Patent Publication Number: US-8526622-B2

Title: Configurable filter for processing television audio signals

Description:
RELATED APPLICATIONS 
     This present application is continuation of U.S. application Ser. No. 12,471,946, filed May 26, 2009, which is a continuation of U.S. application Ser. No. 11/089,385, filed Mar. 24, 2005 (now U.S. Pat. No. 7,539,316), which claims priority under 35 U.S.C. §119 (e) of U.S. Provisional Patent Application Ser. No. 60/555,583, filed Mar. 24, 2004; the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     In 1984, the United States, under the auspices of the Federal Communications Commission, adopted a standard for the transmission and reception of stereo audio for television. This standard is codified in the FCC&#39;s Bulletin OET-60, and is often called the BTSC system after the Broadcast Television Systems Committee that proposed it, or the MTS (Multi-channel Television Sound) system. 
     Prior to the BTSC system, broadcast television audio was monophonic, consisting of a single “channel” or signal of audio content. Stereo audio typically requires the transmission of two independent audio channels, and receivers capable of detecting and recovering both channels. In order to meet the FCC&#39;s requirement that the new transmission standard be ‘compatible’ with existing monophonic television sets (i.e., that mono receivers be capable of reproducing an appropriate audio signal from the new type of stereo broadcast), the Broadcast Television Systems Committee adopted an approach similar to FM radio systems: stereo Left and Right audio signals are combined to form two new signals, a Sum signal and a Difference signal. 
     Monophonic television receivers detect and demodulate only the Sum signal, consisting of the addition of the Left and Right stereo signals. Stereo-capable receivers receive both the Sum and the Difference signals, recombining the signals to extract the original stereo Left and Right signals. 
     For transmission, the Sum signal directly modulates the aural FM carrier just as would a monophonic audio signal. The Difference channel, however, is first modulated onto an AM subcarrier located 31.768 kHz above the aural carrier&#39;s center frequency. The nature of FM modulation is such that background noise increases by 3 decibel (dB) per octave, and as a result, because the new subcarrier is located further from the aural carrier&#39;s center frequency than the Sum or mono signal, additional noise is introduced into the Difference channel, and hence into the recovered stereo signal. In many circumstances, in fact, this rising noise characteristic renders the stereo signal too noisy to meet the requirements imposed by the FCC, and so the BTSC system mandates a noise reduction system in the Difference channel signal path. 
     This system, sometimes referred to as dbx noise reduction (after the company that developed the technique) is of the companding type, comprising an encoder and decoder. The encoder adaptively filters the Difference signal prior to transmission such that amplitude and frequency content, upon decoding, hide (“mask”) noise picked up during the transmission process. The decoder completes the process by restoring the Difference signal to original form and thereby ensuring that noise is audibly masked by the signal content. 
     The dbx noise reduction system is also used to encode and decode Secondary Audio Programming (SAP) signals, which is defined in the BTSC standard as an additional information channel and is often used to e.g., carry programming in an alternative language, reading services for the blind, or other services. 
     Cost is, of course, of prime concern to television manufacturers. As a result of intense competition and consumer expectations, profit margins on consumer electronics products, especially television products, can be vanishingly small. Because the dbx decoder is located in the television receiver, manufacturers are sensitive to the cost of the decoder, and reducing the cost of the decoder is a necessary and worthwhile goal. While the encoder is not located in a television receiver and is not as sensitive from a profit standpoint, any development which will decrease manufacturing costs of the encoder also provides a benefit. 
     SUMMARY OF THE DISCLOSURE 
     In accordance with an aspect of the disclosure, a television audio signal encoder includes a matrix that sums a left channel audio signal and a right channel audio signal to produce a sum signal. The matrix also subtracts one of the left and right audio signals from the other to produce a difference signal. The encoder also includes a configurable infinite impulse response digital filter that selectively uses one or more sets of filter coefficients to filter the difference signal. Each selectable set of filter coefficients is associated with a unique filtering application to prepare the difference signal for transmission. 
     In one embodiment, the configurable infinite impulse response digital filter may include a selector that selects one of the one or more sets of filter coefficients. The configurable infinite impulse response digital filter may include a selector that selects an input signal from a group of input signals. One input signal from the group of input signals may include an output signal of the configurable infinite impulse response digital filter. The configurable infinite impulse response digital filter may be a second order infinite impulse response filter. Furthermore, the configurable infinite impulse response digital filter may be configured as a low pass filter, a high pass filter, bandpass filter, an emphasis filter, etc. The selection of the filter coefficients may based on a rate that the television audio signal is sampled. The sets of filter coefficients may be stored in a memory or in a look-up table that is stored in memory. The television audio signal may comply to the Broadcast Television System Committee (BTSC) standard, the Near Instantaneously Companded Audio Muliplex (NICAM) standard, the A2/Zweiton standard, the EIA-J standard, or other similar audio standard. The configurable infinite impulse response digital filter may be implemented in an integrated circuit. 
