JPH0352698B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an amplitude modulation (AM) stereo broadcast system identification device.

The AM stereo broadcast system identification device of the present invention receives broadcast radio waves sent out from broadcast stations with different stereo broadcast systems, identifies the broadcast system, selects a demodulation circuit according to the broadcast system, and broadcasts stereo broadcasts. It can be applied to, for example, a car radio installed in a car.

[Conventional technology]

In the United States, Canada, etc., the AM stereo broadcasting system is adopted as a stereo broadcasting system. Various systems are known as this AM stereo broadcasting system, such as Magnabox's AM-PM.
system, Harris' CPM system, Kern/Hazelchin's ISB system, Motorola's C-QUAM system,
Examples include Beller's AM-FM system. In the United States, the first four of these systems are currently approved, and broadcast stations are currently operating and broadcasting in stereo, but stereo broadcasting using each of these systems overlaps in some regions. The current situation is that

On the other hand, although the configuration of the receiver differs depending on each of the above-mentioned stereo broadcasting systems, a receiver of a certain system cannot receive stereo broadcasting of another system. For this reason, even though a variety of stereo broadcast programs are being broadcast in a certain area, the stereo broadcast system for those programs is different, so you may not be able to watch the programs on your receiver. Currently, it is only possible to enjoy programs that are based on the system.

[Problem that the invention seeks to solve]

In order to be able to receive all stereo broadcast programs of each stereo broadcast system with a single receiver, the receiver must be equipped with a demodulation circuit for each system, and the receiver must be able to identify the stereo broadcast system of the broadcast program and convert it according to that system. The demodulation circuit may be selected appropriately.

A method using a pilot signal has been proposed as a method for automatically identifying each method. That is,
The broadcast radio waves of each system include a pilot signal for indicating that it is a stereo broadcast with a stereo indicator, and the frequency of this pilot signal differs depending on the system. In other words, the AM-PM method has 5
Hz, ISB method is 15Hz, C-QUAM method is 25Hz,
The CPM method is 55Hz. Therefore, each system can be identified by identifying the pilot signal frequency.

Therefore, it is necessary to extract the pilot signal from the demodulated signal. In order to separate the pilot signal, four types of pilot signal frequency bands (5
Hz to 55Hz) are available. In this case, the pilot signal after passing through the filter contains audio modulation frequency components, external noise components, noise components due to electric field fluctuations, and the like. These components cause erroneous identification of the pilot signal frequency and therefore the identification of the four methods.

Although it is conceivable to use filters tuned to each of the four pilot signal frequencies, it is technically difficult to construct such a filter, and even if it were realized, it would be expensive. Furthermore, in addition to the frequency components of the pilot signal of the target system, such a filter also extracts noise components in the pilot signal frequency bands of the other three systems, and these noise components can also cause malfunctions as described above. Become.

[Means for solving problems]

In order to solve the above-mentioned problems, the present invention provides a plurality of demodulation circuits for different amplitude modulation stereo broadcast systems, each of which has a different pilot signal frequency for stereo display; a circuit for separating a pilot signal from the demodulated output of the demodulating circuit; a frequency identifying circuit for identifying the frequency of the pilot signal separated by the separating circuit; and a plurality of demodulating circuits based on the pilot signal frequency identified by the frequency identifying circuit. The amplitude modulation stereo broadcast system identification device includes a circuit for selecting a demodulation circuit corresponding to the pilot signal frequency from among the demodulation circuits, which further comprises: a hysteresis comparator for shaping the waveform of the pilot signal from the separating circuit; It includes a differential judgment circuit that differentiates the input to the comparator and detects and judges excessive noise input, and a memory holding circuit that holds the identification result of the frequency identification circuit for a predetermined period according to the detection output of the differential judgment circuit. An amplitude modulation stereo broadcast system identification device is provided.

