Method of controlling RDS receiver

Abstract: To permit a user to listen to a broadcasting program of the same network in good receiving conditions when a multipath occurs on an RDS broadcasting wave that is being received at an intermediate field intensity, it is judged whether or not a lock detection signal is produced from a lock detector every unit time during reception of a broadcasting wave; and when the frequency of judgement in a set time that no lock detection signal is produced is greater than a reference value, frequency data of one station of that group of network stations to which a broadcasting station of the receiving broadcasting wave belongs is read out from a memory and the reception frequency is switched to the frequency indicated by the current frequency data read out. To ensure quick transition to the reception of the RDS broadcasting wave that can assure good receiving conditions even when a vehicle is driving in a tunnel or a mountainous area, an average level of a reception signal level of a currently receiving broadcasting wave may be calculated in a predetermined time at every given timing, and when a previous average level is higher than a first predetermined value and a current average level is lower than a second predetermined value smaller than the first predetermined value, or when the previous average level is lower than the second predetermined value and the current average level is higher than the first predetermined value, frequency data of one station of that network station group to which a broadcasting station broadcasting the currently receiving broadcasting wave belongs is read out from the memory, and the reception frequency is switched to the frequency indicated by the read out frequency data. Further, to ensure quick transition to the reception of the RDS broadcasting wave that can assure good receiving conditions even when the reception of the currently receiving RDS broadcasting wave becomes poor, when such poor reception of the currently receiving RDS broadcasting wave occurs, those pieces of frequency data which can assure a field intensity equal to or higher than a set level are first extracted from a frequency data list stored in the memory, then that piece of the extracted frequency data which has the highest reception signal level is selected, and the reception frequency is switched to the frequency indicated by the selected frequency data.

Description: The present invention relates to a method of controlling receivers of a radio data system (hereinafter called "RDS receivers"). There is a radio data system (RDS) which can provide radio listeners with its services by transmitting broadcasting information, such as information about the contents of programs, as data in a multiplexed modulation form at the time a broadcasting station broadcasts the programs, and permitting the listeners to select the desired program based on data acquired by demodulating the transmitted data on the receiver side. This radio data system uses, as a subcarrier, a 57-kHz wave, that is the third harmonic of a stereo pilot signal of 19 kHz, outside of the frequency band of FM modulation waves, subjects this subcarrier to amplitude modulation with a data signal representing broadcasting information, such as filtered and bi-phase-coded contents of programs, to yield a radio data signal, and subjects this amplitude-modulated subcarrier to frequency modulation into a main carrier before broadcasting it. As is apparent from Fig. 1 illustrating the baseband coding structure of a radio data signal, the radio data signal is repeatedly transmitted in a multiplexed form with 104 bits as one group. One group consists of four 26-bit blocks, and each block consists of a 16-bit information word and a 10-bit check word. Referring to Fig. 2, a block 1 includes a program identification (PI) code representing a network, a block 2 includes a traffic program identification (TP) code and a traffic announcement identification (TA) code, a block 3 includes frequency (AF) data of a group of network stations that are broadcasting the same program, and a block 4 includes program service name information (PS), such as a broadcasting station name and a network name. Each group is classified into one of 16 types, namely types 0 to 15, by four bits in accordance with the contents of that group, with two versions, A and B, defined for each type (0-15). These type and version identification codes are located in the block 2. It is to be noted that the AF data of a network station is transmitted only in a type A group, while the PS data is transmitted in type A and B groups. With the use of an on-vehicle receiver, the reception of a broadcasting wave may become poorer as the vehicle travels. Since receiving one RDS broadcast can yield AF data of a group of network stations that are broadcasting the same program, there is a strong demand for the function to permit radio listeners to listen to the broadcasted program from the station in the same network group with the best receiving conditions utilizing the AF data. To fulfill the demand, a system capable of following up the same program in different channels has been developed as disclosed in Japanese Patent Application Provisional Publication No. 64-55916. According to this program follow-up system, AF data of a group of network stations that are broadcasting the same program is previously stored in a memory, and when the field intensity of the received RDS broadcasting wave drops to a set level or below for a predetermined period of time or longer, the AF data of another station in the network station group broadcasting the same program is selectively read out from the memory to actually receive the program at the frequency specified by that AF data, and when an RDS broadcasting wave having the field intensity equal to or higher than the set level can be received, the reception of this RDS broadcasting wave would immediately take place. However, the field intensity does not drop to or below a set level when a multipath occurs with respect to an RDS broadcasting wave that is being received with a field of medium to high intensity. In this case, therefore, even if the reception of an RDS broadcasting wave is poor, it is not possible to receive the same RDS broadcasting wave from a different station in good receiving conditions using the AF data of a group of network stations that are broadcasting the same program. There may be a case where, in a tunnel or the like, reception of a broadcasting wave is possible only from a specific RDS broadcasting station in a group of network stations that are broadcasting the same program. When a vehicle actually enters a tunnel, the radio waves it has been receiving from some RDS broadcasting station can hardly be received further. When that happens, the prior art system starts the program follow-up operation. When the reception signal level of the RDS broadcasting wave receivable in a tunnel is lower than a set level, however, the program follow-up operation will always be performed, and during a period in which the reception frequency varies by that program follow-up operation, the program which has been listened to will be abruptly cut off, becoming very disagreeable to the ears. As a counter-measure to this shortcoming, when the reception signal level is equal to or below a set level, the program follow-up operation will be interrupted for a given period of time for any RDS broadcasting wave of the frequency received by the AF data of the same network station group stored in the memory. If the program follow-up operation is to be interrupted for a given period of time, however, even when the vehicle comes out of the tunnel to be able to receive another RDS broadcasting wave bearing the same program at a sufficient signal level, the program follow-up operation may not start immediately, resuming the poor wave reception. This could happen not only in driving through tunnels but also in driving in mountainous areas. Further, according to the above control method, unless the reception signal level of the RDS broadcasting wave received by the AF data read out from the memory is simply equal to or below a set level, the reception of this RDS broadcasting wave would immediately take place, and no further AF data will be read out from the memory to detect the reception signal levels of other RDS broadcasting waves. Therefore, the reception of that RDS broadcasting wave which can ensure the best receiving conditions will not necessarily take place. EP-A-0326747 concerns an RDS receiver and the problem of making a judgement as to whether the received signal strength is too low. A measurement is made of the length of time that the lock signal is not produced. It is therefore an object of the present invention to provide a method of controlling an RDS receiver capable of permitting a listener to listen to a broadcasting program of the same network in good receiving conditions when a multipath occurs with respect to an RDS broadcasting wave that is being received with a field of medium to high