Quantizer overload prevention for feed-back type delta-sigma modulators

1. A method of preventing quantizer overload in a delta sigma modulator, wherein a closed loop of the delta sigma modulator includes one or more filter stages of the loop filter coupled to the output of the initial filter stage, a quantizer coupled to an output of the loop filter, and a feedback path coupled between an output of the quantizer and one or more of the filter stages coupled to the output of the initial filter stage, the method comprising:

providing a delta sigma modulator input signal minus a quantizer output feedback signal to an initial filter stage of a loop filter of the delta sigma modulator;

providing quantizer feedback to at least one additional filter stage of the loop filter;

limiting an output of the initial filter stage using a limiter in an integrator of the initial filter stage of the loop filter;

providing an output signal of the initial filter stage to a second filter stage of the loop filter; and

maintaining a low pass transfer function of the closed loop when limiting the output of the initial filter stage of the loop filter.

2. The method of claim 1 further comprising:

progressively limiting one or more successive filter stages coupled to the initial filter stage of the loop filter using limiters in each of the one or more successive filter stages if limiting the output of an immediately preceding filter stage is insufficient to prevent quantizer overload.

3. The method of claim 2 further comprising:

providing an output signal from each filter stage, except a last filter stage in the loop filter, to an immediately successive stage; and

delaying a forward path signal in each filter stage.

4. The method of claim 2 wherein a closed loop of the delta sigma modulator includes one or more filter stages of the loop filter coupled to the output of the initial filter stage, a quantizer coupled to an output of the loop filter, and a feedback path coupled between an output of the quantizer and one or more of the filter stages coupled to the output of the initial filter stage, the method further comprising:

maintaining a low pass transfer function of the closed loop when limiting the output of any of the filter stages.

5. The method of claim 1 further comprising:

delaying a forward path signal of the initial filter stage.

6. The method of claim 5 wherein a z-domain transfer function of the initial filter stage comprises (z −1 )/(1−z −1 ).

7. The method of claim 1 wherein the quantizer output feedback is individually weighted for each filter stage.

8. The method of claim 1 wherein the limiter is a clipper and limiting an output of the initial filter stage comprises clipping an output of the initial filter stage using the limiter during quantizer overload prevention conditions.

9. A signal processing system comprising:

a delta-sigma modulator having a feedback topology to convert an input signal into an output signal, the delta-sigma modulator comprising:

a delta sigma modulator input to receive the input signal;

a quantizer to quantize a quantizer input signal and provide a quantizer output signal via a quantizer output; and

a loop filter to provide the quantizer input signal to the quantizer, the loop filter comprising:

an initial filter stage having a limiter L 0 in an integrator of the initial filter stage to limit an output signal of the initial filter stage, wherein the initial filter stage is coupled to the delta sigma modulator input and the quantizer output; and

a filter section coupled to an output of the initial filter stage and the quantizer output;

wherein a transfer function of a closed loop, comprising the filter section coupled to the quantizer, has a low pass transfer function when limiter L 0 is active.

10. The signal processing system of claim 9 wherein the filter section includes at least a first filter stage having a limiter L 1 to limit an output signal of the first filter stage.

11. The signal processing system of claim 10 wherein the limiter L 0 is active to limit the output signal of the initial filter stage before any limiter of any subsequent filter stage of the loop filter limits an output signal of any of the subsequent filter stages.

12. The signal processing system of claim 11 wherein the limiter L 0 limits the output signal of the initial filter stage to approximately 1 dB to 3 dB above a normal full-scale value of the input signal to the initial filter stage.

13. The signal processing system of claim 10 wherein the transfer function of the closed loop maintains a low pass transfer function when limiters L 0 and L 1 are active.

14. The signal processing system of claim 10 wherein the first filter stage of the filter section includes an integrator having a feedback loop coupled to an output of the limiter L 1 .

15. The signal processing system of claim 10 wherein the limiter L 0 limits an output signal of the first filter stage of the filter section to approximately 6 dB above a normal full-scale value of an input signal to the first filter stage.

16. The signal processing system of claim 9 wherein the integrator of the initial filter stage includes a feedback loop connected to an output of the limiter L 0 .

17. The signal processing system of claim 9 wherein the loop filter further comprises:

a first delay component in a forward path of the initial filter stage.

18. The signal processing system of claim 17 wherein a z-domain transfer function of the initial filter stage comprises (z −1 )/(1−z −1 ).

19. The signal processing system of claim 17 wherein the loop filter further comprises:

delay components in forward paths of each filter stage of the filter section.

20. The signal processing system of claim 9 comprises an audio signal processing system.

21. The signal processing system of claim 9 wherein the limiter L 0 is a clipper, and the clipper is configured to clip an output signal of the initial filter stage during quantizer overload prevention conditions.

