JPS61237538A - If band automatic changeover device for am stereophonic receiver - Google Patents

Abstract

PURPOSE:To express stereophonic sense by passing an IF signal through a broad band pass filter at the stereophonic mode and applying the signal to an IF amplifier to ensure the level of a high frequency sound. CONSTITUTION:When a pilot signal is detected by an AM stereophonic recorder 7, diodes D2, D4 in an IF filter selection circuit 5 are turned on and diodes D1, D2 are turned off and the broad band pass filter F2 is selected. As a result, the IF signal from a mixing circuit 3 is fed to the IF amplifier 6 via the filter F2. In this case, a switch circuit 19 is operated in the decoder 7 to form the stereophonic mode, and a matrix circuit receives a difference signal from a synchronous detector via the switch circuit 19 and a sum signal from an envelope detector, and processes the result and L/R sound signals are outputted respectively from output terminals 22L, 22R.

Description

DETAILED DESCRIPTION OF THE INVENTION <Field of Industrial Application> The present invention relates to an automatic IF band switching device for an AM stereo receiver for receiving compatible stereo signals.

Summary> The present invention provides an IF filter selection circuit having narrowband and wideband filters in the front stage of an IF amplifier, and causes the IF filter selection circuit to select the wideband filter when the broadcast is in stereo, and also to select the wideband filter when the broadcast is in stereo. In such cases, a narrow band filter should be selected.In particular, in stereo, by passing the IF multiplied signal through a wide band filter and supplying it to the IF amplifier, the cut in the high frequency range can be minimized, and the high frequency level can be increased. This is to ensure a stereo effect.

<Prior art> Currently, AM stereo broadcasting is being carried out in some countries such as the United States, but in radio receivers that receive this AM stereo broadcasting, an IF amplifier is installed after the IF amplifier of a normal AM monaural radio receiver. It is designed to incorporate an AM stereo decoder (demodulator).

<Problems to be Solved by the Invention> Therefore, in the conventional AM stereo receiver described above, the IF filter has a relatively narrow bandwidth intended to improve the S/NA, as in ordinary AM monaural radio receivers. (with the frequency characteristics shown in the solid line in Figure 4), it has a spectrum as shown in Figure 7.2 General AM
When the modulated wave is passed through the above-mentioned IF filter, the spectrum of the AM modulated wave becomes as shown in FIG. 8, with the high frequency range on both sides of the modulation frequency being cut.

Therefore, if the above-mentioned IF filter is used as it is for stereo broadcasting, there is a drawback that the level of the treble range is insufficient and a stereo feeling cannot be obtained. This was done in view of the drawbacks of the conventional AM stereo receiver mentioned above, and it is possible to convert the AM modulated wave having the spectrum shown in Fig. 7 above into a broadband I/O signal as shown by the dotted line in Fig. 4.
When passed through the F filter, the spectrum of the AM modulated wave becomes the 9th
As shown in the figure, the high frequency range on both sides is not cut as much as in the case of the IF filter with the characteristics shown in Figure 8 above, and focusing on the fact that the high frequency range remains, AM stereo reception An IF filter selection circuit stage including a narrow band filter and a wide band filter and capable of selectively supplying the IF multiplied signal from the front edge section to the IF amplifier through the narrow band or wide band filter is provided in the front stage of the IF amplifier in the machine. AM after the amplifier
An IF of an AM stereo receiver, comprising a switching circuit that operates depending on whether or not a pilot signal is detected by a pilot signal detection means in a stereo decoder circuit, and causes the IF filter selection circuit stage to select the narrowband or wideband filter. The present invention provides an automatic band switching device.

Therefore, according to the present invention, when the pilot signal detector detects a pilot signal in AM stereo broadcasting, the switching circuit causes the IF filter selection circuit to select the broadband signal. By selecting a filter to minimize the cut in the high frequency range level, the high frequency range level is released and a stereo feeling is created.Also, when the broadcast is AM monaural broadcasting, the pilot signal detection means detects the pilot signal. If not detected, the switching circuit causes the IF filter selection circuit to select a narrow band filter and cut the high frequency range, thereby reducing the S/Nli.
! This will improve the results.