[Effect]

According to the present invention, a pilot signal separated from a received radio wave is compared with a certain hysteresis width by a hysteresis comparator and its waveform is shaped. Thereby, noise components within this hysteresis width are removed, and it is possible to prevent the frequency identification circuit from malfunctioning in its identification operation due to the noise components.

Furthermore, even if a noise component with an excessive voltage level that cannot be removed even by the hysteresis comparator is input, the noise component is detected by the detection circuit, and the frequency identification state is determined by the holding circuit. Since the previous state is maintained, it is possible to prevent identification errors due to excessive level of noise.

〔Example〕

An AM stereo broadcast system identification device as an embodiment of the present invention is shown in FIG. In FIG. 1, received radio waves are received and detected by a high frequency receiver (not shown) and converted into an intermediate frequency (IF) signal. The IF signal is guided to the input terminals of four demodulation circuits 11 to 14, each using a different method.

Demodulation circuits 11, 12, 13, and 14 are each
AM-PM method, ISB method, C-QUAM method,
It is a CPM method demodulation circuit, and each demodulation circuit 11,
Pilot signal frequencies 12, 13, and 14 are 5Hz, 15Hz, 25Hz, and 55Hz, respectively. These demodulation circuits 11 to 14 extract L from the input IF signal.
The channel signal and the R channel signal are separated and guided to the input terminal of the system switching circuit 5.

The SUB signal (subchannel signal) obtained by the CPM demodulation circuit 14 is guided to the pilot filter 2. The pilot filter 2 operates in a frequency band including four types of pilot signals, that is, from 5Hz to
This is a bandpass filter that passes the 55Hz band, and removes the L and R channel signal components to extract the pilot signal component.

The output of the pilot filter 2 is led to the input terminals of a hysteresis comparator 3 and a differential determination circuit 5. Hysteresis comparator 3
is a circuit that shapes the pilot filter output signal, which is the signal to be identified, into a pulse waveform, and has the function of shaping the waveform while removing the influence of low-voltage noise components superimposed on the pilot signal. . The output of the hysteresis comparator 3 is led to an input terminal of a frequency discrimination circuit 4.

The frequency identification circuit 4 includes a pilot filter 2
This circuit identifies which of the four types of pilot signals 5Hz, 15Hz, 25Hz, and 55Hz the frequency of the pulsed pilot signal inputted through the waveform shaping circuit 3 corresponds to. The identification result is sent to the system switching circuit 7 in the form of an identification signal that sets one of the four output lines corresponding to each system to a high level. The system switching circuit 7 is composed of, for example, an analog switch, and selectively switches one of the four systems of demodulation circuits 11 to 14 based on an identification signal.

The differential determination circuit 5 is a circuit that detects excessive voltage level noise by determining whether the differential value of the input signal exceeds a certain constant value, and the detection output is led to the input terminal of the memory holding circuit 6. .

The memory holding circuit 6 is a circuit that continuously outputs a "0" level output signal S(q) to its output terminal when a "0" level signal is applied to its input terminal. The output signal S(q) is led to the latch control terminal of the frequency identification circuit 4, thereby holding the identification output of the frequency identification circuit 4 at the value of the previous measurement cycle. A reset signal S(r) is led from the frequency identification circuit 4 to the reset terminal of the memory holding circuit 6,
This reset signal S(r) causes the memory holding circuit 6 to
The output signal of is reset from the "0" level to the "1" level.

An example of the configuration of the hysteresis comparator 3 in the device shown in FIG. 1 is shown in FIG. In FIG. 2, the signal from pilot filter 2 is conducted via resistor R ₃₁ to the non-inverting input terminal of voltage comparator Q ₃₁ and via resistor R ₃₂ to capacitor C ₃₁ and voltage comparator Q. ₃₁ is led to the inverting input terminal. Capacitor _C31 is for creating a DC reference voltage level and applying it to the inverting input terminal of voltage comparator _Q31 . A feedback resistor R ₃₃ is connected between the non-inverting input terminal and the output terminal of the voltage comparator Q ₃₁ to provide hysteresis characteristics to the output.

