the actual level of the input signal and the level operating point T to be taken into consideration, so that here, T is a level value based on a signal level value 0. Thus, advantageously, T can be set, even without a known input signal level. 
     Many changes, modifications, variations and other uses and applications of the subject invention will become apparent to those skilled in the art after considering the specification and the accompanying drawings which disclose preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the scope of the invention are deemed to be covered by this invention. 
     The elements and characteristics described in the various forms of preferred embodiments can be mutually combined without departing from the scope of the invention. 
     Further implementation details will not be described, as the man skilled in the art is able to carry out the invention starting from the teaching of the above description.