Telecommunications – Receiver or analog modulated signal frequency converter – Noise or interference elimination
Reexamination Certificate
1998-06-29
2001-01-09
Bost, Dwayne (Department: 2681)
Telecommunications
Receiver or analog modulated signal frequency converter
Noise or interference elimination
C455S065000, C375S349000
Reexamination Certificate
active
06173166
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates in general to detecting and correcting for multipath interference in an FM radio receiver, and more specifically, to faster, more accurate detection of the existence of multipath events and to more effective correction without adding other distortion or undesirable noises.
The problem of multipath distortion in radio receivers is well known. Multipath events occur when radio frequency (RF) signals following a direct path and one or more indirect (i.e., reflected) paths from a transmitter to a receiver interfere with each other at the receiver. Reflections can be caused by hills and buildings, for example.
Constructive and destructive interference of signals caused by interaction between the reflections and the direct line of sight transmission causes both signal distortion and rapid fluctuations in the received field intensity, especially in moving vehicles.
Multipath is a particularly annoying problem in reception of FM stereo broadcasts due to the greater bandwidths of FM stereo broadcasts, corruption of the stereo pilot carrier, and sensitivity to phase errors. Prior art systems respond to multipath conditions by adjusting stereo separation (i.e., blend) and/or adding a high frequency cutoff for the reproduced audio signal. In order to avoid having these measures become noticeable, they are gradually introduced and then gradually eliminated after the multipath conditions cease. Thus, these prior art measures degrade the overall audio signal fidelity even when there is no multipath disturbance present at a particular moment. Thus, prior systems tend to be slow to react at the onset of multipath so that corrections are not put in place in time to prevent multipath distortion from reaching the audio output. Furthermore, the decreased fidelity resulting from the correction measures continue to negatively impact reception at times when they are no longer necessary.
SUMMARY OF THE INVENTION
The present invention has the advantage of more quickly and accurately detecting the presence of multipath and then applying a signal correction which is more coordinated with the multipath event itself.
In one aspect of the invention, a method of detecting multipath events in a received radio broadcast signal allows much quicker identification of multipath events. An instantaneous noise energy of the broadcast signal is determined. A derivative of the instantaneous noise energy is then determined. A modulation energy of the broadcast signal is also determined. Finally, a multipath detection signal is generated if the derivative is greater than a threshold which has a value proportional to the modulation energy. By using the derivative of the noise energy, the multipath detector can detect a rising noise floor before the noise is actually perceivable by the listener.
In a second aspect of the invention, improved signal correction is obtained using a panning technique. The broadcast signal is demodulated to generate a demodulated signal. The demodulated signal is split into first and second paths. A multipath smoothing filter is applied to the second path. The first and second paths are recombined by panning between the first and second paths in response to the multipath detection signal.
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Bost Dwayne
Gelin Jean A
Mollon Mark
Visteon Global Technologies Inc.
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