JPS6125280B2 - - Google Patents

Description

[Detailed description of the invention] In a public address system that installs one or more speakers in a certain venue, listens to attendees' speech with a microphone, and amplifies the sound from the speakers, the microphone is placed near the speaker. Howling occurs when
For this reason, a transmitter that sends a certain control signal is placed near the microphone, and a receiver is placed near the speaker, so that when the microphone approaches the speaker, the receiver receives the control signal from the transmitter. A method has been considered to prevent howling by attenuating the signal supplied from the microphone to the speaker.

Specifically, the one shown in FIG. 1 has been proposed.

That is, a signal from a microphone 1 is supplied to a speaker 5 through a microphone amplifier 2, an attenuator 3, and a power amplifier 4. An inductive transmitter 6 is arranged near the microphone 1. The inductive transmitter 6 is
A current with a constant frequency is passed through the transmitting coil, so that an induced magnetic flux with a constant frequency is radiated near the microphone 1. On the other hand, an inductive receiver 7 is arranged near the speaker 5. In the inductive receiver 7, a receiving coil 7a and a capacitor 7b are connected in parallel to form a tuned circuit, and the tuned frequency is selected as the frequency of the induced magnetic flux radiated from the inductive transmitter 6, and the microphone 1 is connected to the speaker 5. When approaching , the induced magnetic flux is detected by this tuned circuit, and the detection signal is sent to the amplifier 8.
The signal is supplied to the detection circuit 9 and detected. Then, the attenuator 3 is turned on by the detection output,
The signal supplied from the microphone 1 to the speaker 5 is attenuated to prevent howling.

However, this device has a receiver 7 having a receiving coil 7a, a high gain amplifier 8, and a detection circuit 9.
and is expensive.

By the way, among existing audio equipment, there is a so-called cassette recorder with radio, in which an AM radio receiving section and a cassette recorder are housed in one case. This cassette recorder with radio has a speaker housed in its case, and also has a built-in device for recording level control.
Some have AGC circuits.

The present invention focuses on this point and makes it possible to use such existing audio equipment as a public address system having a howling prevention function by slightly modifying it.

FIG. 2 shows an example of the audio equipment according to the present invention.
This is a case of modifying a cassette recorder with a radio.

That is, first, to explain the existing parts, 11 is an external microphone jack, 12 is a microphone amplifier, 13 is a line amplifier, 14 is a recording amplifier,
15 is a recording/playback head, 16 is a playback amplifier, 17 is a volume, 18 is a power amplifier, and 19 is a speaker. Also, 21 is an AGC circuit, for example,
It consists of a buffer amplifier 22, a rectifier circuit 23, and an FFT 24 for control. On the other hand, 31 is an AM radio receiving section, which includes a bar antenna 32 and a variable capacitor 33 that forms an antenna tuning circuit together with the bar antenna 32.
It has a mixer 34, a local oscillator 35, an intermediate frequency amplifier 36, and a detection circuit 37, and the local oscillator 35 is provided with a tuning circuit consisting of a coil 38 and a variable capacitor 39. Furthermore, 41 is a radio selection switch, and 51, 52, and 53 are recording/reproduction changeover switches. These are housed in one case, so the AM radio receiving section 31 is placed near the speaker 19.

For example, the following configuration is added to such an existing part. That is, an inductive transmitter 62 is placed near a microphone 61 connected to an external microphone jack 11. The inductive transmitter 62 is similar to the inductive transmitter 6 described in FIG. 1, and in this case, for example, an inductive magnetic flux of 500 kHz, which is slightly out of the AM radio broadcast band, is emitted. Further, in the AM radio receiving section 31, capacitors 73 and 79 are provided in parallel with the variable capacitors 33 and 39, respectively, and changeover switches 81 and 82 are provided. The capacitances of the capacitors 73 and 79 are set to such a value that a 500 kHz signal can be received when the switches 81 and 82 are switched to the contact A side as shown in the figure. Further, as shown in the figure, changeover switches 83, 84, and 85 are provided, and a smoothing circuit 91 is also provided.

When this audio equipment is used as an original cassette recorder with a radio, switches 81 to 85 are switched to the contact N side.In this case, if switch 41 is switched to the contact X side, switches 51 to 5 are switched to the contact X side.
You can listen to or record the radio depending on the connection status of the microphone 661 to the external microphone jack 11 by switching the switch 41 to contact Y side.
can be connected to record or play tapes. During recording, the output audio signal of the buffer amplifier 22 of the AGC circuit 21 is supplied to the rectifier circuit 23. The DC output of the rectifier circuit 23 is supplied to the control FET 24 through the switches 52 and 85, and the FET 24 is turned on. As a result, the audio signal supplied to the recording amplifier 14 through the line amplifier 13 is attenuated, and the recording current supplied from the recording amplifier 14 to the recording/reproducing head 15 is attenuated to an appropriate level, thereby preventing recording distortion due to excessive input. Ru.

Then, by switching the switches 81 to 85 to the contact A side, the system can be used as a public address system having a howling prevention function. That is, in this case,
By connecting the microphone 61 to the external microphone jack 11, a signal from the microphone 61 is sequentially supplied to the speaker 19 through the microphone amplifier 12, switch 83, line amplifier 13, volume 17, and power amplifier 18, and is amplified. . Then, when the microphone 61 approaches this audio equipment, and therefore the speaker 19, the induced magnetic flux from the inductive transmitter 62 placed near the microphone 61 is transferred from the bar antenna 32 and capacitor 73 near the speaker 19. The received signal is converted to a radio intermediate frequency by a mixer 34, and then supplied to a detection circuit 37 through an intermediate frequency amplifier 36 for detection. That is, a 500kHz control signal from the guidance transmitter 62 is received by the AM radio receiving section 31. Then, the detected output is supplied to the smoothing circuit 91 through the switch 84 and smoothed, and the smoothed output is supplied to the AGC FET 24 through the switch 85, turning on the FET 24, thereby supplying the signal from the microphone 61 to the speaker 19. Howling is prevented by attenuating the signal.

The control signal emitted from the inductive transmitter 62 may be at an appropriate frequency within the AM radio broadcast band, for example, which is not actually broadcast. In this case, the capacitors 73 and 79 and the switches 81 and 82 are not necessary. Instead, by adjusting the variable capacitors 33 and 39 to this frequency in advance, the AM radio receiving section 31 can similarly receive the control signal from the guidance transmitter 62.

According to this invention, the AM radio receiving section of an existing audio device is used as an inductive receiver for preventing howling, and the control element of the AGC circuit for controlling the recording level of a tape recorder is switched and used for preventing howling. , with just a few modifications such as adding a switch to existing audio equipment,
It is possible to realize a loudspeaker device having a howling prevention function, thereby reducing costs and making the device smaller and lighter.

[Brief explanation of the drawing]

FIG. 1 is a system diagram of a public address system, and FIG. 2 is a system diagram of an example of audio equipment according to the present invention. 61 is a microphone, 62 is an inductive transmitter, 11 is a microphone jack, 19 is a speaker, 21 is an AGC circuit for recording level control, and 31 is an AM radio receiving section.

Claims (1)

[Claims] 1. In an audio device in which a tape recorder equipped with a microphone input terminal, a speaker, and an AGC circuit for recording level control, and a radio reception section are integrated, when a microphone is connected to the microphone input terminal. , a signal from the microphone is supplied to the speaker, and when the microphone approaches the speaker, a predetermined control signal from a transmitter placed near the microphone is transmitted to the radio receiver. An audio device, wherein the received signal is detected and supplied to a control element of the AGC circuit, and the control element attenuates the signal supplied from the microphone to the speaker.