JPH0358212B2 - - 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.
There are various methods such as Harris' V-CPM method, Kahn/Hazelchin's ISB method, Motorola's C-QUAM method, and Beller's AM-FM method. 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 V-CPM method is 55Hz. Therefore, each system can be identified by identifying the pilot signal frequency.

On the other hand, when the electric field of the received radio wave is weak, white noise increases, but this white noise is evenly distributed in the frequency band of the pilot signal. Therefore, when the received radio waves are weak, this white noise makes it impossible to identify the pilot signal and therefore the stereo broadcasting system, making it impossible to enjoy stereo broadcasting.

As a way to deal with this, a weak electric field is detected by measuring the S (signal) level and when this voltage level becomes below a certain reference voltage, and when the electric field is weak, the receiver mode is changed from stereo mode to monaural mode.
One possible method is to forcibly switch to the mode to prevent malfunction of the system identification circuit. however,
In car-mounted receivers, the received electric field tends to be unstable and fluctuate greatly, and instantaneous weak electric fields can frequently occur. For this reason, the above method has the problem of frequently switching between monaural and stereo, making the sound difficult to hear.

[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. A counter circuit that counts signals and identifies their frequencies, and a pilot signal that is supplied to the counter circuit in one measurement while stopping supply of the pilot signal to the counter circuit when the signal level is below a predetermined value. There is provided an amplitude modulation stereo broadcast system identification device characterized by comprising a circuit that performs control so that the time period during which the signal is displayed is a certain period of time.

[Effect]

The pilot signal separated by the pilot signal separating circuit is input to the frequency identification circuit. In the frequency identification circuit, the input pilot signal is
For each measurement, the pilot signal is supplied to a counter circuit for a certain period of time to count the pilot signal and identify its frequency. At this time, the supply of the pilot signal to the counter circuit is stopped during the period when the signal level detected by the signal level detection circuit is below a predetermined value, and the total supply time excluding this stop period is constant. It is controlled so that

〔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, V
- It is a CPM method demodulation circuit, and each demodulation circuit 11,
The 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.

SUB obtained by the V-CPM demodulation circuit 14
The signal (subchannel signal) is guided to a pilot filter 2. Pilot filter 2 is 4
This is a band pass filter that passes a frequency band including the pilot signal of the system, that is, a band of 5 Hz to 55 Hz, and extracts the pilot signal component by removing the L and R channel signal components.

The output of the pilot filter 2 is guided to a waveform shaping circuit 3. The waveform shaping circuit 3 is a circuit that shapes the pilot filter output signal as the signal to be identified into a pulse waveform. This waveform shaping circuit 3
can be constructed from a so-called hysteresis comparator, thereby making it possible to perform waveform shaping by removing the influence of low voltage level noise components superimposed on the pilot signal. The output signal of the waveform shaping circuit 3 is guided to the input terminal of the 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 5 Hz, 15 Hz, 25 Hz, and 55 Hz corresponds to the frequency of the pulsed pilot signal inputted through the waveform shaping circuit 3. The identification result is sent to the system switching circuit 5 in the form of an identification signal that sets one of the four output lines 6 corresponding to each system to a high level. The system switching circuit 5 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.

A detection circuit 7 for detecting the signal level of received radio waves is connected to the frequency identification circuit 4.

The block configuration of the frequency identification circuit 4 in the device shown in FIG. 1 is shown in FIG. In FIG. 2, the pilot signal from waveform shaping circuit 3 is guided to one input terminal of AND gate 47. In FIG. A comparison output signal from the comparator 49 is introduced to the other input terminal of the AND gate 47 . The level value signal S(L) from the signal level detection circuit 7 is guided to the comparison side input terminal of the comparator 49.
(L) is compared with the reference voltage value V (ref) of the reference power source 45, and the comparison output signal S(a) is applied to the input terminal of the AND gate 47, and also to one input terminal of the AND gate 48. give.

The output of AND gate 47 is guided to one input terminal of AND gate 41. A gate signal S(g) from a gate pulse generator 43 is introduced to the other input terminal of the AND gate 41. The gate pulse generator 43 is connected to a reference oscillator 44 via an AND gate 48, and generates a gate signal S(g), a reset signal S(r), and a latch based on the reference frequency signal provided from the reference oscillator 44. Generates a signal S(1) etc. at a predetermined timing, and also generates a gate signal S(1) etc.
The pulse width of (g) is made to be a unit time width (1 second). These signals S(g), S(1), S(r)
The signal waveform and generation timing of
2 and 3, respectively.

