Pulse or digital communications – Equalizers – Automatic
Patent
1996-09-05
1998-11-10
Pham, Chi H.
Pulse or digital communications
Equalizers
Automatic
375321, 455204, 358725, H04N 704
Patent
active
058355324
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention concerns a digital signal processing system. In particular, the invention concerns a blind equalizer for a vestigial side band (VSB) video signal such as may be modulated with high definition television (HDTV) information, for example.
BACKGROUND OF THE INVENTION
A VSB signal and a Quadrature Amplitude Modulated (QAM) signal are both forms of a Pulse Amplitude Modulated (PAM) signal. A QAM signal conveying digital information is represented by a two-dimensional data symbol constellation defined by Real and Imaginary axes. In contrast, a VSB signal is represented by a one-dimensional data symbol constellation wherein only one axis contains quantized data to be recovered at a receiver.
The recovery of data from VSB or QAM signals at a receiver requires the implementation of three functions: timing recovery for symbol synchronization, carrier recovery (frequency demodulation) and equalization. Timing recovery is the process by which the receiver clock (timebase) is synchronized to the transmitter clock. This permits the received signal to be sampled at the optimum point in time to reduce the chance of a slicing error associated with decision-directed processing of received symbol values. Carrier recovery is the process by which a received RF signal, after being frequency shifted to a lower intermediate frequency passband (eg., near-baseband), is frequency shifted to baseband to permit recovery of the modulating baseband information.
One example of a VSB system is the Grand Alliance HDTV transmission system recently proposed for the United States. This system employs a VSB digital transmission format for conveying a packetized datastream, and is being evaluated in the United States by the Federal Communications Commission through its Advisory Committee of Advanced Television Service (ACATS). A description of the Grand Alliance HDTV system as submitted to the ACATS Technical Subgroup on Feb. 22, 1994 (draft document) is found in the 1994 Proceedings of the National Association of Broadcasters, 48th Annual Broadcast Engineering Conference Proceedings, Mar. 20-24, 1994.
Many modern digital data communications systems employ adaptive equalization to compensate for the effects of changing conditions and disturbances on the signal transmission channel. Equalization removes baseband intersymbol interference (ISI) caused by transmission channel disturbances including the low pass filtering effect of the transmission channel. ISI causes the value of a given symbol to be distorted by the values of preceding and following symbols. Equalization is typically performed on the near-baseband signal, before carrier recovery removes any residual frequency offsets from the near-baseband signal to produce a true baseband output signal. This function is typically performed by digital receiver circuits. Equalization is performed prior to carrier recovery baseband demodulation because the carrier recovery process typically is a decision-directed process (as known) that requires at least a partially open "eye" which is provided by the equalizing function.
An adaptive equalizer is essentially an adaptive digital filter. In systems using an adaptive equalizer, it is necessary to provide a method of adapting the filter response so as to adequately compensate for channel distortions. Several algorithms are available for adapting the filter coefficients and thereby the filter response. One widely used method employs the Least Mean Squares (LMS) algorithm. In this algorithm, by varying the coefficient values as a function of an error signal (E), the equalizer output signal is forced to approximately equal a reference data sequence. This error signal is formed by subtracting the equalizer output signal from the reference data sequence. As the error signal approaches zero, the equalizer approaches convergence whereby the equalizer output signal and the reference data sequence are approximately equal.
When the equalizer operation is initiated, the coefficient values (filter tap weights) are usually no
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Jaffe Steven Todd
Strolle Christopher Hugh
Herrmann Eric P.
Kurdyla Ronald H.
Luther William
Pham Chi H.
RCA Thomson Licensing Corporation
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