Signal distinction circuit

Abstract: A signal distinction circuit comprising a noise detector for detecting a noise included in a received signal; and a maintaining circuit for maintaining the condition of a signal outputted from a detector for detect­ ing a discrimination signal according to a signal outputted from the noise detector so that since the noise detector and the maintaining circuit prevent the condition of a signal outputted from the distinction circuit from being changed when an interference such as a multipath interference occurs, an output corresponding to a received signal can be reliably obtained.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to a signal distinction circuit for detecting a discrimination signal having a predetermined frequency, and more particularly, to a signal distinction circuit capable of preventing a malfunction in distinguishing the discrimination signal due to, for example, a multi-path interference. 2. Description of the related art An FMX stereophonic broadcasting is proposed to improve the S/N of an FM stereophonic broadcasting and enlarge a service area. According to the FMX stereophonic broadcasting, a compressed stereophonic difference signal is transmitted simultaneously with the transmission signal of the FM stereophonic broadcasting. The compressed stereophonic difference signal modulate a subcarrier of 38KHz in quardrature with respect to a subcarrier of 38 KHz which is AM-modulated by a stereophonic difference signal. A discrimination signal of 10 Hz is included in the transmission signal of the FMX stereophonic broadcasting so that the FMX stereophonic broadcasting is distinguished from the FM stereophonic broadcasting. Therefore, in order to receive the FMX stereophonic broadcasting, it is necessary to detect the discrimination signal and switch the receiving mode of a receiver to the FMX stereophonic broadcasting receiving mode. The FMX stereophonic broadcasting is described in detail on pages from 11 through 15 of a magazine Journal of the Audio Engineering Society 88-9 "JAS JOURNAL" for September, 1986. The discrimination signal of 10 Hz can be detected by a signal distinction circuit as shown in Fig. 2. Referring to Fig. 2, a stereophonic pilot signal of 19 KHz applied to a first input terminal 1 is divided into approximately 74 Hz by a first frequency divider 2 and then, applied to a counter 3 as a clock signal. The discrimination signal of 10 Hz applied to a second input terminal 4 is applied to a pulse generator 6 through a bandpass filter 5. Therefore, a pulse signal having the same frequency as that of the discrimination signal applied to the input terminal 4 is generated by the pulse generator 6. The pulse signal thus produced is divided by a second frequency divider 7, and then, supplied to the counter 3 as an actuating signal. Consequently, the counter 3 counts the pulse number of the clock signal while the level of the pulse signal is maintained to be an "H" level. A detector 8 detects whether or not a value counted by the counter 3 equals to a predetermined value. However, the signal distinction circuit shown in Fig. 2 generates a signal indicating that it is receiving the transmission signal of the FM stereophonic broadcasting when the discrimination signal of 10 Hz is subjected to an interference such as a multipath interference and thus the average frequency thereof is changed. As such, although the receiver is receiving the signal of an FMX stereophonic broadcasting, the receiver is switched to receive the signal of the FM stereophonic broadcasting. In particular, the interference such as the multipath interference continues for a short period of time. Therefore, when the interference is terminated, the receiver is switched to receive the FMX stereophonic broadcasting. Thus, the receiver is unnecessarily switched two times. In addition, when the receiver is switched to listen to a broadcasting which transmits a signal including a discrimination signal, the discrimination signal is distorted and consequently, its level is lowered when the discrimination signal is subjected to the multipath interference or when an external noise is mixed therewith. It is desirable that the receiving mode of the receiver is not changed as long as the listener desires to keep listening to the above-described broadcasting. The following problem occurs according to the known receiver. That is, for example, if the level of the discrimination signal of 10 Hz instantaneously falls while an FMX stereophonic broadcasting is being received, the receiver is switched to receive an FM stereophonic broadcasting. As a result, a noise level fluctuates, which causes a listener to feel uncomfortable. SUMMARY OF THE INVENTION The present invention has been made to solve the above described disadvantages and has for its essential object to provide a signal distinction circuit. The signal distinction circuit in accordance with the present invention comprises a pulse generator for generating a pulse in accordance with the frequency of a discrimination signal; a counter for counting the pulse number of a clock signal according to a signal outputted from the pulse generator; a detector for detecting the discrimination signal based on a value counted by the counter; a level detector for detecting the level of the discrimination signal, thus generating a reset signal for resetting the discrimination signal detector; and a gate circuit for interrupting the reset signal according to the condition of a signal outputted from the discrimination signal detector. According to the above described construction, the gate circuit is interrupted in response to a first signal applied from the detector to the gate circuit while a signal transmitted from a broadcasting station being received by the signal distinction circuit includes a discrimination signal and a reset signal is prohibited from being applied from the level detector to the detector. Therefore, even though the discrimination signal is interrupted for a short period of time or the level thereof lowers, the detector can be prevented from being reset. When a signal transmitted from the broadcasting station to the receiver includes no discrimination signals, the gate circuit is opened in response to a second signal applied from the detector to the gate circuit. Accordingly, while a signal not including the discrimination signal is being received by the signal distinction circuit, the detector is kept to be reset. When a tuning condition is changed, i.e., when the receiver is switched from a condition in which a signal not including the discrimination signal is received to a condition in which a signal including the discrimination signal is received, and consequently, when the discrimination signal whose level is higher than a predetermined level is applied to the level detector, the reset signal is not applied the gate circuit and the detector detects a value counted by t counter. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features of the present invention will become apparent from the following description taken in conjunction with preferred embodiments thereof with reference to the accompanying drawings, in which: Fig. 1 is a circuit diagram showing an embodiment of the present invention; Fig. 2 is a circuit diagram showing a known sign distinction circuit. DETAILED DESCRIPTION OF THE INVENTION Referring to Fig. 1 showing a circuit diagram of a signal distinction circuit of an embodiment according to the present invention, the embodiment is described below. The signal distinction circuit according to the present invention detects, using a counter, a discrimination signal having a predetermined frequency and has a construction which does not malfunction even though a signal is interrupted. An FMX stereophonic broadcasting is proposed to improve the S/N of an FM stereophonic broadcasting and enlarge a service area. According to the FMX stereophonic broadcasting, a compressed stereophonic difference signal is transmitted simultaneously with the transmission signal of the FM stereophonic broadcasting. The compressed stereophonic difference signal is quadrature with respect to a subcarrier of 38 KHz which is produced by AM-modulating a stereophonic difference signal. A discrimination signal of 10 Hz is included in the transmission signal of the FMX stereophonic broadcasting so that the FMX stereophonic broadcasting is distinguished from the FM stereophonic broadcasting. Therefore, in order to receive the FMX stereophonic broadcasting, it is necessary to detect the discrimination signal and suitably switch the receiving mode of a receiver to the FMX stereophonic broadcasting receiving mode. The discrimination signal of 10 Hz can be detected by a signal distinction circuit as shown in Fig. 1. Referring to Fig. 1, a stereophonic pilot signal of 19 KHz applied to a first input terminal 10 is divided into approximately 74 Hz by a first frequency divider 12 and then, applied to a counter 13 as a clock signal. The discrimination signal of 10 Hz applied to a second input terminal 14 is applied to a pulse generator 16 through a bandpass filter 5. Therefore, a pulse signal having the same frequency as that of the discrimination signal applied to the input terminal 14 is generated by the pulse generator 16. The pulse signal thus produced is divided by a second frequency divider 17, and then, supplied to the counter 13 as an actuating signal. Consequently, the counter 13 counts the pulse number of the clock signal while the pulse signal is maintained to be an "H" level. A detector 18 detects whether or not a value counted by the counter 13 equals to a predetermined value. The signal distinction circuit further comprises a level detector 19 for detecting the level of a signal outputted from the output terminal of the bandpass filter 15. The level detector 19 generates an "L" level signal when the level of the signal outputted from the bandpass filter 15 is more than a predetermined value and generates an "H" level signal when when the level of the signal outputted therefrom is less than the predetermined value. Therefore, when the level of the above described signal falls for some reason or other, the level of the signal outputted from the level detector 19 becomes "H". As a result, the detector 18 is reset. The signal distinction circuit further comprises an AND gate 20. A reset signal outputted from the level detector 19 is applied to the first input terminal of the AND gate 20. A signal outputted from the detector 18 is applied to the second input terminal thereof. The output of the AND gate 20 is applied to the reset terminal of the detector 18. A broadcasting station capable of transmitting FMX radio waves is always transmitting the FMX radio waves. The signal transmitted from the broadcasting station includes an ID signal of 10Hz. On the other hand, some broadcasting stations cannot transmit FMX radio waves, but the FM radio waves. Thus, broadcasting stations transmit signals having different frequencies which range within a predetermined band. Accordingly, every time a receiver is tuned to a certain frequency, the receiver capable of receiving FM and FMX stereophonic broadcastings is required to distinguish the FM stereophonic broadcasting and the FMX stereophonic broadcasting form each other. To this end, the provision of the signal distinction circuit shown in Fig. 1 is essential. However, when the receiver receives a broadcasting of a certain broadcasting station and when it is detected that the broadcasting being received is transmitted from an FMX stereophonic broadcasting station, the FMX stereophonic broadcasting receiving condition is not changed unless the receiver is tuned to a different frequency. Therefore, it is unnecessary to provide the signal distinction circuit with a decision means for detecting the level of a signal. The operation of the signal distinction circuit shown in Fig. 1 is described in correspondence with signal receipt conditions. Supposing that an FM stereophonic broadcasting is being received, the stereo pilot signal of 19 KHz applied to the first input terminal 10 is divided by the first frequency divider 12 and the divided signal is applied to the counter 13 as a clock signal. At this time, since the receiver is receiving the FM stereophonic broadcasting, the discrimination signal of 10 Hz is not applied to the second input terminal 14. Consequently, the actuating signal is not applied from the second frequency divider 17 to the counter 13. Accordingly, the counter 13 does not perform a counting operation and the level of a signal outputted from the detector 18 becomes "H". The "H" level signal outputted from the detector 18 is transmitted from the output terminal 11 to a circuit (not shown) such as a display circuit and also to the second input terminal of the AND gate 20. As a result, the AND gate 20 is opened. Since the discrimination signal is not applied to the second input terminal 14, the level of a signal inputted to the level detector 19 becomes "L" and the level of a signal outputted therefrom becomes "H". The "H" level signal outputted from the level detector 19 passes through the AND gate 20 which is opened and applied to the reset terminal of the detector 18. As a result, the detector 18 is reset and the level of the output terminal 11 is maintained to be "H". When the tuning condition is changed, that is, when an FMX stereophonic broadcasting is received, the stereo pilot signal of 19 KHz is applied to the first input terminal 10 and the discrimination signal of 10 Hz is applied to the second input terminal 14. If the level of the discrimination signal of 10 Hz is sufficiently high, the level of a signal outputted from the level detector 19 becomes "L" and the "L" level signal is applied to the detector 18 through the AND gate 20. As a result, the reset condition of the detector 18 is terminated. Then, the counter 13 counts the pulse number of a clock signal outputted from the first frequency divider 12 in correspondence with the actuating signal outputted from the second frequency divider 17. Consequently, the result counted by the counter 13 is detected by the detector 18. If the value counted by the counter 13 equals to the predetermined value, the detector 18 generates an "L" level signal indicating that the receiver is receiving the FMX stereophonic broadcasting. The "L" level signal outputted from the detector 18 is transmitted from the output terminal 11 to the circuit such as the display circuit as described above and applied to the second input terminal of the AND gate 20. Accordingly, the AND gate 20 is closed and the level of a signal outputted therefrom becomes "L", which prevents the detector 18 from being reset. The detector 18 is not reset in this condition and the level of a signal outputted from the detector 18 is changed only by the value counted by the counter 13. Accordingly, when the signal distinction circuit detects the FMX stereophonic broadcasting receiving condition, the signal distinction circuit does not respond to the change of the level of the discrimination signal. Thus, an erroneous switching can be prevented. When the tuning condition is changed again, i.e., when the FM stereophonic broadcasting is received, the discrimination signal of 10 Hz is not inputted to the signal distinction circuit. Therefore, the counter 13 stops its operation. At this time, the detector 18 maintains the FMX stereophonic broadcasting receiving condition. This condition can be released by detecting that the discrimination signal of 10 Hz is not present during a predetermined period of time and by resetting the detector 18. By doing so, the level of a signal outputted from the detector 18 becomes "H" and the AND gate 20 is opened. As a result, the "H" level signal outputted from the level detector 19 is applied to the detector 18. Thus, the signal distinction circuit is in the FM stereophonic broadcasting receiving condition. The level detector 19 has a predetermined threshold and detects the level of a signal applied thereto from the bandpass filter 15. That is, when the level of the signal outputted from the bandpass filter 15 is more than the threshold, the level detector 19 generates an "L" level signal and when the level of the signal outputted therefrom is less than the threshold, the level detector 19 generates an "H" level signal. In order to prevent the detector 19 from malfunctioning due to, for example, a noise, the signal distinction circuit is so constructed that level detector 19 outputs either the "H" or "L" level signal after the level detector 19 performs a plurality of detections. The above description of the present invention is concerned with the application of the signal distinction circuit to the FMX stereophonic broadcasting and the FM stereophonic broadcasting. But utilizing the signal distinction circuit, a receiver is capable of receiving both a signal including a discrimination signal and a signal not including the discrimination signal. For example, the present invention can be applied to a broadcasting system, widely adopted in Europe, in which an ARI (traffic information) broadcasting (corresponding to FMX broadcasting) and an FM stereophonic broadcasting are received. As described above, the signal distinction circuit according to the embodiment is capable of reliably distinguishing a discrimination signal included in a signal transmitted from a broadcasting station. Further, once the discrimination signal is detected, the gate is closed so that the output of the level detector is interrupted. Accordingly, the signal distinction circuit can reliably distinguish a signal including the discrimination signal from a signal not including the discrimination signal.

Claims: A signal distinction circuit having a pulse generator (16) for generating a pulse signal in correspondence with the frequency of a discrimination signal; a counter (13) for counting the number of pulses of a clock signal according to the pulse signal outputted from said pulse generator; a detector (18) for detecting said discrimination signal based on a value counted by said counter; characterized by a level detector (19) for detecting the level of said discrimination signal, thus generating a reset signal for resetting said discrimination signal detector; and a gate circuit (20) for interrupting said reset signal according to the condition of a signal outputted from said discrimination signal detector (18). A signal distinction circuit as claimed in claim 1, wherein said discrimination signal detector (18) generates a first level output signal when the value counted by said counter reaches a predetermined value, and said gate circuit (20) interrupts the signal outputted from said level detector (19) according to said first level signal.

Inventors: TANNO MASAYA, TANNO, MASAYA
Applicants: SANYO ELECTRIC CO, SANYO ELECTRIC CO., LTD.
Application year: 1989
Main classification: H04B1
IPC codes: H04B1