Patent Application
20070032217
This invention relates in general to receivers and more specifically to techniques and apparatus in receivers that are arranged and constructed for receiving radio data system signals.
The Radio Data System (RDS) is used to broadcast information together with Frequency Modulated (FM) radio signals for automobile radios as well as home based FM receivers. The FM broadcast signal with the embedded RDS signal is known as a multiplex (MPX) signal. This signal includes information such as program identification including type of program (news, music, etc.), traffic information, title of a song, artist, and the like. In some automotive radios, the radio can switch to another station with the same programming when a given signal deteriorates. The RDS signal may also be accompanied by a Motorist Information System (referred to commonly as ARI) signal. Both the RDS and ARI signals are relatively narrowband signals spaced at 57 KHz (see
Various problems (interference or anomalies) have been observed in radios using RDS. For example, when a signal rapidly deteriorates the user may be presented with low quality information due to the weak signal conditions. It is important that the RDS minimizes the occurrence of low quality information. In those instances when the FM signal combined with noise results in an amplitude modulated (AM) signal that exceeds 100% modulation, large impulse noise components may be introduced into the RDS signal after FM demodulation and thus interfere with proper demodulation of the RDS signal. Normally the RDS signal is a suppressed carrier signal, however some broadcasters do not observe this convention and broadcast an MPX signal with an unsuppressed subcarrier. This results in another form of interference in attempts to properly demodulate the RDS signal.
The accompanying figures where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention.
In overview, the present disclosure concerns receivers, and more specifically techniques and apparatus for use in a receiver arranged and configured to demodulate signals including embedded data signals, e.g. a radio data system (RDS) signal, in order to mitigate various forms of interference or other anomalies that may be associated with such signals and corresponding demodulation systems. More particularly various inventive concepts and principles embodied in methods and apparatus, e.g., receivers, radio data systems, demodulation systems, integrated circuits, and the like for receiving, demodulating, decoding, etc. data signals, such as RDS signals, while mitigating interference, will be discussed and disclosed.
The apparatus in various embodiments of particular interest may be or include receivers or the like for receiving and otherwise processing broadcast Frequency Modulated (FM) signals or similar signals that comprises the normal broadcast signal together with a data signal. These receivers may be employed in various transportation vehicles, such as automobiles, trucks, or similar vehicles as well as other forms of equipment such as construction or agricultural equipment and the like. These receivers may be found in various forms of entertainment equipment, including portable and home based receivers and the like. Such receivers or the data system portion thereof may be subject to loss of signal and various forms of interference or out of specification data signals. Systems, equipment and devices constructed and operating to receive multiplexed signals including decoding data signals, e.g., RDS signals, may advantageously utilize one or more of the methods and apparatus described below when practiced in accordance with the inventive concepts and principles as taught herein.
The instant disclosure is provided to further explain in an enabling fashion the best modes, at the time of the application, of making and using various embodiments in accordance with the present invention. The disclosure is further offered to enhance an understanding and appreciation for the inventive principles and advantages thereof, rather than to limit in any manner the invention. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.
It is further understood that the use of relational terms, if any, such as first and second, top and bottom, and the like are used solely to distinguish one from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions.
Much of the inventive functionality and many of the inventive principles are best implemented with or in integrated circuits (ICs) including possibly application specific ICs or ICs with integrated processing controlled by embedded software or firmware. It is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation. Therefore, in the interest of brevity and minimization of any risk of obscuring the principles and concepts according to the present invention, further discussion of such software and ICs, if any, will be limited to the essentials with respect to the principles and concepts of the various embodiments.
Referring to
The IF signal is coupled to an IF and analog to digital converter (A/D) function 107. The IF and A/D function are also known. The IF portion operates to attenuate all signals other than the desired signal centered at the IF frequency, e.g., 10.8 MHz, amplify the desired signal, and down convert the desired signal from the IF frequency to a base band (near zero) frequency. The A/D converts the base band signal from an analog format to a digital format and this digital signal is provided at the output 109 of the IF function. In various exemplary embodiments this digital signal may be a multiplexed signal (i.e., FM broadcast signal along with an RDS signal) and comprises 24 bit complex samples at a rate of 480 thousand samples per second (KS/s).
This digital signal at output 109 is coupled to a baseband processing unit 111. Much of all of the baseband processing unit can be implemented in an integrated circuit form comprising hardware or hardware together with some form of a known processor (digital signal processor, reduced instruction set processor, or the like) executing firmware and performing numerical processing on the samples of the signal at output 109. The base band processing unit 111 includes an FM demodulator 113 for demodulating the programming portion of the multiplex signal as well as an audio processing block 115 for various audio processing. The output signal(s) from the audio processing block 115 is passed at 117 to digital to analog converters, then to audio amplifiers and from there to speakers or the like (not specifically shown). The FM demodulator and audio processing are known functions that are not relevant to the present disclosure and thus will not be further discussed.
