Abstract:
A system for decoding an electrical input signal includes a filter that impresses a variable gain on a portion of the input signal to deemphasize a spectral region of the input signal. The variable gain is set as a function of a variable gain control signal. A frequency detector generates the variable gain control signal in accordance with a frequency value wherein approximately one-half of the energy of the input signal is below the frequency value.

Description:
PRIORITY INFORMATION 
     This application is a continuation of Ser. No. 09/897,722 filed Jul. 2, 2001 now U.S. Pat. No. 7,046,750. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to signal conditioning systems, and in particular to a signal decoder with adaptive signal weighting. 
     Various techniques for encoding and decoding a data signal (e.g., video or audio) are known. For example, U.S. Pat. Nos. 4,101,849 and 4,136,314 disclose an encoding technique that compresses the data signal with high frequency preemphasis. The signal is then stored or transmitted onto or across a medium, and the received data signal is expanded and deemphasized in a complementary manner. Preemphasis involves altering the magnitude of select frequency components of the signal with respect to the magnitude of frequency components, to reduce noise and thus improve the signal-to-noise ratio (SNR). Similarly, deemphasis involves altering select frequency components of a received encoded signal (e.g., a signal that was transmitted across a communication channel or read from a storage medium), in either a negative or positive sense in a complementary manner to the preemphasis applied to the signal. 
     U.S. Pat. Nos. 4,101,849 and 4,136,314 disclose controlling the amount of preemphasis as a function of the ratio of the energy in high and low frequency portions of the data signal. Of course the amount of deemphasis is also controlled by the ratio of the energy in the high and low frequency portions of the signal. However, a problem with this ratio technique is that it does not the bandwidth of the transmission channel or the storage medium. 
     U.S. Pat. No. 4,539,526 discloses an adaptive signal weighting system for encoding and decoding a data signal. The technique disclosed therein preemphasizes only the high frequency signal components during encoding as a function of the spectral energy contained in the high frequency portion of the signal. On the decoder side the received signal is deemphasized in a complementary manner by providing a gain to the high frequency signal components based upon the spectral energy within the high frequency portion of the spectrum of the received signal. One problem with this technique is that it controls the amount of preemphasis/deemphasis based upon the spectrum of only the high frequency components. 
     Therefore, there is a need for an improved technique for encoding and/or decoding a data signal that is transmitted across a data channel or stored and retrieved from a storage medium. 
     SUMMARY OF THE INVENTION 
     Briefly, according to an aspect of the present invention, a system for decoding an electrical input signal includes a filter that impresses a variable gain on a portion of an input signal to deemphasize a spectral region of the input signal. The variable gain is set as a function of a variable gain control signal. A frequency detector generates the variable gain control signal in accordance with a frequency value wherein approximately one-half of the energy of the input signal is below the frequency value. 
     In one embodiment, the frequency detector includes a variable notch filter that receives and filters the input signal and provides a notch filtered signal value, wherein the notch filter includes a notch set as a function of the variable gain control signal. A mixer receives and mixes the notch indicative thereof to an integrator, which integrates the mixed signal to provide the variable gain control signal. 
     In a second embodiment, the frequency detector includes a low pass filter that filters the input signal to provide a first filtered signal. A first absolute value detector receives the first filtered signal and provides a first absolute filtered signal indicative thereof. An amplifier amplifies the first absolute value signal to provide an amplified first absolute filtered signal. A second absolute value detector receives the input signal and provides a second absolute filtered signal. A comparator compares the amplified first absolute filtered signal and the second absolute filtered signal that provides a control signal, which is indicative of the second control signal. 
     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  is a block diagram illustration of a data communications system; 
         FIG. 2  is a block diagram illustration of the expander of the system illustrated in  FIG. 1 ; 
         FIG. 3  is a block diagram illustration of the frequency detector of the expander; 
         FIG. 4  is an alternative embodiment frequency detector; and 
         FIG. 5  is a block diagram illustration of the compressor of the system illustrated in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a block diagram illustration of a data communications system  10 . The system includes a signal source  12  that provides a signal (e.g., audio and/or video) on a line  14  to a compressor  16  (i.e., an encoder), which provides a compressed (i.e., companded) signal on a line  18  that is transmitted over a communications channel (i.e., a communication medium or a storage media such as for example a tape, a CD, or an electronic memory device). The system also includes an expander  20  that receives the encoded/compressed signal from the communications medium or storage media, and decompresses the signal to provide a decompressed signal on a line  22 . As known, transmitting a signal over a communications medium or writing a signal to a storage media often results in the coupling of some undesirable noise to the compressed signal on the line  18 . The compressor  16  provides preemphasis to the high frequency portion of the input signal on the line  14 , and the expander  20  provides a complementary deemphasis to the high frequency portion of its input signal. 
       FIG. 2  is a block diagram illustration of the expander  20 . The expander  20  includes a first bandpass filter (BPF)  32  that receives the compressed signal (e.g., a compressed audio signal) on the line  18 . The bandpass filter  32  preferably includes a passband in the frequency range of about 20 Hz-20 kHz. The bandpass filter  32  provides a bandpass filtered signal on a line  34  to a variable filter  36 , a frequency detector  38  and a second bandpass filter  40 . The variable filter  36  receives a control signal on a line  42  that controls the gain impressed on the high frequency portion of the signal transmitted through the variable filter  36 . The details of how this control signal is generated shall be discussed in detail hereinafter. 
     The second bandpass filter  40  includes a passband (e.g., 50 Hz to 5 kHz) that provides a second bandpassed signal on a line  44  to a level detector  46 . The level detector  46  generates a gain control signal on a line  47  to the gain control  37 , that provides a decoder output signal. 
     According to an aspect of the present invention, the decoder  20  includes the frequency detector  38  that provides the control signal on the line  42 .  FIG. 3  is a block diagram illustration of the frequency detector  38 . In this digital embodiment, the frequency detector  38  includes a notch filter  50  and a delay  52  that each receive the first bandpassed signal on the line  34 . The notch filter  50  provides a notch filtered output signal on a line  54  to a mixer  56 , which also receives, on a line  58 , a delayed version of the input signal. The mixer  56  mixes the signals on the lines  54 ,  58  and provides a mixed signal on a line  60 . A multiplier  62  multiplies the mixed signal on the line  60  with a time constant value τ, and the resultant product is provided on a line  64 . A summer  66  then sums the past value of a control value C n  with the signal value on the line  64 . As a result, the control value C n  on the line  42  can be expressed as:
 