The operation of this hysteresis comparator 3 will be explained below. Now, the input signal to the hysteresis comparator 3 has a DC voltage component V at point a.
(a) and an AC voltage component v(a), at point C, only the DC voltage component V(a) appears and the AC voltage component v(a) becomes zero, and the DC voltage component v(a) is input as a reference voltage to the inverting input terminal of voltage comparator _Q31 .

The output signal voltage level at the output terminal (ie, point d) of voltage comparator Q ₃₁ is either high level V (dh) or low level V (dl). Therefore, when the output signal of comparator _Q31 is at a high level V(dh), the DC voltage V(bh) at point b is V(bh)= _R31・V(dh)+ _R33・V(a)/ On the other hand, when the output signal is at a low level V (dl), the DC voltage V ( _bl ) at point b is V (bl) = R ₃₁ · V (dl) + R _{33 ·} V (a) / It becomes R ₃₁ + R ₃₃ .

In this way, the DC bias voltage at point b changes according to the output voltage level of comparator Q ₃₁ , and in the end, comparator Q ₃₁ detects the difference V (H) between V (bh) and V (bl). It has a hysteresis characteristic with a hysteresis width.

V(H)=R ₃₁ /R ₃₁ +R ₃₃ (V(dh)-V(dl)) To make the above operation easier to understand, the signals at each point a, b, c, and d in Figure 2 are shown below. The waveform is the fifth
As shown in the figure. FIG. 51 shows a signal waveform S(a) at point a, and shows a pilot signal when no noise is superimposed. 5. FIG. 2 shows the signal waveform S(b) at point b and the signal waveform S(c) at point C.
FIG. 3 shows a signal waveform S(d) at point d.

An example of the configuration of the differential determination circuit 5 in the apparatus shown in FIG. 1 is shown in FIG. In FIG. 3, the signal from the pilot filter 2 is conducted to the bases of transistors Q ₅₁ and Q ₅₂ via a delay circuit 51 consisting of a resistor R ₅₁ and a capacitor C ₅₁ , and via a resistor R ₅₂ . Transistor Q ₅₁
and guided by the emitsuta of Q ₅₂ . transistor
Q ₅₁ and Q ₅₂ have their emitters connected together and are inserted in series with resistors R ₅₃ and R ₅₄ between the power supply V _cc and ground. The transistors Q ₅₁ and Q ₅₂ have both the function of detecting an excessive input signal and the function of amplifying the detected signal, and the gain G is G=R ₅₃ /R ₅₄ .

The collector of transistor Q ₅₁ is a transistor
Connected to the base of Q ₅₃ . The collectors of transistors Q ₅₂ and Q ₅₃ are connected to the base of transistor Q ₅₄ . Transistor _Q54 is a circuit for interfacing with the next stage digital circuit,
The detection signal is pulsed.

The operation of this differential discrimination circuit 5 will be explained below. The input voltage of the delay circuit 51 is v ₁ and the output voltage is v ₂
and the delay time is T(d). This delay time T(d)
is expressed as T(d)=C ₅₁・R ₅₁ . Input signal voltage to differential discrimination circuit 5
v ₁ is connected to transistor Q ₅₁ and through resistor R ₅₁
It is immediately applied to the emitter of Q ₅₂ and, after being delayed by time T(d) in delay circuit 51, applied to the bases of transistors Q ₅₁ and Q ₅₂ .
Therefore, the difference voltage between the input voltage v ₁ and the output voltage v ₂ is applied between the base and emitter of transistors Q ₅₁ and Q ₅₂ .