     In accordance with another aspect of the disclosure, a television audio signal decoder includes a configurable infinite impulse response digital filter that selectively uses one or more sets of filter coefficients to filter a difference signal. The difference signal is produced by subtracting one of a left channel and a right channel audio signal from the other audio signal. Each selectable set of filter coefficients is associated with a unique filtering application to prepare the difference signal for separating the left channel and right channel audio signals. The decoder also includes a matrix that separates the left channel and right channel audio signals from the difference signal and a sum signal. The sum signal includes the sum the left channel audio signal and the right channel audio signal. 
     In one embodiment, the configurable infinite impulse response digital filter may include a selector that selects one of the one or more sets of filter coefficients. The configurable infinite impulse response digital filter may include a selector that selects an input signal from a group of input signals. One input signal from the group of input signals may include an output signal of the configurable infinite impulse response digital filter. The configurable infinite impulse response digital filter may be a second order infinite impulse response filter. Furthermore, the configurable infinite impulse response digital filter may be configured as a low pass filter, a high pass filter, bandpass filter, an emphasis filter, etc. The selection of the filter coefficients may based on a rate that the television audio signal is sampled. The sets of filter coefficients may be stored in a memory or in a look-up table that is stored in memory. The television audio signal may comply to the Broadcast Television System Committee (BTSC) standard, the Near Instantaneously Companded Audio Muliplex (NICAM) standard, the A2/Zweiton standard, the EIA-J standard, or other similar audio standard. The configurable infinite impulse response digital filter may be implemented in an integrated circuit. 
     In accordance with another aspect of the disclosure, a digital BTSC signal encoder for encoding digital left and right channel audio signals so that the encoded left and right channel audio signals can be subsequently decoded so as to reproduce the digital left and right channel audio signals with little or no distortion of the signal content of the digital left and right channel audio signals includes, a matrix that sums the left channel audio signal and the right channel audio signal to produce a sum signal. The matrix also subtracts one of the left and right audio signals from the other to produce a difference signal. The BTSC encoder also includes a configurable infinite impulse response digital filter that selectively uses one or more sets of filter coefficients to filter the difference signal. Each selectable set of filter coefficients is associated with a unique filtering application to prepare the difference signal for transmission and to comply with the BTSC standard. 
     In one embodiment, the configurable infinite impulse response digital filter may include a selector that selects one of the one or more sets of filter coefficients. The configurable infinite impulse response digital filter may include a selector that selects an input signal from a group of input signals. One input signal from the group of input signals may include an output signal of the configurable infinite impulse response digital filter. The configurable infinite impulse response digital filter may be a second order infinite impulse response filter. Furthermore, the configurable infinite impulse response digital filter may be configured as a low pass filter, a high pass filter, bandpass filter, an emphasis filter, etc. The selection of the filter coefficients may based on a rate that the television audio signal is sampled. The sets of filter coefficients may be stored in a memory or in a look-up table that is stored in memory. 
     In accordance with another aspect of the disclosure, a digital BTSC signal decoder for decoding digital left and right channel audio signals with little or no distortion of the signal content of the digital left and right channel audio signals, includes, a configurable infinite impulse response digital filter that selectively uses one or more sets of filter coefficients to filter a difference signal that complies with the BTSC standard. The difference signal is produced by subtracting one of a left channel and a right channel audio signal from the other audio signal. Each selectable set of filter coefficients is associated with a unique filtering application to prepare the difference signal for separating the left channel and right channel audio signals. BTSC signal decoder also includes a matrix that separates the left channel and right channel audio signals from the difference signal and a sum signal. The sum signal includes the sum the left channel audio signal and the right channel audio signal. 
     In one embodiment, the configurable infinite impulse response digital filter may include a selector that selects one of the one or more sets of filter coefficients. The configurable infinite impulse response digital filter may include a selector that selects an input signal from a group of input signals. One input signal from the group of input signals may include an output signal of the configurable infinite impulse response digital filter. The configurable infinite impulse response digital filter may be a second order infinite impulse response filter. Furthermore, the configurable infinite impulse response digital filter may be configured as a low pass filter, a high pass filter, bandpass filter, an emphasis filter, etc. The selection of the filter coefficients may based on a rate that the television audio signal is sampled. The sets of filter coefficients may be stored in a memory or in a look-up table that is stored in memory. 
     In accordance with another aspect of the disclosure, a computer program product residing on a computer readable medium has stored instructions that when executed by a processor, cause the processor to sum a left channel audio signal and a right channel audio signal to produce a sum signal. Executed instructions also cause the processor to subtract one of the left and right audio signals from the other signal to produce a difference signal. Furthermore, executed instructions cause the processor to select one or more sets of filter coefficients to filter the difference signal with a configurable infinite impulse response digital filter. Each selectable set of filter coefficients is associated with a unique filtering application to prepare the difference signal for transmission. 