The output of the AND gate 41 is led to the input terminal of the counting section 42. The counting unit 42 counts the number of pulses of the input pilot signal, identifies whether the pilot signal frequency of each method is 5, 15, 25, or 55 Hz, and selects the corresponding one of the four output lines. is set to the "1" level and sent to the latch circuit 46.

A reset signal S(r) is introduced from the gate pulse generator 43 to the reset terminal of the counting section 42, and the count value is reset by the erroneous reset signal S(r). A latch signal S(1) is introduced from the gate pulse generator 43 to the latch control terminal of the latch circuit 46, and the latch timing is controlled by the latch signal S(1). Latch circuit 46
The output is led to the system switching circuit 5 via the output line 6.

An example of the configuration of the counting section 42 is shown in FIG. In FIG. 4, the output from AND gate 41 is directed to the input terminal of 1/5 frequency divider 420. The frequency divided output of the frequency divider 420 is led to the input terminal of the AND gate 427, and the frequency divided output of the frequency divider 420 is led to the input terminal of the AND gate 427.
423 input terminals. counter 421
423 are circuits that output a "1" level output signal when the number of input pulse signals is 2 to 4, 4 to 6, and 9 to 13, respectively.

The output of counter 423 is led to the input terminal of latch circuit 46, and also to each input terminal of AND gates 427-429 via inverter 426. Similarly, the output of the counter 422 is led to the input terminal of an AND gate 429, and is passed through an inverter 425 to AND gates 427, 4.
28 input terminals, and the output of the counter 421 is guided to the input terminal of an AND gate 428 and, via an inverter 424, to the input terminal of an AND gate 427. and gate 42
Outputs 7 to 429 are respectively led to each input terminal of the latch circuit 46 via four output lines.

In the counter shown in FIG. 4, the input pilot signal is once divided into 1/5 by a frequency divider 420. The pilot signal frequencies to be identified are therefore converted from 5 Hz to 1 Hz, 15 Hz to 3 Hz, 25 Hz to 5 Hz, and 55 Hz to 11 Hz. counter 42
1 sends an output to the latch circuit when the number of input pulses is 2 to 4, counter 422 is 4 to 6, and counter 423 is 9 to 13.

When each counter 421 to 423 sends an output, the AND gates 427 to 429 are closed via an inverter connected to its output terminal, so that a lower counter with a lower number of counters than itself outputs to the latch circuit. is prevented from being sent. This can prevent a situation in which the lower counter also outputs an integer number of counts even though the upper count is transmitting an output. If the pilot signal is once divided by 1/5 and counted, the counters 421 to 4
The number of connected stages of binary counters constituting 23 can be significantly reduced.

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

The received radio waves are IFed by a high frequency receiver (not shown).
The IF signal is converted into a signal, and the IF signal is sent to the demodulation circuits 11 to 1.
4 respectively. The SUB signal obtained by the demodulation circuit 14 is input to a pilot filter 2 to extract a pilot signal, which is shaped into a pulse waveform by a waveform shaping circuit 3 and then input to a frequency discrimination circuit 4.

In the frequency identification circuit 4, when the signal level of the received radio wave is sufficiently large, a high level output is sent from the comparator 49 and the AND gate 4
Since gates 7 and 48 are open, the input pilot signal passes through AND gates 47 and 41 and is input to the counting section 42. In this case, the opening time of the AND gate 41 is adjusted to one second by the gate signal S(g) from the gate pulse generator 43. Therefore, the number of pulses of the pilot signal counted by the counting section 42 during this one second becomes the pilot signal frequency.

When the counting section 42 identifies whether the frequency of the input pilot signal is 5Hz, 15Hz, 25Hz, or 55Hz, the counting section 42 transmits the identification result to the latch circuit 46.
Send to. The latch circuit 46 has a latch signal S
At the timing (L), the identification result is latched and the identification result is sent to the method switching circuit 5.
As a result, the system switching circuit 5 selects a demodulation circuit suitable for the stereo broadcasting system of the received radio waves.