intensity. It is another object of the present invention to provide a method of controlling an RDS receiver which can allow for quick transition to reception of an RDS broadcasting wave that can be received in good receiving conditions even in driving through tunnels or driving in mountainous areas. It is a further object of the present invention to provide a method of controlling an RDS receiver which can permit quick transition to reception of an RDS broadcasting wave that can be received in good receiving conditions when the receiving conditions for the currently receiving RDS broadcasting wave become poor. According to the present invention there is provided a method of controlling an RDS receiver having a memory holding frequency data of a group of stations in the same network, and a lock detector for detecting a lock status of a digital PLL circuit for producing a clock for demodulation of a radio data signal extracted from an FM detection output and producing a lock detection signal, the method characterized by: a first step of judging whether or not the lock detection signal is produced every unit time during reception of a broadcasting wave; a second step of incrementing a count value of a counter by "1" when the frequency of judgement in the first step that the lock detection signal is produced within a first set time is smaller than a first predetermined value, keeping the count value of the counter when the frequency of judgement is greater than the first predetermined value and is equal to or lower than a second predetermined value larger than the first predetermined value, and resetting the count value of the counter to an initial value when the frequency of judgement is greater than the second predetermined value; a third step of selectively reading out frequency data of one station of that group of network stations to which a broadcasting station of the receiving broadcasting wave belongs from the memory when the count value of the counter in a second set time is greater than a reference value; and a fourth step of switching a reception frequency to a frequency indicated by the frequency data read out in the third step. According to the above control method of the present invention, whether the digital PLL circuit that produces a data demodulation clock signal is in a lock state is determined from the detection output of the lock detector, the status of multipath-induced audible noise at the time of reception of an RDS broadcasting wave with an electric field of an intermediate intensity is judged from the unlock status of the PLL circuit, and when the reception of the currently receiving broadcasting wave becomes poor by a multipath, making it difficult to listen to the program, the reception frequency is automatically switched to the frequency of another station in the same network station group. According to another aspect of the present invention, there is provided a method of controlling an RDS receiver having a memory containing frequency data of a group of stations in the same network, comprising: a first step of detecting an average level of a reception signal level of a currently receiving broadcasting wave in a predetermined time at every given timing; and characterized by a second step of producing a frequency check signal when a previous average level is higher than a first predetermined value and a current average level is lower than a second predetermined value smaller than the first predetermined value, or when the previous average level is lower than the second predetermined value and the current average level is higher than the first predetermined value; and a third step of selectively reading out frequency data of one station of that network station group to which a broadcasting station broadcasting the currently receiving broadcasting wave belongs from a memory in accordance with the frequency check signal, and switching a reception frequency to a frequency indicated by the read frequency data. According to this control method, for example, although the previous average level exceeds the first predetermined value due to a drop of the reception signal level of the currently receiving broadcasting wave as a vehicle enters or comes out of a tunnel, if the current average level is below the second predetermined value, frequency data of one station of that network station group to which a broadcasting station broadcasting the currently receiving broadcasting wave belongs is selectively read out from the memory in accordance with the frequency check signal, and switching the reception frequency to the frequency specified by the read frequency data starts. Accordingly, with regard to the frequency received by the frequency data of the same network station group stored in the memory, when the reception signal level of any RDS broadcasting wave is equal to or lower than a set level, the reception of the currently receiving broadcasting wave continues, and the program follow-up operation for actually receiving another broadcasting wave based on the AF data stored in the memory to detect the reception signal level will not be repeated unless the reception signal level of the currently receiving broadcasting wave substantially varies. Further, although the previous average level is lower than the second predetermined value due to an increase in the reception signal level of the currently receiving broadcasting wave as a vehicle comes out of a tunnel, if the current average level exceeds the first predetermined value, frequency data of one station of that network station group to which a broadcasting station broadcasting the currently receiving broadcasting wave belongs is selectively read out from the memory in accordance with the frequency check signal, and switching the reception frequency to the frequency specified by the read frequency data starts. This feature can permit prompt transition to the reception of an RDS broadcasting wave with the frequency that ensures better reception. According to a preferred embodiment of the present invention, the method further comprises a fifth step of judging whether or not a reception signal level of the reception frequency switched in the fourth step is greater than a set level, and returning to the third and fourth steps after the judgement until all frequency data is read out from the memory; and a sixth step of selecting that piece of frequency data, among all of those pieces of frequency data judged in the fifth step to have reception signal levels greater than the set level, which has a highest reception signal level and switching the reception frequency to a frequency indicated by the selected frequency data. According to this control method, when the receiving conditions of the currently receiving RDS broadcasting wave become poor, only those pieces of AF data which can provide an electric field intensity equal to or higher than a set level are extracted first from a frequency data list stored in the memory, then that piece of AF data whose reception signal level is highest is selected from the read AF data, and the reception frequency is switched to the frequency specified by the selected AF data. When the receiving conditions of the RDS broadcasting wave become poor, it is possible to promptly shift to the reception of the RDS broadcasting wave having the frequency selected from the AF data list, which has an electric field equal to or higher than a set level and can ensure the best reception. Preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings wherein: Fig. 1 is a diagram illustrating the baseband coding structure of a radio data signal; Fig. 2 is a diagram illustrating the format of a type 0A group; Fig. 3 is a structural block diagram showing an RDS receiver to which a control method according to the present invention is applied; Fig. 4 is a flowchart showing a lock detecting routine as a procedure of the control method according to a first embodiment of the present invention; Fig. 5 is a flowchart showing an AF check routine; Fig. 6 is a flowchart showing a lock detecting routine as a procedure of the control method according to a second embodiment of the present invention; Fig. 7 is a flowchart showing an AF check routine; Fig. 8 is a flowchart showing the continuation of the AF check routine in Fig. 7; Fig. 9 is a flowchart showing the continuation of the AF check routine in Fig. 8; Fig. 10 is a flowchart showing the lock detecting routine; Fig. 11 is a flowchart showing a field intensity detecting routine; Fig. 12 is a flowchart showing the AF check routine; and Fig. 13 is a flowchart showing the continuation of the AF check routine in Fig. 12. An RDS receiver, shown in Fig. 3, where the control method of the present invention is applied,is capable of receiving not only an RDS broadcasting wave but also the broadcasting wave of a traffic information broadcasting system as well as a broadcasting wave carrying an RDS signal multiplexed with the signal of the traffic information broadcasting system. The traffic information broadcasting system provides traffic information to car radio