     In one embodiment, the computer program product further includes instructions that, when executed, may select an input signal from a group of input signals. 
     In accordance with another aspect of the disclosure, a computer program product residing on a computer readable medium stores instructions which, when executed by a processor, cause that processor to select one or more sets of filter coefficients to filter a difference signal with an infinite impulse response digital filter. The difference signal is produced by subtracting one of a left channel and a right channel audio signal from the other audio signal. The selectable set of filter coefficients is associated with a unique filtering application to prepare the difference signal for separating the left channel and right channel audio signals. Executed instructions also cause the processor to separate the left channel and right channel audio signals from the difference signal and a sum signal. The sum signal includes the sum the left channel audio signal and the right channel audio signal. 
     In one embodiment, the computer program product further includes instructions that, when executed, may select an input signal from a group of input signals. 
     In accordance with another aspect of the disclosure, a television audio signal encoder includes an input stage that receives a secondary audio programming signal. The television audio signal encoder also includes a configurable infinite impulse response digital filter that selectively uses one or more sets of filter coefficients to filter the secondary audio programming signal. Each selectable set of filter coefficients is associated with a unique filtering application to prepare the secondary audio programming signal for transmission. 
     In one embodiment, the configurable infinite impulse response digital filter may include a selector that selects one of the one or more sets of filter coefficients. The configurable infinite impulse response digital filter may include a selector to select an input signal from a group of input signals. One input signal from the group of input signals may include an output signal of the configurable infinite impulse response digital filter. The configurable infinite impulse response digital filter may be a second order infinite impulse response filter. 
     In accordance with another aspect of the disclosure, a television audio signal decoder includes a configurable infinite impulse response digital filter that selectively uses one or more sets of filter coefficients to filter a secondary audio programming signal. Each selectable set of filter coefficients is associated with a unique filtering application to prepare the secondary audio programming signal for a television receiver system. 
     In one embodiment, the configurable infinite impulse response digital filter may include a selector that selects one of the one or more sets of filter coefficients. The configurable infinite impulse response digital filter may include a selector to select an input signal from a group of input signals. One input signal from the group of input signals may include an output signal of the configurable infinite impulse response digital filter. The configurable infinite impulse response digital filter may be a second order infinite impulse response filter. 
     Additional advantages and aspects of the present disclosure will become readily apparent to those skilled in the art from the following detailed description, wherein embodiments of the present invention are shown and described, simply by way of illustration of the best mode contemplated for practicing the present invention. As will be described, the present disclosure is capable of other and different embodiments, and its several details are susceptible of modification in various obvious respects, all without departing from the spirit of the present disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as limitative. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram representing a television signal transmission system that is configured to comply with the BTSC television audio signal standard. 
         FIG. 2  is a block diagram representing a portion of a BTSC encoder included in the television signal transmission system shown in  FIG. 1 . 
         FIG. 3  is a block diagram representing a television receiver system that is configured to receive and decode BTSC television audio signals sent by the television signal transmission system shown in  FIG. 1 . 
         FIG. 4  is a block diagram representing a portion of a BTSC decoder included in the television receiver system shown in  FIG. 3 . 
         FIG. 5  is a diagrammatic view of a configurable second-order infinite impulse response filter with selectable inputs. 
         FIG. 6  is a graphical representation of a transfer function of the second-order infinite impulse response filter shown in  FIG. 5 . 
         FIG. 7  is a block diagram of a portion of a BTSC encoder that highlights operations that may be performed by the configurable second-order infinite impulse response filter shown in  FIG. 5 . 
         FIG. 8  is a block diagram of a portion of a BTSC decoder that highlights operations that may be performed by the configurable second-order infinite impulse response filter shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a functional block diagram of a BTSC compatible television signal transmitter  10  includes five lines (e.g., conductive wires, cables, etc.) that provide signals for transmission. In particular, left and right audio channels are provided on respective lines  12  and  14 . An SAP signal is provided by line  16  in which the signal has content to provide additional channel information (e.g., alternative languages, etc.). A fourth line  18  provides a professional channel that is typically used by broadcast television and cable television companies. Video signals are provided by a line  20  to a transmitter  22 . The left, right, and SAP channels are provided to a BTSC encoder  24  that prepares the audio signals for transmission. Specifically, the left and right audio channels are provided to a matrix  26  that calculates a sum signal (e.g., L+R) and a difference signal (e.g., L−R) from the audio signals. Typically operations of matrix  26  are performed by utilizing a digital signal processor (DSP) or similar hardware or software-based techniques known to one skilled in the art of television audio and video signal processing. Once produced, sum and difference signals (i.e., L+R and L−R) are encoder for transmission. In particular, the sum signal (i.e., L+R) is provided to a pre-emphasis unit  28  that alters the magnitude of select frequency components of the sum signal with respect to other frequency components. The alteration may be in a negative sense in which the magnitude of the select frequency components are suppressed, or the alteration may be in a positive sense in which the magnitude of the select frequency components are enhanced. 
