JPH0358211B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to a system identification device used in AM stereo broadcasting to identify systems based on pilot signal frequencies that differ for each system. Conventional configurations and their problems Various AM stereo broadcasting systems have been proposed, and an AM stereo receiver that can support each system is required. This requires a system identification device that selects demodulated signals from different demodulation means for each system. In the system currently being proposed in the United States, a pilot signal indicating stereo broadcasting is superimposed on the difference signal between the left and right channel signals and sent out.
Since the frequency and modulation level of the pilot signal differ depending on the method, this information is used to identify the method. A conventional method identification device will be explained below with reference to the drawings. FIG. 1 is a block diagram of a pilot signal detection device for detecting the presence or absence of a one-sided pilot signal. Filter 1, which takes out the pilot signal superimposed on the difference signal a, is composed of a bandpass filter or a low-pass filter that passes the frequency of the pilot signal, and the pilot signal selected by filter 1 is amplified by amplifier 2 and output to It is input to circuit 3. The pilot signal b input to the Schmitt circuit 3 is converted into a rectangular wave c and input to the CR integration circuit 4. The CR integration circuit 4 sequentially performs integration, and its integral output d is input to the level detection circuit 5, where it is determined whether the integrated value exceeds a specified value. If it exceeds the specified value, it is determined that a pilot signal exists. It outputs a detection signal e indicating that it has occurred. The operation of the pilot signal detection device configured as described above will be explained. FIG. 2 is a timing chart for detecting a pilot signal. In FIG. 2, the horizontal axis t indicates the passage of time, and when stereo broadcasting starts at time _T1 , a difference signal a on which a pilot signal is superimposed is output. By passing the difference signal a through the filter 1, a pilot signal is taken out and a pilot signal b is obtained which is amplified by the amplifier 2. In amplifier 2, the detection level of Schmitt circuit 3
Amplify to a level suitable for V ₁ and V ₂ . In the Schmitts circuit 3, when the pilot signal b becomes higher than the level of _V1 , it outputs H (high level) and _V2
When it becomes smaller than the VU level, it outputs L ((low level) and converts it into a rectangular wave c.The rectangular wave c is integrated by the CR integration circuit 4, and its integral output d is sent to the level detection circuit 5.If it becomes larger than the _VU level, it becomes H. ((or L)
is output as the pilot detection signal e. Second
At time _T2 in the figure, the integral output d becomes greater than the _VU level, indicating that the pilot detection signal e becomes H. On the other hand, if stereo broadcasting changes to monaural broadcasting at time _T3 , pilot signal b is no longer detected, rectangular wave c is also not output, and the level of integral output d decreases with the discharge time constant of CR. When the voltage drops below the _VL level, the pilot detection signal e becomes L at time _T4 in Figure 2.
become. In this way, the pilot signal can be detected. FIG. 3 is a block diagram of a method identification device using a pilot signal detection device. Filter 11
0, 120, 130, and 140 are composed of bandpass filters whose center frequency is the frequency of the pilot signal superimposed on the difference signal of each method, and the selected pilot signal is passed through the amplifier 210,
The signals are amplified by 220, 230, and 240 and converted into rectangular waves by Schmitt circuits 310, 320, 330, and 340. The converted square wave has a CR time constant suitable for the frequency of the pilot signal of each method.
It is integrated by the CR integration circuits 410, 420, 430, and 440, and the integrated output is sent to the level detection circuit 51.