When this differential voltage exceeds the base-emitter voltage V _BE of transistor Q ₅₁ or Q ₅₂ and forward biases their base-emitter, transistor Q ₅₁ or Q ₅₂ becomes conductive, thereby causing
An output signal is delivered from transistor _Q54 . That is, when a positive excessive input voltage is applied, transistors Q ₅₁ , Q ₅₃ and Q ₅₄ are turned on and a "0" level pulse signal is output, and when a negative excessive input voltage is applied, a "0" level pulse signal is output. Transistor Q ₅₂
and _Q54 are made conductive, and a pulse signal of the same "0" level is output.

The difference voltage between the input voltage v ₁ and the output voltage v ₂ is the base voltage.
The condition for exceeding the emitter voltage V _BE is that the time rate of change of the input signal voltage is equal to or higher than V _BE /T(d). The delay time T(d) is selected such that the pilot signal frequency itself does not cause the detection operation of the circuit. Therefore, the cutoff frequency needs to be 55Hz or higher.

An example of the configuration of the frequency identification circuit 4 in the apparatus shown in FIG. 1 is shown in FIG. In FIG. 4, the pulse waveform signal from the hysteresis comparator 3 is guided to the input terminal of a counting section 42 via an AND gate 41. A gate signal S(g) having a pulse waveform having a unit time width is introduced from a gate pulse generator 43 to the other input terminal of the AND gate 41.

A reference oscillator 44 is connected to the gate pulse generator 43, and based on the reference frequency signal from the oscillator 44, the gate pulse generator 43 determines the pulse width of the gate signal S(g) or the gate signal S(g). ,
The timing of generating the latch signal S(l) and reset signal S(r) is controlled. These gate signals S
(g), latch signal S(l) and reset signal S(r)
The signal waveform and generation timing of is shown in FIG. The gate signal S(g) is shown in FIG. 61, and the gate signal S(g) is shown in FIG.
The latch signal S(l) is shown in FIG. 6, and the reset signal S(r) is shown in FIG.

Reset signal S from gate pulse generator 43
(r) is led to the reset terminal of the counting section 42. Further, the latch signal S(l) from the gate pulse generator 43 is passed through the AND gate 45 to the latch circuit 4.
6 latch timing control terminal. The output signal S(g) from the memory holding circuit 6 is introduced to the other input terminal of the AND gate 45.

The counting unit 42 measures the frequency of the input signal by counting the number of pulses input through the AND gate 41 within a unit time determined by the gate signal S(g), and determines whether the input signal frequency is the same as that of the four types of pilots. This is a circuit that identifies which of the signal frequencies corresponds to, and sends out a "1" level signal to the output line of the corresponding method among the four output lines 47. The latch circuit 46 latches the identification signal from the identification section 42 at the timing of the latch signal S(l).
The output signal of the latch circuit 46 is led to the switching control input terminal of the system switching circuit 7.

The operation of the apparatus of FIG. 1 will now be described with reference to FIG.

The received broadcast radio wave is converted into an IF signal by a high frequency receiving section (not shown), and the IF signal is input to demodulation circuits 11 to 14, respectively.

The SUB signal obtained by the demodulation circuit 14 is guided to the pilot filter 2, and the pilot signal frequency band components are extracted by the pilot filter 2. The output signal waveform of this pilot filter 2 is shown in FIG. This output signal waveform A is a waveform when a high frequency noise component is superimposed on a low frequency pilot signal. The output signal of this pilot filter 2 has hysteresis.
It is input to comparator 3.

The hysteresis comparator 3 compares the output signal waveform A with the reference voltage V(a) and shapes the waveform into a pulse waveform. In the case of a circuit in which the resistor _R33 is removed), the waveform after shaping becomes waveform B as shown in FIG. 72. That is, the pulse waveform is subdivided by noise components other than the pilot signal component, which causes the frequency identification circuit 4 in the next stage to malfunction.

