JPH0418270Y2 - - Google Patents

Description

[Detailed explanation of the idea] <Technical field> The present invention relates to an AM stereo receiver.

<Prior Art> Some conventional AM stereo receivers use the CQUAM method (compatible quadrature modulation method). this
As shown in Figure 3, in a CQUAM type AM stereo receiver, a received AM stereo signal is converted to an intermediate frequency through a tuning circuit a and a mixing circuit b, and then inputted into an AM stereo decoder section c. Ru.
This AM stereo signal includes an L+R signal in amplitude and an LR signal and a pilot signal in phase.

Then, the AM stereo signal input to the AM stereo decoder section c is branched, and one side is detected by the envelope detection circuit d as an L+R signal.
The signal is applied to the matrix circuit e and the amplitude comparison circuit g. Further, the other AM stereo signal passes through a variable gain control circuit h, and an L+R signal is synchronously detected by an I detection circuit k, and is applied to the amplitude comparison circuit g. The amplitude comparison circuit g is an envelope detection circuit d
L+R given from and I detection circuit k, respectively.
The amplitudes of the signals are compared, and a correction signal 1/cosφ is output so that the amplitudes of both signals are equal, and the gain of the variable gain control circuit h is controlled by this correction signal.

Further, the AM stereo signal that has passed through the variable gain control circuit h is subjected to synchronous detection of the L-R signal and the pilot signal in the Q detection circuit m, and these signals are respectively applied to the switch circuit n and the pilot signal detection circuit o. It will be done. When the pilot signal detection circuit o detects the pilot signal, it drives the switch circuit n, so that the LR signal detected by the Q detection circuit m is inputted to the matrix circuit e via the switch circuit n. As a result, the L signal and the R signal are separated and taken out from the matrix circuit e.

By the way, since the I detection circuit k and the Q detection circuit m perform synchronous detection, the AM signal converted to the intermediate frequency is
After inputting the stereo signal to the voltage controlled oscillator (hereinafter referred to as VCO) p and dividing the constant frequency signal generated from the VCOp to 450kHz,
The phases are set to 0° and 90° and are applied to the I detection circuit k and the Q detection circuit m, respectively.

On the other hand, in the manual tuner type, the local frequency of the mixing circuit is continuously changed in order to tune to the desired signal frequency, but if an AM stereo signal of the desired signal frequency is received in a detuned state,
The intermediate frequency created by the desired signal frequency and the local frequency varies. In this case, although the VCO is controlled to follow the fluctuations in the intermediate frequency and oscillate at a constant frequency, the frequency range of the VCO's lock range is approximately ±4kHz relative to 450kHz, so if the VCO is significantly detuned, In this case, the frequency range of the VCO is outside the above lock range. Then,
The VCO loses its lock and its oscillation frequency becomes the free-run frequency, causing a sudden change in frequency. At this time, the I detection circuit and the Q detection circuit receive and detect the change in the frequency of the VCO as phase information, so this becomes an out-of-lock noise. As described above, conventional CQUAM type stereo receivers, especially manual tuner type ones, have the disadvantage that even a slight detuning causes the VCO to go out of lock range and generate noise.

<Purpose of the invention> The invention was developed in view of the above-mentioned problems. In a CQUAM type AM stereo receiver, when the AM stereo signal is tuned and the electric field is strong, the frequency is high. You can get a sound with a stereo feel that extends to the band, and even if the stereo signal becomes a weak electric field due to detuning, it prevents noise from being mixed in due to the VCO losing lock, resulting in sound with less interference. The purpose is to obtain the following.

<Structure of the invention> In order to achieve the above-mentioned purpose, the present invention provides a CQUAM type AM stereo equipped with an AM stereo decoder section that separates an AM stereo signal converted into an intermediate frequency into an L signal and an R signal and extracts the signal. In the receiver, a wideband filter and a narrowband filter are sequentially connected in series before the input side of the AM stereo decoder section, the AM stereo decoder section is connected to the output section of the wideband filter, and an envelope detection circuit is connected to the output section of the narrowband filter. a level detection circuit that detects the signal level of the carrier wave detected by the envelope detection circuit and outputs a selection signal for selecting the L+R signal and the L,R signal according to the detected signal level; an L+R signal that has passed through the narrow band filter and has been envelope detected by an envelope detection circuit in response to the selection signal from the level detection circuit;
A signal switching circuit that switches and outputs the L signal and R signal separated by the AM stereo decoder section.