The base band processing unit 111 also includes a data demodulator, e.g., RDS demodulator or demodulation system 119. This system in one or more embodiments is coupled to the FM demodulator and receives a multiplex signal at 240 KS/s where the samples are 20 bits, demodulates this signal and provides a clock and a data signal, e.g., RDS signal data (outputs 121, 123 respectively) to a decoder 125. The RDS demodulation system also provides in one or more embodiments a signal strength indication, e.g., RDS signal strength, at output 126. A more detailed discussion of the demodulator or demodulation system 119 is provided below with reference to
The decoder is configured to decode the RDS signal data in accordance with the appropriate radio data standard, e.g., known RDS standards, to provide decoded signals corresponding to information that was embedded in the RDS signal. The decoded signals or data is coupled to a display driver 127 and used to drive a display 129. Note that the decoded signals or data typically comprises information for user consumption, where this information may be displayed to a user or perhaps otherwise used to control some function of the receiver (for example, control channel scanning looking for a particular station name or for particular programming).
This RDS signal level indication is coupled to a controller 133 and used by the controller 133 to interrupt user consumption of information that may be decoded when the RDS signal level is not satisfactory, i.e., when the RDS signal level is low implying low quality or low confidence in the decoded data. For example, when the level is too low, the controller may operate via the path 135 to the display driver to either blank the display or alternative freeze the display. This avoids presenting the user with unreliable and likely erroneous data. The particular value for RDS signal level indication that is deemed appropriate may be experimentally determined and may vary depending on whether the level is used to control display updates or decide to switch to another station with the same programming.
Referring to
A radio data system signal is represented by the spectra 211 with a suppressed subcarrier 213 located at 57 KHz. Note that the spectra 211 may contain an RDS as well as an ARI signal (Autofahrer-Rundfunk-Information-System referred to usually as a Motorist Information System in the United States). The ARI signal component when present is a narrowband amplitude modulated signal with a carrier frequency of 57 KHz while the RDS signal is a binary signal that consists of a continuous binary data stream with a bit rate of 1.1875 K bits/s and a bandwidth generally limited to +/−2.4 KHz of the 57 KHz carrier. The RDS signal is a suppressed carrier signal where the suppressed carrier is phase shifted by 90 degrees relative to the ARI carrier, thereby minimizing interference between the RDS and ARI components.
Note that the relative amplitudes and bandwidths shown in
Referring to
The outputs (I and Q) from the quadrature mixer 303 are coupled to a low pass filter 305 that in certain embodiments has a cutoff frequency around 24 KHz. It is noted that while
The signal level detector 311 is configured to provide an indication corresponding to a level of the RDS signal. The signal level detector essentially takes the average of the sum of the squares of the I and Q components of the RDS signal and provides an RDS strength indication 313 (corresponds to RDS strength 126 in
Therefore as will be further discussed, when a demodulator 315 that is configured to demodulate the RDS signal and provide RDS data, where the RDS data corresponds to information for user consumption (see
The demodulation system 300 also in some embodiments includes a second filter 317 (FIR) that is coupled to the RDS signal at the first sample rate and in some embodiments has a cutoff frequency near or less than 6 KHz. The second filter is further series coupled to a second down sampler 319, e.g., that down samples by a factor of four (4), i.e., discards 3 of 4 samples of the RDS signal at the first sample rate. The down sampled 319 thus provides the RDS signal at a second sample rate, e.g., 12 KS/s, and the demodulator 315 is configured to demodulate the RDS signal at the second sample rate, e.g., in these embodiments 12 KS/s.
In some exemplary embodiments, the demodulation system 300 further comprises a gain stage 321 that adjusts the gain of the RDS signal, specifically the RDS signal at the second sample rate, and a blanker 323 that is coupled to the RDS signal (e.g., from the gain stage 321). The blanker 323 is configured and arranged to remove impulse noise from the RDS signal prior to the demodulator demodulating the RDS signal. Note that in those embodiments where the RDS signal is a digital signal, the blanker can operate to set a predetermined number of bits (e.g., 3 most significant bits) in each sample of the digital signal to a predetermined value (e.g., set to 0), thereby removing impulse noise from the RDS signal. Impulse noise may result from excess (100% or more) AM modulation of the FM envelope due, for example, to strong noise, and the resultant phase jump (180 degree phase shift) will produce large spikes in the RDS signal. By removing the most significant bits, the impulse noise can likewise be removed or reduced enough to avoid destroying or masking the RDS signal. As is further described below with reference to
In one or more exemplary embodiments, the demodulation system 300 further comprises a subcarrier detector 325 and a switch 327. The subcarrier detector is coupled to the RDS signal, e.g., prior to or ahead of the blanker 323, and is configured to control the switch to alternatively couple the RDS signal (at or prior to input to blanker) and the RDS signal with the impulse noise removed (at or after output from blanker) to the demodulator 315. The subcarrier detector 325 in various embodiments detects an unsuppressed subcarrier, e.g. a carrier for the RDS signal that has not been suppressed. When the unsuppressed subcarrier is detected, the switch 327 is controlled, e.g., by the output from the detector 325, to couple the RDS signal to the demodulator and otherwise to couple the RDS signal with the impulse noise removed to the demodulator. Thus, in the presence of an unsuppressed RDS subcarrier, when the blanker might otherwise “blank” or eliminate the RDS signal, the blanker is effectively disabled, i.e., the blanker is bypassed thereby preserving the embedded or underlying RDS data.
