JPH0431208B2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field The present invention relates to an AM stereo receiver,
In particular, the present invention relates to an AM stereo receiver that can accurately discriminate a stereo identification signal included in an AM stereo broadcast signal. (b) Conventional technology Currently, three AM stereo broadcasting systems have been proposed.
Implemented in the United States. The AM stereo broadcasting system includes a method (first method) proposed by Motorola of the United States that is a quadrature modulation type and includes a 25Hz first identification signal, and an ISB (independent side band) modulation type that includes a 15Hz second identification signal. There is a method (second method) proposed by Hazeltine, Inc. of the United States, and a method (third method) proposed by Magnabox, Inc. of the United States, which is a phase modulation type and includes a third identification signal of 5 Hz. By detecting the first to third identification signals, automatic switching of the receiving circuit and received AM
It is possible to display the stereo broadcast system. As described in Japanese Patent Application No. 59-100038, the detection of the first to third identification signals is performed using a plurality of bandpass filters whose center frequencies are respectively the frequencies of the first to third identification signals. If you use
It's easy to do. Figure 2 shows a detection circuit for detecting such an identification signal.
The identification signal included in the output signal of the detection circuit 1 is
The output signals of the first to third band pass filters 2 to 4 are applied to the first to third band pass filters 2 to 4, and are detected by the first to third detection circuits 5 to 7, respectively. The third display devices 8 to 10 are driven to turn on. For example, if the receiver is now receiving AM stereo broadcast of the first method,
The first identification signal is detected by the first band pass filter 2 having a center frequency of 25 Hz, and the first indicator 8 lights up, indicating that the first system AM stereo broadcast is being received. Furthermore, if the receiver receives AM stereo broadcasting of the second system, the second identification signal is detected by the second bandpass filter 3 having a center frequency of 15Hz, the second indicator 9 lights up, and the It will be displayed that you are receiving two types of AM stereo broadcasts. Third method AM
A similar operation is performed when receiving stereo broadcasts,
A similar display will be made. Therefore, by using the circuit shown in FIG. 2, it is possible to determine what type of broadcast is currently being received. (c) Problems to be Solved by the Invention However, the circuit shown in FIG. 2 requires multiple bandpass filters that cannot be integrated into an IC (integrated circuit). have Furthermore, since the frequencies of the first to third identification signals are close to each other, it is necessary to increase the Q of the band pass filter to prevent malfunction. A drawback is that the characteristics tend to change depending on variations in the elements constituting the bandpass filter. (d) Means for solving the problems The present invention has been made in view of the above points.
A PLL circuit that locks to the IF signal frequency and the PLL
A quadrature synchronous detection circuit that synchronously detects an IF signal in accordance with an orthogonal synchronous signal obtained from the circuit; a bandpass filter that passes a stereo identification signal included in the output signal of the orthogonal synchronous detection circuit; a pulse generation circuit that generates a pulse according to the frequency of the stereo identification signal that has passed;
a clock signal generation circuit that generates a clock signal based on the signal obtained from the PLL circuit; a counting circuit that counts the clock signal according to the pulses generated from the pulse generation circuit; and a discrimination circuit for discriminating the stereo signal. (E) Effect According to the present invention, a clock signal is created based on a broadcast carrier wave whose frequency is accurately specified to be transmitted from a broadcast station, and the stereo identification signal is determined by counting the clock signal. Because it is on,
The accuracy of discrimination can be improved. (F) Embodiment FIG. 1 is a circuit diagram showing an embodiment of the present invention.
11 amplifies the IF signal of the AM stereo broadcast signal
The IF amplifier circuit 12 includes a phase comparator 13 to which the output signal of the IF amplifier circuit 11 is applied, a loop filter 14, a VCO (voltage controlled oscillator) 15, and a first
A PLL (phase locked loop) circuit 17 includes a frequency divider 16 and a PLL (phase locked loop) circuit 17 which converts the output signal of the IF amplifier circuit 11 into the output signal of the first frequency divider 16 of the PLL circuit 12 (which has a frequency equal to the IF signal frequency). , a signal whose phase is shifted by 90 degrees), 18 is a bandpass filter that passes the stereo identification signal included in the output signal of the orthogonal synchronous detection circuit 17, and 19 is the bandpass filter 18. 20 is a first frequency divider 1 of the PLL circuit 12 ;
21 is a counter that counts the clock signal obtained from the second frequency divider 20 during the output pulse generation period of the pulse generating circuit 19; A circuit 22 is a discrimination circuit 23 to 25 that discriminates which stereo identification signal the count value of the counting circuit 21 corresponds to.
are the first to third display devices driven by the output signal of the discrimination circuit 22. Next, the operation will be explained. The 450KHz IF signal obtained at the output end of the IF amplifier circuit 11 is applied to the quadrature synchronous detection circuit 17 and also to the PLL circuit 12.
is applied to the phase comparator 13 of . The PLL circuit 12 includes a phase comparator 13, a VCO 15 having a free run frequency of 3.6 MHz, and a first frequency divider 16 that divides the free run frequency by 8 to create a 450 Hz signal. In the phase comparator 13, the IF signal and the output signal of the first frequency divider 16 are compared, and the PLL 12 is locked to the IF signal. As a result, the frequency of the output signal of the first frequency divider 16 becomes equal to the frequency of the IF signal, and the phase of the output signal is exactly 90 degrees out of phase with the IF signal. The output signal of the first frequency divider 16 is applied to the orthogonal synchronous detection circuit 17 as an orthogonal synchronous signal, and the IF
The signal is detected by the orthogonal synchronous detection circuit 17.
