Decimation of baseband DTV signals prior to channel equalization in digital television signal receivers

Abstract

A radio receiver uses the same tuner for receiving a selected digital television (DTV) signal, irrespective of whether it is a quadrature-amplitude-modulation (QAM) or a vestigial sideband (VSB) signal. The final IF signal is digitized at a rate that is a multiple of both the symbol frequencies of the QAM and VSB signals, for synchrodyning to baseband. The carrier frequencies of the QAM and VSB final IF signals are regulated to be submultiples of the multiple of both the symbol frequencies of the QAM and VSB signals by applying automatic frequency and phase control (AFPC) signals developed in the digital circuitry to a local oscillator of the tuner. Baseband DTV signals obtained by synchrodyning the final IF signals have a sample rate higher than symbol rate to facilitate symbol synchronization. The baseband DTV signals are decimated to symbol rate before performing channel equalization to reduce the number of multipliers required in the channel equalization filter.

Description

The invention relates to radio receivers having the capability of receiving digital television (DTV) signals such as digital high-definition television (HDTV) signals, transmitted using quadrature amplitude modulation (QAM) of the principal carrier wave or transmitted using vestigial sideband (VSB) amplitude modulation of the principal carrier wave.BACKGROUND OF THE INVENTIONA Digital Television Standard published Sep. 16, 1995 by the Advanced Television Systems Committee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals in 6-MHz-bandwidth television channels such as those currently used in over-the-air broadcasting of National Television System Committee (NTSC) analog television signals within the United States. The VSB DTV signal is designed so its spectrum is likely to interleave with the spectrum of a co-channel interfering NTSC analog TV signal. This is done by positioning the pilot carrier and the principal amplitude-modulation...

AI Technical Summary

Benefits of technology

In accordance with an aspect of the invention, the digitized DTV signal is decimated before performing channel equalization thereof, which reduces the number of samples in the kernels of the digital filters for performing channel equalization and reduces the cost of the DTV receiver substantially.

The invention is embodied in a digital television (DTV) receiver including a radio receiver portion for selecting a channel for reception, for converting DTV signal in the selected channel to intermediate frequencies for filtering and amplification, and for synchrodyning an analog final intermediate-frequency output signal resulting from that filtering and amplification to baseband, thereby to generate a baseband signal. The DTV receiver may be one designed for receiving QAM DTV signal, VSB DTV signal or both types of DTV signal. An analog-to-digital converter (ADC) is included in this radio receiver portion for sampling one of the signals therein and digitizing it so that the baseband signal is supplied from the radio receiver portion as a first stream of digital samples descriptive of the baseband signal. A sample clock generator is connected for supplying a sample clock signal to time the sampling by the ADC so that the first stream of digital samples has a sample rate substantially equal to a prescribed multiple MN times the symbol rate of the DTV signal. MN is the product of a positive number M greater than one and of a positive integer N at least two. A decimator is connected for receiving the first stream of digital samples and generating in response thereto a second stream of digital samples at a sample rate that is one-N.sup.th that of the first stream of digital samples. The number of taps required in a channel equalizer for performing channel equalization to generate a channel equalizer response is reduced by the N:1 decimation of the second stream of digital samples. The resultant saving in digital multipliers provides a substantial benefit in cost and reliability. A symbol synchronizer is included in the DTV receiver for correcting the symbol phase error in the channel equalizer response; and a symbol decoder is included in the DTV receiver for decoding symbols in the channel equalizer response, as corrected for symbol phase error, to recover groups of bits corresponding to decoded symbols.

Problems solved by technology

This presents television receiver designers with the challenge of designing receivers that are capable of receiving either type of transmission and of automatically selecting suitable receiving apparatus for the type of transmission currently being received.

The zero-intermediate-frequency (ZIF) receivers, which perform amplification and channel selection at baseband, that are used for receiving QAM DTV signals are not well suited for receiving VSB DTV signals.

This is because of problems with securing adequate adjacent-channel rejection in a ZIF receiver when the carrier is not at the center frequency of the channel.

The challenge is in optimally constructing the circuitry for synchrodyning to baseband and for symbol decoding to accommodate both DTV transmission standards and in arranging for the automatic selection of the appropriate mode of reception for the DTV transmission currently being received.

Separately digitizing in-phase and quadrature-phase synchronous detection results generated in the analog regime presents problems with regard to the synchronous detection results satisfactorily tracking each other after digitizing; quantization noise introduces pronounced phase errors in the complex signal considered as a phasor.

These types of known DTV signal radio receiver present some problem in the design of the tuner portion of the receiver because the respective carrier frequencies of VSB DTV signals and of QAM DTV signals are not the same as each other.

The radio-frequency switching that must be done reduces the reliability of the tuner.

The RF switching and the additional frequency-stabilizing crystal for the third oscillator increase the cost of the tuner appreciably.

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