Television – Receiver circuitry – Demodulator
Patent
1996-09-06
1998-09-08
Powell, Mark R.
Television
Receiver circuitry
Demodulator
375355, 375321, 375270, 455204, 455 47, 348426, 348720, 348369, H03D 124, H04L 2716
Patent
active
058052429
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention concerns a digital signal processing system. In particular, the invention concerns a timing recovery system for use in a receiver of a vestigial side band (VSB) signal such as may be modulated with high definition television (HDTV) information, for example.
BACKGROUND OF THE INVENTION
The recovery of data from a VSB or a QAM (Quadrature Amplitude Modulated) signal 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, is frequency shifted to baseband to permit recovery of the modulating baseband information. Equalization is a process which compensates for the effects of transmission channel disturbances upon the received signal. More specifically, equalization removes intersymbol interference (ISI) caused by transmission channel disturbances. ISI causes the value of a given symbol to be distorted by the values of preceding and following symbols.
For QAM signals, timing recovery is usually the first function implemented in a receiver. The timing is recovered from either the intermediate passband signal or from a near-baseband signal, i.e., a baseband signal with a carrier offset that is corrected by the carrier recovery network. In either case, timing can be established prior to baseband demodulation.
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. Timing recovery for a baseband VSB signal can be achieved using periodic synchronizing (sync) symbols or components. This technique is used by a terrestrial broadcast Grand Alliance HDTV system recently proposed for the United States. A significant drawback of this technique is that the use of such sync symbols reduces the payload capacity of the data transmission channel.
The Grand Alliance HDTV system employs a vestigial sideband (VSB) digital transmission format for transmitting a packetized datastream. This HDTV system is a proposed transmission standard that is under consideration in the United States by the Federal Communications Commission through its Advisory Committee of Advanced Television Service (ACATS). In this system, data is arranged as a sequence of data fields. Each field includes 313 segments: a field sync segment (which does not contain payload data) followed by 312 data segments. A sync component is associated with each data segment. 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.
SUMMARY OF THE INVENTION
A disclosed timing recovery system, suitable for use with VSB signals, advantageously achieves timing recovery without reliance upon sync components. This is accomplished using band edge timing recovery with respect to a received VSB signal to develop double sideband amplitude modulated signals from which timing information is extracted. In an illustrated embodiment, a band edge filter network filters upper and lower band edges of the VSB signal. The disclosed system advantageously operates at the symbol rate, does not rely on a received signal containing bandwidth limiting synchronizing components to facilitate timing recovery, and is capable of achieving timing synchronism inde
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Jaffe Steven Todd
Strolle Christopher Hugh
Herrmann Eric P.
Kurdyla Ronald H.
Le Uyen
Powell Mark R.
Thomson Consumer Electronics Inc.
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