Digital video receiver

• US 5,995,168 A
• Filed: 01/30/1997
• Issued: 11/30/1999
• Est. Priority Date: 01/31/1996
• Status: Expired due to Term

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Yagi, Manabu
• Primary Examiner(s): Lee, Michael H.
• Assistant Examiner(s): DESIR, JEAN WICEL
• Application Number:
  US08/790,801
• Time in Patent Office: 1,034 Days
• Field of Search: 348/720, 348/725, 348/726, 348/558, 348/426, 348/461, 348/731, 348/554, 348/507, 348/508
• US Class Current: 348/725
• CPC Class Codes: H04N 21/426 Internal components of the client ; Characteristics thereof H04N5/44 takes precedence
• CPC Class Codes: H04N 21/4382 Demodulation or channel decoding, e.g. QPSK demodulation analog front ends or means for connecting modulators, demodulators or transceivers to a transmission line H04L27/0002
• CPC Class Codes: H04N 5/211 Ghost signal cancellation H04N23/81 takes precedence
• CPC Class Codes: H04N 5/455 Demodulation-circuits

Abstract

A video receiver includes a tuner, a QAM demodulator including a selective gated PLL carrier recovery circuit, and a digital equalizer. The selective gated PLL carrier recovery circuit has a normal operation mode and a selection control mode. The channel changing of the tuner is performed after the selective gated PLL carrier recovery circuit is set to the selection control mode and the digital equalizer is held at its current state.

Claims

1. A video receiver comprising:

receiving means for receiving a modulation signal through a user-designated channel selected from a plurality of channels which are multiplexed in frequency division multiplexing, the modulation signal conveying a digital video signal according to 2^N (N is an even integer) quadrature amplitude modulation (2^N -QAM);

detecting means for detecting the digital video signal from the modulation signal using a reproduced carrier signal;

carrier generating means for generating the reproduced carrier signal from the digital video signal received through the user-designated channel, the carrier generating means having a first operation mode and a second operation mode which are selectable, the first operation mode providing the reproduced carrier signal based on 2^N level ranges corresponding to possible signal points on a 2^N -QAM signal constellation, respectively, and the second operation mode providing the reproduced carrier signal based on selected level ranges corresponding to signal points selected from the possible signal points on the 2^N -QAM signal constellation, respectively;

equalization means for compensating for channel distortions included in the digital video signal to produce an equalized digital video signal; and

control means for controlling such that the carrier generating means is set to the second operation mode and the equalization means is held at a current state before changing the channel of the receiving means from a presently received channel to a newly selected user-designated channel.

2. The video receiver according to claim 1, wherein the second operation mode provides the reproduced carrier signal based on the selected level ranges corresponding to signal points existing on diagonal lines with respect to orthogonal axes of the 2^N -QAM signal constellation.

3. The video receiver according to claim 1, wherein the carrier generating means comprises a PLL (phase-locked loop) circuit,

the PLL circuit comprising:

an extracting circuit for extracting a PLL control signal from the digital video signal;

a phase detection circuit for detecting the selected level ranges from the digital video signal to producing a detection signal;

phase control means for generating a phase control signal from the PLL control signal according to control of the control means such that the phase control signal is generated based on the PLL control signal in the first operation mode selected and that the phase control signal is generated based on the PLL control signal and the detection signal in the second operation mode selected; and

an oscillator for generating the reproduced carrier signal based on the phase control signal.

4. The video receiver according to claim 3, wherein the PLL circuit further comprises a sync detection circuit for detecting synchronization of the PLL circuit based on the phase control signal, wherein the phase control signal is generated based on the PLL control signal and the detection signal when the synchronization of the PLL circuit is not detected.

5. The video receiver according to claim 1, wherein the equalization means comprises a transversal filter for compensating for the channel distortions by adjusting a plurality of tap coefficients thereof based on the digital video signal, wherein the equalization means is held at the current state by fixing the tap coefficients to current values according to control of the control means.

6. The video receiver according to claim 5, wherein the tap coefficients are fixed to the current values when the carrier generating means is set to the second operation mode.

7. The video receiver according to claim 3, wherein the PLL circuit further comprises a sync detection circuit for detecting synchronization of the PLL circuit based on the phase control signal,

wherein the PLL circuit generates the phase control signal based on the PLL control signal and the detection signal when operating in the second operation mode or out of synchronization, and

the PLL circuit generates the phase control signal based on the PLL control signal when operating in the first operation mode and in synchronization.

8. The video receiver according to claim 7, wherein the equalization means comprises a transversal filter for compensating for the channel distortions by adjusting a plurality of tap coefficients thereof based on the digital video signal,

wherein the equalization means is held at the current state by fixing the tap coefficients to current values when the PLL circuit operating in the second operation mode or out of synchronization, and

the equalization means adjusts the tap coefficients when the PLL circuit operating in the first operation mode and in synchronization.