 C   n   =C   n−1 +( Q *τ)  EQ. 1
 
     where Q is equal to the signal on the line  60 . 
     The control value C n  is input to the notch filter  50  to control the location of the notch. For example, as the value of the control value C n  increases, the frequency value of the notch increases. Similarly, as the control value C n  decreases, the frequency value of the notch also decreases. Of course, one of ordinary skill will recognize the system may be configured such that the opposite is true. 
     Referring still to  FIG. 3 , the transfer function for the notch filter  50  may be expressed as:
 
1+2C n z −1 +z −2   EQ. 2
 
The mean frequency detector  38  operates to drive the control value C n  to a value that causes the value of the signal on the line  60  to approach zero. For example, in steady state (i.e., the DC value of the signal on the line  60  is zero), the notch of the notch filter  50  is located at a frequency value such that approximately one-half of the energy of the signal on the line  34  is located less than the notch filter value, while the other half of the energy of the signal on the line  34  is located above the notch filter value.
 
     Consider for example if the frequency spectrum of the input signal on the line  18  includes only frequency components of equal energy at 1 kHz and 3 kHz. In this case the mean frequency detector  38  would shift the notch of the notch filter  50  such that approximately one-half of the energy is above the notch while one-half the energy is below the notch (e.g., the notch is located at about 2 kHz). Once the mean frequency detector  38  reaches steady state in response to this input signal, the DC component of the signal on the line  60  will be zero and as a result the control signal value C n  will be relatively constant. 
     Referring again to  FIG. 1 , the compressor  16  and the expander  20  are similar to those disclosed in U.S. Pat. No. 4,539,526, with the principal exception that the amount of preemphasis and deemphasis applied to the signal is determined by a novel frequency detection technique. Specifically, unlike the system disclosed in U.S. Pat. No. 4,539,526, the expander of the present invention utilizes the entire signal spectrum to determine the amount of deemphasis to be applied by the variable filter  36 . The decoder disclosed in U.S. Pat. No. 4,539,526 looks at only the high frequency energy of the signal to determine the amount of deemphasis to be applied to the high frequency portion of the signal. In contrast, the expander/decoder of the present invention looks at entire signal spectrum to determine the frequency at which one-half of the input signal energy is below. A signal indicative of this frequency value is then used to determine the amount of deemphasis to be applied. Of course a complementary technique is used to compress the signal. In the interest of brevity elements that are substantially the same as those disclosed in U.S. Pat. No. 4,539,526 shall not be discussed herein. For example, the first and second BPFs  32 ,  40 , the variable filter  36 , the level detector  46  and the gain control  37  are similar to the corresponding elements disclosed in U.S. Pat. No. 4,539,526. Accordingly, U.S. Pat. No. 4,539,526 is hereby incorporated by reference. 
       FIG. 4  is an alternative embodiment mean frequency detector  70 . This embodiment includes a variable low pass filter  72  having an adjustable corner frequency. This filter  72  provides a low pass filtered signal on a line  74  to a gain function  76  having a value of two and the resultant amplified signal is provided to a first absolute value unit  77  that provides a first absolute value signal on a line  78 . A second absolute value unit  80  receives the input signal on the line  34 , and provides a second absolute value signal on a line  82 . A comparator  84  receives the first and second absolute value signals, and provides an output signal on a line  86  indicative of the difference between the signals. Specifically, if the value on the line  82  is greater than the value on the line  78 , then the comparator  84  provides an output signal on the line  86  to increase the value of the corner frequency of the low pass filter  72 . The signal on the line  86  may be input to a low pass filter  88  to smooth the signal prior to providing it to the low pass filter  72  to set the corner frequency thereof. Similarly, if the value on the line  82  is less than the value on the line  78 , then the comparator  84  provides an output signal on the line  86  to decrease the value of the corner frequency of the low pass filter  72 . In general, the value of the signal on the line  86  is driven such that the corner frequency of the low pass filter  72  is set to the one-half energy point of the signal on the line  34 . That is, if the signal values on the lines  78  and  82  are equal, then the output value of the low pass filter  72  is one-half the value of the input signal on the line  34 . As the input signal on the line  34  changes, the mean frequency detector  70  tracks the change to shift the corner frequency of the low pass filter  72  such that the value of the signals on the lines  78  and  82  are equal. 
       FIG. 5  is a block diagram illustration of the compressor  16  of the system illustrated in  FIG. 1 . This compressor  16  is similar to the compressor disclosed in U.S. Pat. No. 4,539,526, with the principal exception that the amount of preemphasis applied to the signal is determined by a novel mean frequency detector  102 , which is complementary to the mean frequency detector  38  ( FIG. 2 ). Specifically, unlike the system disclosed in U.S. Pat. No. 4,539,526, the compressor of the present invention utilizes the entire signal spectrum to determine the amount of preemphasis. Again, in the interest of brevity elements that are substantially the same as those disclosed in U.S. Pat. No. 4,539,526 shall not be discussed at length herein. 
     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.