Abstract: A television signal ghost cancellation system is described in which a microprocessor is used to develop a mathematical model of the transmission channel which produced a ghosted video signal. The microprocessor determines the coefficient values and delay values for a digital IIR filter from this model. The system uses the IIR filter to cancel ghost signals from the ghosted video signal. The IIR filter includes circuitry for interpolating between successively delayed sample values to cancel ghost signals which are delayed by a non-integer number of sample periods.

Abstract: An adaptive television deghosting system operates on modulated video signals including a direct signal component and one or more ghost signal components. The system uses synchronously demodulated in-phase and quadrature-phase baseband video signals as the respective real and imaginary input signals to a complex IIR filter. The filter coefficients are developed adaptively from preset initial values using the signals provided by the filter during a training interval. The training interval includes the interval between the leading edge of the vertical sync pulse and the first serration pulse of each field. The filtered training signals are subtracted from a sync-tip reference value to develop a signal which is proportional to the error in the filter coefficient values. The error signal values corresponding to ghost signals are multiplied by the complex conjugate of the training signal values which represent the analogous sampling points of the direct signal.

Abstract: A television deghosting system includes an IIR filter having programmable filter coefficients which processes the input signals to remove ghost signal components. The IIR filter may be reconfigured as an FIR filter, having its coefficient and delay values changed to operate as a matched correlation filter. In this mode the filter is used to correlate a training signal to determine the relative delays of ghost signals with respect to the direct signal. The training signal used is the sum of the squared differentials of the in-phase and quadrature-phase components of the video IF signal during an interval defined by the leading edge of the vertical sync pulse and the first equalization pulse.

Abstract: A television receiver which includes an automatic deghosting system synchronously demodulates IF television signals to generate baseband signals representing the in-phase and quadrature phase modulation components of the IF signals. A signal representing the phase difference between the carrier of the signal as broadcast and the carrier of the received signal, which may include ghosts, is generated from the in-phase and quadrature signals. This signal is applied to circuitry which develops signals proportional to the sine and cosine of the phase angle separating the broadcast carrier phase and the received carrier phase. The demodulated in-phase and quadrature phase baseband signals are multiplied by the sine and cosine signals in a complex multiplier to correct distortion resulting from the synchronous demodulation using the carrier of the received signal.

Abstract: An automatic deghosting system synchronously demodulates IF television signals to generate baseband composite video signals. A strong RF ghost signal having a carrier phase that differs from that of the desired signal may cause demodulation phase errors, causing quadrature distortion in the baseband video signals. The circuitry shown in this disclosure synchronously demodulates the quadrature component of the IF television signals and develops a signal proportional to the difference between the values of the baseband quadrature signal immediately before and immediately after the leading edge of the vertical sync pulse. This signal, which is proportional to the demodulation phase error, is used to control a phase locked loop that regenerates the in-phase and quadrature carrier signals used by the synchronous demodulators to substantially eliminate any demodulation phase error.

Abstract: A digital television signal deghosting system includes an IIR filter responsive to the in-phase and quadrature-phase components of modulated television signals for cancelling ghost signals irrespective of their phase. A double-conversion tuner develops modulated composite video signals having a 3f.sub.c carrier frequency. An analog-to-digital converter responsive to a two-phase 3f.sub.c clock signal develops digital samples which alternately represent the baseband in-phase and quadrature-phase components of the modulated video signals. These samples are separated to develop digital signals representing the in-phase and quadrature-phase components of the modulated television signals.

Abstract: The line rate of a video input signal is doubled for display in progressive scan fashion by dual mode processor which generates added lines for the display by interpolation when displaying normal television pictures and by replication (repeating lines) when displaying characters. The processor mode is automatically controlled by an image analyzer which identifies fields of text by counting "major" transitions of the video input signal occurring within a field and comparing the count with a threshold value. Errors in identification are minimized by a further counter which integrates the field count and a hysteresis detector which controls the processor operating mode in accordance with high and low limits of the integrated field count.

Abstract: A recursive filter for noise reducing composite video signal separately processes the luminance and chrominance components of the composite signal but requires only a single frame of storage. Input composite video signal is separated into luminance and chrominance component signals. The component signals are coupled to separate signal recursion scaling/combining circuits. The outputs from the recursion circuits are summed in an adder to produce noise-reduced composite video signal which is applied to a frame delay element. The output of the delay element is applied to a chroma/luma separator which separates the luminance and chrominance components and applies them to the respective recursion circuits.

Abstract: A digital deghosting system uses the interval between the sixth pre-equalization pulse and the first serration as a training signal to determine the timing and amplitude of ghost signals relative to the desired signal. The system includes level shifting circuitry to change the amplitude of the video signals during the training interval. This amplitude shift ensures that all of the information concerning the ghost signals is within the dynamic range of the analog to digital converter.

Abstract: In a television receiver having digital signal processing, an automatic chrominance gain control apparatus employs a combination of analog gain control and digital gain control stages. Specifically, the analog gain control stage provides a range of fine gain control by altering the frequency response characteristic of the intermediate frequency (IF) amplifier circuits. This is sometimes referred to as "IF tilt" control. The digital gain control stage provides coarse gain control in predetermined increments using a shifter in the digital signal processing circuits. The total gain required is calculated and is apportioned between the coarse and fine gain control stages by a microprocessor.