Next, the operation of the frequency discrimination circuit 4 when the signal level of the received radio wave momentarily becomes weak will be explained below with reference to FIG. FIG. 5 is a signal waveform diagram of each part of the frequency identification circuit 4 of FIG. 2. In FIG. 5, S(L) is the level value signal from the signal level detection circuit 7, S(a) is the comparison output signal of the comparator 49, and S(b) is the level value signal from the reference oscillator 44 which has passed through the AND gate 48. The reference frequency signal S(c) is a pilot signal passed through an AND gate 47.

Level value signal S from signal level detection circuit 7
(L) is larger than the reference voltage value V(ref) of the reference power supply 45, the comparison output signal S from the comparator 49
(a) is high level and AND gate 47, 48
has been opened and the operations described above are taking place.

The level value signal S(L) momentarily changes to the reference voltage value V
(ref) or less, it becomes impossible to identify the pilot signal. In this case, the comparison output signal S(a) of the comparator 49 becomes low level, and the AND gates 47 and 4
8 will be closed. As a result, the pilot signal S(c)
is no longer supplied to the counting section 42, and
The reference frequency signal S(b) is no longer supplied to the gate pulse generator 43.

The gate pulse generator 43 generates a reference frequency signal S(b)
The pulse length of the gate signal S(g) is determined by counting the pulse length of the gate signal S(g), but this counting is stopped while the reference frequency signal S(b) is cut off.
Therefore, the gate signal S(g) does not become low level even if the pulse width exceeds 1 second.

The level value signal S(L) is again the reference voltage value V(ref)
When it returns to above, the comparison output signal S of the comparator 49
(a) becomes high level again and gate 47,
48 is opened, and the supply of the pilot signal S(c) to the counting section 42 and the supply of the reference frequency signal S(b) to the gate pulse generator 43 are restarted. The gate pulse generator 43 sets the gate signal S(g) to a low level and activates the AND gate 41 when the count value of the pulse width of the gate signal S(g) is added to the value before counting stops and reaches 1 second. Close.

As a result of the above operation, when the received electric field becomes weak and it becomes impossible to identify the pilot signal (in the section where the level value signal S(1) is less than or equal to the reference voltage value V(ref)), the counting unit 42 Since counting of signals can be interrupted and restarted again after the received electric field becomes strong, it is possible to prevent malfunctions in frequency identification caused by momentary weakening of the received electric field.

〔Effect of the invention〕

According to the present invention, even when the received radio waves momentarily fluctuate to a weak electric field, it is possible to properly identify the pilot signal frequency and therefore each stereo broadcasting system without error, and the radio waves can be properly identified. Even when reception conditions are poor, you can enjoy stereo broadcasts without switching to monaural mode. This is particularly useful when the present device is used in a vehicle-mounted radio where radio wave reception conditions are not necessarily good.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an amplitude modulation stereo broadcast system as an embodiment of the present invention, FIG. 2 is a block diagram of a frequency discrimination circuit in the device shown in FIG. 1, and FIG. 3 is a gate pulse generator in the circuit shown in FIG. 4 is a block diagram of the counting section in the circuit of FIG. 2, and FIG. 5 is a diagram of signal waveforms of various parts of the circuit of FIG. 11-14...Demodulation circuit, 2...Pilot
Filter, 3... Waveform shaping circuit, 4... Frequency identification circuit, 5... System switching circuit, 7... Signal level detection circuit, 42... Count section, 43... Gate pulse generator, 45... Reference power supply , 46...latch circuit, 49... comparator.

Claims (1)

[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, a circuit for separating the pilot signal from the demodulated output of the demodulation circuit, and a circuit for separating the pilot signal from the demodulated output of the demodulation circuit. a frequency identification circuit that identifies the frequency of the pilot signal separated by the circuit; and a demodulation circuit corresponding to the pilot signal frequency is selected from among the plurality of demodulation circuits based on the pilot signal frequency identified by the frequency identification circuit. The amplitude modulation stereo broadcast system identification device includes a signal level detection circuit for detecting the signal level of a received radio wave, and the frequency identification circuit includes a counter circuit that counts pilot signals and identifies the frequency thereof; When the signal level falls below a predetermined value, the supply of the pilot signal to the counter circuit is stopped, and control is performed so that the time during which the pilot signal is supplied to the counter circuit is a fixed time in one measurement. An amplitude modulation stereo broadcast system identification device comprising a circuit.