     The difference signal (i.e., L−R) is provided to a BTSC compressor  30  that adaptively filters the signal prior to transmission such that when decoded, the signal amplitude and frequency content suppress noise imposed during transmission. Similar to the difference signal, the SAP signal is provided to a BTSC compressor  32 . An audio modulator stage  34  receives the processed sum signal, difference signal, and SAP signal. Additionally, signals from the professional channel are provided to audio modulator stage  34 . The four signals are modulated by audio modulator stage  34  and provided to transmitter  22 . Along with the video signals provided by the video channel, the four audio signals are conditioned for transmission and provided to an antenna  36  (or an antenna system). Various signal transmitting techniques known to one skilled in the art of television systems and telecommunications may be implemented by transmitter  22  and antenna  36 . For example, transmitter  22  may be incorporated into a cable television system, a broadcast television system, or other similar television system. 
     Referring to  FIG. 2 , a block diagram representing operations performed by a portion of BTSC compressor  30  is shown. In general, the difference channel (i.e., L−R) processing performed by BTSC compressor  30  is considerably more complex than the sum channel (i.e., L+R) processing by pre-emphasis unit  28 . The additional processing provided by the difference channel processing BTSC compressor  30 , in combination with complementary processing provided by a decoder (not shown) receiving a BTSC signal, maintains the signal-to-noise ratio of the difference channel at acceptable levels even in the presence of the higher noise floor associated with the transmission and reception of the difference channel. BTSC compressor  30  essentially generates the encoded difference signal by dynamically compressing, or reducing the dynamic range of the difference signal so that the encoded signal may be transmitted through a limited dynamic range transmission path, and so that a decoder receiving the encoded signal may recover substantially all the dynamic range in the original difference signal by expanding the compressed difference signal in a complementary fashion. In some arrangements, BTSC compressor  30  is a particular form of the adaptive signal weighing system described in U.S. Pat. No. 4,539,526, incorporated by reference herein, and which is known to be advantageous for transmitting a signal having a relatively large dynamic range through a transmission path having a relatively narrow, frequency dependent, dynamic range. 
     The BTSC standard rigorously defines the desired operation of BTSC encoder  24  and BTSC compressors  30  and  32 . Specifically, the BTSC standard provides transfer functions and/or guidelines for the operation of each component included e.g., in BTSC compressor  30  and the transfer functions are described in terms of mathematical representations of idealized analog filters. Upon receiving the difference signal (i.e., L−R) from matrix  26 , the signal is provided to an interpolation and fixed pre-emphasis stage  38 . In some digital BTSC encoders, the interpolation is set for twice the sample rate and the interpolation may be accomplished by linear interpolation, parabolic interpolation, or a filter (e.g., a finite impulse response (FIR) filter, an infinite impulse response (IIR) filter, etc.) of n-th order. The interpolation and fixed pre-emphasis stage  38  also provides pre-emphasis. After interpolation and pre-emphasis, the difference signal is provided to a divider  40  that divides the difference signal by a quantity determined from the difference signal and described in detail below. 
     The output of divider  40  is provided to a spectral compression unit  42  that performs emphasis filtering of the difference signal. In general, spectral compression unit  42  “compresses”, or reduces the dynamic range, of the difference signal by amplifying signals having relatively low amplitudes and attenuating signals having relatively large amplitudes. In some arrangements spectral compression unit  42  produces an internal control signal from the difference signal that controls the pre-emphasis/de-emphasis that is applied. Typically, spectral compression unit  42  dynamically compresses high frequency portions of the difference signal by an amount determined by the energy level in the high frequency portions of the encoded difference signal. Spectral compression unit  42  thus provides additional signal compression toward the higher frequency portions of the difference signal. This is done because the difference signal tends to be noisier in the higher frequency portion of the spectrum. When the encoded difference signal is decoded with a spectral expander in a decoder, respectively in a complementary manner to the spectral compression unit of the encoder, the signal-to-noise ratio of the L−R signal is substantially preserved. 
     Once processed by spectral compression unit  42 , the difference signal is provided to an over-modulation protection unit  44  and band-limiting unit  46 . Similar to the other components, the BTSC standard provides suggested guidelines for the operation of over-modulation protection unit  44  and band-limiting unit  46 . Generally, band-limiting unit  46  and a portion of over-modulation protection unit  44  may be described as low pass filters. Over-modulation protection unit  44  also performs as a threshold device that limits the amplitude of the encoded difference signal to full modulation, where full modulation is the maximum permissible deviation level for modulating an audio subcarrier in a television signal. 
