JPS6312414B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an improvement of an AM stereo receiver, and in particular, to preventing a stereo display element from malfunctioning due to noise (unnecessary signals) generated during tuning operation. purpose. AM stereo broadcasting is known in which a carrier wave is amplitude-modulated by a sum signal (L+R) of left and right stereo signals L and R, and phase-modulated by a difference signal (L-R) and a stereo display signal ID. be. A receiver for receiving such AM stereo broadcasting is shown in FIG. In FIG. 1, 1 is an antenna for receiving AM broadcasting, 2 is a radio frequency amplification stage, 3 is a frequency conversion stage, 4 is an intermediate frequency amplification stage, 5 is an amplitude detection stage for detecting the sum signal (L+R), 6 is a phase detection stage for detecting the difference signal (L-R) and the stereo display signal ID; 7 is an equalizer for level correction of the output signal from the phase detection stage 6; 8 is a preamplifier; 9 is the aforementioned Delay circuit for delaying sum signal (L+R), 1
0 is a detection circuit for detecting the stereo display signal ID included in the output signal from the phase detection stage 6;
Reference numeral 11 denotes a drive circuit for driving a display element 12 such as a light emitting diode, which turns on when receiving an AM stereo broadcast and turns off when receiving an AM monaural broadcast, and 13.
is a matrix stage for matrixing the sum signal (L+R) from the delay circuit 9 and the difference signal from the preamplifier 8 to obtain stereo left and right signals L and R. Now, if an AM stereo broadcast radio wave is received by the antenna 1, the radio wave is amplified by the radio frequency stage 2, mixed with the signal from the local oscillation stage 14 by the frequency conversion stage 3, and converted into an AM intermediate frequency signal (center frequency is converted to 455KHz). The AM intermediate frequency signal is amplified by the intermediate frequency amplification stage 4 and subjected to AM detection by the amplitude detection stage 5 to generate a sum signal (L+R) at the output end of the delay circuit 9. On the other hand, the AM intermediate frequency signal obtained at the output end of the intermediate frequency amplification stage 4 is applied to the phase detection stage 6 and subjected to phase detection. For that reason,
The output terminal of the phase detection stage 6 receives a difference signal (L-
A mixed signal of R) and the stereo display signal ID is obtained. The difference signal (L-R) in the mixed signal is applied to the equalizer 7, subjected to level correction, amplified by the preamplifier 8, and applied to the matrix stage 13. Also, the stereo display signal in the mixed signal
The ID is detected by the detection circuit 10 and the drive circuit 11
Since the signal is applied to the display element 12 through the channel, the display element 12 lights up to indicate that AM stereo broadcasting is being received. Furthermore, the sum signal (L+R) obtained at the output terminal of the delay circuit 9 and the difference signal (L-R) obtained at the output of the preamplifier 8 are matrixed at the matrix stage 13 and output to the first output terminal 15. stereo left signal L
However, the stereo sum signal R is obtained at the second output terminal 16, and AM stereo broadcast reception is achieved. Good reception is achieved in exactly the same way when receiving AM monaural broadcast waves. AM monaural broadcast radio waves are received by antenna 1, amplified by radio frequency amplification stage 2, and then sent to frequency conversion stage 3 to 455kHz.
The signal is converted into an intermediate frequency signal, further amplified by an intermediate frequency amplification stage 4, and then detected by an amplitude detection stage 5.
