FM analog demodulator compatible with IBOC signals
View Patent ↗A method includes: receiving an FM radio signal including an analog-modulated portion; digitally sampling an analog-modulated portion of the radio signal to produce a plurality of samples; using a ratio between an average magnitude and an RMS magnitude of a block of the samples to compute a signal quality metric; detecting sum and difference components of the baseband multiplex signal content; using the baseband content to produce an output signal; and blending the output signal from stereo to monaural as the signal quality metric falls below a threshold value.
1. A method comprising:
receiving an FM radio signal including an analog-modulated portion;
digitally sampling an analog-modulated portion of the radio signal to produce a plurality of samples;
using a ratio between an average magnitude and an RMS magnitude of a block of the samples to compute a signal quality metric;
detecting sum and difference components of a baseband multiplex signal content;
using the baseband content to produce an output signal; and
blending the output signal from stereo to monaural as the signal quality metric falls below a threshold value.
2. The method of claim 1 , wherein blending the output signal to monaural is performed through reduction or elimination of a stereo difference signal.
3. The method of claim 2 , wherein a stereo blend control signal weights the stereo difference signal by a value ranging from 0 to 1.
4. The method of claim 3 , wherein a stereo metric value is determined using a pilot power signal and the signal quality metric.
5. The method of claim 4 , wherein an in-phase component of the pilot signal is filtered, the power of the filtered pilot signal is measured, and then the pilot signal is filtered again to provide a measure of the pilot power, having a normalized range from zero to approximately 1.
6. The method of claim 3 , wherein the stereo blend control signal is formed as a function of both a filtered and unfiltered stereo metric signal.
7. An apparatus comprising:
an input for receiving an FM radio signal including an analog-modulated portion; and
processing circuitry for digitally sampling an analog-modulated portion of the radio signal to produce a plurality of samples,
using a ratio between an average magnitude and an RMS magnitude of a block of the samples to compute a signal quality metric,
detecting sum and difference components of a baseband content, using the baseband content to produce an output signal, and
blending the output signal from stereo to monaural as the signal quality metric falls below a threshold value.
8. The apparatus of claim 7 , wherein blending the output signal to monaural is performed through reduction or elimination of a stereo difference signal.
9. The apparatus of claim 8 , wherein a stereo blend control signal weights the stereo difference signal by a value ranging from 0 to 1.
10. The apparatus of claim 9 , wherein a stereo metric value is determined using a pilot power signal and the signal quality metric.
11. The apparatus of claim 10 , wherein an in-phase component of the pilot signal is filtered, the power of the filtered pilot signal is measured, and then the pilot signal is filtered again to provide a measure of the pilot power, having a normalized range from zero to approximately 1.
12. The apparatus of claim 9 , wherein the stereo blend control signal is formed as a function of both a filtered and unfiltered stereo metric signal.