Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1999-07-26
2001-10-23
Pham, Chi (Department: 2731)
Pulse or digital communications
Receivers
Interference or noise reduction
C708S322000
Reexamination Certificate
active
06307903
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to digital signal processing (DSP), and more specifically to the implementation of a low-pass filter for filtering undesired high frequency components from a signal.
2. Related Art
Low pass filters are often used for eliminating high frequency spectral components from different types of signals. For example, as is well known in the relevant arts, low pass filters may be used in modems to eliminate (or substantially attenuate) spectral components having a frequency greater than 3.4 kHz in an analog signal. Such analog signals are generally generated at another modem (source modem) to encode digital data. A receiving modem may receive the analog signal and filter high frequency spectral components to recover the digital data as described below.
To recover the digital data from a received analog signal, a modem often needs to filter high frequency signal components (over 3.4 kHz bandwidth) typically present in a received analog signal. Ideally, only the frequency components having less than 3.4 kHz bandwidth should be passed, and the higher frequency components should be eliminated completely.
However, for many practical applications substantial attenuation (e.g., 60 dB) of the undesired components is sufficient. Typically, the transition from zero attenuation to 60 dB attenuation in 1 kHz presents complex challenges as is well known in the relevant arts.
Low pass filters are often implemented using digital signal processing (DSP) circuits as the circuits provide a cost-effective, compact (i.e., less space consuming), and flexible (i.e., designer can implement desired computational steps) way of eliminating the undesired frequency components. In a typical configuration using a DSP circuit, an analog to digital converter (ADC) may sample an analog signal to generate a high frequency data stream.
The ADC may be implemented using sigma-delta modulation, and generate a high frequency single bit output. A low pass filter implemented as a DSP circuit may process the digital samples to eliminate the high frequency components. The encoded digital data may be recovered from the digital samples representing the filtered signal.
A prior filtering system may be implemented using non-recursive polyphase filters (also known as finite impulse response filters). The output of a finite impulse response (FIR) filter always returns to zero when excited with a unit impulse. However, as is well known in the relevant arts, the FIR filter may need to perform a large number of computations to perform the filtering operation.
The large number of computations may be problematic in some situations as the filtering operation may need to be performed in “real time,” that is, as the analog signal is being received. By filtering in real-time, large buffers may be avoided. The time required for computations may be decreased by employing complex hardware, but the required number of gates (“gate count”) may be too numerous for integration on a single integrated circuit. In addition, circuits generally require more electrical power for performing a large number of computations, and accordingly such solutions may not be suitable at least in portable devices, which are often powered by electrical sources storing a limited amount of electrical power.
Therefore, what is needed is a circuit which optimizes one or more of the gate count, the number of computations, cost, and/or electrical power required while performing a filtering operation.
SUMMARY OF THE INVENTION
The present invention may be used to provide a low pass filter, particularly suited for use in a modem. The modem may contain a first low pass filter (interpolating transmit filter) used in the transmit direction, and a second low pass filter (decimating receive filter) used in the receive direction. Several features of the two filters are described below in further detail.
A decimating receive filter provided in accordance with the present invention filters high frequency components from an input signal represented by a digital data stream containing digital data elements. Each digital data element may contain a small number of bits. The low pass filter may contain a converter for receiving the digital data stream and for generating 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 decimates the output of the converter to generate a decimated output, wherein the decimated output exhibits a non-flat pass-band spectral response. An equalizer filter may process the decimated output to flatten the non-flat pass-band spectral response. A polyphase decimation filter may process the output of the equalizer filter, and a spectral cleanup filter processes the output of the decimation filter to modify the spectrum at band edges of the decimating filter.
The output of the spectral cleanup filter represents the filtered data. As the Hogenauer decimating filter is used in an early stage of the receive filter, the processing and hardware requirements may be minimized in the subsequent stages.
A low pass transmit filter may also be provided similarly. The low pass transmit filter may generate a high frequency digital data stream representing samples of a low pass signal from a lower frequency sampled digital data stream. Each of the two digital data streams contain a plurality of data elements, wherein each data element in the lower frequency data stream containing more number of bits than the data elements in the high frequency digital data stream. The high frequency digital data stream represents samples of a low pass signal suitable for transmission on a telephone line.
The transmit filter may contain a spectral cleanup filter for processing lower frequency digital data so that the spectrum of the lower frequency digital data meets a spectral mask (i.e., substantially attenuate components over the 3.4 kHz bandwidth) constraint at the band edges. A polyphase interpolating filter may be coupled to the output of the spectral clean up filter, and a pre-equalizer filter may be coupled to the output of the polyphase interpolating filter. A Hogenauer interpolating filter may be coupled to the pre-equalizer filter.
The Hogenauer interpolating filter increases the sample rate of the equalizer filter. The pre-equalizer pre-distorts the spectrum obtained from the polyphase interpolating filter to compensate for the spectral droop generated by the Hogenauer interpolating filter. The output of the Hogenauer interpolating stream may be a data stream composed of data words extending over a large number of bits. A converter receives the 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.
The output of the transmit filter may be sent to a transmit modulator, which along with a DAC/filter and a data access arrangement transmits the data (encoded in analog signal) on a telephone line. Therefore, the present invention provides a modem suited for transmitting and receiving data on a telephone line.
The modem may be implemented while minimizing the number of gates as a Hogenauer decimating filter is used in the receive direction and a Hogenauer interpolating filter is used in the transmit direction.
The present invention allows a Hogenauer receive filter to be used in the receive direction as an equalizer filter may process the output of the Hogenauer decimation filter to flatten the non-flat pass-band spectral response.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.
REFERENCES:
patent: 5309484 (1
Bril Vlad
Harris Fred
Bayard Emmanuel
Pham Chi
TeleCruz Technology, Inc.
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