listeners, etc. in a time sharing manner while a program of a general broadcasting station is broadcast, and is usually called "ARI system." In this system as well as an RDS system, a subcarrier of 57 kHz is treated as a traffic information station identification (ID) signal (SK signal) which indicates a station for sending the traffic information on the air. When the traffic information is broadcast, the 57-kHz subcarrier is subjected to amplitude modulation by an area ID signal (in the range of 23.75 to 53.98 kHz) indicating for which area the traffic information is, and a message ID signal (a single tone of 125 kHz) which indicates that the traffic information is being broadcast. The amplitude-modulated subcarrier is then subjected to frequency modulation into a main carrier before broadcasting it. As described above, the 57-kHz subcarrier is used in both the RDS and traffic information broadcasting system. In the RDS, therefore, the subcarrier of the RDS has a phase difference of about π/2 from that of the traffic information broadcasting system, so that the subcarrier (RDS signal) amplitude-modulated by the data signal is distinguished from the subcarrier (SDK signal) amplitude-modulated by the message signal (DK signal) of the traffic information broadcasting system. In the RDS receiver of the above system, an FM multiplexed broadcasting wave received at an antenna 1 is sent to a front end 2 where a desired station is selected, and is converted into a wave of an intermediate frequency (IF), which is in turn supplied through an IF amplifier 3 to an FM detector 4. The front end 2 generates a transmission signal to a mixer 2b by a PLL synthesizer system using a PLL circuit 2a including a programmable frequency divider. The front end 2 performs channel selection based on the frequency dividing ratio of the programmable frequency divider controlled by a controller 14 to be described later. The output of the FM detector 4 is supplied to an MPX (multiplex) demodulator 5, and, in the case of a stereophonic broadcast, it is separated into audio signals of R (right) and L (left) channels. The output of the FM detector 4 is sent through a filter 6, so as to extract a 57-kHz subcarrier, that is, a radio data signal amplitude-modulated by a biphase-coded data signal. This subcarrier is then demodulated in a PLL circuit 7. An SDK detector, provided in the PLL circuit 7 though not shown, detects when the RDS and SDK signals are present at the same time. The output of the SDK detector is sent to the controller 14. The circuit disclosed in Japanese Patent Application Provisional Publication No. 63-87052 is used as the PLL circuit 7, for example. The demodulation output of the PLL circuit 7 is supplied to a digital (D) PLL circuit 8 and a decoder 9. Based on the demodulation output from the PLL circuit 7, the D-PLL circuit 8 generates a clock for data demodulation. The generated clock is supplied to a gate circuit 10. A lock detector 11, which may be the circuit disclosed in, for example, Japanese Patent Application Provisional Publication No. 63-87039 or No. 63-87040, generates a lock detection signal of a high level when it detects that the D-PLL circuit 8 is locked, generates an unlock detection signal of a low level when it detects that the D-PLL circuit 8 is unlocked. An output signal from the lock detector 11 is sent to the gate circuit 10 so as to control the gate circuit 10 to be enabled when the D-PLL circuit 8 is unlocked. The output signal of the lock detector 11 is also sent to the controller 14 as well as the PLL circuit 7 and D-PLL circuit 8 as a lock range select signal for selecting the lock ranges of the PLL circuit 7 and the D-PLL circuit 8. When the lock detector 11 detects the lock status of the D-PLL circuit 8, the lock ranges of the PLL circuit 7 and the D-PLL circuit 8 become narrower so that the clock for data demodulation can always be supplied as a stable clock, free of external influences. The biphase-coded data signal as the demodulation output from the PLL circuit 7 is decoded by the decoder 9 in synchronism with the clock generated by the D-PLL circuit 8. The output data from the decoder 9 is separated into groups each consisting of four 26-bit blocks, a total of 104 bits. The output data is sequentially supplied to a group-block synchronizing/error-detecting circuit 12. The circuit 12 performs a group-to-block synchronization on the basis of 10-bit offset words which are respectively assigned to 10-bit check words in the blocks, and detects an error in 16-bit information words on the basis of the check words. The error detected data is corrected in an error correcting circuit 13 at the next stage, and then is supplied to the controller 14. The controller 14, constituted by a microcomputer, fetches code information of each block in radio data which is received group by group, i.e., radio data information (above-described PI code, AF data, PS data, etc.) which is associated with the contents of the program from a broadcasting station currently being received, and stores the information in the memory 15. Through this operation, an AF data list (AF data f 1 , f 2 , ..., f n ) of stations in the same network as the station currently broadcasting the program is prepared. On the basis of a station or channel select instruction from an operation section 16, the controller 14 controls the value of received frequency data to determine the frequency-dividing ratio for the programmable frequency divider (not shown) in the PLL circuit 2a, provided as a part of the front end 2, and selects the desired station or channel. The value of the received frequency data is a count value of a counter, for example. A level detector 19 detects the level (field intensity) of a receive signal on the basis of the level of the IF signal in the IF amplifier 3. A station detector 20 outputs a station detection signal when the level of the IF signal in the IF amplifier 3 is equal to or above a determined or set level and the detection output having a so-called "S" curved characteristic from the FM detector 4 is within a predetermined level range. The output signals from the level detector 19 and the station detector 20 are supplied to the controller 14. The memory 15 includes an nonvolatile RAM where data, such as the reception frequency data, the PI code and the AF data, are to be written, and a ROM where programs and data are previously written. A description will now be given of the operation of a processor in the controller 14 which specifically represents the procedures of the control method according to the first feature of the present invention, referring to the flowcharts in Figs. 4 and 5. When the processor detects that a program follow-up button (not shown) in the operation section 16 has been operated, the processor sets a program follow-up mode. A lock detecting routine shown in Fig. 4 is performed at every predetermined timing in program follow-up mode. In the lock detecting routine, the processor resets a timer value T of the timer and a count value C of the counter to their initial values (e.g., both to "0") (step S101). The timer value T and the count value C are determined by executing steps S101 to S104, etc. The timer value T of the timer is incremented by unit time "T 1 " (step S102), and then it is determined whether or not the lock detection signal is sent from the lock detector 10 (step S103). If the lock detection signal is not generated but the unlock detection signal is generated, the D-PLL " circuit 8 is considered to be in the unlock state by generation of a multipath. The count value C of the counter is thus incremented by "1" (step S104). It is then determined if the timer value T has overflowed (step S105), i.e., whether or not the timer value T has reached a set time T MAX . If the lock detection signal is supplied from the lock detector 10, the process in step S105 is immediately performed. If T < T MAX , the flow returns to step S102. Though not shown in the flowchart, if T < T MAX , the process in step S102 is performed after the lapse of the unit time T 1 is determined. If T ≥ T MAX in step S105, the processor fetches the reception signal level V s received from the level detector 19 (step S106). It is then determined from the output of the SDK detector in the PLL circuit 7 if the received broadcasting wave is an RDS-broadcast and also ARI-broadcast wave by the ARI system (step S107). If the received broadcasting wave is an RDS- and an ARI-broadcast wave, the processor searches a first data map for a reference value C r corresponding to the reception signal level V s , and sets the value C r (step S108). When the received broadcasting wave is RDS-broadcasted only, the processor searches a second data map for the reference value C r , and sets the value C r (step S109). The first and second data maps referred to in searching for the reference value C r are previously stored in the memory 15. The processor then determines if the count value C of the counter is greater than the reference value C r (step S110). If C > C r , the processor considers