System for, and method of, processing quadrature amplitude modulated signals
Analog signals encoded with quadrature amplitude modulation (QAM) pass through a coaxial cable at a particular baud rate. These signals have a carrier frequency individual to the TV station being received. They are mixed with signals from a variable frequency oscillator to produce signals at a particular intermediate frequency (IF). An analog-digital converter (ADC) converts the IF signals to corresponding digital signals which are demodulated to produce two digital signals having a quadrature phase relationship. After being filtered and derotated, the digital signals pass to a symmetrical equalizer including a feed forward equalizer (FFE) and a decision feedback equalizer (DFE) connected to the FFE in a feedback relationship. The DFE may include a slicer providing amplitude approximations of increasing sensitivity at progressive times. Additional slicers in the equalizer combine the FFE and DFE outputs to provide the output data without any of the coaxial cable noise or distortions. The equalizer outputs and initially the derotation outputs, and the slicer outputs, servo (1) the oscillator frequency to obtain the IF frequency, (2) the ADC sampling clock to obtain the digital conversion at a rate related to the particular baud rate and (3) the derotator. The servos may have (1) first constants initially after a change in the station selection and (2) second time constants thereafter. The ADC gain is also servoed (1) initially in every ADC conversion and (2) subsequently in every nth ADC conversion where n=integer>1. The above recover the QAM data without any of the coaxial cable noise or distortions.
1. An equalizer configured to correct for distortions in an in-phase (I) signal and a quadrature phase (Q) signal, comprising:
a feed forward equalizer (FFE) configured to correct for distortions in a left half of the I signal and in a left half of the Q signal;
a decision feedback equalizer (DFE) configured to correct for distortions in a right half of the I signal and in a right half of the Q signal;
a first slicer configured to approximate a first combination based upon an output of the FFE and an output of the DFE to provide data; and
a second slicer configured to approximate a second combination of the output of the FFE and the output of the DFE to provide an error feedback.
2. The equalizer of claim 1 , further comprising:
a first adder configured to provide an input to the first slicer and the second slicer based upon the output of the FFE and the output of the DFE, wherein the first slicer and the second slicer determine a corresponding one of a plurality of progressive values that is substantially closest to an output of the first adder.
3. The equalizer of claim 1 , wherein the FFE is operative in one of a T-spaced function or a T/2-spaced function.
4. The equalizer of claim 1 , wherein the FFE is configured to correct for distortions in the left half of the I signal and in the left half of the Q signal based upon the error feedback, and
wherein the DFE is configured to correct for distortions in the right half of the I signal and in the right half of the Q signal based upon the error feedback.
5. The equalizer of claim 1 , wherein the second slicer is configured to approximate the second combination based upon a control input.
6. The equalizer of claim 5 , wherein the control input is configured to receive successive binary indications of two, four, eight, and sixteen binary values.
7. The equalizer of claim 6 , wherein the second slicer is configured to select the second combination that is closest to two progressive binary values of the binary indication and to substitute the closest of the two progressive binary values as the output of the second slicer when the control input receives the binary indication of two binary values.
8. The equalizer of claim 6 , wherein the second slicer is configured to select the second combination that is closest to four progressive binary values of the binary indication and to substitute the closest of the four progressive binary values as the output of the second slicer when the control input receives the binary indication of four binary values.
9. The equalizer of claim 6 , wherein the second slicer is configured to select the second combination that is closest to eight progressive binary values of the binary indication and to substitute the closest of the eight progressive binary values as the output of the second slicer when the control input receives the binary indication of eight binary values.
10. The equalizer of claim 6 , wherein the second slicer is configured to select the second combination that is closest to sixteen progressive binary values of the binary indication and to substitute the closest of the sixteen progressive binary values as the output of the second slicer when the control input receives the binary indication of sixteen binary values.
11. A method to correct for distortions in an in-phase (I) signal and a quadrature phase (Q) signal, the method comprising:
(a) correcting, in a communications receiver, for distortions in a left half of the I signal and in a left half of the Q signal;
(b) correcting, in the communications receiver, for distortions in a right half of the I signal and in a right half of the Q signal based upon an output of step (a);
(c) approximating, in the communications receiver, a first combination based upon an output of step (a) and an output of step (b) to provide data;
(d) approximating, in the communications receiver, a second combination of the output of step (a) and an output of step (b); and
(e) providing, in the communications receiver, an error feedback based on the second combination.
12. The method of claim 11 , further comprising:
(f) providing an input to step (c) based upon the output of step (a) and the output of step (b),
wherein step (c) comprises:
(c)(i) determining a corresponding one of a plurality of progressive values that is substantially closest to an output of step (f).
13. The method of claim 11 , wherein step (a) comprises:
(a)(i) correcting for distortions in the left half of the I signal and in the left half of the Q signal based upon the error feedback, and wherein step (b) comprises:
(b)(i) correcting for distortions in the right half of the I signal and in the right half of the Q signal based upon the error feedback.
14. The method of claim 11 , wherein step (d) comprises:
(d)(i) approximating the output of step (a) and the output of step (b) based upon a control input.
15. The method of claim 14 , wherein the control input is configured to receive successive binary indications of two, four, eight, and sixteen binary values.
16. The method of claim 15 , wherein step (d) comprises:
(d)(i) selecting the second combination that is closest to two progressive binary values of the binary indication; and
(d)(ii) substituting a closest of the two progressive binary values as an output of step (d) when the control input receives the binary indication of two binary values.
17. The method of claim 15 , wherein step (d) comprises:
(d)(i) selecting the first second combination that is closest to four progressive binary values of the binary indication; and
(d)(ii) substituting a closest of the four progressive binary values as an output of step (d) when the control input receives the binary indication of four binary values.
18. The method of claim 15 , wherein step (d) comprises:
(d)(i) selecting the second combination that is closest to eight progressive binary values of the binary indication; and
(d)(ii) substituting a closest of the eight progressive binary values as an output of step (d) when the control input receives the binary indication of eight binary values.
19. The method of claim 15 , wherein step (d) comprises:
(d)(i) selecting the second combination that is closest to sixteen progressive binary values of the binary indication; and
(d)(ii) substituting a closest of the sixteen progressive binary values as an output of step (d) when the control input receives the binary indication of sixteen binary values.