FM analog demodulator compatible with IBOC signals

1. A method comprising:

receiving an in-band on-channel radio signal including an analog-modulated portion and a digitally modulated portion;

approximating a derivative of a phase of the radio signal by sampling the radio signal to produce a plurality of successive complex signal samples and determining a phase difference between successive ones of the complex signal samples representing the analog FM signal;

using the phase difference to obtain an FM baseband multiplex signal, wherein approximating the derivative results in an error in a non-flat frequency response of a differentiator output; and

compensating for gain loss in a stereo difference signal portion located in a bandwidth from 23 to 53 kHz of the FM baseband multiplex signal with a constant gain to correct for the error resulting from approximating the derivative of the phase of the radio signal.

2. The method of claim 1 , further comprising:

conjugate multiplying pairs of successive samples; and

computing the angle over ±π of the result.

3. The method of claim 1 , further comprising:

scaling the phase difference angle to be within a range of ±1 with 100% modulation or frequency deviation to obtain the FM baseband multiplex signal.

4. The method of claim 1 , further comprising:

decimating a rate of the complex signal samples, wherein the compensation step applies a different gain for different decimation factors.

5. The method of claim 1 , further comprising:

filtering the in-band on-channel radio signal prior to detection of baseband content of the radio signal.

6. A method comprising:

receiving an in-band on-channel radio signal including an analog-modulated portion and a digitally modulated portion;

filtering the in-band on-channel radio signal prior to detection of baseband content of the radio signal to produce a predetection signal;

sampling the predetection signal to produce a plurality of successive complex signal samples;

determining a phase difference between successive ones of the complex signal samples representing the analog FM signal;

using the phase difference to obtain an FM baseband multiplex signal, wherein the FM baseband multiplex signal is distorted due to limiting a bandwidth of the predetection signal; and

compensating a stereo difference signal portion located in a bandwidth from 23 to 53 kHz of the FM baseband multiplex signal to correct for distortion due to limiting a bandwidth of the predetection signal, wherein non-linearity in the stereo difference signal portion is compensated with a complementary quadratic function.

7. The method of claim 6 , wherein the filtering is performed using a filter having a passband of about ±90 kHz and a stopband of about ±100 kHz.

8. The method of claim 5 , wherein the filtered radio signal is sampled at approximately 186 kHz and the sampled signal is used to produce an output signal.

9. An apparatus comprising:

an input for receiving an in-band on-channel radio signal including an analog-modulated portion and a digitally modulated portion; and

processing circuitry for approximating a derivative of a phase of the radio signal by sampling the radio signal to produce a plurality of successive complex signal samples and determining a phase difference between successive ones of the complex signal samples representing the analog FM signal, and wherein the processing circuitry uses the phase difference to obtain an FM baseband multiplex signal, wherein determining the phase difference results in an error in the non-flat frequency response of a differentiator output; and compensates for gain loss in a stereo difference signal portion located in a bandwidth from 23 to 53 kHz of the FM baseband multiplex signal with a constant gain to correct for an error resulting from approximating the derivative of the phase of the radio signal.

10. The apparatus of claim 9 , further comprising:

conjugate multiplying pairs of successive samples; and

computing the angle over ±ÿ of the result.

11. The method of claim 9 , further comprising:

scaling the phase difference angle to be within a range of ±1 with 100% modulation or frequency deviation to obtain the FM baseband multiplex signal.

12. The apparatus of claim 9 , wherein the processing circuitry decimates a rate of the complex signal samples, and applies a different gain for different decimation factors.

13. The apparatus of claim 9 , further comprising:

filtering the in-band on-channel radio signal prior to detection of baseband content of the radio signal.

14. The apparatus of claim 13 , wherein the filtering is performed using a filter having a passband of about ±90 kHz and a stopband of about ±100 kHz.

15. The apparatus of claim 13 , wherein the filtered radio signal is sampled at approximately 186 kHz and the sampled signal is used to produce an output signal.

16. An apparatus comprising:

an input for receiving an in-band on-channel radio signal including an analog-modulated portion and a digitally modulated portion;

a filter for filtering the in-band on-channel radio signal prior to detection of baseband content of the radio signal to produce a predetection signal; and

processing circuitry for sampling the radio signal to produce a plurality of successive complex signal samples, determining a phase difference between successive ones of the complex signal samples representing the analog FM signal, and using the phase difference to obtain an FM baseband multiplex signal, wherein the FM baseband multiplex signal is distorted due to limiting a bandwidth of the predetection signal, and wherein non-linearity in a stereo difference signal located in a bandwidth from 23 to 53 kHz of the FM baseband multiplex signal is compensated with a complementary quadratic function to correct for distortion due to limiting a bandwidth of the predetection signal.