Abstract: Methods and apparatus for performing spectral filtering of channel estimates corresponding to a communications channel used to transmit a multi-tone signal are described. A channel estimate is examined to identify portions where significant multi-path interference is present. Real, as opposed to complex, low pass filters are used to perform spectral filtering on the channel estimate to produce a filtered channel estimate. Values corresponding to portions of the channel estimate determined to correspond to areas where significant multi-path interference is present are replaced with the original unfiltered channel estimate values to generate a selectively filtered channel estimate. By using unfiltered channel estimate values in areas of multi-path interference, the errors introduced in such areas by real filtering are avoided without the need to resort to complex filtering.

Abstract: Methods and apparatus for estimating and correcting carrier frequency offsets in a bust multi-tone receiver are described. Course and fine carrier frequency estimates are generated from the signal's preamble. Decision directed carrier frequency offset estimates are then generated from the signal field and data fields of the multi-tone signal. Frequency error estimates are generated for each tone of the signal and combined using a weighted average to generate the frequency error estimate used to perform the correction operation. Error estimates corresponding to noisy data tones are weighted less then estimates corresponding to less noisy data tones. In cases of low SNR frequency error estimates corresponding to pilots are weighted by an extra amount as compared to error estimates corresponding to tones used to transmit data symbols. During times of high SNR error estimates corresponding to pilot tones are weighted in the same manner as error estimates corresponding to data tones.

Abstract: Methods and apparatus for estimating and correcting carrier frequency offsets in a bust multi-tone receiver are described. Course and fine carrier frequency estimates are generated from the signal's preamble. Decision directed carrier frequency offset estimates are then generated from the signal field and data fields of the multi-tone signal. Frequency error estimates are generated for each tone of the signal and combined using a weighted average to generate the frequency error estimate used to perform the correction operation. Error estimates corresponding to noisy data tones are weighted less then estimates corresponding to less noisy data tones. In cases of low SNR frequency error estimates corresponding to pilots are weighted by an extra amount as compared to error estimates corresponding to tones used to transmit data symbols. During times of high SNR error estimates corresponding to pilot tones are weighted in the same manner as error estimates corresponding to data tones.

Abstract: Methods and apparatus for performing spectral filtering of channel estimates corresponding to a communications channel used to transmit a multi-tone signal are described. A channel estimate is examined to identify portions where significant multi-path interference is present. Real, as opposed to complex, low pass filters are used to perform spectral filtering on the channel estimate to produce a filtered channel estimate. Values corresponding to portions of the channel estimate determined to correspond to areas where significant multi-path interference is present are replaced with the original unfiltered channel estimate values to generate a selectively filtered channel estimate. By using unfiltered channel estimate values in areas of multi-path interference, the errors introduced in such areas by real filtering are avoided without the need to resort to complex filtering.

Abstract: Methods and apparatus for converting a relatively low frequency signal, e.g., a 1.5 MHz signal, to a high frequency signal, e.g., a 30-100 MHz signal, in the digital domain without the need for a digital mixer operating at the high frequency are described. The high frequency represents, e.g., the ultimate digital to analog conversion frequency. In accordance with the present invention an interpolation technique is used to convert the low rate digital signal to a high rate signal and to shift the carrier to a desired frequency. This is accomplished, by first positioning the information signal, e.g., the digital waveform to be modulated on a carrier at a relatively low rate using a digital mixer operating at a fraction of the ultimate digital to analog conversion frequency. The relatively low rate signal generated by the mixing operation is then converted to a high rate signal by one or more interpolator stages. An adjustable passband filter circuit is included in each interpolation stage.

Abstract: Methods and apparatus for converting a relatively low frequency signal, e.g., a 1.5 MHz signal, to a high frequency signal, e.g., a 30-100 MHz signal, in the digital domain without the need for a digital mixer operating at the high frequency are described. The high frequency represents, e.g., the ultimate digital to analog conversion frequency. In accordance with the present invention an interpolation technique is used to convert the low rate digital signal to a high rate signal and to shift the carrier to a desired frequency. This is accomplished, by first positioning the information signal, e.g., the digital waveform to be modulated on a carrier at a relatively low rate using a digital mixer operating at a fraction of the ultimate digital to analog conversion frequency. The relatively low rate signal generated by the mixing operation is then converted to a high rate signal by one or more interpolator stages. An adjustable passband filter circuit is included in each interpolation stage.

