Low pass digital filter implemented in a modem of a television system
AssigneeTeleCruz Technology, Inc.
InventorsFred Harris, Vlad Bril
Life of this patent
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Abstract
A low pass digital filter using cascade polyphase recursive digital signal processing circuit. The filter in the receive direction may contain a re-sampling Hogenauer filter followed by an equalizer filter to correct for its non-flat pass-band spectral response. The output of the spectral equalizer may be processed by a poly-phase re-sampling filter to reduce the sample rate. A clean-up filter may insert two stop-band spectral intervals in the frequency response of the earlier stages. The filter in the receive direction may be implemented by using similar components in the reverse order.
Claims43
A low pass receive filter for filtering high frequency components from an input signal represented by a digital data stream, said digital data stream containing a plurality of digital data elements, with each digital data element containing a small number of bits, said low pass filter comprising:
- a converter to receive said digital data stream and to generate as output an integer in the interval K to −K, wherein K is an integer representing the range of the final output data stream;
- a Hogenauer decimating filter coupled to said converter, said Hogenauer decimating filter to decimate the output of said converter to generate a decimated output, wherein said decimated output exhibits a non-flat pass-band spectral amplitude response;
- an equalizer filter coupled to said Hogenauer decimating filter, said equalizer to process said decimated output to flatten said non-flat pass-band spectral amplitude response;
- a polyphase decimation filter coupled to said equalizer, said polyphase decimation filter to process the output of said equalizer filter;
- and
- a spectral cleanup filter coupled to said polyphase decimation filter, said spectral cleanup filter to process the output of said decimation filter to modify the spectrum at band edges of said decimating filter.
A low pass transmit filter for generating a high frequency output digital data stream representing samples of a low pass signal from a lower frequency input sampled digital data stream, wherein each of said high frequency output digital data stream and said lower frequency input sampled digital data stream comprises a plurality of data elements, wherein each data element in said lower frequency data stream contains more number of bits than the data elements in said high frequency digital data stream, said high frequency digital data stream representing samples of a low pass signal suitable for transmission on a telephone line, said transmit filter comprising;
- a spectral cleanup filter to process lower frequency digital data so that the spectrum of the lower frequency digital data meets a spectral mask constraint at the band edges;
- a polyphase interpolating filter coupled to the output of the spectral clean up filter;
- a pre-equalizer filter coupled to said polyphase interpolating filter;
- a Hogenauer interpolating filter coupled to said pre-equalizer filter, said Hogenauer interpolating filter increasing the sample rate of said equalizer filter, wherein said pr-equalizer pre-distorts the amplitude spectrum obtained from said polyphase interpolating filter to compensate for the spectral droop generated by said Hogenauer interpolating filter;
- and
- a converter to receive a digital data stream from said Hogenauer filter, said data stream being composed of data words extending over a large number of bits, said converter to receive said digital data stream and generating as output an integer +K or −K, where K is an integer representing the range of the input data stream.
A system comprising:
- a memory;
- a processor coupled to said memory, said processor causing a plurality of data elements to be stored in said memory;
- and
- a modem coupled to said processor, said modem generating said plurality of data elements, said modem comprising:
- a data access arrangement to generate signals proportionate to the strength of analog signals received on a phone line;
- an analog to digital converter (ADC) to sample said signals generated by said data access arrangement to generate a first data stream containing a plurality of multi-bit digital values;
- a receive demodulator generating a second data stream based on said first data stream, wherein said second data stream contains a plurality of single-bit digital values;
- and
- a low pass receive filter for filtering high frequency components from an input signal represented by said second data stream, said low pass filter comprising:
- a converter to receive said second data stream and to generate as output an integer in the interval K to −K, wherein K is an integer representing the range of the final output data stream;
- a Hogenauer decimating filter coupled to said converter, said Hogenauer decimating filter to decimate the output of said converter to generate a decimated output, wherein said decimated output exhibits a non-flat pass-band spectral amplitude response;
- an equalizer filter coupled to said Hogenauer decimating filter, said equalizer to process said decimated output to flatten said non-flat pass-band spectral amplitude response;
- a polyphase decimation filter coupled to said equalizer, said polyphase decimation filter to process the output of said equalizer filter;
- and
- a spectral cleanup filter coupled to said polyphase decimation filter, said spectral cleanup filter to process the output of said decimation filter to modify the spectrum at band edges of said decimating filter.
A system comprising:
- a memory storing a plurality of data elements;
- a processor coupled to said memory, said processor causing said plurality of data elements to be transmitted on a line;
- and
- a modem coupled to said processor, said modem for transmitting said plurality of data elements on said line, said modem comprising a low pass it filter for generating a high frequency output digital data stream representing samples of a low pass signal from a lower frequency input sampled digital data stream, wherein each of said high frequency output digital data stream and said lower frequency input sampled digital data stream comprises a plurality of data elements, wherein each data element in said lower frequency data stream contains more number of bits than the data elements in said high frequency digital data stream, said high frequency digital data stream representing samples of a low pass signal suitable for transmission on a telephone line, said low pass transmit filter comprising;
- a spectral cleanup filter to process lower frequency digital data so that the spectrum of the lower frequency digital data meets a spectral mask constraint at the band edges;
- a polyphase interpolating filter coupled to the output of the spectral clean up filter;
- a pr-equalizer filter coupled to said polyphase interpolating filter;
- a Hogenauer interpolating filter coupled to said pre-equalizer filter, said Hogenauer interpolating filter increasing the sample rate of said equalizer filter, wherein said pre-equalizer pre-distorts the amplitude spectrum obtained from said polyphase interpolating filter to compensate for the spectral droop generated by said Hogenauer interpolating filter;
- and
- a converter to receive a digital data stream from said Hogenauer filter, said data stream being composed of data words extending over a large number of bits, said converter to receive said digital data stream and generating as output an integer +K or −K, where K is an integer representing the range of the input data stream.
Dependencies are inferred from the claim text.