Therefore, like the stereo sub signal of the first method of the three types of AM stereo broadcasting methods mentioned above, the IF
The orthogonally modulated signal contained in the signal is demodulated by the orthogonal synchronous detection circuit 17 and transmitted from the output terminal 26 to the subsequent stage. The demodulated signal is used for stereo demodulation in the subsequent stage. In the orthogonal synchronous detection circuit 17, when the IF signal is synchronously detected by the orthogonal synchronous signal, the stereo identification signal is also synchronously detected. Then, the stereo identification signal passes through a bandpass filter 18,
It is applied to the pulse generation circuit 19. The pulse generation circuit 19 includes a conversion circuit that detects the zero crossing point of the stereo identification signal and converts it into a rectangular wave signal, a first frequency division circuit that divides the output pulse of the conversion circuit into two, and a first frequency division circuit that divides the output pulse of the conversion circuit into a rectangular wave signal. It includes a second frequency dividing circuit that further divides the frequency of the output pulse of the frequency circuit by two, and generates a rectangular pulse having a frequency of 1/4 of the frequency of the stereo identification signal at the output terminal. FIG. 3 is a waveform diagram showing the relationship between the input signal and the output signal of the pulse generating circuit 19. FIG. Output signals; FIG. 3C is the output signal of the first frequency divider circuit, and FIG. 3D is the output signal of the second frequency divider circuit. obtained from the first frequency divider 16 of the PLL circuit 12
The 450 KHz signal is divided by the second frequency divider 20 into a 220 Hz clock signal. Since the clock signal is input as a signal to be counted to the counting circuit 21, the counting circuit 21 receives a signal corresponding to the stereo identification signal applied from the pulse generating circuit 19 to the counting circuit 21. When the clock signal becomes "L", the counting operation of the clock signal is started, and when the clock signal becomes "L", the counting operation is stopped. Therefore, during the period when the pulse generating circuit 19 is generating an "H" output signal, the counting circuit 21 counts the clock signals. When the output pulse of the pulse generating circuit 19 becomes "L" and the counting operation of the counting circuit 21 is stopped, the count value of the counting circuit 21 is inputted to the discriminating circuit 22, and the discriminating operation is started. In the discrimination operation, digital data corresponding to each stereo identification signal is stored in advance in the discrimination circuit 22, and each digital data and the counting circuit 21 are
This is done by comparing the counted value of Further, the discrimination operation may be performed by performing a logical operation on the state of each bit of data inputted from the counting circuit 21 to the discrimination circuit 22, and generating an output signal at a predetermined output terminal when a predetermined value is reached. . When the stereo identification signal is determined to be the first identification signal through the determination operation, the first indicator 23 lights up to indicate that AM stereo broadcasting of the first system is being received. Similarly, when the second or third identification signal is determined, the second or third indicator 24 or 25 lights up to display reception of AM stereo broadcasting of the second or third system. still,
The output signal of the discrimination circuit 22 is used not only for display purposes but also for
It can be used for various purposes such as automatic circuit switching of various circuits suitable for the reception method, stereo/monaural switching, etc. The frequency of the output clock signal of the second frequency divider 20 is 220Hz, and the output signal frequency of the pulse generation circuit is 1/4 of the frequency of the stereo identification signal, so when the first identification signal of 25Hz is detected, counting is performed. Circuit 21 counts 17.6 clock pulses and 15
28.6 clock pulses are counted for the second identification signal of Hz, and 88 clock pulses are counted for the third identification signal of 5 Hz. In addition, since the frequencies of each identification signal are sufficiently spaced apart, sufficient tolerance can be given to the counted value, and in the embodiment, when 16 to 20 clock signals are counted, the first identification signal and When counting 27 to 33 clock signals, the second
It is regarded as an identification signal, and when 80 to 98 clock signals are counted, it is regarded as a third identification signal. (G) Effects of the Invention As described above, according to the present invention, it is possible to provide an AM stereo receiver that automatically determines the broadcasting system of AM stereo broadcasting. Furthermore, according to the present invention, a clock signal is created based on an IF signal with an accurate frequency, and the broadcasting system is determined by counting the clock signal. It is possible to make a judgment. Furthermore, according to the present invention, the clock signal is obtained from the PLL circuit that locks to the IF signal originally required for the AM stereo receiver, and the stereo identification signal is obtained from the quadrature synchronous detection circuit, so the circuit configuration can be simplified. Measure,
We can provide an AM stereo receiver suitable for IC.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a circuit diagram showing an identification signal detection circuit of a conventional AM stereo receiver, and FIGS. It is a waveform diagram for explaining a circuit. Explanation of main figure numbers, 12 ... PLL circuit, 16... First frequency divider, 17... Quadrature synchronous detection circuit, 19... Pulse generation circuit, 20... Second frequency divider, 21... Counting circuit, 22... Discrimination circuit .

Claims (1)

[Claims] 1 An AM stereo receiver capable of receiving AM stereo broadcasts, which includes a PLL circuit that locks to the IF signal frequency of the received AM stereo broadcast signal, and
IF according to the orthogonal synchronization signal obtained from the PLL circuit.
A quadrature synchronous detection circuit that synchronously detects a signal, a bandpass filter that passes a stereo identification signal included in the output signal of the orthogonal synchronous detection circuit, and a frequency-divided and output stereo identification signal that has passed through the bandpass filter. A pulse generation circuit that generates pulses, a clock signal generation circuit that divides the frequency of the signal obtained from the PLL circuit to generate a clock signal, and counts the clock signal according to the pulses generated from the pulse generation circuit. a counting circuit;
and a discrimination circuit that discriminates the stereo identification signal according to the count value of the counting circuit.