Timing recovery using the pilot signal in high definition TV

• US 6,222,891 B1
• Filed: 11/03/1999
• Issued: 04/24/2001
• Est. Priority Date: 11/03/1998
• Status: Expired due to Term

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Broadcom Corporation (Broadcom, Inc.)
• Inventors: Tan, Loke Kun, Jaffe, Steven T., Liu, Tian-Min
• Primary Examiner(s): Ghebretinsae, Temesghen
• Application Number:
  US09/433,734
• Time in Patent Office: 538 Days
• Field of Search: 375/355, 375/321, 375/326, 348/726, 348/725
• US Class Current: 375/326
• CPC Class Codes: H04B 1/68 for wholly or partially suppressing the carrier or one side band
• CPC Class Codes: H04L 2025/03382 Single of vestigal sideband
• CPC Class Codes: H04L 2025/0342 QAM
• CPC Class Codes: H04L 2025/0349 as a feedback filter
• CPC Class Codes: H04L 2025/03503 as a combination of feedback and prediction filters
• CPC Class Codes: H04L 2027/0032 at baseband and passband
• CPC Class Codes: H04L 2027/0042 the equaliser providing the offset correction per se
• CPC Class Codes: H04L 2027/0057 quadrature phase
• CPC Class Codes: H04L 2027/0067 Phase error detectors
• CPC Class Codes: H04L 2027/0087 Out-of-band signals, (e.g. pilots)
• CPC Class Codes: H04L 25/03057 with a recursive structure H04L25/03031 takes precedence
• CPC Class Codes: H04L 27/02 Amplitude-modulated carrier systems, e.g. using on-off keying; Single sideband or vestigial sideband modulation H04L27/32 takes precedence
• CPC Class Codes: H04L 27/066 Carrier recovery circuits H04L27/2271 takes precedence
• CPC Class Codes: H04L 27/2273 associated with quadrature demodulation, e.g. Costas loop
• CPC Class Codes: H04L 27/3427 in which the constellation is the n - fold Cartesian product of a single underlying two-dimensional constellation
• CPC Class Codes: H04L 27/3827 in which the carrier is recovered using only the demodulated baseband signals
• CPC Class Codes: H04N 11/24 High-definition television systems
• CPC Class Codes: H04N 21/426 Internal components of the client ; Characteristics thereof H04N5/44 takes precedence
• CPC Class Codes: H04N 5/21 Circuitry for suppressing or minimising disturbance, e.g. moiré or halo
• CPC Class Codes: H04N 5/211 Ghost signal cancellation H04N23/81 takes precedence
• CPC Class Codes: H04N 5/44 : Receiver circuitry for the reception of television signals according to analogue transmission standards H04N5/14 takes precedence
• CPC Class Codes: H04N 5/455 : Demodulation-circuits
• CPC Class Codes: H04N 5/46 : for receiving on more than one standard at will deflecting circuits of multi-standard receivers H04N3/27

Abstract

Improved carrier recovery and symbol timing systems and methods suitable for use in connection with a dual-mode QAM/VSB receiver system is disclosed. Carrier and symbol timing acquisition and tracking loops are phase/frequency locked to an inserted pilot signal provided in an input VSB spectrum at a given frequency. An input spectrum is centered about baseband and the pilot is extracted by an equivalent filter which functions as a bandpass filter having pass bands centered about the pilot frequency. Since the pilot signal'"'"'s frequency is given, its position in the frequency domain for any sampling frequency, is deterministic. The receiver'"'"'s sampling frequency is provided such that the relationship is expressed as f_c=f_S/4. When tracked by a phase-lock loop, the pilot signal will appear at the correct location in the spectrum if the sampling frequency f_S is correct, and will be shifted in one direction or the other if the sampling frequency f_S is too high or too low.

Claims

1. A method for operating a receiver having a particular sampling frequency related to a symbol rate to recover carrier and timing information from a received spectrum including a pilot signal, comprising:

centering the received spectrum at a known position relative to baseband;

tracking the pilot signal with a phase-lock-loop;

evaluating the frequency of the pilot signal with respect to the particular sampling frequency;

adjusting the centering of the received spectrum until the evaluated frequency of the pilot is in integral relationship with the particular sampling frequency;

filtering the received spectrum in a low pass filter having a cut-off frequency related to the particular sampling frequency;

processing the filtered signal in a high pass filter having a cut-off frequency related to the particular sampling frequency; and

wherein the low pass and high pass filters define an equivalent bandpass filter, the equivalent bandpass filter defining upper and lower sideband regions each centered at an expected position of the pilot signal.

2. The method according to claim 1, wherein the received spectrum exhibits a raised cosine response characteristic, the upper and lower sideband regions of the equivalent filter including transition regions of the spectra defined by the high pass and low pass filters.

3. The method according to claim 2, wherein the receiver'"'"'s sampling frequency is determined such that the pilot frequency is equal to about one-fourth the sampling frequency.

4. The method according to claim 3, wherein the high pass filter is a Nyquist prefilter having a lower cut-off frequency at about one-fourth the sampling frequency, the high pass filter centering the spectrum at a frequency about one half the sampling frequency.

5. The method according to claim 4, wherein the low pass filter is a square root Nyquist filter having an upper cutoff frequency at about one-fourth the sampling frequency, the square root Nyquist filter centering the spectrum at about baseband.

6. The method according to claim 5, the desired position of the pilot signal being substantially centered in each transition region, thereby causing the desired position of the pilot signal to be centered in each of the upper and lower sideband regions of the equivalent filter at a frequency substantially equal to one-fourth the sampling frequency, thereby augmenting a signal occurring at one-fourth the sampling frequency.