that the multipath has greatly affected the count value, and executes an AF check routine (step S111). If C ≤ C r , the influence of the multipath is comparatively small, and the processor terminates the lock detecting routine. In the AF check routine, for example, as shown in Fig. 5, the processor reads AF data fAF from an AF data list of AF data f 1 , f 2 ..., f n , which is written into the memory 15, and sets the read data to the frequency divider in the PLL circuit 2a (step S121). Accordingly, the reception frequency changes to the frequency of another station in the same network. The processor determines if the reception signal level V s at the new reception frequency selected with reference to the AF data list is a set level V 1 or lower (step S122). Though not illustrated in the flowchart, the process in step S122 is performed upon the lapse of a time which is sufficient for the reception frequency to become stable from the point of time at which the AF data has been set in step S121. If the reception signal level V s is higher than the set level V 1 , the reception frequency data in the memory 15 is updated to the AF data fAF to maintain the reception condition (step S123). The AF check routine is then terminated. In the next execution of the AF check routine, the frequency of the AF data fAF will become a new reception frequency f D for the program follow-up operation. The processor newly fetches AF data in and rewrites the data f 1 , f 2 , ..., f n on the AF data list in the memory 15. When the reception signal level V s is the set level V 1 or lower, the processor reads the reception frequency data from the memory 15 to receive the original broadcasting wave again, and sets the read data to the frequency divider in the PLL circuit 2a (step S124). It is then determined if the previous broadcasting wave has been received in a predetermined time (step S125). If a predetermined time has elapsed, it is determined whether or not all the AF data on the AF data list has been read out (step S126). For example, if reading of the final AF data f n is completed and all the AF data has thus been read out, the AF check routine will be terminated. If all the AF data on the AF data list has not yet been read out, the flow returns to step S121 to read a new piece of AF data. The processes in the lock detecting routine are repeated at every given timing, and the D-PLL circuit 8 is unlocked by the generation of the multipath in the reception of the RDS broadcasting wave with the frequency f D . If the number (count value of the counter) C of the unlock states, which are detected by determining the lock or unlock state at every unit time T 1 within the set time T MAX , is larger than the reference value C r , the influence of the multipath is considered great. The AF data of the frequency f 1 is then read from the AF data list of the data f 1 , f 2 , ..., f n for the same network, which are written into the memory 15, and sent to the frequency divider of the PLL circuit 2a. As a result, the reception frequency changes to the frequency f 1 of another station in the same network. If the reception signal level V s at the new frequency f 1 is higher than the set level V 1 , the reception conditions are retained. When the reception signal level V s is equal to the set level V 1 or lower, the reception of the broadcasting wave at the previous frequency f D starts. After a predetermined time has elapsed, AF data of the frequency f 2 is read from the AF data list of the data f 1 , f 2 , ..., f n and is sent to the frequency divider of the PLL circuit 2a. If the influence of the multipath is considered great under the reception condition for the broadcasting wave at the frequency f D , the reception frequency changes: f D → f 1 → f D → f 2 .... This frequency change is repeated until detection of such a broadcasting wave that the reception signal level V s of that network station group to which the station for the frequency f D belongs is higher than the set level V 1 . When the AF check routine is executed, providing the RDS broadcasting wave of V s > V 1 from the data f 1 , f 2 , ..., f n on the AF data list of the same network, the lock detecting routine will be performed on the RDS broadcasting wave. If the broadcasting wave whose reception signal level V s is higher than the set level V 1 cannot be detected even when the reception is tried at every frequency, from f 1 to f n , the program follow-up mode is canceled and the reception condition at the frequency f D is maintained. In a period where the reception frequency has changed to a frequency on the AF data list, a muting circuit (not shown) is activated to cut off the output of an audio signal. The muting state is released when the broadcasting wave having the reception signal level V s higher than the set level V 1 is detected. Further, according to the above-described embodiment, when V s ≤ V 1 in step S122, the flow advances to step S124 to receive the previous broadcasting wave. The flow however may return to step S121 when V s ≤ V 1 , to thereby read out a new piece of AF data fAF among the data f 1 , f 2 , ..., f n on the AF data list and sent it to the frequency divider of the PLL circuit 2a. In other words, instead of receiving the previous broadcasting wave again, all the AF data f 1 , f 2 , ..., f n on the AF data list may be checked to detect the RDS broadcasting wave of V s > V 1 , and only when that RDS broadcasting wave cannot be detected, the reception of the previous broadcasting wave should be restarted. Alternatively, all the pieces of the AF data on the AF data list, f 1 , f 2 , ..., f n , may be checked to read the reception signal level of the individual data, and when the RDS broadcasting waves of V s > V 1 are detected, the RDS broadcasting wave having the highest field intensity may be received thereafter. According to this embodiment of the present invention, the frequency data of the same network station group has been stored in the memory by acquiring the AF data from the RDS broadcasting wave being received. The frequency data of the same network station group for each receivable broadcasting station may be previously stored in the memory. According to this embodiment, while the count value C of the counter is incremented by "1" when no lock detection signal is generated, this increment of the count value may be performed when the lock detection signal is generated and the influence of a multipath is considered significant when the count value C in the set time T MAX is smaller than the reference value, followed by the execution of the AF check routine. Fig. 6 illustrates a lock detecting routine according to a second embodiment of this invention. This lock detecting routine, like the one shown in Fig. 4, is executed by the processor in the controller 14 in the receiver shown in Fig. 3. The timer value and count value are to be determined by performing steps described below. In the lock detecting routine, first, the processor resets a timer value T of the timer and a count value C of a lock counter to their initial values (e.g., both to "0") (step S131). The timer value T of the timer is incremented by unit time "T 1 " (step S132), and then it is determined whether or not the lock detection signal is sent from the lock detector 10 (step S133). The steps up to this point are identical to steps S101 to S103 in the routine shown in Fig. 4. When generation of the lock detection signal is determined in step S133, the count value C of the lock counter is incremented by "1" (step S134). It is then determined if the timer value T becomes equal to a first set time T MAX or greater and has overflowed (step S135). If the lock detection signal is not produced and the unlock detection signal is supplied from the lock detector 10, the process in step S135 is immediately performed. If T < T MAX , the flow returns to step S132. Though not shown in the flowchart, if T < T MAX , the process in step S132 is performed after the lapse of the unit time T 1 is determined. After execution of step S135, it is determined whether or not the count value C of the lock counter is greater than a predetermined value C 2 (step S136). When C ≤ C 2 , it is then determined whether or not the count value C of the lock counter is smaller than a predetermined value C 1 (C 2 > C 1 ) (step S137). When C > C 2 , a count value R of an AF trigger counter is reset to an initial value (e.g., "0") (step S138). When C < C 1 , the count value R of the AF trigger counter is incremented by "1" (step S139). After execution of step S138 or S139, the flow advances to step S140. If C 1 ≤ C ≤ C 2 , the flow goes to step S140 while maintaining the count value R of the AF trigger counter at that time. After incrementing a count value U of a set time counter by "1" in step S140, it is determined whether or not the count value U has reached a predetermined value m indicating a second set time U m (step S141). Since step S140 is executed upon each lapse of nearly the time T MAX due to the execution of step S135, m × T MAX becomes the second set time U m . When U < m, this routine will be terminated. When U ≥ m, which means that the second set time U m has elapsed, the count value U of the set time counter is reset to an initial value (e.g., "0") (step S142). Then it is determined whether or not the count value R of the AF trigger counter at that time has reached a reference value R r (step S143). When R ≥ R r , the AF check routine will be executed, considering that the influence of a multipath is prominent (step S144). When R < R r , this routine will be terminated, judging that the influence of the multipath is relatively small. For each unit time T 1 , it is determined whether or not the D-PLL circuit 8 is in a lock state and the number of discriminations C (the count value of the lock counter) of the lock state within the first set time T MAX can be acquired. When C < C 1 , it is judged that the D-PLL circuit 8 has been locked many times by a multipath, and the count value R of the AF trigger counter is incremented by "1." When C > C 2 , the count value R of the AF trigger counter is reset, while when C 1 ≤ C ≤ C 2 , the count value R of the AF trigger counter at that time is maintained. When the count value R of the AF trigger counter has reached the reference value R r within the second set time U m , the influence of the multipath is judged prominent, and the AF check routine shown in Fig. 5 will be executed. As described above, in the RDS receiver control method according to the first embodiment of the present invention, it is determined whether or not the lock detection signal is produced every unit time during reception of a broadcasting wave, the frequency data of one station of that group of network stations to which a broadcasting station of the receiving broadcasting wave belongs is read out from the memory when the number of times it is determined, in a set time,that the lock detection signal is not produced is greater than a reference value, and the reception frequency is switched to the frequency specified by the read frequency data. In the RDS receiver control method according to the second embodiment, it is determined whether or not the lock detection signal is produced every unit time during reception of a broadcasting wave, the count value of the AF trigger counter is incremented by "1" when the number of times it is determined that the lock detection signal is produced within the first set time is smaller than the first predetermined value, the count value of the counter is retained when the number of determinations is greater than the first predetermined value and is equal to or lower than the second predetermined value which is larger than the first predetermined value, the count value of the counter is reset to the initial value when the number of determinations is greater than the second predetermined value, the frequency data of one station of that group of network stations to which a broadcasting station of the receiving broadcasting wave belongs is selectively read out from the memory when the count value of the counter in the second set time is greater than the reference value, and the reception frequency is switched to the frequency specified by the read frequency data. According to the first and second embodiments of the present invention, whether the digital PLL circuit that produces a data demodulation clock is in a lock state is determined from the detection output of the lock detector, and the status of multipath-induced audible noise at the time of reception of an RDS broadcasting wave with an electric field of an intermediate intensity is determined from the frequencies of the unlock status of the PLL circuit. When the reception of the currently receiving broadcasting wave becomes poor by a multipath, making it difficult for listeners to listen to the program, therefore, the reception frequency is automatically switched to the frequency of another station in the same network station group. Accordingly, even in the case where a multipath disturbance occurs, the same program broadcasted by another station in the network station group can continuously be listened to in a good receiving condition. A description will now be given of the operation of a processor in the controller 14 which specifically represents the procedures of the control method according to another embodiment of the present invention, referring to the flowcharts in Figs. 7 through 9. When the processor detects that the program follow-up button (not shown) in the operation section 16 has been operated, the processor sets the program follow-up mode, and the AF check routine shown in Figs. 7 and 8 is performed at every given timing. In the AF check routine, the processor resets the timer value T of the timer to an initial value (e.g., "0") (step S201). An AF check trigger flag F TR and a PI search request flag F RQ are both reset to "0" (step S202). The timer value T is determined by executing steps S201, S204 and S206. After execution of step S202, a variable m is set equal to "1" (step S203), the timer value T of the timer is incremented by unit time "T 1 " (step S204), and the reception signal level V s output from the level detector 19 is latched and stored as a latch value V s (m) in the memory 15 (step S205). It is then determined if the timer value T has overflowed (step S206), i.e., whether or not the timer value T has reached a set time T MAX . If T < T MAX , the variable m is incremented by "1" (step S207) and the flow returns to step S204. Though not shown in the flowchart, if T < T MAX , the process in step S204 is performed after the lapse of the unit time T 1 is determined. If T ≥ T MAX in step S206, the processor computes an average level V AV of the reception signal levels V s (1), V s (2), ..., V s (m) (step S208). That is, the reception signal levels V s (1), V s (2), ..., V s (m) are read out from the memory 15 and are added together to acquire the total value, and this total value is divided by the variable m. After the compution of the average level V AV , this average level V AV is read out as V AV-1 (step S209), and it is determined whether or not the average level V AV is smaller than a predetermined value V L (second predetermined value) (step S210). When V AV < V L , it is determined whether or not the previous average level V AV-1 is greater than a predetermined value V H (first predetermined value) (step S211). When V AV < V L , it is determined whether or not the previous average level V AV-1 is greater than a predetermined value V H (first predetermined value) (step S211). The predetermined value V H is greater than the predetermined value V L . When V AV-1 > V H , the PI search request flag F RQ is set to "1" (step S212), the AF check trigger flag F TR is set to "1" (step S213), and the current average level V AV is stored in the memory 15 (step S214). Even if V AV < V L , when V AV-1 ≤ V H is determined in step S211, the flow advances to step S214. Setting the flag F TR to "1" generates the frequency check signal. When V AV ≥ V L in step S210, it is determined whether or not the average level V AV is greater than the predetermined value V H (step S215). When V AV > V H , it is determined whether or not the previous average level V AV-1 is smaller than the predetermined value V L (step S216). When V AV-1 < V L , the flow returns to step S213 to set the AF check trigger flag F TR to "1". Even if V AV ≥ V L , when V AV ≤ V H is determined in step S215, the flow moves to step S214. Further, even if V L ≤ V AV ≤ V H , when V AV-1 ≥ V L is determined in step S216, the flow also advances to step S214. After execution of step S214, it is determined whether or not the AF check trigger flag F TR is equal to "1" (step S217). When F TR = 1, a variable u is set equal to "1" (step S219), and AF data f (u) from the AF data list of AF data f (1) , f (2) , ..., f (n) , which is written into the memory 15, is read out and is set to the frequency divider in the PLL circuit 2a (step S220). Accordingly, the reception frequency changes to the frequency of another station in the same network as the current broadcasting station (which is a broadcasting station whose broadcasting wave has been received immediately before the mode change to the program follow-up mode). Then, the processor reads the lock signal V s at the new reception frequency selected by one piece of AF data f (m) on the AF data list (step S221), and determines if V s is greater than a set level V 1 (step S222). Though not illustrated in the flowchart, the process in step S221 is performed upon the lapse of a time sufficient for the reception frequency to become stable from the point of time at which the AF data has been set in step S220. When V s ≤ V 1 , it is determined whether or not the variable u has reached the number of pieces of the AF data, n, on the AF data list (step S223). When u < n, the variable u is incremented by "1" (step S224) and the flow returns to step S220. As this operation is repeated, AF data that can ensure the reception signal level V s > V 1 can be selected from the individual pieces of AF data f (1) , f (2) , ..., f (n) on the AF data list. In other words, when V s > V 1 in step S222, the reception frequency data in the memory 15 is updated to the AF data f (u) to maintain the reception conditions (step S225) before terminating this routine. Consequently, the broadcasting wave will be received at the frequency specified by the AF data f (u) That is, the same program as the currently receiving broadcasting wave which has been received immediately before the mode change to the program follow-up mode, is broadcasted, that broadcasting wave whose reception signal level is higher than the set level V 1 is received, that broadcasting station employing this frequency becomes a new current broadcasting station, and the program follow-up mode is terminated. When u = n, which means that checking the reception signal level for every piece of the AF data f (1) , f (2) , ..., f (n) has been completed in step S222, the flow advances to step S226 to be described below. It is determined in step S226 whether or not the PI search request flag F RQ is equal to 1. If F RQ = 0, the reception signal level V s > V 1 is not provided by any other broadcasting station specified by the AF data list, which is broadcasting the same program as the current broadcasting station, and the PI search is not required. The reception frequency data in the memory 15 is therefore read out and is set to the frequency divider in the PLL circuit 2a to receive again the currently receiving broadcasting wave, which has been received immediately before the mode change to the program follow-up mode (step S227). The reception conditions become those under which that broadcasting wave which has been received immediately before the mode change to the program follow-up mode. This routine is then terminated to shift the mode from the program follow-up mode to the usual reception mode. When F RQ = 1, the PI search operation starts. The reception frequency data of the current broadcasting station is read out as search frequency data from the memory 15 (step S229). The value of the search frequency data is then increased by a predetermined number (e.g., 100 kHz as the reception frequency) (step S230). When the value of the search frequency data is above a value corresponding to the upper frequency limit within the reception band, the data value then becomes equal to a value corresponding to the lower frequency limit. It is determined if the search frequency data is equal to the reception frequency data of the current broadcasting station (step S231). When the former frequency data is not equal to the latter, the search frequency data is set to the frequency divider in the PLL circuit 2a (step S232). It is then determined whether or not a station detecting signal is generated from the station detector 20 (step S233). Though not shown in the flowchart, step S233 is executed upon the lapse of a time sufficient for the reception frequency to become stable after the setting of the AF data in step S232. When the station detecting signal is generated, the reception signal level V s is latched (step S234), and it is determined whether or not V s is higher than the set level V 1 (step S235). If V s > V 1 , it is determined if the searched and received broadcasting wave is an RDS broadcasting wave (step S236). This determination is made by checking whether or not various data can be obtained from the error correcting circuit 12. If the searched and received broadcasting wave is the RDS broadcasting wave, the PI code of the received broadcasting wave is latched (step S237), and the PI code of the current broadcasting station is read from the memory 15 (step S238). It is then determined if both PI codes of the searched and received broadcasting wave and the current broadcasting station coincide with each other (step S239). If both PI codes are matched with each other, the reception frequency data in the memory 15 is updated in accordance with the searched frequency data to maintain the reception condition (step S240) before this routine is terminated. The broadcasting wave is therefore received at the frequency determined by the searched frequency data. In other words, that broadcasting wave bearing the same program as the currently receiving broadcasting wave that has been received immediately before the mode shift to the program follow-up mode, and having a reception signal level higher than the set level V 1 , is received, and the station employing that frequency becomes a new current broadcasting station. The program follow-up mode is ended together with the search operation. When it is determined in step S233 that the station detecting signal has not been produced, or when V s ≤ V 1 in step S235, or when it is determined in step S236 that the received broadcasting wave is not an RDS broadcasting wave, or when both PI codes are not judged to coincide with each other in step S239, the flow advances to step S230 where the value of the search frequency data is increased by a predetermined number. Thereafter, the above-described operation will be repeated. When it is determined in step S231 that the search frequency data is identical to the reception frequency data of the current broadcasting station, the RDS broadcasting wave whose PI code is matched with that of the current broadcasting station at the reception signal level of V s > V 1 cannot be received, although complete searching within the reception band has been finished. The flow therefore moves to step S227, and the reception frequency data is read out from the memory 15 and is set to the frequency divider in the PLL circuit 2a in order to try receiving that broadcasting wave which has been received immediately before the mode change to the program follow-up mode. In a period where the program follow-up mode is set and the reception frequency has changed to the frequency specified by a frequency data on the AF data list other than the frequency of the current broadcasting station or the frequency specified by the search frequency data, a muting circuit (not shown) is activated to cut off the output of an audio signal. The muting state is released when the program follow-up mode is terminated. According to the above-described embodiment of the present invention, when the reception signal level V s is determined higher than the set level V 1 based on one piece of AF data on the AF data list, the operational transition immediately takes place to be ready for reception of the broadcasting wave for that piece of AF data, thus terminating the program follow-up mode. It is however possible that the reception signal levels V s for all the pieces of AF data on the AF data list are acquired, then that piece of AF data which has the highest reception signal level V s greater than the set level V 1 is extracted, and the operational transition to the reception of the broadcasting wave for that AF data takes place, thus terminating the program follow-up mode. Further, although the frequency data of the same network station group has been stored in the memory by acquiring the AF data from the RDS broadcasting wave being received according to this embodiment, the frequency data of the same network station group for each receivable broadcasting station may be previously stored in the memory. As described above, in the RDS receiver control method according to this embodiment of the present invention, the average level of the reception signal level of a currently receiving broadcasting wave in a predetermined time is detected at every given timing, and when the previous average level is higher than the first predetermined value and the current average level is lower than the second predetermined value smaller than the first predetermined value, or when the previous average level is lower than the second predetermined value and the current average level is higher than the first predetermined value, frequency data of one station of that network station group to which a broadcasting station broadcasting the currently receiving broadcasting wave belongs is selectively read out from the memory and the reception frequency is switched to the frequency specified by one piece of the read frequency data. Unless the reception signal level of the currently receiving broadcasting wave significantly changes, therefore, the program follow-up operation for actually receiving another broadcasting wave based on the frequency data stored in the memory to detect the reception signal level would not be performed. In other words, since the program follow-up operation is not repeated even if the reception signal level of the currently receiving broadcasting wave is kept lower than the set level, the reception frequency does not change, thus preventing the program from becoming very difficult to listen to. Even when a vehicle is running in a tunnel or a mountainous area, the program follow-up operation will start at the proper timing, permitting prompt transition to the reception of that RDS broadcasting wave which can ensure good reception. A description will now be given of the operation of a processor in the controller 14 