At that time, since it is an AM monaural broadcast radio wave, there is no difference signal or stereo display signal, so the output of the preamplifier 8 becomes zero, and the output signals obtained at both output terminals 15 and 16 of the matrix stage are the output signals of the delay circuit 9. Only the output signal and the display element 12
does not turn on. As mentioned above, the receiver shown in FIG. 1 can receive AM stereo broadcasts and AM monaural broadcasts well. However, the receiver shown in FIG. 1 has the following drawbacks. In other words, when receiving AM monaural broadcasting and performing a tuning operation by operating the tuning knob, there will be a large DC level fluctuation at the output end of the phase detection stage 6 that corresponds to the S-curve shift according to changes in tuning and detuning. Then, a frequency component that is the same as or near the stereo display signal frequency is generated in this fluctuating component, which is detected by the detection circuit 10, and the display element 12 turns on even though it is receiving an AM monaural broadcast. This will cause a malfunction. The present invention has been made in view of the above points, and will be described below based on one embodiment of the present invention with reference to the drawings. In FIG. 2, 17 is a DC cut capacitor for cutting the DC component contained in the output signal of the phase detection stage 6, and 18 is a resistor 19 and a capacitor 20. a low-pass filter 21 for removing relatively high frequency components such as the included difference signal (LR);
22 is an amplifier circuit, and 22 is a detection device for detecting a signal near the stereo display signal frequency, which is composed of a pair of diodes 23 and 24 connected with opposite polarities, smoothing capacitors 25 and 26, and offset bias resistors 27, 28, and 29. circuit, 30 is the detection circuit 22
A level detection comparator 31 detects the level of a signal detected by a rectifier diode. DC components and components of a predetermined frequency or higher in the output signal of the phase detection stage 6 are removed by a DC cut capacitor 17.
and removed by a low pass filter 18,
Only signals within a predetermined frequency range including the AM stereo display signal frequency are transmitted to the subsequent stage and amplified by the amplifier circuit 21. The output signal of the amplifier circuit 21 is applied to the detection circuit 22 via the DC cut capacitor 32, the positive signal is applied to the positive input terminal of the comparator 30 via the first diode 23, and the negative signal is applied to the positive input terminal of the comparator 30 via the first diode 23. 24 to the negative input terminal of the comparator 30. Offset bias resistors 27, 28 and 29 are connected to the first
and the second diodes 23 and 24 to remove the offset and comparator 3
This is to give an offset to the 0 input end. In the comparator 30, the signal applied to the positive or negative input terminal exceeds the offset, the voltage applied to the positive input terminal is higher than the negative input terminal voltage, or the voltage applied to the negative input terminal is higher than the negative input terminal voltage. When the voltage becomes lower than the positive input terminal voltage, an output signal is generated at the output terminal of the comparator 30. Furthermore, the output signal generated at the output terminal of the comparator 30 is held at its peak for a certain period of time by a diode 31 and a capacitor 33, and is applied as a control signal to the drive circuit 11 of the display element 12. The AM stereo display signal ID indicating AM stereo broadcasting has a very low frequency of 5 Hz and a constant magnitude (approximately 5 mV at the output end of the phase detection stage 6). On the other hand, an unnecessary signal containing a 5 Hz signal component generated due to the S-curve characteristic of the phase detection stage 6 during a tuning operation has a level several hundred times higher than the AM stereo display signal ID. For that reason,
By appropriately setting the reference level of the comparator 30, the circuit shown in FIG. It can be configured to occur. Therefore, according to the circuit shown in FIG. 2, when an unnecessary 5Hz signal component is generated during a tuning operation, the drive circuit 11 of the display element 12 is controlled by the control signal, and the unnecessary 5Hz signal component is generated. A malfunction in which the display element 12 turns on can be reliably prevented. As described above, the present invention can reliably prevent malfunctions of the stereo reception status display element of an AM stereo broadcast receiver, and greatly enhance the functionality of the AM stereo broadcast receiver.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of an AM stereo broadcast receiver for explaining the present invention, and FIG. 2 is a circuit diagram showing an embodiment of the present invention. Explanation of main figure numbers, 5... Amplitude detection stage, 6... Phase detection stage, 12... Display element, 22 ... Detection circuit, 30... Comparator.

Claims (1)

[Claims] 1. In an AM stereo broadcast receiver for receiving an AM stereo signal amplitude modulated by a stereo sum signal and phase modulated by a stereo difference signal and an AM stereo display signal, the stereo difference signal and the AM stereo a detection stage for obtaining a display signal; a display signal detection circuit connected to the detection stage and detecting the AM stereo display signal;
Unwanted signal detection connected to the output end of the detection stage, having a frequency equal to or near the AM stereo display signal frequency, and having a higher level than the AM stereo display signal. a circuit that controls the display signal detection circuit according to the output signal of the unnecessary signal detection circuit, and is driven by the display signal detection circuit.
An AM stereo broadcast receiver characterized in that it is configured to prevent malfunction of an AM stereo display element.