Abstract: Apparatus is disclosed for demodulating sampled data chrominance signals that include a first component that modulates a carrier to produce asymmetrical sidebands, and a second component modulated in quadrature to produce double sidebands. Samples representing the second component are derived and subtracted from the chrominance samples. The samples produced represent the asymmetrical sideband component. These samples are passed through a Nyquist filter to produce samples representing the first component signal but having the frequency spectrum of a vestigial sideband modulated signal. Demodulation of these samples by conventional means produces samples of the first component signal having a uniform range of amplitude values over the frequency band of the first signal.

Abstract: Gain control of signals processed in a digital TV receiver is split between serially connected coarse and fine gain blocks. The coarse gain block consists of bit shifters to shift the sample bits to greater or lesser significant bit positions which provides a 6 dB gain change for each bit position change. Fine gain control is performed in the digital-to-analog converters which ultimately produce the analog signals for driving the display tube. The digital-to-analog converters are selectively reset each sample period with reset pulses that are developed by a fine gain control. The output signal from the digital to analog converter is an amplitude and pulse width modulated signal. The average amplitude of the signal over a sample period is determined by the pulse width and is thus a function of the fine gain control.

Abstract: In a digital signal processing system, an analog-to-digital converter producing digital representations of an analog signal is dithered to increase its apparent quantizing resolution. Certain processing of the digital signals, however, can result in the loss of the dither information, because of either the form of digital filtering employed or the number of bits of the digital signals processed. To avoid the effects of such loss, the least significant bit of the digital signals is delayed and is applied to an analog output device to reintroduce a dither signal thereat having a magnitude substantially equivalent to that of the lost bit. The present invention is useful in processing digital television signals in a television receiver having a digital signal processing section.

Abstract: A digital signal processing system applies numbers to the multiplied or divided to address a memory storing a logarithm table to produce the logarithms of the numbers. These logarithms are added for multiplication or substracted for division to produce the logarithm of the result number which is applied to address a memory storing an antilogarithm table to produce the result number. The base of the logarithms is selected in accordance with the magnitudes of the numbers to be multiplied or divided so as to utilize substantially the entire range of magnitudes of the digital representation of the logarithms.

Abstract: In a digital television receiver, a signal processing stage generates color mixture digital signals representing color picture information as components in a first color mixture coordinate system such as the I-Q system. The I and Q digital signals are applied to a read only memory look-up table in which memory locations are stored the logarithmic values of the signals in order to convert the signals into digital words represented in logarithmic form. A microprocessor or other digital circuit generates or retrieves digital coefficients in logarithmic form that enable the I and Q digital signals to be transformed into second color mixture digital signals representing the color picture information as components in a second color mixture coordinate system such as the R-Y, B-Y, color difference coordinate system. The I and Q digital signals in logarithmic form are applied to a first input of an adder.

Abstract: A deghosting system incorporated in a digital television signal processing system includes a number of channels for producing digital samples of psuedo ghost signals for cancelling digital samples of respective ghost components of a contaminated video signal. Each channel includes a random access memory into which digital video samples are read and later read out of at a time corresponding to the delay between the respective ghost component and the main (desired) component of the ghost contaminated video signal. The resulting delayed digital samples are multiplied with an appropriate coefficient to generate the digital samples of the psuedo ghost signal.

Abstract: Digital TV receivers which digitally process chroma and luma signals in different channels and convert the digital signals to analog form before matrixing will include low pass analog filters after the DAC's to remove clock switching components. The luma and chroma analog filters will impose differing group delay on their respective signals. This creates a differential phase error between the analog signals which is corrected by predictively delaying the signal which leads in phase in the DAC. This is accomplished by using clocked digital-to-analog converters forced to operate synchronously with the data stream and appropriately delaying the clock signal applied to one of the DAC's relative to the other.

Abstract: In a color TV receiver, color saturation errors associated with flesh tone correction circuits operating on demodulated color mixture signals are corrected. This is accomplished by adding the absolute value of a fraction of one of the color mixture signals to the color mixture signal in quadrature phasing therewith.

Abstract: In processing color video signals in sampled data format, the sample rate is nominally far in excess of that required to process the baseband color signals. Thus, when the chrominance signal is demodulated only certain ones of the chrominance signal samples need to be demultiplexed from the data stream to reconstruct the color mixture signals. It is convenient to phase lock a clock to the data stream and select like samples from every sequence of samples. However, the NTSC video format provides a chrominance signal which has a 180 degree phase displacement from line-to-line. Thus, if like samples are selected from all lines, there will be a line-to-line spatial translation of image points on the displayed image giving rise to fine dot crawl along vertical edges of color transitions. The present invention includes a demodulator which demultiplexes sample points which are vertically aligned line-to-line and complements (inverts the polarity) of the samples on every other line.

Abstract: In a digital television signal processing system, first digital samples containing picture information are supplied to a digital processor. The processor operates on the samples to derive second samples that contain picture information modified from the original picture information. The digital processor includes within it processing chain a stage that produces an output digital word that is subject to truncation error when the processor is supplied with selected ones of the first digital samples. The truncation error results in the derivation of second digital samples that contain erroneously modified picture information. The truncation error is detected, and the values of the second digital samples are changed to different ones that contain picture information corrected for the effects of the truncation error.