which specifically represents the procedures of the control method according to a further feature of the present invention, referring to the flowcharts in Figs. 10 through 13. When the processor detects that the program follow-up button (not shown) in the operation section 16 has been operated, the processor sets a program follow-up mode. A lock detecting routine and a field intensity detecting routine, as shown respectively in Figs. 10 and 11, are performed at every predetermined timing in this program follow-up mode. In the lock detecting routine, first, the processor resets the timer value T of the timer and the count value C of the counter to their initial values (e.g. both to "0") (step S301). The timer value T and the count value C are determined by executing steps S301 to S304, etc. The timer value T of the timer is incremented by unit time "T 1 " (step S302), and then it is determined whether or not the lock detection signal is sent from the lock detector 10 (step S303). If the lock detection signal is not generated but the unlock detection signal is generated, the D-PLL circuit 8 is judged to be in the unlock state due to the occurence of a multipath. The count value C of the counter is thus incremented by "1" (step S304). It is then determined if the timer value T has overflowed (step S305). That is, it is determined whether or not the timer value T has reached a set time T MAX . If the lock detection signal is supplied from the lock detector 10, the process in step S305 is immediately performed. If T < T MAX , the flow returns to step S302. Though not shown in the flowchart, if T < T MAX , the process in step S302 is performed after the lapse of the unit time T 1 is determined. If T ≥ T MAX in step S305, the processor reads the reception signal level V s received from the level detector 19 (step S306), and searches a data map for the reference value C r corresponding to the reception signal level V s , and sets the value C r (step S307). The data map referred to in searching for the reference value C r is previously stored in the memory 15. The processor then determines if the count value C of the counter is greater than the reference value C r (step S308). If C > C r , the processor sets a flag F 1 to "1", indicating that the unlock state of the D-PLL circuit 8 has frequently occurred and the influence of a multipath is significant (step S309), and executes an AF check routine (step S310). If C ≤ C r , the influence of the multipath is relatively small, and the processor resets the flag F 1 to "0" (step S311), thus terminating this lock detecting routine. The setting of the flag F 1 to "1" generates a first frequency check signal. In the field intensity detecting routine, the processor latches the reception signal level V s output from the level detector 19 (step S321), and determines whether or not V s is smaller than a set level V r (step S322). When V s < V r , it is then determined if a low-level (low field intensity) reception has continued for a predetermined time t 1 or longer (step S323). When the low-level reception of V s < V r has continued for the predetermined time t 1 or longer, the processor sets a flag F 2 to "1", indicating that the currently receiving broadcasting wave is in low-level receiving condition (step S324), and executes the AF check routine (step S325). When V s ≥ V r , which means no low-level reception, the processor resets the flag F 2 to "0" (step S326), thus terminating this routine. In the AF check routine, as shown in Fig. 12, the processor first determines if the flag F 1 is equal to "1" or not (step S331). When F 1 = 1, which indicates that the unlock state of the D-PLL circuit 8 has frequently occurred, the set level V 1 is set to a predetermined level V A (step S332). When F 1 = 0, it is determined whether or not the flag F 2 equals "1" (step S333). When F 2 = 1, which means that the currently receiving broadcasting wave is in low-level receiving condition, the set level V 1 is set to a predetermined level V B (step S334). When F 2 = 0, this routine will be terminated. It is to be noted that the predetermined level V A is higher than the predetermined level V B . The setting of "1" to the flag F 2 generates a second frequency check signal. After execution of step S332 or S334, the variable m is set equal to "1" (step S335) and the variable u to "0" (step S336). And AF data f (m) from the AF data list of AF data f (1) , f (2) , ..., f (n) , which is written into the memory 15, is read out and is set to the frequency divider in the PLL circuit 2a (step S337). Accordingly, the reception frequency changes to the frequency of another station in the same network as the current broadcasting station. Then, the processor latches the reception signal level V s at the new reception frequency selected by one piece of AF data f (m) on the AF data list (step S338), and determines if V s is greater than the set level V 1 or not (step S339). Though not illustrated in the flowchart, the process in step S338 is performed upon the lapse of a time sufficient for the reception frequency to become stable from the point of time at which the AF data has been set in step S337. When V s ≤ V 1 , the flow advances to step S343 to be described later. When V s > V 1 , the PI code representing the network of the broadcasting station associated with the AF data f (m) is latched (step S340), and the variable u is incremented by "1" (step S341). Then, the AF data f (m) the reception signal level V s and the latched PI code are stored respectively as check AF data cf (u) , reception signal level V s (u) and PI(u) into the memory 15 (step S342). It is then determined if the variable m has reached the number of pieces of AF data, n, on the AF data list (step S343). When m < n, the variable m is incremented by "1" (step S344), and the flow returns to step S337. Repeating this operation, all of those pieces of the AF data f (1) , f (2) , ..., f (n) on the AF data list which can ensure the reception signal level V s > V 1 , or all the check AF data is written in the memory 15. When m = n, meaning that checking the reception signal level has been done on every piece of AF data f (1) , f (2) , ..., f (n) in step S339, the flow then advances to step S345. The value of the variable u at that time indicates the number of pieces of AF data which have ensured the reception signal level V s > V 1 . In step S345, it is determined whether or not the variable u equals "0". When u = 0, meaning that the reception signal level V s > V 1 could not be acquired at another broadcasting station that is broadcasting the same program as the current broadcasting station, the flow then advances to step S358 to be described later. When u ≠ 0, the value of the maximum level, V MAX , is set equal to the initial value (e.g., "0") (step S346), and a variable w to "1" (step S347). Then, the PI code of the current broadcasting station (broadcasting station whose broadcasting wave has been received immediately before the mode change to the program follow-up mode) is read out from the memory 15 (step S348), and a PI code PI(w) is read out from the memory 15 (step S349). It is then determined if the PI code of the current broadcasting station is matched with the PI code PI(w) (step S350). When these PI codes differ from each other, the flow moves to step S355 to be described later. When the PI codes coincide with each other, the reception signal level V s (w) is read out from the memory 15 (step S351), and it is determined whether or not V s (w) is greater than the maximum level value V MAX (step S352). When V s (w) ≤ V MAX , the flow goes to step S355, while when V s (w)> V MAX , the reception signal level V s (w) is set to the maximum level value V MAX ) (step S353) and the variable w is treated as a select number x (step S354). After executing step S354, it is determined if the variable w has reached the variable u (step S355). When w < u, the variable w is incremented by "1" (step S356), and the flow returns to step S349. When w = u, it is determined whether or not the maximum level value V MAX equals the initial value (step S357). When V MAX equals the initial value, it means that the broadcasting station having the PI code PI(w) matched with the PI code of the current broadcasting station has not been detected, the reception frequency data is read out from the memory 15 and is set to the frequency divider in the PLL circuit 2a in order to return to the reception of the currently receiving broadcasting wave that has been received immediately before the mode change to the program follow-up mode (step S358). This routine is then terminated to change the mode to the normal reception mode from the program follow-up mode. When the maximum level value V MAX is not equal to the initial value, the check AF data cf(x) that has provided the maximum level value V MAX as the reception signal level is read out from the memory 15 and is set to the frequency divider in the PLL circuit 2a (step S359). Then, the reception frequency data in the memory 15 is updated to the check AF data cf(x) (step