     Two feedback paths  48  and  50  are included in BTSC compressor  30 . Feedback path  50  includes a spectral control bandpass filter  52  that typically has a relatively narrow pass band that is weighted towards higher audio frequencies to provide a control signal for spectral compression unit  42 . To condition the control signal produced by spectral control bandpass filter  52 , feedback path  50  also includes a multiplier  54  (configured to square the signal provided by spectral control bandpass filter  52 ), an integrator  56 , and a square root device that provides the control signal to spectral compression unit  42 . Feedback path  48  also includes a bandpass filter (i.e., gain control bandpass filter  60 ) that filters the output signal from band-limiting unit  46  to set the gain applied to the output signal of interpolation and fixed pre-emphasis stage  38  via divider  40 . Similar to feedback path  50 , feedback path  48  also includes a multiplier  62 , an integrator  64 , and a square root device  66  to condition the signal that is provided to divider  40 . 
     Referring to  FIG. 3 , a block diagram is shown that represents a television receiver system  68  that includes an antenna  70  (or a system of antennas) to receive BTSC compatible broadcast signals from television transmission system  10  (shown in  FIG. 1 ). The signals received by antenna  70  are provided to a receiver  72  that is capable of detecting and isolating the television transmission signals. However, in some arrangements receiver  72  may receive the BTSC compatible signals from another television signal transmission technique known to one skilled in the art of television signal broadcasting. For example, the television signals may be provided to receiver  72  over a cable television system or a satellite television network. 
     Upon receiving the television signals, receiver  72  conditions (e.g., amplifies, filters, frequency scales, etc.) the signals and separates the video signals and the audio signals out of the transmission signals. The video content is provided to a video processing system  74  that prepares the video content contained in the video signals for presentation on a screen (e.g., a cathode ray tube, etc.) associated with the television receiver system  68 . Signals containing the separate audio content are provided to a demodulator stage  76  that e.g., removes the modulation applied to the audio signals at television transmission system  10 . The demodulated audio signals (e.g., the SAP channel, the professional channel, the sum signal, the difference signal) are provided to a BTSC decoder  78  that appropriately decodes each signal. The SAP channel is provided a SAP channel decoder  80  and the professional channel is provided to a professional channel decoder  82 . After separating the SAP channel and the professional channel, a demodulated sum signal (i.e., L+R signal) is provided to a de-emphasis unit  84  that processes the sum signal in a substantially complementary fashion in comparison to pre-emphasis unit  28  (shown in  FIG. 1 ). Upon de-emphasizing the spectral content of the sum signal, the signal is provided to a matrix  88  for separating the left and right channel audio signals. 
     The difference signal (i.e., L−R) is also demodulated by demodulation stage  76  and is provided to a BTSC expander  86  included in BTSC decoder  78 . BTSC expander  86  complies with the BTSC standard, and as described in detail below, conditions the difference signal. Matrix  88  receives the difference signal from BTSC expander  86  and with the sum signal, separates the right and left audio channels into independent signals (identified in  FIG. 3  as “L” and “R”). By separating the signals, the individual right and left channel audio signals may be conditioned and provided to separate speakers. In this example, both the left and right audio channels are provided to an amplifier stage  90  that applies the same (or different) gains to each channel prior to providing the respective signals to a speaker  92  for broadcasting the left channel audio content and another speaker  94  for broadcasting the right channel audio content. 
     Referring to  FIG. 4 , a block diagram identifies some of the operations performed by BTSC expander  86  to condition the difference signal. In general, BTSC expander  86  performs operations that are complementary to the operations performed by BTSC compressor  32  (shown in  FIG. 2 ). In particular, the compressed difference signal is provided to a signal path  96  for un-compressing the signal, and to two paths  98  and  100  that produce a respective control and gain signal to assist the processing of the difference signal. To initiate the processing, the compressed difference signal is provided to a band-limiting unit  102  that filters the compressed difference signal. The band-limiting unit  102  provides a signal to path  98  to produce a control signal and to path  100  to produce a gain signal. Path  100  includes a gain control bandpass filter  104 , a multiplier  106  (that squares the output of the gain control bandpass filter), an integrator  108 , and a square root device  110 . Signal path  98  also receives the signal from band-limiting unit  102  and processes the signal with a spectral control bandpass filter  112 , a squaring device  114 , an integrator  116 , and a square root device  118 . Path  98  then provides a control signal to a spectral expansion unit  120  that performs an operation that is complementary to the operation performed by spectral compression unit  42  shown in  FIG. 2 . The gain signal produced by path  100  is provided to a multiplier  122  that receives an output signal from spectral expansion unit  120 . Multiplier  122  provides the spectrally expanded difference signal to a fixed de-emphasis unit  124  that filters the signal in a complementary manner in comparison to filtering performed by BTSC compressor  30 . In general, the term “de-emphasis” means the alteration of the select frequency components of the decoded signal in either a negative or positive sense in a complementary manner in which the original signal is encoded. 