Abstract: In accordance with the present invention an interpolation technique is used to convert a low rate digital signal to a high rate signal and to shift the carrier to a desired frequency. This is accomplished, by first positioning the information signal, e.g., the digital waveform to be modulated on a carrier at a relatively low rate using a digital mixer operating at a fraction of the ultimate digital to analog conversion frequency. The relatively low rate signal generated by the mixing operation is then converted to a high rate signal by one or more interpolator stages. An adjustable passband filter circuit is included in each interpolation stage. One feature of the present invention is directed to a control circuit which is response to an H bit frequency control word representing a desired output carrier frequency. The control circuit generates individual filter control signals for each adjustable filter circuit from the single H bit frequency control word.

Abstract: Methods and apparatus for converting a relatively low frequency signal, e.g., a 1.5 MHz signal, to a high frequency signal, e.g., a 30-100 MHz signal, in the digital domain. An interpolation technique is used to convert the low rate digital signal to a high rate signal and to shift the carrier to a desired frequency. This is accomplished, by first positioning the information signal, e.g., the digital wave form to be modulated on a carrier at a relatively low rate using a digital mixer operating at a fraction of the ultimate digital to analog conversion frequency. The relatively low rate signal generated by the mixing operation is then converted to a high rate signal by one or more interpolation stages. An adjustable passband filter circuit is included in each interpolation stage for selectively filtering out the signal or images created as a result of a signal padding operation performed as part of each interpolation stage.

Abstract: A common transceiver circuit for use as either a modulator or demodulator and that is implemented through a shared resource approach. This approach is particularly, though not exclusively, suited for vestigial sideband (VSB) signals. Specifically, a VSB transceiver circuit (700), through strategically located multiplexing stages, physically re-uses a complex vestigial Nyquist filter (610), a complex mixer (620) and an equalizer (785) during demodulation and modulation. The VSB transceiver also selects a particular configuration of a common complex Hilbert transform circuit (720) for use during either demodulation or modulation. In addition, the same equalizer selectively provides both channel equalization, during de-modulation, and (sin x)/x compensation, during modulation, through use of differing corresponding sets of tap coefficients.

Abstract: A common transceiver circuit for use as either a modulator or demodulator and that is implemented through a shared resource approach. This approach is particularly, though not exclusively, suited for with quadrature amplitude modulated (QAM) or vestigial sideband (VSB) signals. Specifically, a QAM transceiver circuit (400), through strategically located multiplexing stages, physically re-uses both a complex Nyquist filter (310, 320) and an equalizer (140) for demodulation and modulation. Additionally, tap coefficients of the complex Nyquist filter are set such that a center frequency of an otherwise baseband Nyquist filter is translated upward to a symbol rate in order to eliminate a separate complex mixer (250, 260). Similarly, a VSB transceiver circuit (700), also through strategically located multiplexing stages, physically re-uses a complex vestigial Nyquist filter (610), a complex mixer (620) and an equalizer (785) during demodulation and modulation.

Abstract: Method and apparatus for providing a QAM, a VSB, and a joint QAM/VSB demodulator are described. The describe demodulators are designed to minimize the amount of duplicated circuitry required to implement a joint QAM/VSB demodulator capable of performing the functions of the disclosed individual QAM and VSB demodulators. All digital architectures are used for each of the described demodulators to facilitate their combination into the described joint VSB/QAM demodulator. The described demodulators are suitable for demodulating, e.g, advanced or high definition television signals modulated using QAM or VSB modulation techniques.

Abstract: Methods and apparatus for automaticly distinguishing between QAM and VSB modulated signals and, for implementing an automatic VSB/QAM modulation recognition circuit are described. In accordance with various described embodiments, a narrow digital passband filter is used to sweep across the frequency region of a received HDTV signal where a VSB pilot tone would be located if the received signal is a VSB signal. The power of the filtered HDTV signal is estimated and compared to various preselected thresholds. If a power threshold value indicative of the presence of a VSB pilot tone is exceeded VSB is declared present. If no VSB pilot tone is detected, as indicated by the measured signal power levels, receipt of a QAM signal is declared. Various methods of insuring that transient changes in a received signal do not result in an erroneous decision with regard to the type of demodulation to perform are also described.

Abstract: An apparatus and method for varying the slew rate of a digital AGC circuit is disclosed. A gain amplifier receiving an analog signal from a tuner is converted by an A/D converter into a digital form. An ABS circuit then obtains an absolute value level of the signal, which is then low pass filtered. The filtered signal is compared to a reference level to determine if the gain should be increased or decreased. The filtered signal is also communicated to a lock detect circuit to determine how far out of the desired range the signal is, thereby requiring large step changes for a fast, coarse adjustment or smaller step changes fine adjustment of the gain. An integrator combines the two results to determine the varying slew rate of the gain signal, which is converted back to the analog domain to control the amplifier.