The demodulator is generally known and includes a matched filter that is configured to essentially provide a complementary (mirror image) response to whatever response, vis-à-vis channel that the RDS signal was subjected to in transmission and receiving processes. After the matched filter the RDS signal is coupled to is an AGC system that compensates for or normalizes the RDS signal to a known level. The RDS signal is then coupled to a phase locked loop (PLL) demodulator that is used to detect frequency variations of the RDS signal (i.e., the data that was modulated onto the RDS Carrier). As is known the output of a loop filter portion of the PLL is a good indication of these frequency variations and thus RDS data. A known clock recovery scheme is then used to determine and recover bit or clock transitions. The RDS data and clock are provided at outputs 329, 331, respectively.
The demodulation system 300 in certain embodiments further comprises or may be viewed as further comprising a decoder (see
It is noted that
The Radio Data System can additionally comprise in one or more embodiments the subcarrier detector 325 coupled to the radio data signal and the switch 327 coupled to the RDS signal and the RDS signal without impulse noise. The subcarrier detector is configured to control the switch to alternatively couple the RDS signal and the RDS signal without impulse noise to the demodulator. As will be further discussed below with reference to
The Radio Data System often also comprises a decoder to decode the data from the demodulator 315 to provide RDS data or decoded data and a display driver coupled to the RDS data (see
Referring to
Referring to
Thus when the blanker performs a left shift, i.e., exercises the left shift control etc., the contents of the shift register are shifted to the left, a zero is input at the right end or LSB position of the shift register, and the MSB is shifted out the left end of the register and thus discarded. If this left shift is followed by a right shift, i.e., the right shift control 509 is exercised, etc., the MSB will be loaded with a zero and the zero that was in the LSB position is shifted out and discarded. After the left shift followed by the right shift has occurred the contents of the shift register are the same as before the shifting operations, other than the MSB has been set to zero. It will be appreciated that two shifts to the left followed by two to the right would set the two MSB positions to zero. Alternative implementations of the blanker can include merely loading the least significant bits of the sample into a register, i.e., use a register that is smaller than the sample width and simply discard the predetermined number of bits that would otherwise be set to zero or the like.
Referring to
In more detail, the method 600 starts at 601 followed by providing or receiving a multiplex signal 603. The multiplex signal is down converted 605 via a single step mixing process in one or more embodiments. The resultant down converted multiplex signal in various embodiments is a digital signal, which at 607 is low pass filtered and down sampled. The resultant RDS signal is applied to a detector where the signal level is detected and an indication thereof is provided 609. The RDS signal is further filtered and down sampled 611. Then 613 is a process for removing impulse noise from the RDS signal and in one or more embodiments this amounts to setting a predetermined number of the most significant bits of each sample of the RDS signal to some predetermined value, e.g., zero. This may be accomplished by performing a plurality of shifts on the sample, e.g., one left shift followed by a right shift for each of the predetermined number of most significant bits. For example to set 3 bits equal to zero can be accomplished by 3 left shifts followed by 3 right shifts. Note that shifts may be performed in the opposite direction, i.e., right shift followed by left shift, if the most significant bits are stored toward the right end of the shift register.
In some embodiments 615 is performed to detect any unsuppressed subcarrier and when an unsuppressed subcarrier is detected, selecting the RDS signal rather than the RDS signal with the impulse noise removed is performed 617. The RDS signal or RDS signal without impulse noise (depending on result at 617) is then demodulated 619 and decoded 621 to provide data corresponding to the RDS signal in a form (visual display) suitable for user consumption 623. Note that providing this data may be interrupted if the indication of the signal level from 609 is not satisfactory. The method 600 ends at 625 but is continuously repeated as needed.
The processes, apparatus, and systems, discussed above, and the inventive principles thereof are intended to and can alleviate various forms of interference and other anomalous issues in Radio Data Systems in, e.g., FM Broadcast systems for automotive or home entertainment systems. Using these principles of eliminating impulse noise when appropriate and rapidly determining signal level and conditioning availability of RDS data on quality of that data can quickly enhance user satisfaction with relatively minimal costs and the like.
This disclosure is intended to explain how to fashion and use various embodiments in accordance with the invention rather than to limit the true, intended, and fair scope and spirit thereof. The foregoing description is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications or variations are possible in light of the above teachings. The embodiment(s) was chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims, as may be amended during the pendency of this application for patent, and all equivalents thereof, when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.