0, 520, 530, and 540, it is detected which type of pilot signal is present, and a pilot detection signal is output. The pilot detection signal is held in a latch circuit 6 and output as a system identification device. On the other hand, the latch signal of the latch circuit 6 is outputted from the latch pulse generating circuit 7 at the rising edge of each type of pilot detection signal. The pilot signal detection operation in each method is the same as that described in FIGS. 1 and 2. However, in the method identification device having the above-described configuration, each filter is required to have a steep selectivity characteristic. In other words, the pilot signals of each system use extremely low frequencies; for example, the Magnabox system uses 5Hz, the Kahn system uses 15Hz, the Motorola system uses 25Hz, and the Harris system uses 55Hz, so the pilot signal frequencies of each system are close to each other. Therefore, there may be a problem that pilot signal detection devices of different types operate erroneously. In addition, if the time constants of C and R in the CR integration circuit are small, the output of the pilot signal detection device will be an intermittent output corresponding to the pilot signal.Increasing the time constant will solve the above problem, but the pilot Even if a signal is input, the detection output is sent out with a delay of a time constant, which poses a problem in that a quick detection operation cannot be performed. Purpose of the Invention The object of the present invention is to provide a system for an AM stereo receiver that can quickly and accurately identify the system by detecting the arrival of pilot signals of each system in AM stereo broadcasting and identifying the frequency. An object of the present invention is to provide an identification device. Structure of the Invention The system identification device of the present invention includes a filter for selecting a low-frequency pilot signal included in a difference signal of AM stereo broadcasting, a Schmitt circuit for rectangulating the signal after passing through the filter, and a reference signal. a reference signal generator that generates a frequency, an AND circuit that takes the logical product of the output of the Schmitt circuit and the output of the reference signal generator, a counter circuit that inputs the output of the AND circuit, and an output state of the counter circuit. a first latch circuit that holds, a logic circuit that inputs the output of the first latch circuit, and a second latch circuit that holds the output state of the logic circuit;
a first pulse generating circuit that generates a reset pulse for the counter circuit and a latch pulse for the first latch circuit based on the output of the Schmitt circuit; It is composed of a second pulse generation circuit which inputs the output and generates a latch pulse to the second latch circuit, thereby identifying each AM stereo broadcast system. DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 4 shows a block diagram of an automatic method identification device according to an embodiment of the present invention. In FIG. 4, the difference signals of each method are added together and input to a filter 8. Here, as for the frequency characteristics of the filter 8, if method 1 is the Magnabox method, method 2 is the Kahn method, method 3 is the Motorola method, and method 4 is the Harris method, the respective pilot signal frequencies are 5Hz for the Magnabox method, Since the Kern method is 15Hz, the Motorola method is 25Hz, and the Harris method is 55Hz, a low pass filter with a cutoff frequency of 55Hz is sufficient. However, since 50Hz or higher is a region where broadcast signals exist, it is desirable that the filter characteristic be as steep as possible. Also, since the pilot signal level differs depending on the method, it is desirable that the filter 8 has amplitude characteristics such that the output of the filter 8 has the same pilot signal level. The pilot signal f obtained by passing through the filter 8 is converted into a rectangular wave g by the Schmitt circuit 9.
and is input to the AND circuit 11. on the other hand,
The signal h of the reference frequency generator 10 is also connected to the AND circuit 11
The signal is input to , and the logical product with the rectangular wave g is taken. The ANDed signal i is input to the counter circuit 12, and its output is held by the first latch circuit 13. Also, the latch pulse j of the first latch circuit 13
is outputted from the pulse generating circuit 16 at the falling edge of the rectangular wave g, and furthermore, the reset pulse k of the counter circuit 12 is outputted at the falling edge of the latch pulse j. The logic circuit 14 determines the pilot signal frequency based on the value held in the latch circuit 13, and its output is held in the latch circuit 15 and becomes a system identification signal. This latch circuit 15 latches only when a pilot signal frequency of one of the systems is detected in the logic circuit. pulse generation circuit 17
A latch pulse is output. Therefore, the method identification signal is always in a state in which one of the methods is selected. The operation of the method identification device of this embodiment configured as described above will be described below. FIG. 5 shows a timing chart of the pilot signal f, the rectangular wave g, the signal h of the reference frequency generator 10, the output i of the AND circuit 11, the latch pulse j, and the reset pulse k. The pilot signal f that has passed through the filter 8 is applied to V _H of the Schmitt circuit 9.
If the level becomes higher than the level of VL, the output g goes to H (high level), and if it goes below the level of _VL , the output g goes down to L (low level). As a result, the output g becomes a rectangular wave. On the other hand, the output h of the reference frequency generator 10 is output to the counter circuit 12 only when the rectangular wave g is H.
is input. That is, the output h is counted during a half cycle of the pilot signal. On the other hand, at the falling edge of the rectangular wave g, a latch pulse j is output to the latch circuit 13, which holds the counter value, and then uses the falling edge of the latch pulse j to output the latch pulse j to the latch circuit 13.
