Use of 64-QAM circuitry for receiving and decoding 8-VSB signals

• US 6,292,518 B1
• Filed: 10/21/1999
• Issued: 09/18/2001
• Est. Priority Date: 10/21/1999
• Status: Expired due to Term

• Current Assignee: NBC Universal Media LLC (Comcast Corporation)
• Original Assignee: General Electric Company
• Inventors: Hershey, John Erik, Grabb, Mark Lewis, Welles, Kenneth Brakeley II
• Primary Examiner(s): Ghebretinsae, Temesghen
• Application Number:
  US09/422,446
• Time in Patent Office: 698 Days
• Field of Search: 375/321, 375/261, 375/298, 375/301, 375/324, 332/103, 329/304, 329/357, 348/641, 348/726
• US Class Current: 375/321
• 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 27/38 Demodulator circuits; Receiver circuits

Abstract

An N-VSB (vestigial sideband) modulation signal is converted into an M-QAM (quadrature amplitude modulation) signal, where M=N², by shifting the symbol rate frequency of a received N-VSB modulation signal to center the waveform spectrum about zero Hertz prior to complex demodulation so that data symbols will alternately appear on demodulated I and Q channels. A pilot tone of the received N-VSB modulation signal is removed to eliminate any bias in the both I and Q channels. Symbol timing between I and Q channels is offset, and quadrature amplitude demodulation of the I and Q channel signals generate alternating I and Q channel data symbols. Alternating inversion of the alternating I and Q channel data symbols recovers the N-VSB symbol data.

Claims

1. A method for recovering an N-VSB (vestigial sideband) modulation signal from M-QAM (quadrature amplitude modulation) signal, where M=N², comprising the steps of:

shifting a symbol rate frequency of a received N-VSB modulation signal containing symbol data to center the waveform spectrum about zero Hertz prior to complex demodulation so that data symbols will alternately appear on demodulated I and Q channels;

removing a pilot tone from the received N-VSB modulation signal to eliminate any bias in both the I and Q channels;

offsetting symbol timing between the I and Q channels;

quadrature amplitude demodulating signals on the I and Q channels to generate alternating I and Q channel data symbols; and

alternating inversion of the alternating I and Q channel data symbols to recover the N-VSB symbol data.

2. The method of claim 1, wherein N=8, M=64 and the symbol rate of the received N-VSB modulation signal is shifted by ¼

symbol rate.

3. The method of claim 1, further comprising the step of combining the alternating inversion of the I and Q channel data.

4. A digital television receiver which recovers an N-VSB (vestigial sideband) modulation signal from an M-QAM (quadrature amplitude modulation) signal, where M=N², comprising:

a first circuit for shifting a symbol rate frequency of a received N-VSB modulation signal to center the waveform spectrum about zero Hertz prior to complex demodulation so that data symbols will alternately appear on demodulated I and Q channels;

a second circuit for removing a pilot tone of the received N-VSB modulation signal to eliminate any bias in both I and Q channels;

a third circuit for offsetting symbol timing between the I and Q channels;

a quadrature amplitude detector for demodulating the I and Q channel signal to generate alternating I and Q channel data symbols; and

an inverter circuit for inverting alternating I and Q channel data symbols to recover the N-VSB symbol data.

5. The digital television receiver recited in claim 4, wherein the first circuit comprises first and second mixers for receiving an intermediate frequency signal and, respectively, an in phase and quadrature phase local oscillator signal for generating I and Q channel output signals at baseband.

6. The digital television receiver recited in claim 5, wherein the second circuit comprises, in each of said I and Q channels, a pilot signal detector and a subtractor for subtracting a detected pilot signal from respective I and Q channel output signals.

7. The digital television receiver recited in claim 6, wherein the third circuit comprises a first delay in the I channel.

8. The digital television receiver recited in claim 7, wherein the first delay comprises a surface acoustic wave device.

9. The digital television receiver recited in claim 7, further comprising a sign generator and a symbol clock generator responsive to one of said pilot signal detectors and a second delay responsive to a symbol clock from the symbol clock generator to delay a sign output signal of the sign generator.

10. The digital television receiver recited in claim 9, wherein the quadrature amplitude demodulator is adapted to demodulate signals in the I and Q channels in response to the symbol clock generator, said inverter circuit being responsive to the delayed sign output signal to generate I and Q output signals.

11. The digital television receiver recited in claim 10, further comprising a multiplexer responsive to the symbol clock generator for combining the I and Q output signals of the inverter to recover the N-VSB symbol data.

12. The digital television receiver recited in claim 11, wherein N=8 and M=64.