<Example> Hereinafter, the present invention will be described in detail based on an example shown in the drawings.

FIG. 1 is a block diagram of the main parts of a CQUAM type AM stereo receiver showing an embodiment of the present invention. In the same figure, numeral 1 indicates the entire AM stereo receiver, and 2 is an AM stereo decoder section that separates and extracts the AM stereo signal converted into an intermediate frequency into an L signal and an R signal, and this AM stereo decoder The structure and operation of section 2 are the same as those of the conventional example shown in FIG.

In this embodiment, a wideband filter 4 and a narrowband filter 6 having frequency selection characteristics as shown in FIG. An AM stereo decoder section 2 is connected via an intermediate frequency amplifier 8, and an envelope detection circuit 10 that performs envelope detection is connected to the output section of the narrowband filter 6.

12 is a voltage conversion circuit that converts the signal level of the carrier wave detected by the envelope detection circuit 10 into a corresponding voltage value; 14 is a voltage conversion circuit that compares the voltage value from the voltage conversion circuit 12 with a reference value; This is a level detection circuit that outputs a selection signal for selecting the L+R signal and the L,R signal according to the signal level of the carrier wave detected by the comparator 16 and a plurality of light emitting diodes 18 connected to the output pin of the selection signal. Equipped with.

20 is a mode selection switch for manually selecting each mode of monaural and stereo; 22 is a selection signal outputted from the level detection circuit 14, passed through the narrow band filter 6, and envelope detected by the envelope detection circuit 10; This is a signal switching circuit that switches and outputs the L+R signal and the L signal and R signal separated by the AM stereo decoder section 2.

This signal switching circuit 22 includes first and second switching transistors 24 and 26, and first and second RS flip-flops 28 and 30 that are individually connected to the collectors of the first and second switching transistors 24 and 26, respectively. , is turned on and off by the output of the first RS flip-flop 28, and is turned on and off by the output of the first switch 32 connected to the L signal output section of the matrix circuit of the AM stereo decoder section 2 and the first RS flip-flop 28, and A.M.
A second switch 34 connected to the R signal output section of the matrix circuit of the stereo decoder section 2;
The third switch 36 is turned on and off by the output of the 2RS flip-flop 30 and is connected to the output of the envelope detection circuit 10, and the third switch 36 is turned on and off by the output of the 2RS flip-flop 30 and The connected fourth switch 38 and the first RS flip-flop 28
The FET 40 has a gate connected to the output section of the FET 40.

In addition, 41 and 42 are the left and right output terminals, 43 is an intermediate frequency transformer, and 44 is the drain of FET40.
An indicator indicating the stereo broadcast reception state is provided between the switch circuit of the AM stereo decoder section 2, 46 is the matrix circuit of the AM stereo decoder section 2, and 48 is the AM stereo decoder section 2.
This is a switch circuit.

Next, the operation of the AM stereo receiver of this embodiment will be explained.

The AM stereo signal converted to an intermediate frequency by a mixing circuit (not shown) passes through an intermediate frequency transformer 43 and a wideband filter 4, and is branched into two.One AM stereo signal passes through a narrowband filter 6 and is subjected to envelope detection. The other AM stereo signal is applied to the circuit 10 via the intermediate frequency amplifier 8 to the AM stereo decoder section 2, respectively. The AM stereo signal input to the envelope detection circuit 10 is subjected to envelope detection and an L+R signal is extracted.
Commonly applied to fourth switches 36 and 38. Further, the signal level of the carrier wave detected by the envelope detection circuit 10 is converted into a voltage value by the voltage conversion circuit 12, and the signal level of the carrier wave detected by the envelope detection circuit 10 is converted into a voltage value by the comparator 1 of the level detection circuit 14.
Join one side of 6. The comparator 16 compares this input voltage with a reference voltage and outputs a selection signal according to the difference between the input voltage and the reference voltage.

