Television – Image signal processing circuitry specific to television – Noise or undesired signal reduction
Reexamination Certificate
2001-01-18
2004-11-09
Lee, Michael H. (Department: 2614)
Television
Image signal processing circuitry specific to television
Noise or undesired signal reduction
C348S611000, C375S231000, C375S233000
Reexamination Certificate
active
06816204
ABSTRACT:
The invention relates to receivers for broadcast digital television signals and, more particularly, to filtering for the cancellation of multipath distortion in the received signals, which filtering is adaptive responsive to training signals inserted into the broadcast digital television signals.
BACKGROUND OF THE INVENTION
The Advanced Television Systems Committee (ATSC) published its A/53 Standard for Digital Television Broadcasting in 1995; and that standard is referred to simply as “A/53” in the rest of this specification. In 1995 ATSC also published its A/54 Guide to the Use of the ATSC Digital Television Standard, which guide is referred to simply as “A/54” in the rest of this specification.
The broadcast digital television signal to which the receiver synchronizes its operations is called the principal signal, and the principal signal is usually the direct signal received over the shortest transmission path. The multipath signals received over other paths are thus usually delayed with respect to the principal signal and appear as lagging ghost signals. It is possible however, that the direct or shortest path signal is not the signal to which the receiver synchronizes. When the receiver synchronizes its operations to a (longer path) signal that is delayed respective to the direct signal, there will be a leading ghost signal caused by the direct signal, or there will a plurality of leading ghost signals caused by the direct signal and other reflected signals of lesser delay than the signal to which the receiver synchronizes. While the term “ghost” was usually used by workers in the analog television art to refer to a multipath signal component other than the principal signal, many workers in the digital television art customarily refer to the multipath signal component using the term “echo signal” or the shorter term “echo” because of its similarity to a reflection on a transmission line. The leading ghost signals are referred to as “pre-ghosts” or “pre-echoes”, and the lagging ghost signals are referred to as “post-ghosts” or “post-echoes”. The ghost or echo signals vary in number, amplitude and delay time from location to location and from channel to channel at a given location. On Jan. 19, 2000, A. L. R. Limberg filed U.S. provisional application Ser. No. 60/177,080 titled “GHOST CANCELLATION REFERENCE SIGNALS FOR BROADCAST DIGITAL TELEVISION SIGNAL RECEIVERS AND RECEIVERS FOR UTILIZING THEM”, which application is incorporated herein by reference and is referred to simply by its serial number in following portions of this specification. At the time 60/177,080 was filed, it was generally assumed that post-ghosts with significant energy are seldom delayed more than forty microseconds from the reference signal and that pre-ghosts with significant energy seldom precede the reference signal more than three to four microseconds.
Ghost signals that are displaced in time from the principal signal substantially less than a symbol epoch, so as to affect channel frequency response, but not enough to overlap symbols with ghosts of symbols more than a symbol epoch away are sometimes referred to as “microghosts”. These short-delay or close-in microghosts are most commonly caused by unterminated or incorrectly terminated radio frequency transmission lines such as antenna lead-ins or cable television drop cables. Ghost signals that are displaced in time from the principal signal by most of a symbol epoch or by more than one symbol epoch are sometimes referred to as “macroghosts” to distinguish them from “microghosts”.
The transmission of the digital television (DTV) signal to the receiver is considered to be through a transmission channel that has the characteristics of a sampled-data time-domain filter that provides weighted summation of variously delayed responses to the transmitted signal. In the DTV signal receiver the received signal is passed through channel-equalization and ghost-suppression filtering that compensates at least partially for the time-domain filtering effects that originate in the transmission channel. This channel-equalization and ghost-suppression filtering is customarily sampled-data filtering that is performed in the digital domain. Time-domain filtering effects differ for the channels through which broadcast digital television signals are received from various transmitters. Furthermore, time-domain filtering effects change over time for the broadcast digital television signals received from each particular transmitter. Changes referred to as “dynamic multipath” are introduced while receiving from a single transmitter when the lengths of reflective transmission paths change, owing to the reflections being from moving objects. Accordingly, adaptive filtering procedures are required for adjusting the weighting coefficients of the sampled-data filtering that provides ghost-cancellation and equalization.
Determination of the weighting coefficients of the sampled-data filtering that provides channel equalization and ghost suppression is customarily attempted using a method that relies on analysis of the effects of ghosting on all portions of the transmitted signal or using a method that relies on analysis of the effects of ghosting on a training signal or ghost-cancellation reference (GCR) signal included in the transmitted signal specifically to facilitate such analysis. While the data field synchronizing (DFS) signals in the initial data segments of the data fields in the DTV signal specified by A/53 were originally proposed for use as a training signal sequence, they are not well-designed for such purpose. So, most DTV manufacturers have attempted to use decision-feedback methods that rely on analysis of the effects of ghosting on all portions of the transmitted signal for adapting the weighting coefficients of the sampled-data filtering. Decision-feedback methods that utilize least-mean-squares (LMS) method or block LMS method can be implemented in an integrated circuit of reasonable size. These decision-feedback methods provide for tracking dynamic multipath conditions reasonably well after the channel-equalization and ghost-suppression filtering has initially been converged to substantially optimal response, providing that the sampling rate through the filtering is appreciably higher than symbol rate, and providing that the rates of change of the dynamic multipath do not exceed the stewing rate of the decision-feedback loop. However, these decision-feedback methods tend to be unacceptably slow in converging the channel-equalization and ghost-suppression filtering to nearly optimal response when initially receiving a ghosted DTV signal. Worse yet, convergence is too slow when tracking of dynamic multipath conditions must be regained after the stewing rate of the decision-feedback loop has not been fast enough to keep up with rapid change in the multipath conditions. Data-dependent equalization and ghost-cancellation methods that provide faster convergence than LMS or block-LMS decision-feedback methods are known, but there is difficulty in implementing them in an integrated circuit of reasonable size. Since 60/177,080 was filed, progress has been made with regard to initializing the parameters of the adaptive filter used for echo suppression by data-directed methods, particularly by the “constant amplitude modulus” method. However, it is still desirable to introduce into the A/53 DTV signal a training signal which does not interfere with the operation of DTV signal receivers already in the field and which will rapidly adjust the channel-equalization and ghost-suppression filtering for substantially optimal response.
A/53 specifies the last twelve symbols of the initial data segment of each data field repeat the last twelve symbols of the final data in the preceding data field as a precode signal. This precode signal is specified to implement resumption of trellis coding in the second data segment of each field proceeding from where trellis coding left off processing the data in the preceding data field. This relationship between the initial and second data s
Lee Michael H.
Tran Trang U.
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