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Technique for determining a tuning frequency of an audio receiverRelated Patent Categories: Telecommunications, Receiver Or Analog Modulated Signal Frequency Converter, Signal Selection Based On Frequency (e.g., Tuning), Channel Or Station Selection, With Comparing Desired And Existing Tuning ConditionsTechnique for determining a tuning frequency of an audio receiver description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070149154, Technique for determining a tuning frequency of an audio receiver. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATION [0001] This application is a divisional application of U.S. Ser. No. 10/740,332, filed Dec. 18, 2003. TECHNICAL FIELD [0002] The present invention is generally directed to an audio receiver and, more specifically, determining a tuning frequency of an audio receiver. BACKGROUND OF THE INVENTION [0003] Various commercially available after-market devices have been designed to intercept radio frequency (RF) transmissions at an input of an RF receiver and inject a different transmission signal into the RF receiver. In general, these devices use existing RF receivers, e.g. frequency modulation (FM) receivers, to implement additional audio functions, e.g., mobile phones, satellite digital audio radio systems (SDARS) and compact disc (CD) players. [0004] Many of these devices detect a local oscillator (LO) frequency of an FM receiver to determine a current channel of the receiver (tuner). In general, such after-market devices have been inserted between the receiver and its associated antenna. [0005] These devices have typically relied on the presumption that FM receivers use the same tuner design or the same intermediate frequencies. Such devices work relatively well with FM receivers that have sufficient LO leakage such that the intermediate frequencies can be detected. However, there are many FM receivers that suppress LO leakage and/or use different tuner architectures and, in these situations, the currently available after-market devices do not function adequately. In receivers that suppress LO leakage, an operator of the receiver has been required to tune to a fixed frequency of operation for the operation for the after-market device. Unfortunately, this eliminates any ability for an added device to interrupt the current receiver channel, unless a user is on that channel. [0006] What is needed is a device that, when placed between a radio frequency (RF) receiver and its associated antenna, determines a current channel of the receiver. SUMMARY OF THE INVENTION [0007] One embodiment of the present invention is directed to a technique for determining a tuning frequency of an audio receiver. Initially, a received radio frequency (RF) signal at an input of an audio receiver is interrupted. Next, a modulated audio signal is injected into the input of the audio receiver. Then, it is determined whether a demodulated audio signal that corresponds to the modulated audio signal is present at the output of the audio receiver. If not, the frequency of the modulated audio signal is changed until the demodulated audio signal is detected at the output of the audio receiver. [0008] According to another embodiment of the present invention, the modulated audio signal may be initially injected at a last known channel of the audio receiver. According to still another embodiment of the present invention, the modulated audio signal is one of a tone, a sequence of tones, a set of tones and a pseudo random noise (PRN). According to yet another embodiment of the present invention, the received RF signals are interrupted responsive to an audio source. In another embodiment of the present invention, the modulated audio signal is one of an amplitude modulated (AM) signal and a frequency modulated (FM) signal. [0009] According to various embodiments of the present invention, audio content from an audio source is provided at the output of the audio receiver when the demodulated audio signal is present at the output of the audio receiver, thus, indicating that the modulated audio signal is tuned to the current channel of the audio receiver. [0010] According to a different embodiment of the present invention, a modulated audio signal is superimposed on a received radio frequency (RF) signal at an input of an audio receiver. Next, it is determined whether a demodulated audio signal that corresponds to the modulated audio signal is present at the output of the audio receiver. If the audio signal is not present at the output of the audio receiver, the frequency of the modulated audio signal is changed. [0011] These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. BRIEF DESCRIPTION OF THE DRAWINGS [0012] The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: [0013] FIG. 1 is an electrical block diagram of an exemplary audio system and an audio subsystem for implementing the present invention; [0014] FIG. 2 is a flow chart of an exemplary routine for implementing one embodiment of the present invention; and [0015] FIG. 3 is a flow chart of an exemplary routine for implementing another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS [0016] According to one embodiment of the present invention, an audio subsystem is implemented that injects an audio signal between a radio frequency receiver and its associated antenna. When commanded, via for example an additional audio source, the subsystem interrupts a received RF signal and injects an audio signal at the last known frequency, which statistically speeds up tuning, or may step through a sequence of frequencies. The subsystem then looks for the expected audio signal at the output of the audio receiver, e.g., at the speakers of the audio system. If the expected audio signal is not present at the output of the audio receiver, the system changes to the next frequency and repeats until the correct frequency is found. The subsystem may use a microphone (or other device) to detect the audio signal at the output of the audio receiver. It should be appreciated that the known audio signal may be a tone, a sequence of tones, a set of tones, a pseudo random noise (PRN) or other detectable input. [0017] According to another embodiment of the present invention, an audio subsystem continually monitors and tracks a current radio frequency (RF) channel and superimposes an audio signal on top of received RF signals. In general, the superimposed audio signal is low in power, as compared to the received RF signal currently on the channel, such that a listener does not detect the intrusion but the audio signal is detectable at the output of the audio receiver. As with the above-described technique, a known pseudo random noise (PRN) may be superimposed that is unnoticeable to a user, but detectable. For example, if the PRN sequence is long enough, the superimposed signal may readily be detectable. As with the above technique, the subsystem may, in general, step through a series of channels until the audio signal is detected at the output of the audio receiver. [0018] With reference to FIG. 1, an exemplary audio system 100 includes a processor 102 that is coupled to a memory subsystem 104, a display 120, an AM/FM tuner 124 and a plurality of audio sources 130. The processor 102 controls audio content provided to a listener, via a the speaker 112, and may also supply various information to a user, via the display 120 and/or the speaker 112. As used herein, the term processor may include a general purpose processor, a microcontroller (i.e., an execution unit with memory, etc., integrated within a single integrated circuit), an application specific integrated circuit (ASIC), a programmable logic device (PLD) or a digital signal processor (DSP). The memory subsystem 104 includes an application appropriate amount of memory (e.g., volatile and non-volatile memory), which provides storage for various routines that are used to tune the tuner AM/FM 124 and may also provide a storage area for one or more speech recognition applications. 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