Now, when receiving a stereo broadcast, when the mode selection switch 20 is switched to the stereo mode, the mode selection switch 20 is turned on. Moreover, when the AM stereo signal is in tune and has a strong electric field, the output voltage of the voltage conversion circuit 12 becomes large, so the output level from the level detection circuit 14 also becomes high, and each light emitting diode 18 becomes equal to the reference voltage. The light is turned on according to the difference, and this high level selection signal is sent to the second signal switching circuit 22.
It is added to the base of switching transistor 26. Therefore, the second switching transistor 2
6 is turned on. Then, the first switching transistor 24 enters a cut-off state because no current flows through its base. As a result, the first RS flip-flop 28 is set and outputs a high level signal, and the second RS flip-flop 30 is reset and outputs a low level signal. As a result, the first and second switches 32, 34
is turned on, and the third and fourth switches 36 and 38 are turned off. Therefore, the AM
The L signal and R signal separated by the stereo decoder section 2 are taken out from the left and right output terminals 41 and 42 via the first and second switches 32 and 34, respectively. As a result, the L signal and the R signal are extracted from the AM stereo signal that includes a directional high frequency, so that a stereophonic sound can be obtained. Also, if synchronization is achieved, FET4
A high level signal is added to the gate of 0, and the switch circuit 4 of the AM stereo decoder section 2
At this time, the terminal 48a of No. 8 becomes low level, so the indicator 44 indicating stereo signal reception lights up.

On the other hand, when the stereo signal becomes a weak electric field due to detuning, the output voltage of the voltage conversion circuit 12 becomes small, so that the output level from the level detection circuit 14 becomes a low level. Therefore, the light emitting diode 18 is turned off and a low level selection signal is applied to the base of the second switching transistor 26 of the signal switching circuit 22. Therefore, the second switching transistor 26 is in a cut-off state. Then, current flows through the base of the first switching transistor 24, turning it on. As a result, the first RS flip-flop 28 is reset and outputs a low level signal.
The 2RS flip-flop 30 is set and outputs a high level signal. As a result, the third and fourth
Only the switches 36 and 38 are turned on, and the first and second switches 32 and 34 are turned off. For this reason,
Envelope detection circuit 1 passes through narrowband filter 6
The L+R signal detected at
taken from. Furthermore, in this case, even if noise is generated in the AM stereo decoder section 2 due to the VCO being out of lock, both the first and second switches 32 and 34 are off, so
This prevents noise from entering and provides sound with less interference. Furthermore, since a low level signal is applied to the gate of the FET 40, the indicator 44 is turned off.

Note that when receiving monaural broadcasting, when the mode selection switch 20 is switched to monaural mode, the mode selection switch 20 is turned off, so the second switching transistor 26 of the signal switching circuit 22 is turned off. Only the third and fourth switches 36 and 38 are turned on, and only the detected output of the envelope detection circuit 10 is always obtained. Further, the indicator 44 also does not light up.

<Effects of the invention> As described above, according to the invention, tuning can be achieved in a CQUAM type AM stereo receiver.
When the AM stereo signal has a strong electric field, it passes through a wideband filter and is taken out as an L signal and an R signal separated by the AM stereo decoder section, so that a sound with a stereo feel with a frequency extending to a high band can be obtained. In addition, when the stereo signal becomes a weak electric field due to detuning, the L+R signal that has passed through the narrow band filter and is detected by the envelope detection circuit is extracted, so there is no noise introduced due to the VCO being out of lock. This provides excellent effects such as being able to obtain sound with less interference.

[Brief explanation of the drawing]

Figure 1 is a block diagram of the main parts of an AM stereo receiver showing an embodiment of the present invention, Figure 2 is a characteristic diagram of frequency selection of the wideband filter and narrowband filter, and Figure 3 is the configuration of a conventional AM stereo receiver. It is a diagram. 1...AM stereo receiver, 2...AM stereo decoder section, 4...wideband filter, 6...narrowband filter, 10...envelope detection circuit,
14... Level detection circuit, 22... Signal switching circuit.

Claims (1)

[Claims for Utility Model Registration] In a CQUAM type AM stereo receiver equipped with an AM stereo decoder section that separates and extracts an AM stereo signal converted into an intermediate frequency into an L signal and an R signal, the AM stereo decoder section A wideband filter and a narrowband filter are sequentially connected in series at the front stage of the input side, and the AM stereo decoder section is connected to the output part of the wideband filter, and the envelope detection circuit is connected to the output part of the narrowband filter, while the envelope detection circuit a level detection circuit that detects the signal level of the carrier wave detected by the carrier wave and outputs a selection signal for selecting the L+R signal and the L,R signal according to the detected signal level; In response, an L+R signal passed through the narrowband filter and envelope-detected by an envelope detection circuit;
An AM stereo receiver comprising: a signal switching circuit that switches and outputs an L signal and an R signal separated by the AM stereo decoder section.