A reset pulse k is output. From the above, period detection is performed in half the period of the pilot signal, so even if the detection time is long, the Magnabox method is
100msec, the fastest is the Harris method.
It is 9msec. In FIG. 5, the waveform of the signal f is different between the first half and the second half, but this indicates a difference in the method and indicates that the operation is the same. Next, the algorithm of the logic circuit will be explained. In particular, the case where the reference frequency is 680Hz+10Hz will be explained. Note that the reason why the reference frequency is set to 680 Hz is that the configuration of the logic circuit becomes simpler as will be described later. It should be noted that the logic circuit shall be constructed to prevent malfunctions by considering fluctuations in the range of ±2 Hz regarding the pilot signal. Table 1 shows the decimal values of the counters for each method.

[Table] FIG. 6 shows the conversion of this counter decimal value into a 7-bit binary value. In FIG. 6, characteristic parts for identifying each method are enclosed in squares. In other words, one of the top two bits is 1.
Range A, where the upper 2 bits are 0 and the 3rd bit is 1, is the Kern method; range C, where the upper 3 bits are 0 and the upper 4th and 5th bits are both 1, is the Motorola Method, top 4
If the range D where the bit is 0 and the 5th bit is 1 is based on the Harris method, the result will be a detection range as shown in Table 2.

[Table] FIG. 7 shows an embodiment of the logic circuit 14. Fourth
The same parts as in the block diagram in the figure are given the same numbers. FIG. 7 shows a circuit configuration in which characteristic parts of each method of counter binary value shown in FIG. 6 can be identified. Although the operation has been described above, the logical sum of the pilot signal detection signals output from the logic circuit 14 can also be used for information on a monaural/stereo switching signal. In addition, a microcomputer with a built-in counter circuit allows
It is also possible to replace all of the Schmitt circuit 9 and subsequent parts with microcomputer software, and even more accurate method identification can be performed. Effects of the Invention As is clear from the above explanation, the present invention can reduce the time required to identify the different pilot signal frequencies for each AM stereo broadcasting formula in half the period of the pilot signal frequency. , frequency identification accuracy can be improved by the configuration of the reference signal generator and the logic circuit, and a system without malfunction can be configured. Furthermore, it can be said that the system is versatile in that it can be adapted to different detection frequencies by changing the reference signal generator and logic circuit. Furthermore, since the system can be automatically identified, the operator does not have to worry about switching the system every time he or she tunes in.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a conventional pilot signal detection device, Fig. 2 is a timing chart for detecting a pilot signal, Fig. 3 is a block diagram of a method identification device using a conventional pilot signal detection device, and Fig. 4 is a block diagram of a method identification device using a conventional pilot signal detection device.
Figure 5 is a block diagram of a system identification device according to an embodiment of the present invention, Figure 5 is a timing chart of the system identification system of the invention, and Figure 6 shows the relationship between decimal values and binary values of counter values. Diagram showing the characteristics of the formula, 7th
The figure is a logic circuit diagram in one embodiment of the present invention. 8...Filter, 9...Schmitt circuit, 10
...Reference frequency generator, 11...AND circuit,
12... Counter circuit, 13, 15... Latch circuit, 14... Logic circuit, 16, 17... Pulse generation circuit.

Claims (1)

[Claims] 1. A filter for selecting a low-frequency pilot signal included in the difference signal of AM broadcasting, a Schmitt circuit for shaping the waveform of the signal after passing through the filter, a reference frequency generator for generating a reference frequency, and the above-mentioned an AND circuit that takes the logical product of the output of the Schmitt circuit and the output of the reference frequency generator, and a counter circuit that inputs the output of the AND circuit;
a first latch circuit that holds the output state of the counter circuit; a logic circuit that determines the output state of the first latch circuit; a second latch circuit that holds the output state of the logic circuit; a first pulse generating circuit that generates a reset pulse for the counter circuit and a latch pulse for the first latch circuit according to an output of the circuit; and a reset pulse for the counter circuit in the first pulse generating circuit and an output of the logic circuit. and a second pulse generation circuit that generates a latch pulse to the second latch circuit, and the method is identified by the output state of the second latch circuit.
System identification device in AM stereo receiver.