     Both BTSC encoder  24  and BTSC decoder  78  include multiple filters that adjust the amplitude of audio signals as a function of frequency. In some prior art television transmission systems and reception systems, each of the filters are implemented with discrete analog components. However, with advancements in digital signal processing, some BTSC encoders and BTSC decoders may be implemented in the digital domain with one or more integrated circuits (ICs). Furthermore, multiple digital BTSC encoders and/or decoders may implemented on a single IC. For example, encoders and decoders may be incorporated into a single IC as a portion of a very large scale integration (VLSI) system. 
     A significant portion of the cost of an IC is directly proportional to the physical size of the chip, particularly the size of its ‘die’, or the active, non-packaging part of the chip. In some arrangements filtering operations performed in digital BTSC encoders and decoders may be executed using general purpose digital signal processors that are designed to execute a range of DSP functions and operations. These DSP engines tend to have relatively large die areas, and are thereby costly to use for implementing BTSC encoders and decoders. Additionally the DSP may be dedicated to executing other functions and operations. By sharing the this resource, the processing performed by the DSP may overload and interfere with the processing of the BTSC encoder and decoder functions and operations. 
     In some arrangements, BTSC encoders and decoders may incorporate groups of basic components to reduce cost. For example, groups of multipliers, adders, and multiplexers may be incorporated to produce the BTSC encoder and decoder functions. However, while the groups of nearly identical components may be easily fabricated, the components represent significant die area and add to the total cost of the IC. Thus, a need exists to reduce the number of duplicated circuits components used to implement a digital BTSC encoder and/or decoder. 
     Referring to  FIG. 5 , a block diagram of a configurable infinite impulse response (IIR) filter  126  is shown that is capable of performing multiple filtering operations for a digital BTSC encoder or decoder. By providing selectable filtering coefficients, configurable IIR filter  126  may be configured for various filtering operations. For example, filtering coefficients may be selected so that configurable IIR filter  126  operates as a low pass filter, a high pass filter, a band pass filter, or other type of filter known to one skilled in the art of filter design. Thus, one or a relatively small number of configurable IIR filters may be used to provide most or all of the filtering needs of a BTSC encoder or a BTSC decoder. By reducing the number of decoder and encoder filters, the implementation area of an IC chip is reduced along with the production cost of the BTSC encoders and decoders. 
     To allow configurable IIR filter  126  to perform multiple types of filtering operations, the filter includes an input selector  128  that controls which input (e.g., Input  1 , Input  2 , . . . , Input N) provides an input signal to the filter. Referring briefly to  FIG. 2 , some of the inputs to selector  128  may be connected to provide input signals for each of the filtering operations performed within BTSC compressor  30 . For example, the input to gain control bandpass filter  60  may be connected to input  2  of selector  128 . Similarly, the input to spectral control bandpass filter  52  may be connected to another input (e.g., input N) of selector  128 . Then, selector  128  may control which particular filtering operation is performed by configurable IIR filter  126 . For example, during one time period, one input (e.g., input  2 ) may be selected and configurable IIR filter  126  is configured to provide the filtering function of gain control bandpass filter  60 . Then, at another time period, selector  128  is used to select another input (e.g., input N) to perform a different filtering operation. Along with selecting the other input (e.g., input N), configurable IIR filter  126  is also configured to provide the different type of filtering function, such as the filtering provided by spectral control bandpass filter  52 . 
     In order to perform multiple filtering operations e.g., for a BTSC compressor or a BTSC expander, configurable IIR filter  126  operates at a clock speed substantially faster than the other portions of the digital compressor or expander. By operating at a faster clock speed, configurable IIR filter  10  may perform one type of filtering without causing other operations of the digital compressor or expander to be delayed. For example, by operating configurable IIR filter  126  at a substantially fast clock speed, the filter may first be configured to perform filtering for gain control bandpass filter  60  without substantially delaying the execution of the next filter configuration (e.g., filter operations for spectral control bandpass filter  52 ). 