<Example> Hereinafter, an example of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a block electrical circuit diagram of an AM stereo receiver equipped with an embodiment of the automatic IF band switching device according to the present invention, in which 1 is an antenna, 2 is an RF stage, 3 is a mixing circuit,
4 is a local oscillation circuit, 5 is a narrow band filter and a wide band filter, and an IF that can selectively pass the IF multiplied signal to these filters.
Filter selection circuit, 6 IF amplifier, 7 well-known AM
Stereo decoder, 8 is audio amplifier, 9L, 9R
is a speaker, and 10 is a switching circuit that operates depending on whether or not a pilot signal is detected by a pilot signal detector in the AM stereo decoder 7, and when the pilot signal detector detects a pilot signal, It operates to cause the IF filter selection circuit 5 to select a wideband filter, and when the pilot signal detector does not detect all pilot signals, to cause the IF filter selection circuit 5 to select a narrowband filter. It has become.

FIG. 2 shows the IF filter selection circuit 5 and the switching circuit 10.
The IF filter selection circuit 5 receives the IF multiplied signal from the mixing circuit 3 in the primary coil.
) Lance T and this IF Switch diode D inserted between the secondary coil of Lance T and the above IF iron 6
A series circuit consisting of I + D 3 and a narrowband filter F1, and a switch diode D 2 + D 4 and a wideband filter F2 connected in parallel to this series circuit.
The switching circuit 1° has resistors R6+R9 and a light-emitting diode D5 in a loop drawn in a circular shape that receives power supply voltage from a 10B power supply.
are inserted in series with each other, and the monaural/stereo identification terminal 21 of the AM stereo decoder 7 is connected to the midpoint e between the cathode of the light emitting diode D5 and the resistor R6, and the The base of a switching transistor QI with a common emitter is connected to the middle point e2 between the middle point e1 and the resistor R6 via a resistor R7, and the collector of the transistor Q1 is connected to the above-mentioned 10B power supply via a resistor R8. It is made up of.

This switching circuit 10 has a resistance R at its midpoint e2.
3, connected to both ends of the narrow band filter F via R4, and connected to both ends of the wide band filter F2 via a resistor R21 and R4 to the collector of the switching transistor Q1, thereby selecting the IF filter. It is connected to circuit 5.

In addition, in the IF filter selection circuit 5, C+ + 02
is a bypass capacitor.

Next is the AM stereo decoder 7, which is a well-known circuit, and its main components include, as shown in FIG. 3, an input terminal 11, which receives the IF multiplied signal output from the IF amplifier. An envelope detector 18 receives the IF signal (CCQUM signal) inputted from the input terminal II through the branch point 12, performs envelope detection, and outputs a sum signal of I+L+H; An amplifier 14 that amplifies the IF multiplied signal, a synchronous detector 15 that synchronously detects the output signal from the amplifier I4 and outputs an L-R difference signal and a pilot signal (25 Hz), and an amplifier 16 that amplifies a signal that is amplified by the amplifier 16; a band amplifier 17 that amplifies only the pilot signal among the signal components amplified by the amplifier 16; , a pilot decoder 18 outputs a switch drive signal when determining that it is a regular pilot signal, and a difference signal (
L-R) is supplied to the IJ circuit 20, and the transistor Q provided inside the circuit as shown in FIG.
2 to turn on the monaural sound in the AM stereo decoder.
It includes a switch circuit 19 that sets the stereo identification terminal 21 to L (low: zero voltage) level, and output terminals 22L and 22R of the matrix circuit 20.

In addition, in the AM stereo decoder 7, if the broadcast wave is related to AM monaural broadcast, the synchronous detector I5
Since the pilot decoder 18 does not output an equivalent output signal including the pilot signal, the pilot decoder 18 is also connected to the switch circuit 1.
9 also does not operate, so the transistor Q2 in the switch circuit 19 is turned off and the voltage at the output terminal 20 is set to H (high) level, and the matrix circuit 20 does not separate the signals and remains in the L+R monaural state. Output.