S360) before terminating this routine. As a result, the broadcasting wave is received at the frequency specified by the check AF data cf(x). In other words, the broadcasting wave of that one of the broadcasting stations broadcasting the same program as the currently receiving broadcasting wave that has been received immediately before the mode change to the program follow-up mode, which has the highest field intensity, is received, the broadcasting station using that frequency becomes a new current broadcasting station, and the program follow-up mode is then terminated. In a period where the mode is changed to the program follow-up mode and the reception frequency has changed to a frequency on the AF data list, a muting circuit (not shown) is activated to cut off the output of an audio signal. The muting state is released when the program follow-up mode is terminated. According to this embodiment, the check AF data that can assure the reception signal level V s > V 1 is written in the memory 15. However, a modification may be made so that when V s > V 1 is judged in step S339, it is then determined if the PI code of the current broadcasting station coincides with the PI code PI(w) in question, and when the PI codes coincide with each other, it is determined whether or not the reception signal level V s is greater than the maximum level value V MAX , thus yielding the AF data of that one of the broadcasting station broadcasting the same program as the currently receiving broadcasting wave which has the highest field intensity, eliminating the need to write the check AF data in the memory 15. In addition, if the PI codes of the broadcasting stations in the same network station group which correspond to the individual pieces of AF data on the AF data list are known in advance to be identical, coincidence between the PI code of the current broadcasting station and the PI code PI(w) should not necessarily be judged. According to this embodiment of the present invention, the frequency data of the same network station group has been stored in the memory by acquiring the AF data from the RDS broadcasting wave being received. The frequency data of the same network station group for each receivable broadcasting station may be previously stored in the memory. Although the AF data that can assure good reception is selected in accordance with the reception signal level according to this embodiment, the selection is not limited to this particular way. For instance, the noise level may be detected together with the reception signal level so that the AF data which ensures good reception is selected according to these two levels. As described above, in the RDS receiver control method according to this embodiment of the present invention, when the receiving condition of the currently receiving RDS broadcasting wave becomes poor, only those pieces of AF data which can provide an electric field intensity equal to or higher than a set level is extracted first from a frequency data list stored in the memory, then that piece of AF data whose reception signal level is highest is selected from the read AF data, and the reception frequency is switched to the frequency specified by the selected AF data. When poor reception occurs, therefore, it is possible to promptly shift to the reception of the RDS broadcasting wave having the frequency selected from the AF data list, which has an electric field equal to or higher than a set level and can ensure the best reception.

Claims: A method of controlling an RDS receiver having a memory holding frequency data of a group of stations in the same network, and a lock detector for detecting a lock status of a digital PLL circuit (8) for producing a clock for demodulation of a radio data signal extracted from an FM detection output and producing a lock detection signal, the method characterized by: a first step (S133) of judging whether or not the lock detection signal is produced every unit time during reception of a broadcasting wave; a second step (S134) of incrementing a count value of a counter by "1" when the frequency of judgement in the first step that the lock detection signal is produced within a first set time is smaller than a first predetermined value, keeping the count value of the counter when the frequency of judgement is greater than the first predetermined value and is equal to or lower than a second predetermined value larger than the first predetermined value, and resetting the count value of the counter to an initial value when the frequency of judgement is greater than the second predetermined value; a third step (S220) of selectively reading out frequency data of one station of that group of network stations to which a broadcasting station of the receiving broadcasting wave belongs from the memory when the count value of the counter in a second set time is greater than a reference value; and a fourth step (S225) of switching a reception frequency to a frequency indicated by the frequency data read out in the third step. A method of controlling an RDS receiver having a memory containing frequency data of a group of stations in the same network, comprising: a first step (S208) of detecting an average level of a reception signal level of a currently receiving broadcasting wave in a predetermined time at every given timing; and characterized by a second step (S213) of producing a frequency check signal when a previous average level is higher than a first predetermined value and a current average level is lower than a second predetermined value smaller than the first predetermined value, or when the previous average level is lower than the second predetermined value and the current average level is higher than the first predetermined value; and a third step (S220) of selectively reading out frequency data of one station of that network station group to which a broadcasting station broadcasting the currently receiving broadcasting wave belongs from a memory in accordance with the frequency check signal, and switching a reception frequency to a frequency indicated by the read frequency data. A method according to claim 2, wherein when the reception signal level at the reception frequency switched in the third step is greater than the set level, the current receiving conditions are maintained, when the reception signal level is equal to or below the set level, new frequency data is read out, and when the reception signal levels at frequencies specified by all pieces of frequency data of the same network station group are equal to or below the set level, returning to the receiving conditions of the broadcasting wave in the first step takes place. A method according to claim 2 or 3, wherein in a case where the frequency check signal is generated when a previous average level in the second step exceeds the first predetermined value and a current average level is lower than the second predetermined value smaller than the first predetermined value, if the reception signal levels at frequencies specified by all pieces of frequency data of the same network station group are equal to or below the set level in the third step, that broadcasting wave whose reception signal level is equal to or higher than the set level and whose program identification code acquired therefrom coincides with that of the current receiving broadcasting wave is detected while sweeping the reception frequency. A method according to claim 1, further comprising: a fifth step of judging whether or not a reception signal level of the reception frequency switched in the fourth step is greater than a set level, and returning to the third and fourth steps after the judgement until all frequency data is read out from the memory; and a sixth step of selecting that piece of frequency data, among all of those pieces of frequency data judged in the fifth step to have reception signal levels greater than the set level, which has a highest reception signal level and switching the reception frequency to a frequency indicated by the selected frequency data. A method according to claim 2, further comprising: a fourth step of judging whether or not a reception signal level of the reception frequency switched in the third step is greater than a set level, and returning to the third step after the judgement until all frequency data is read out from the memory; and a fifth step of selecting that piece of frequency data, among all of those pieces of frequency data judged in the fourth step to have reception signal levels greater than the set level, which has a highest reception signal level and switching the reception frequency to a frequency indicated by the selected frequency of data.

Inventors: KASA KOICHI, KUNUGI YOSHIRO, KUROSU TAKAAKI, KAMIYA, KAZUHIRO, KASA, KOICHI, KUNUGI, YOSHIRO, KUROSU, TAKAAKI, MIYAKE, TAKASHI, KASA, KOICHI, C/O PIONEER ELECTRONIC CORPORATION, Kunugi, Yoshiro, c/o Pioneer Electronic Corp., Kurosu, Takaaki, c/o Pioneer Electronic Corp.
Applicants: PIONEER ELECTRONIC CORP, PIONEER ELECTRONIC CORPORATION
Application year: 1991
Main classification: H04H1
IPC codes: H04H20