7. The method according to claim 6, further comprising:

receiving a sideband region from the equivalent filter in a tracking loop;

sweeping the sideband region to identify an augmented frequency component;

comparing the augmented frequency component to the expected frequency of the pilot signal; and

shifting the sampling frequency in a direction and an amount such that the augmented frequency component coincides with the expected frequency of the pilot signal.

8. The method according to claim 7, further comprising the step of using the shifted sampling frequency to define a symbol timing reference signal.

9. A digital communication system, comprising:

a front end receiving an input spectrum at an intermediate frequency, the input spectrum including an inserted predetermined frequency component;

first and second nested tracking loops, the first loop acquiring carrier frequency lock in operative response to the predetermined frequency component of the received spectrum, the second loop providing a signal adapted to position the spectrum at a predetermined location relative to baseband in operative response to said predetermined frequency component; and

a third tracking loop coupled to define a symbol timing parameter in operative response to said same predetermined frequency component.

10. The digital communication system according to claim 9, further comprising an equivalent filter operating on the received spectrum in order to define a pair of symmetric signals, each of the symmetric signals centered at the characteristic frequency of the predetermined frequency component when the received spectrum is at baseband.

11. The digital communication system according to claim 10, wherein symbol timing is performed at a sampling frequency, the system further comprising:

a first high pass filter having a lower cut-off frequency related to the sampling frequency;

a second low pass filter having an upper cut-off frequency related to the sampling frequency; and

wherein the first and second filters define an equivalent bandpass filter having symmetric passband regions centered about a frequency related to the sampling frequency.

12. The digital communication system according to claim 11, wherein the received spectrum exhibits a raised cosine response characteristic, the passband regions including the transition regions of the high and low pass filtered spectra.

13. The digital communication system according to claim 12, wherein the sampling frequency is chosen such that the predetermined frequency component is disposed at a frequency one-fourth the sampling frequency.

14. The digital communication system according to claim 13, wherein the high pass filter has a lower cut-off of about one-fourth the sampling frequency and a passband center of about one half the sampling frequency.

15. The digital communication system according to claim 14, wherein the low pass filter has an upper cut-off of about one-fourth the sampling frequency, the equivalent filter passbands thereby centered at a frequency about one-fourth the sampling frequency.

16. The digital communication system according to claim 15, wherein the inserted predetermined frequency component is a pilot signal disposed at a location along the spectrum normally reserved for a suppressed carrier when the spectrum is a conventional terrestrial broadcast spectrum, the pilot signal centered in the equivalent filter'"'"'s passband regions when the sampling frequency is one-fourth the frequency of the pilot signal.

17. The digital communication system according to claim 16, further comprising:

a phase/frequency detector coupled to receive an equivalent filter passband signal;

means for determining whether the pilot is centered in the passband region; and

an oscillator circuit developing a timing reference signal having a frequency related to the sampling frequency, the oscillator circuit increasing or decreasing the timing reference signal frequency in operative response to the position of the pilot signal with respect to a passband region center.

18. In an integrated circuit digital communication system of the type receiving an input spectrum characterized by a raised cosine response, the spectrum including an inserted pilot signal at a characteristic frequency, a symbol timing recovery circuit comprising:

an equivalent bandpass filter defining a passband having a center frequency, the bandpass filter extracting the pilot signal and positioning the pilot at the center frequency of the passband;

a phase/frequency detector coupled to receive the passband signal from the equivalent filter;

means for determining whether the pilot is centered in the passband region; and

an oscillator circuit developing a timing reference signal, the oscillator circuit increasing or decreasing the timing reference signal frequency in operative response to the position of the pilot signal with respect to the passband region center.

19. The symbol timing recovery circuit according to claim 18, the equivalent bandpass filter further comprising:

a square root Nyquist filter; and

a Nyquist-type high pass prefilter, coupled in series relationship with the root Nyquist filter.

20. The symbol timing recovery circuit according to claim 19, the integrated circuit digital communication system determining symbol timing in accordance with a sampling frequency, wherein the pilot signal is inserted into a spectrum at a frequency approximately equal to one-fourth the sampling frequency.

21. The symbol timing recovery circuit according to claim 20, wherein the square root Nyquist filter is a lowpass filter having a cut-off of about one-fourth the sampling frequency.

22. The symbol timing recovery circuit according to claim 21, wherein the Nyquist prefilter is a high pass filter having a cut-off of about one-fourth the sampling frequency, the equivalent filter thereby defining passbands centered at a frequency of about one-fourth the sampling frequency.

23. An integrated circuit communication device comprising:

a filter circuit for isolating an inserted pilot signal;

a detector circuit coupled to receive the isolated pilot signal and compare its frequency value to a predetermined frequency;

a frequency reference generation circuit, the generation circuit increasing or decreasing a reference frequency based upon the comparison result; and

a symbol timing circuit, defining consecutive symbol occurrence intervals in operative response to the reference frequency, the symbol timing circuit operating at a sample frequency having an integer relationship to the predetermined frequency.

24. The integrated circuit communication device according to claim 23, the detector circuit further comprising:

a consecutive detection circuit, for generating frequency error estimates associated with consecutive received pilot signals;

a consecutive sign determination circuit for determining the sign associated with frequency errors of consecutive received pilot signals; and

a multiplication circuit for multiplying the sign of each of the frequency errors of consecutive received pilot signals to thereby define a sign of an average frequency error of consecutive received pilot signals.

25. The integrated circuit communication device according to claim 24, the generation circuit increasing or decreasing a reference frequency based upon the sign of average frequency error of consecutive received pilot signals, the reference frequency increasing if the sign of the average frequency error is negative.