The present invention is constructed as described above, and its operation will be explained below.

During monaural broadcasting: When AM monaural broadcasting waves enter the antenna 1, the monaural broadcasting waves pass through the RF stage 2 and are sent to the IF in the mixing circuit 3.
After the double signal conversion, the IF filter selection circuit 5, the IF
The signal is input to an AM stereo decoder 7 via an amplifier 6.

Then, in this AM stereo decoder 7, since the synchronous detector 15 does not output equivalent signals including the pilot signal, the pyrosoto decoder 18 does not output the switch drive signal, and therefore the transistor Q2 of the switch circuit 19 is in the off state. The voltage level of the collector terminal, that is, the voltage level of the monaural/stereo identification terminal 21 of the AM stereo decoder 7 is set to H level in response to the power supply voltage of the 10B power supply.

As a result, the H level voltage at this monaural/stereo identification terminal 21 is transferred from the midpoint e2 of the switching circuit 10 to the above I
It is applied to the resistor R+ + R3 of the F filter selection circuit 5, and based on it, current flows through the route of the secondary coil of the resistor R1 → switch diode D1 → IF) lance and the route of resistor R3 → switch diode D3 → resistor R5. . Moreover, on the other hand, the monaural/stereo identification terminal 21
Since the voltage at H level of
No voltage is applied to the resistors R2 and R4 of the F filter selection circuit 5, so the switch diodes D 2 + D
No current flows through these diodes D2 + D
4 takes the off state.

That is, when the voltage at the monaural/stereo identification terminal 21 of the AM stereo decoder 7 becomes H level, the switch diode D I + D 3 in the IF filter selection circuit 5
is on, switch diodes D2 + D4 are off, and narrowband filter F is selected.
As a result, in the 1F filter selection circuit 5, the IF multiplied signal from the mixing circuit 8 (IF) lance T→switch diode D1→narrowband filter F1→switch diode D
3, is supplied to the F amplifier 6, is amplified, and reaches the AM stereo decoder 7.

In this AM stereo decoder 7, as mentioned above, the switch circuit 19 does not operate and it is in a monaural operating state, so the matrix circuit 20 is operated at its output terminal 22L based on the sum signal of L + H obtained from the envelope detector 13. ,2
A monaural audio signal is output from 2R, passed through an audio amplifier 8, and reproduced from speakers 9L and 9R.

During stereo broadcasting: When an AM stereo broadcast wave enters the antenna 1, the broadcast wave passes through the RF stage 2, is converted into an IF multiplied signal by the mixing circuit 3, and then passes through the IF filter selection circuit 5 and the IF amplifier 6. The signal is then input to an AM stereo decoder 7.

Then, in this decoder 7, the synchronous detector 15 detects and outputs the L-R difference signal and the pilot signal, so the pilot decoder 18 outputs the switch drive signal, and therefore the switch circuit 19 detects the difference signal ( L-R
) is supplied to the matrix circuit 20, and the transistor Q2 is turned on to change the voltage level of its collector terminal, that is, the monaural signal of the AM stereo decoder 7.
The voltage level of the stereo identification terminal 21 is set to L level.

Therefore, in conjunction with this, the switching transistor Q1, which was on in the monaural state, is turned off, so that the collector terminal voltage of the transistor Q1 becomes H level, and the applied H level voltage is applied to the IF filter selection circuit 5. Based on this, current flows through the route of the secondary coil of the resistor R2 → switch diode D2 → IF) lance and the route of resistor R4 → switch diode D4 → resistor Rr+. Also, at this time,
Since the voltage at the monaural/stereo identification terminal 21 has become L level, no voltage is applied to the resistor R+ + R3, and no current flows through the switch diode DI + D3. is in the off state.

