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NTSC interference rejection filter
| Details |
Inventors: Liu, Tian-Min; Tan, Loke Kun; Jaffe, Steven T.; Hawley, Robert A.;
Assignee: Broadcom Corporation (Irvine, CA)
Primary Examiner: Lee; Michael
Assistant Examiner:
Attorney, Agent or Firm: Christie, Parker & Hale, LLP
An electronic, programmable filter is disclosed which selectively removes interference, noise or distortion components from a frequency band without perturbing any of the other signals of the band. An input frequency band such as a television channel spectrum is initially demodulated to baseband and applied to the input of the filter. The baseband spectrum is combined in a complex mixer with a synthesized frequency signal that shifts the spectrum a characteristic amount, in the frequency domain, so as to position an interference component in the region about DC. Once shifted, the frequency components about DC are removed by DC canceler circuit and the resulting spectrum is mixed with a subsequent synthesized frequency signal which shifts the spectrum back to its original representation and baseband. The frequency signals are developed by a programmable frequency synthesizer which a user may program with an intelligence signal that defines the frequency location of an interference signal within the spectrum. Filter blocks may be added or subtracted in order to optimize the filter response for any number of interference components for which rejection is desired. |
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DETAILED DESCRIPTION One aspect of the present invention is to provide a system and method which enables the removal of unwanted signal components, such as NTSC co-channel interference components, from composite channel information without introducing extraneous noise and without perturbing any of the remaining signal components of the channel.
In one particular aspect of the invention, an electronic programmable filter is configured to selectively reject NTSC co-channel interference components from a composite input signal representing an HDTV channel.
Channel information has been demodulated to baseband prior to being introduced to the filter input.
The programmable filter suitably comprises at least one programmable digital frequency synthesizer which defines a signal oscillating at a specific characteristic frequency corresponding to a frequency characteristic of an NTSC co-channel interference component whose removal is desired.
The frequency signal from the digital frequency synthesizer is combined with the input channel information in a complex mixer, thereby shifting the channel spectrum by an amount equal to the synthesized frequency signal, thereby positioning the NTSC co-channel interference component in a region symmetrical about DC.
A DC cancellation circuit removes a narrow band of frequencies about DC, consequently removing the NTSC co-channel interference component from the channel spectrum.
Following DC cancellation, the channel spectrum is subsequently shifted (upconverted) back to its original baseband representation by combining the shifted spectrum with an additional frequency signal developed by an additional digital frequency synthesizer.
The digital frequency synthesizer is programmable in that particular shift frequencies are defined by a user and communicated to the synthesizer circuit through an intelligence signal; the digital frequency synthesizer faithfully producing a frequency signal oscillating at the user commanded frequency.
In a further aspect of the invention, the programmable filter includes a multiplicity of filter blocks, with each filter block configured to shift an input spectrum by a characteristic frequency amount and then to cancel the frequency components in a narrow band about DC
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Television 