On the other hand, when the output signal waveform A is shaped by the hysteresis comparator 3, the noise component within the range that does not exceed the differential voltage V(H) is generated by hysteresis.
Since the output pulse waveform is removed by the hysteresis width of the comparator, the output pulse waveform is not subdivided by noise components, and only the pilot signal is converted into a pulse waveform, as shown by waveform c in Figure 7. . Therefore, it is possible to prevent a discrimination error in the frequency discrimination circuit 4, and the system switching circuit 7 can select the correct demodulation circuit corresponding to the pilot signal frequency.

On the other hand, if a noise component exceeding the range of the differential voltage V(H) is superimposed on the pilot signal, the hysteresis comparator 3 cannot remove the noise component, which may cause the frequency discrimination circuit 4 to malfunction. Such excessive voltage level noise components are detected by determining the differential value in the differential determining circuit 5, and the output signal S of the memory holding circuit 6 is thereby detected.
(q) is held at the "0" level.

This “0” level output signal S(q) closes the AND gate 45 in the frequency identification circuit 4, and the latch signal S from the gate pulse generator 43 closes.
(l) is no longer applied to the latch timing control terminal of latch circuit 46. Therefore, the contents of the latch circuit 46 are held at the identification contents of the previous measurement cycle, and erroneous identification operations due to excessive level noise components can be prevented.

Note that the memory holding circuit 6 is connected to the gate pulse generator 4.
It is reset by the reset signal S(r) from 3 at the timing shown in FIG.
will be open again in the next measurement cycle, and
Therefore, normal frequency identification operation continues.

Various modifications are possible in implementing the invention. For example, a hysteresis comparator,
The frequency identification circuit, differential determination circuit, etc. are not limited to the configurations shown in FIGS. 2 to 4, but may have other circuit configurations as long as they achieve the intended functions.

〔Effect of the invention〕

According to the present invention, even if there are noise components in the pilot signal frequency band, it is possible to prevent malfunctions due to these noise components, identify the four AM stereo broadcasting systems, and enjoy stereo broadcasting.In particular, it is possible to differentiate the pilot signal. Since a differential judgment circuit is provided to hold the identification content of the frequency identification circuit according to the detection output of this differential judgment circuit, even if an excessive noise component is superimposed on the pilot signal, excessive level noise can be detected. Identification errors due to components can be prevented.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an AM stereo broadcast system identification device as an embodiment of the present invention, and FIG.
3 is a circuit diagram of the differential determination circuit in the device shown in FIG. 1, FIG. 4 is a block diagram of the frequency discrimination circuit in the device shown in FIG. FIG. 6 is a waveform diagram of each part of the hysteresis comparator, FIG. 6 is a waveform diagram of the output signal from the gate pulse generator in the circuit of FIG. 4, and FIG. 7 is a waveform diagram of input/output signals of the hysteresis comparator. 11-14...Demodulation circuit, 2...Pilot
Filter, 3...Hysteresis comparator,
4... Frequency identification circuit, 5... Differential determination circuit, 6
...memory holding circuit, 7...method switching circuit.

Claims (1)

[Scope of Claims] 1. A plurality of demodulation circuits for different amplitude modulation stereo broadcasting systems, each of which has a different pilot signal frequency for stereo display, which separates the pilot signal from the demodulated output of the demodulation circuits. a frequency identification circuit 4 that identifies the frequency of the pilot signal separated by the separating circuit; and a frequency identification circuit 4 that selects the pilot signal from among the plurality of demodulation circuits based on the pilot signal frequency identified by the frequency identification circuit 4. In an amplitude modulation stereo broadcast system identification device comprising a circuit 7 for selecting a demodulation circuit corresponding to a signal frequency, a hysteresis comparator 3 for shaping the waveform of the pilot signal from the separating circuit 2 is provided.
and a differential determination circuit 5 that differentiates the input to the hysteresis comparator 3 to detect and determine excessive noise input; 1. An amplitude modulation stereo broadcast system identification device comprising a memory holding circuit 6 for holding data for a period of time.