That is, when the voltage at the monaural/stereo identification terminal 21 of the AM stereo decoder 7 becomes L level, the switch diode D 2 + D 4 in the F filter selection circuit 5 changes.
is on, DI + D3 is off, and broadband filter F2 is selected. As a result, ■F
In the filter selection circuit 5, the IF multiplied signal from the mixing circuit 3 passes through the route of lance T→switch diode D2→wideband filter F2→switch diode D4, is supplied to the IF amplifier 6, is amplified, and is sent to the AM stereo decoder 7. reach.

At this time, the 4M stereo decoder 7 is in the stereo operating state with the switch circuit 19 operating as described above, so the matrix circuit 20 receives the difference signal (L -R) and the sum signal (L+R) from the envelope detector 13 are received and processed, and separated from the output terminals 22L and 22R.
The audio signal is output and reproduced by the speakers 9L and 9R.

Incidentally, when this stereo broadcast is received, a current flows through the light emitting diode D5, causing the diode 5 to light up, indicating that the stereo operating state is in effect.

In order to better understand the gist of the present invention, FIGS. 4 to 6 show the selectivity characteristics when stereo broadcast radio waves are passed through a filter with a narrow band (see the solid line) and a filter with a wide band (see the dotted line). It shows the relationship between frequency characteristics, distortion rate, degree of separation, and S/H, and it can be seen that the frequency characteristics, distortion rate, and degree of separation are better when the signal is passed through a wideband filter.Only S/N
Although it is slightly inferior to a narrowband filter, it is not a big problem since it is important to extend the high frequency range and obtain a high degree of separation in stereo mode. In the present invention, it is possible to improve the S/N ratio by passing the signal through the narrow band filter when monaural.

<Effects> Since the present invention is configured as described above, when the broadcast radio waves are stereo broadcasting, the high frequency range can be extended to create a stereoscopic feeling, and when the broadcast radio waves are in monaural, the high frequency range can be cut. This makes it possible to improve the S/N ratio and provide an extremely practical AM stereo receiver.

[Brief explanation of the drawing]

FIG. 1 is a block electrical circuit diagram of an AM stereo receiver equipped with an embodiment of an automatic IF band switching device according to the present invention;
Figure 2 is a detailed electrical circuit diagram of the main parts of the same circuit as above, Figure 3 is a diagram of the main part of the AM stereo decoder, and Figures 4 to 6 show how AM stereo broadcast waves are filtered into narrowband filters and wideband filters, respectively. Figure 7 is a diagram showing the spectrum of a general AM modulated wave, Figures 8 and 9 are the same as above. 8 is a diagram showing spectra when the AM modulated wave of FIG. 7 is passed through narrowband and wideband filters, respectively. FIG. ]; Antenna, 5: ■F filter selection circuit, 6: IF
Amplifier, 7: AM stereo decoder, IO; switching circuit. Agent Patent Attorney Aihiko Fuku (and 2 others) lJ Figure IKH! Figure 7 Car θ Figure 9

Claims (1)

[Claims] 1. The front end section converts the broadcast radio waves received by the antenna into an IF signal, and supplies this IF signal to the AM stereo decoder circuit through the IF amplifier.
This is applied to an AM stereo receiver that performs stereo or monaural demodulation depending on whether or not a pilot signal is detected in an M stereo decoder circuit, and is provided before the IF amplifier, and also includes a narrowband filter and a wideband filter. an IF filter selection circuit stage capable of selectively supplying the IF signal from the front end section to the IF amplifier through the narrowband or wideband filter; and pilot signal detection of the pilot signal detection means in the AM stereo decoder circuit. a switching circuit that operates depending on the presence or absence of the IF filter and causes the IF filter selection circuit stage to select the narrowband or wideband filter; and causing the IF filter selection circuit to select the broadband filter,
An automatic IF band switching device for an AM stereo receiver, characterized in that when the pilot signal detection means does not detect a pilot signal, the IF filter selection circuit selects the narrowband filter accordingly. .