     In this particular arrangement, configurable IIR filter  126  is implemented as a second-order IIR filter. Referring to  FIG. 6 , a z-domain signal flow diagram  130  is presented for a typical second-order IIR filter. An input node  132  receives an input signal identified as X(z). The input signal is provided to a gain stage  134  that applies a filter coefficient a 0  to the input signal. In some applications the filter coefficient a 0  has a unity value. Similarly, a filter coefficient b 0  is applied to the input signal at gain stage  136 . At a delay stage  138 , a time delay (i.e., represented in the z-domain as z −1 ) is applied as the input signal enters the first-order portion of the filter and filter coefficients a 1  and b 1  are applied at respective gain stages  140  and  142 . A second delay (i.e., z −1 ) is applied at delay stage  144  for producing the second-order portion of filter  130  and filter coefficients a 2  and b 2  are applied at respective gain stages  146  and  148 . The filtered signal is provided to an output node  150  such that output signal Y(z) may be determined from the transfer function H(z) of the second-order filter  130 , as described in the following Equation (1): 
     
       
         
           
             
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                       - 
                       2 
                     
                   
                 
               
             
           
         
       
     
     Each of the coefficients (i.e., b 0 , a 0 , b 1 , a 1 , b 2 , and a 2 ) included in the transfer function may be assigned particular values to produce a desired type of filter. For example, particular values may be assigned to the coefficients to produce a low-pass filter, a high-pass filter, or a band-pass filter, etc. Thus, by providing the appropriate values for each coefficient, the type and characteristics (e.g., pass band, roll-off, etc) of the second-order filter may be configured and re-configured into another type of filter (dependent upon the application) with a different set of coefficients. While this example describes a second-order filter, in other arrangements an n th -order filter may be implemented. For example, higher order (e.g. third-order, fourth-order, etc.) filters or lower order (e.g., first-order filters) may be implemented. Furthermore, for some applications, filters of the same or different orders may be cascaded to produce an n th -order filter. 
     Referring back to  FIG. 5 , along with using selector  128  to select a particular input for configurable IIR filter  126 , the coefficients used by the filter are selected to implement different types of filters and to provide particular filter characteristics. For example, coefficients may be selected to implement a low-pass filter, a high-pass filter, a band-pass filter, or other similar type of filter used to encode or decode BTSC audio signals. In this example, respective selectors  152 ,  154 ,  156 ,  160  and  162  are used to select each coefficient for the second-order configurable filter  126 . For example, selector  152  provides the a 0  coefficient of the second-order filter from a group of “n” coefficients (i.e., a 0(0) , a 0(1) , a 0(2) , . . . , a 0(n) ) dependent upon the filter type and filter characteristics. Similarly, selectors  154 - 162  also select from respective groups of coefficient values to implement the filters. By providing these selectable coefficients values, configurable IIR filter  126  may be configured to provide filters for both encoding and decoding operations. Returning to the previous example, if selector  128  is placed in a position to select input  2  (i.e., the input for gain control bandpass filter  60 ), selectors  152 - 162  select the respective coefficients (e.g., a 0(0) , b 0(0) , a 1(0) , b 1(0) , b 2(0) , a 2(0) ) so that IIR filter  126  is configured into the appropriate filter type with characteristics to perform as the gain control bandpass filter. Upon completing the filtering, selector  128  may then be placed in a position to provide signals present on input N to configurable IIR filter  126 . Still using the previous example, input N of selector  128  may provide the input signal destined for spectral control bandpass filter  52 . By selecting this input, new filter coefficients may be selected to provide the particular filter type and filter characteristics needed to perform the filtering of spectral control bandpass filter  52 . To provide this filter and filter characteristics, selectors  152 - 162  may be respectively select filter coefficients (e.g., a 0(1) , b 0(1) , a 1(1) , b 1(1) , a 2(1)  and b 2(1) ) associated with the filter type and characteristics of spectral control bandpass filter  52 . 
     In this example, configurable IIR filter  126  is a second-order filter, however, some encoding and/or decoding filtering applications may call for a higher order filter. To provide higher order filters, in this example, one input of selector  128  is connected to an output  164  of IIR filter  126  to form a feed-back path. By providing the output of the IIR filter back to the input, filtered output signals may pass through the IIR filter multiple times using the same (or different) filter coefficients. Thus, signals may be passed through the second-order IIR filter  126  more than one time to produce a higher-order. In this particular example, a conductor  166  provides a feedback path from output  164  of configurable IIR filter  126  to input  1  of selector  128 . 
     Various techniques and components known to one skilled in the art of electronics and filter design may be used to implement selector  128  and selectors  152 - 162 . For example, selector  128  may be implemented by one or more multiplexers to select among the input lines (i.e., Input  1 , Input  2 , . . . , Input N). Multiplexers or other types digital selection devices may be implemented as one or more of selectors  152 - 162  to select appropriate filter coefficients. Various coefficient values may be used to configure IIR filter  126 . For example, coefficients described in U.S. Pat. No. 5,796,842 to Hanna, which is herein incorporated by reference, may be used by configurable IIR filter  126 . In some arrangements, the filter coefficients are stored in a memory (not shown) associated with the BTSC encoder or decoder and are retrieved by selectors  152 - 162  at appropriate times. For example, the coefficients may be stored in a memory chip (e.g., random access memory (RAM), read-only memory (ROM), etc.) or another type of storage device (e.g., a hard-drive, CD-ROM, etc.) associated with the BTSC encoder or decoder. The coefficients may also be stored in various software structures such as a look-up table, or other similar structure. 
