Delta-sigma modulator with selectable noise transfer function

1. An apparatus, comprising:

a delta-sigma modulator having a variable noise transfer function and a fixed sampling frequency, wherein the modulator is coupled to a communication medium carrying a selected channel, wherein the noise transfer function is varied in accordance with at least one of a communication protocol, bandwidth, and oversampling ratio associated with the selected channel.

2. The apparatus of claim 1 wherein the noise transfer function has a first value NTF1 if the selected channel has a first bandwidth BW1, wherein the noise transfer function has a distinct second value NTF2 if the selected channel has a second bandwidth BW2 distinct from BW1.

3. The apparatus of claim 1 wherein the noise transfer function is selected to have a first value NTF1 if the modulator is processing the selected channel at a first oversampling ratio, wherein the noise transfer function is selected to have a second value NTF2 if the modulator is processing the selected channel at a second oversampling ratio.

4. The apparatus of claim 1 wherein the communication medium is a subscriber line.

5. The apparatus of claim 1 wherein the noise transfer function is selectable from a discrete plurality of functions.

6. A method comprising the steps:

a) selecting a communication protocol for a first channel; and

b) providing a delta-sigma modulator of fixed sampling frequency coupled to a communication medium carrying the first channel, the modulator having a variable noise transfer function selected to vary noise shaping within the first channel in accordance with the selected communication protocol.

7. The method of claim 6 further comprising:

c) performing a line test on a line for carrying the first channel and a second channel in order to select the communication protocol.

8. The method of claim 6 wherein the communication medium carries a second channel, wherein an amount of quantization noise in the second channel varies in accordance with the selected noise transfer function.

9. The method of claim 8 wherein the second channel is a voiceband channel having a bandwidth less than or equal to 4 KHz.

10. The method of claim 6 wherein the selected communication protocol is a digital multi-tone communication protocol having a bandwidth no greater than 1.1 MHz.

11. The method of claim 6 wherein the selected communication protocol is a digital multi-tone communication protocol having a bandwidth no greater than 2.2 MHz.

12. The method of claim 7 wherein the line is an analog subscriber line.

13. A method comprising the steps:

a) providing a multi-channel communication medium supporting a first channel and a second channel, wherein the first channel has a bandwidth V; and

b) providing a delta-sigma modulator of fixed sampling frequency coupled to the multi-channel communication medium, wherein the delta-sigma modulator is configured to have a first noise transfer function NTF1 if the second channel has a bandwidth BW1, wherein the delta-sigma modulator is configured to have a second noise transfer function NTF2 if the second channel has a bandwidth BW2.

14. The method of claim 13 wherein NTF1 is distinct from NTF2 if BW1≠BW2.

15. A method comprising the steps:

a) providing a delta-sigma modulator coupled to receive a broadband signal including a selected channel, wherein the delta-sigma modulator has a selectable noise transfer function and a fixed sampling frequency;

b) selecting a first noise transfer function NTF1 if the delta-sigma modulator processes the selected channel at a first oversampling ratio; and

c) selecting a second noise transfer function NTF2, if the delta-sigma modulator processes the selected channel at a second oversampling ratio.

16. method of claim 15 wherein NTF1≠NTF2 if the first and second oversampling ratios are distinct.