This invention relates generally to the field of audio receiver systems, and more specifically to an inventive method for controlling the selectivity of a tuner in a variable bandwidth audio receiver system.
Certain types of audio signals are composed of several categories of signal components that exist at different frequencies. For example, an AM or FM signal may contain in-band-on-channel (IBOC) sub-carrier signals, in addition to the traditional analog AM/FM modulation. In this type of combination IBOC-AM/FM or mixed bandwidth signal, the IBOC sub-carrier frequency components are spectrally farther from the channel center frequency than the analog AM/FM frequency components.
In a typical system for receiving signals containing mixed bandwidth frequency components, selectivity is used to remove the unwanted signal component content, such as the noise outside of the bandwidth of the desired signal or signals present on an adjacent channel. The selectivity may be variable or adaptive depending on the received signal and/or the reception conditions.
When a receiver is tuned to a particular channel, the receiver will generally have no information indicating whether that channel has any wideband modulation or digital signal components, such as IBOC signal components. Traditionally, setting the selectivity had to be determined without this knowledge.
In typical receiver systems, if the selectivity is set too narrow, the presence of a digital signal may or may not be detected and if not detected, the digital signal would not be demodulated. Known methods to detect the sub-carrier components using narrow selectivity are not desirable solutions because the use of narrow selectivity attenuates some or all of the digital sub-carrier signal components thus rendering the detection method ineffective in at least some cases.
On the other hand, if the selectivity is set very wide, the digital signal would be demodulated, and thus its presence confirmed. Any interference that is also present in a strong adjacent channel, however, would also pass through the system and be received, thus creating interference that is audible to the listener.
Thus, when a receiver system is tuned to an AM or FM station that may include a mixed bandwidth signal containing, for instance, IBOC sub-carrier components it would be desirable to control the selectivity of the system such that wider selectivity is used only when such signal components are known to be present and receivable. Otherwise, a narrower selectivity that only passes the center channel analog modulation component would be used, thus removing, as much as possible, any noise and interference that may be present.
One aspect of the present invention is a method for controlling the selectivity of a tuner in a variable bandwidth audio receiver system in which the selectivity of the tuner is set to the wide bandwidth position and the presence of any sub-carrier frequencies are detected through the use of a fast detection method.
Other aspects and advantages of the present invention will become apparent upon reading the following detailed description of the invention in combination with the accompanying drawings.
The following description of the preferred embodiment of the inventive retuning method is not intended to limit the inventive method to this preferred embodiment, but rather to enable any person skilled in the art of audio communication systems to make and use the inventive method.
In
Referring now to
In decision step 140, a decision is made whether or not to reset the selectivity to narrow based whether a sub-carrier frequency component in the signal was detected in detection step 130. If a sub-carrier signal was not detected in detection step 130, then the selectivity is reset from wide selectivity to narrow selectivity in reset step 150. If a digital sub-carrier signal was detected in detection step 140, however, the inventive method skips over reset step 150 such that the selectivity is left set at a wide selectivity and the process continues directly to method step 160, in which the audio output of the receiver is un-muted, and to the final method step 170 in which the retuning process is ended.
The fast detection method used in detection step 130 to detect the presence of multi-carrier signals components may be the method described in patent application [insert serial number for case with attorney docket number 10541-1826] filed [insert filing date], or any one of the other methods that are known in the field of audio receiver systems for achieving this purpose. This method for quickly determining the presence of multiple carrier frequency components includes the steps of creating frequency bins through spectral characterization of at least a portion of the frequency spectrum containing the multiple carrier frequency components creating a series of peaks and valleys, calculating a value for at least one characteristic of the set of peaks, defining a range of values of the characteristic that indicates the presence of multiple carrier components, comparing the calculated characteristic value against the values in the defined range of values and determining that multiple carrier frequency components are present if the calculated value falls within the defined range of values.
Referring now to
In decision step 270, a decision is made whether or not the adjacent channel interference exceeds a certain threshold level. If the adjacent channel interference is determined in decision step 270 to exceed the threshold level, then no multiple sub-carrier detection is attempted because of the noisy or unfavorable conditions and the selectivity remains set to narrow. The method advances to step 160 in which the audio output of the receiver is un-muted and final method step 170 that ends the retuning process.
Alternatively, in decision step 270, if the adjacent channel interference is determined to be below the threshold level, detection of the presence of sub-carrier frequency components in the signal is attempted and the method proceeds to process entry point “A” 110 in the selectivity determination method 100 described above.
The invention is not limited to the embodiments illustrated and described, as it also covers all equivalent implementations insofar as they do not depart form the spirit of the invention. Further, the invention is not yet limited to the combination of features as described herein but may be defined by any other combination of all of the individual features disclosed. Any person skilled in the art of radio frequency receiver systems will recognize from the previous detailed description and from the figures and claims that modifications could be made to the preferred embodiments of the invention without departing from the scope of the invention, which is defined by the following claims.