Abstract: An incoming television signal to a television receiver is switched to either NTSC (National Television Systems Committee) processing circuitry (130) or ATV (Advanced Television) processing circuitry (140) as determined from power measurements in each of two separate sub-bands of the power spectrum of the incoming signal. The first power measurement is obtained from a sub-band (204) wherein the NTSC power spectrum has significant spectral energy, whereas the second power measurement is obtained from another sub-band (205) wherein the NTSC power spectrum has minimal spectral energy. If the first measurement exceeds the second measurement by a prescribed value, the television signal is processed by the NTSC circuitry; otherwise, the ATV circuitry effects processing of the incoming signal.

Abstract: A synchronous pulse generator (100) for producing synchronization pulses that are synchronized with a preamble pattern contained in an input digital data signal. The generator contains a correlator circuit (102), a pulse generator circuit (106) and a flywheel circuit (104). The correlator circuit is an M-bit out of N-bit preamble correlator which periodically generates timing pulses whenever M-bits in a received preamble match M-bits in an N-bit preamble pattern. The pulse generator circuit periodically generates a synchronization pulse at a rate equivalent to an expected rate of occurrence of the preambles. The flywheel circuit monitors the occurrence of the timing pulses from the correlator circuit relative to the synchronization pulse occurrences from the pulse generator circuit. Furthermore, the flywheel circuit resets the pulse generator circuit when the timing pulses are not synchronized with the synchronization pulses for a pre-defined period of time.

Abstract: A complex NTSC interference canceler for eliminating NTSC signal interference from in-phase and quadrature phase received television signals including both a desired HDTV signal and NTSC interference signal. The complex NTSC interference canceler uses a bank of complex recursive notch filters with adjustable center frequencies to isolate the NTSC signal's major components, i.e. the picture carrier signal, the chrominance subcarrier signal and the audio carrier signal. A single complex filter is used to isolate each NTSC interference signal component, with each particular filter's center frequency being adjusted to match the frequency of the particular NTSC interference signal component to be isolated by the filter. Each filter's gain may be either fixed or dynamically adjusted to match the amplitude of the NTSC interference signal component. Once isolated, the interference signal components are subtracted from the in-phase and quadrature phase received television signals.

Abstract: An implementation efficient digital timing recovery circuit capable of being implemented without the use of multipliers. The circuit includes a voltage controlled crystal oscillator ("VCXO"), a signal generator, a non-linear operation circuit, a digital bandpass filter, a bi-quadratic filter, a digital phase lock loop circuit (DPLL) and a zero crossing detector circuit. The circuit implements a spectral line extraction technique for symbol timing recovery. A signal is received by a tuner, passed through an analog to digital converter, whose sampling rate is controlled by the timing recovery circuit, and is then supplied to a Nyquist filter. The In-phase and quadrature-phase signals output by the Nyquist filter are passed through the non-linear operation circuit to produce a signal which is then passed through the bandpass filter and then the bi-quadratic filter which has a passband centered at the symbol rate of the received signal.

Abstract: A digital notch filter apparatus for removing narrowband interference signals from a wideband communication signal. The apparatus includes a recursive digital passband interference acquisition filter, an acquisition mode center frequency control circuit, a recursive digital passband interference tracking filter, a tracking mode center frequency control circuit and an interference detection circuit. The tracking filter has a narrower bandwidth than the acquisition filter. Both the acquisition and tracking filters are gang tuned so that the center frequencies of their passbands are adjusted in unison. During interference acquisition mode, the acquisition mode center frequency control circuit is used is to adjust the center frequency of the filters over the bandwidth covered by the interference canceler.

Abstract: An NTSC interference canceler for eliminating NTSC signal interference from a HDTV signal uses recursive notch filters with adjustable center frequencies to isolate an NTSC signal's major components, i.e. the picture carrier signal, the chrominance subcarrier signal and the audio carrier signal. A single filter is used to isolate each NTSC interference signal component, with each particular filter's center frequency being adjusted to match the frequency of the particular NTSC interference signal component to be isolated by the filter. Each filter's gain may be either fixed or dynamically adjusted to match the amplitude of the NTSC interference signal component. Once isolated, these interference signal components are subtracted from the received television signal which includes both the NTSC interference signal and the HDTV signal. In this manner, the NTSC interference is removed from the HDTV signal.

Abstract: An NTSC interference canceler for eliminating NTSC signal interference from a HDTV signal uses recursive notch filters with independently adjustable gains and center frequencies to isolate an NTSC signal's major components, i.e. the picture carrier signal, the chrominance subcarrier signal and the audio carrier signal. A single filter is used to isolate each NTSC interference signal component, with each particular filter's gain and center frequency being independently adjusted to match the amplitude and frequency of the particular NTSC interference signal component to be isolated by the filter. Once isolated, these interference signal components are substracted from the received television signal which includes both the NTSC interference signal and the HDTV signal. In this manner, the NTSC interference is removed from the HDTV signal.