Television – Bandwidth reduction system – Format type
Patent
1996-09-05
1998-01-06
Peng, John K.
Television
Bandwidth reduction system
Format type
348470, 348537, 348725, 375326, H04N 704
Patent
active
057060576
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention concerns a digital signal processing system. In particular, the invention concerns a phase detector in a carrier recovery network for 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, near baseband, is finally 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 baseband intersymbol interference (ISI) caused by transmission channel disturbances including the low pass filtering effect of the channel. 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 a carrier recovery network. In either case, timing can be established prior to baseband demodulation. The carrier recovery demodulation process is usually a two step process. First, the passband signal is shifted to near-baseband by a frequency shifter which uses a "best guess" as to what the frequency offset is between the incoming passband signal and the desired baseband signal. This frequency shift is usually performed by analog circuits; i.e., prior to analog to digital conversion in the receiver. Next, equalization is performed on this near-baseband signal. Finally, carrier recovery is performed which removes any residual frequency offsets from the near-baseband signal to produce a true baseband output signal. This function is performed by digital receiver circuits. The equalizer is inserted between a first local oscillator which performs the shifting to near-baseband and the carrier recovery loop network. This is 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 equalizer function.
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. Synchronous demodulation of a VSB signal has usually been accomplished with the aid of a pilot signal. The pilot signal facilitates demodulating the VSB signal to baseband in one step, typically without residual phase or frequency errors. Performing the functions of timing recovery, demodulation and equalization in the order they are performed for QAM signals does not work for VSB signals using conventional techniques. For QAM signals, several timing recovery methods are known which are independent of the frequency offset between the near-baseband signal and the baseband signal. However, it is generally accepted that frequency independent timing recovery is not feasible for VSB signals. For this reason, in VSB systems, absolute demodulation to baseband has historically been implemented first.
One example of a VSB system including a pilot component is the G
REFERENCES:
patent: 3795865 (1974-03-01), Armstrong
patent: 5170415 (1992-12-01), Yoshida et al.
patent: 5228060 (1993-07-01), Uchiyama
patent: 5386239 (1995-01-01), Wang et al.
patent: 5410573 (1995-04-01), Taga et al.
patent: 5588025 (1996-12-01), Strolle et al.
Mueller and Muller, Timing Recovery in Digital Synchronous Data Receivers, IEEE Transactions on Communications, May 1976, pp. 516-531.
The Grand Alliance HDTV System Specification (Draft Document) submitted to the ACATS Technical Subgroup, as published in the 1994 Proceedings of the 48th Annual Broadcase Engineering Conf. Proceedings Mar. 20-24, 1994.
Davis et al., "Pseudo-Coherent Phase Shift Keyed Demodulator," ICASSP '89: Acoustics, Speed & Signal Processing Conference, pp. 1388-1391, Feb. 1989.
Kuan et al, "An Adaptive Notch-Filter-Based Frequency-Difference Detector and Its Applications", IEEE Transactions on Communications, vol. 43, No. 11, pp. 2784-2793, Nov. 1995.
Kempez, "A Comparison of QAM and VSB for Hybrid Fiber/Coax Digital Transmission", IEEE Transactions on Broadcasting, vol. 41, No. 1, pp. 9-16, Mar. 1995.
Jaffe Steven Todd
Strolle Christopher Hugh
Herrmann Eric P.
Kurdyla Ronald H.
Murrell Jeffrey S.
Peng John K.
RCA Thomson Licensing Corporation
LandOfFree
Phase detector in a carrier recovery network for a vestigial sid does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Phase detector in a carrier recovery network for a vestigial sid, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Phase detector in a carrier recovery network for a vestigial sid will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2333434