     Configurable second-order IIR filter  126  also includes respective adding devices  168 ,  170 ,  172 ,  174  and  176  are included in configurable IIR filter  126  along with multipliers  178 ,  180 ,  182 ,  184 ,  186  and  188  that apply the filter coefficients to signal values. Various techniques and/or components known to one skilled in the art of electronic circuit design and filter design may be used to implement adding devices  168 - 176  and multipliers  178 - 188  included in configurable IIR filter  126 . For example, logic gates such as one or more “AND” gates may be implemented as each of the multipliers. To introduce time delays that correspond to delay stages  138  and  144  (shown in  FIG. 6 ), registers  190  and  192  provide delays by storing and holding the digitized input signal values for an appropriate number of clock cycles during the filtering process. Additionally, another register  194  is included configurable IIR filter  126  to initially store input signal values. 
     In this example, configurable IIR filter  126  is implemented with hardware components, however, in some arrangements one or more operational portions of the filter may be implemented in software. One exemplary listing of code that performs the operations of configurable IIR filter  126  is presented in appendix A. The exemplary code is provided in Verilog, which, in general, is a hardware description language that is used by electronic designers to describe and design chips and systems prior to fabrication. This code may be stored on and retrieved from a storage device (e.g., RAM, ROM, hard-drive, CD-ROM, etc.) and executed on one or more general purpose processors and/or specialized processors such as a dedicated DSP. 
     Referring to  FIG. 7 , a block diagram of BTSC compressor  30  is provided in which portions of the diagram are highlighted to illustrate functions that may be performed by a single (or multiple) configurable IIR filters such as configurable IIR filter  126 . In particular, filtering performed by interpolation and fixed pre-emphasis stage  38  may be performed by configurable IIR filter  126 . For example, input  1  of selector  128  may be connected to the appropriate filter input within interpolation and fixed pre-emphasis stage  38 . Correspondingly, when input  1  of selector  128  is selected, filter coefficients may be retrieved from memory and used to produce to an appropriate filter type and filter characteristics. Similarly, gain control bandpass filter  60  may be assigned to input  2  of selector  128  in configurable IIR filter  126  and spectral control bandpass filter  52  may be assigned to a third input of selector  128 . Band-limiting unit  46  may be assigned to a fourth input of selector  128 . For each of these selectable inputs, corresponding filter coefficients are stored (e.g., in memory) and may be retrieved by selectors  152 - 162  of configurable IIR filter  126 . In this example, filtering associated with four portions of BTSC compressor  30  is selectively performed by configurable IIR filter  126 , however, in other arrangements, more or less filtering operations of the compressor may be performed by the configurable IIR filter. 
     Referring to  FIG. 8 , portions of BTSC expander  86  are highlighted to identify filtering operations that may be performed by one or more configurable IIR filters such as configurable IIR filter  126 . For example, filtering associated with band-limiting unit  102  may be performed by configurable IIR filter  126 . In particular, input  1  of selector  128  may be assigned to band-limiting unit  102  such that when input  1  is selected, appropriate filtering coefficients are retrieved and used by IIR filter  126 . Similarly, filtering associated with gain control bandpass filter  104  (assigned to a second input of selector  128 ), spectral control bandpass filter  112  (assigned to a third input of selector  128 ), and fixed de-emphasis unit  124  (assigned to a fourth input of selector  128 ) is consolidated onto configurable IIR filter  126 . 
     While the previous example described using configurable IIR filter  126  with BTSC encoders and BTSC decoders, encoders and decoders that comply with television audio standards may implement the configurable IIR filter. For example, encoders and/or decoders associated with the Near Instantaneously Companded Audio Multiplex (NICAM), which is used in Europe, may incorporate one or more configurable IIR filters such as IIR filter  126 . Similarly, encoders and decoders implementing the A2/Zweiton television audio standard (currently used in parts of Europe and Asia) or the Electronics Industry Association of Japan (EIA-J) standard may incorporate one or more configurable IIR filters. 
     While the previous example described using configurable IIR filter  126  to encode and decoder a difference signal produced from right and left audio channel, the configurable IIR filter may be used to encode and decode other audio signals. For example, configurable IIR filter  126  may be used to encode and/or decode an SAP channel, a professional channel, a sum channel, or one or more other individual or combined types of television audio channels. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. Accordingly, other implementations are within the scope of the following claims.