22. A signal processing system comprising:

a delta-sigma modulator to convert an input signal into an output signal, the delta-sigma modulator comprising:

a quantizer to generate a quantizer output signal; and

a loop filter coupled to the quantizer, the loop filter having a feedback topology and comprising:

an initial N successive filter stages (FS) arranged in ascending order FS 0 through FS N−1 , wherein:

filter stage FS 0 receives a difference signal comprised of a difference between the input signal and the quantizer output signal;

each of the initial N−1 successive filter stages FS 1 through FS N−1 receives input signals comprised of a feedback signal from the quantizer and an input signal from a preceding filter stage;

each of the filter stages includes an integrator;

the integrator of filter stage FS 0 includes limiter L 0 to limit output values of filter stage FS 0 to a maximum absolute value;

N is a non-negative integers and N≧2;

a transfer function of a closed loop, comprising the filter stages FS 1 through FS N−1 coupled to the quantizer maintains a low pass transfer function when limiter L 0 is active limiting respective output values of filter stage FS 0 .

23. The signal processing system of claim 22 wherein at least one of filter stages FS 1 through FS N−1 includes a limiter to limit respective output values of filter stages FS 1 through FS N−1 having a limiter to a respective maximum absolute value and the limiter of stage FS 0 is adapted to limit an output value of the integrator of stage FS 0 before any limiter of any subsequent stage of the loop filter limits an output value of any of the subsequent stages.

24. The signal processing system of claim 22 wherein each filter stage FS i further includes a delay component in a forward path of the filter stage FS i , wherein i is a non-negative integer and i is an element of the set {0, 1, . . . , N−1).

25. The signal processing system of claim 24 wherein a z-domain transfer function of each filter stage FS i comprises (z −1 )/(1−z −1 ).

26. The signal processing system of claim 22 wherein the limiter L 0 is a clipper, and the clipper is configured to clip output values of filter stage FS 0 to a maximum absolute value during quantizer overload prevention conditions.

27. A method of preventing quantizer overload in a delta sigma modulator, the method comprising:

receiving a first difference signal in an initial filter stage of a loop filter of the delta sigma modulator, wherein the difference signal comprises a difference between a delta sigma modulator input signal and a first quantizer feedback signal;

limiting the first difference signal with a limiter disposed within an integrator of the initial filter;

integrating the first limited difference signal to produce a first limited integrated output signal;

integrating a second difference signal to produce a second integrated output signal, wherein the second difference signal comprises a difference between a second quantizer output signal and the first limited integrated output signal;

limiting the second integrated output signal if limiting the first difference signal is insufficient to prevent quantizer overload; and

maintaining a low pass transfer function in the delta sigma modulator when limiting the first integrated output signal.

28. The method of claim 27 wherein a z-domain of a transfer function of the initial filter stage comprises (z −1 )/(1−z −1 ).

29. The method of claim 27 wherein limiting the first difference signal during quantizer overload prevention conditions to produce the first limited difference signal generates harmonics in the first limited integrated output signal, the method further comprising:

attenuating the harmonics of the first limited integrated output signal when limiting the first difference signal; and

generating an output signal of a quantizer of the delta sigma modulator in which the harmonics of the first limited integrated output signal are attenuated.

30. The method of claim 27 wherein the limiter is a clipper and limiting the first difference signal with a limiter disposed within an integrator of the initial filter stage comprises clipping the first difference signal with a clipper during quantizer overload prevention conditions.

31. An apparatus for preventing quantizer overload in a delta sigma modulator, wherein a closed loop of the delta sigma modulator includes one or more filter stages of the loop filter coupled to the output of the initial filter stage, a quantizer coupled to an output of the loop filter, and a feedback path coupled between an output of the quantizer and one or more of the filter stages coupled to the output of the initial filter stage, the apparatus comprising:

means for providing a delta sigma modulator input signal minus a quantizer output feedback signal to an initial filter stage of a loop filter of the delta sigma modulator;

means for providing quantizer feedback to at least one additional filter stage of the loop filter;

means for limiting an output of the initial filter stage using a limiter in an integrator of the initial filter stage of the loop filter; and

means for providing an output signal of the initial filter stage to a second filter stage of the loop filter;

wherein a low pass transfer function of the closed loop is maintained when limiting the output of the initial filter stage of the loop filter.

32. An apparatus for preventing quantizer overload in a delta sigma modulator, the apparatus comprising:

means for receiving a first difference signal in an initial filter stage of a loop filter of the delta sigma modulator, wherein the difference signal comprises a difference between a delta sigma modulator input signal and a first quantizer feedback signal;

means for limiting the first difference signal with a limiter disposed within an integrator of the initial filter stage to produce a first limited difference signal while maintaining a low pass transfer function in the delta sigma modulator when limiting the first integrated output signal;

means for integrating the first limited difference signal to produce a first limited integrated output signal;

means for integrating a second difference signal to produce a second integrated output signal, wherein the second difference signal comprises a difference between a second quantizer output signal and the first limited integrated output signal; and

means for limiting the second integrated output signal if limiting the first difference signal is insufficient to prevent quantizer overload.