9. The video receiver according to claim 1, wherein the control means outputs a selection control signal and a hold control signal to the carrier generating means and the equalization means, respectively, when a channel changing request occurs, and the control means outputs a channel change signal to the receiving means after the carrier generating means is set to the second operation mode and the equalization means is held at a current state.

10. A set top box comprising the video receiver according to claim 1.

11. A method for reproducing a digital video signal from a modulation signal, comprising the steps of:

a) selecting a first user-identified channel selected from a plurality of channels which are multiplexed in frequency division multiplexing;

b) receiving the modulation signal through the first user-identified channel selected, the modulation signal conveying a digital video signal according to 2^N (N is an even integer) quadrature amplitude modulation (2^N -QAM);

c) detecting the digital video signal from the modulation signal using a reproduced carrier signal;

d) generating the reproduced carrier signal from the digital video signal received through the channel in a first operation mode providing the reproduced carrier signal based on 2^N level ranges corresponding to possible signal points on a 2^N -QAM signal constellation, respectively;

e) compensating for channel distortions included in the digital video signal by adjusting tap coefficients of a transversal filter to produce the digital video signal;

f) selecting a second operation mode to generate the reproduced carrier signal and holding the tap coefficients at a current state when a channel change request occurs, the second operation mode providing the reproduced carrier signal based on selected level ranges corresponding to signal points selected from the possible signal points on the 2^N -QAM signal constellation, respectively; and

g) selecting a requested second user-identified channel from the channels after the second operation mode is selected and the tap coefficients are held at the current state.

12. The method according to claim 11, wherein, in the step (f), the second operation mode provides the reproduced carrier signal based on the selected level ranges corresponding to signal points existing on diagonal lines with respect to orthogonal axes of the 2^N -QAM signal constellation.

13. The method according to claim 11, wherein the reproduced carrier signal is generated by a PLL circuit, the step (d) comprises the steps of:

extracting a PLL control signal from the digital video signal;

detecting the selected level ranges from the digital video signal to producing a detection signal;

generating a phase control signal from the PLL control signal such that the phase control signal is generated based on the PLL control signal; and

generating the reproduced carrier signal based on the phase control signal.

14. The method according to claim 11, wherein the reproduced carrier signal is generated by a PLL circuit, the step (f) comprises the steps of:

extracting a PLL control signal from the digital video signal;

detecting the selected level ranges from the digital video signal to producing a detection signal;

generating a phase control signal from the PLL control signal such that the phase control signal is generated based on the PLL control signal and the detection signal; and

generating the reproduced carrier signal based on the phase control signal.

15. The method according to claim 11, further comprising the step of detecting PLL synchronization based on the phase control signal, wherein the phase control signal is generated based on the PLL control signal and the detection signal when the PLL synchronization is not detected.

16. The method according to claim 15, wherein the phase control signal is generated based on the PLL control signal and the detection signal when the second operation mode is selected or when the PLL synchronization is not detected, and

the phase control signal is generated based on the PLL control signal when the first operation mode is selected and the PLL synchronization is detected.

17. The method according to claim 16, wherein the tap coefficients is fixed to current values when the second operation mode is selected or when the PLL synchronization is not detected, and

the tap coefficients are adjusted when the first operation mode is selected and the PLL synchronization is detected.

18. A video receiver comprising:

a tuner for receiving a modulation signal through a user-identified channel selected from a plurality of channels which are multiplexed in frequency division multiplexing, one of the channels being selectable according to a channel designation signal, the modulation signal conveying a digital video signal according to 2^N (N is an even integer) quadrature amplitude modulation (2^N -QAM);

a QAM detector for detecting the digital video signal from the modulation signal using a reproduced carrier signal;

a carrier recovery circuit for generating the reproduced carrier signal from the digital video signal received through the channel, the carrier generating means having a first operation mode and a second operation mode wherein the first operation mode is normally selected and the second operation mode is selected by a selection control signal, the first operation mode providing the reproduced carrier signal based on 2^N level ranges corresponding to possible signal points on a 2^N -QAM signal constellation, respectively, and the second operation mode providing the reproduced carrier signal based on selected level ranges corresponding to signal points selected from the possible signal points on the 2^N -QAM signal constellation, respectively;

a digital equalizer for compensating for channel distortions included in the digital video signal by adjusting tap coefficients of the digital equalizer to produce an equalized digital video signal, the tap coefficients being fixed to current values according to a hold control signal; and

a controller for controlling the tuner responsive to a user channel selection input, the carrier recovery circuit and the digital equalizer such that the selection control signal and the hold control signal are output to the carrier recovery circuit and the digital equalizer, respectively, before the channel designation signal is output to the tuner.

19. The video receiver according to claim 18, wherein the second operation mode provides the reproduced carrier signal based on the selected level ranges corresponding to signal points existing on diagonal lines with respect to orthogonal axes of the 2^N -QAM signal constellation.