JPS628079B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for preventing howling development in a public address system.

Howling in a public address system consisting of a microphone, an amplifier, and a speaker occurs when a portion of the sound emitted from the speaker is fed back to the microphone, and the gain of the positive feedback loop of speaker → microphone → amplifier → speaker is less than 1. This is a phenomenon in which the system loses stability and self-oscillation occurs as the noise becomes louder, and as a result, the maximum volume of the loudspeaker is severely limited. To prevent this howling in advance and obtain the maximum volume without causing howling, measure the feedback rate (β) of the sound emitted from the speaker and return it to the microphone, and adjust the amplification factor (μo) according to this value. By configuring a negative feedback loop to control the overall system gain (μo
It is sufficient to adjust so that β) is always smaller than 1. However, the feedback rate (β) sometimes differs depending on the usage conditions of the loudspeaker, and it is extremely difficult to measure only the feedback rate (β) while the loudspeaker is in use. This had to be done by detecting howling once it had occurred and controlling the amplification factor (μo) after the fact. For this reason, conventional methods have been available to lower the amplifier volume or stop operation after the occurrence of howling is detected automatically or by the user, but simply lowering the amplifier volume will not stop the howling once it has occurred. It takes time, and if the operation of the amplifier is stopped, the loudspeaker broadcast will be interrupted each time (Japanese Patent Publication No. 44-25527). Also, since it takes a certain amount of time for howling to grow due to the positive feedback loop, there is a way to prevent howling from growing by temporarily stopping the operation of the amplifier during the initial state of occurrence, Transient phenomena caused by noise appeared as noise, and the sound quality was not necessarily sufficient as a public address system (Kokoku Publication No. 45-21771).

This invention was made in view of the above circumstances, and includes providing a part with particularly large gain on the high frequency side outside the audible band in the frequency characteristics of the amplification system of the loudspeaker.
By detecting the occurrence of howling in this frequency band with large gain (hereinafter referred to as the ultra-audible band) and adjusting the gain of the entire frequency band of the amplification system, howling in the audible band can be prevented in advance and howling can be suppressed. It is an object of the present invention to provide a device capable of obtaining the maximum sound volume without causing noise.

Howling prevention device 2 of the loudspeaker 1 of this invention
An embodiment will be described based on the drawings. The present invention is constructed by connecting a howling prevention device 2 between a preamplifier 4 and a volume 5 of a public address system 1 which includes a microphone 3, a preamplifier 4, a volume 5, a power amplifier 6, and a speaker 7. There is. In addition, 8 is a cancel switch, and by turning on this switch 8, the preamplifier 4 and the volume 5 are activated by the howling prevention device 2.
It can be connected directly without going through it, making it the same as a normal public address system. The howling prevention device 2 includes an equalizer between the preamplifier 4 and the volume 5 that sets a super-audible band by providing a portion in which the gain is particularly larger than the audible band on the high frequency side outside the audible band in the frequency characteristics of the amplification system. 9 is connected to a level adjuster 10 that controls the output level (gain) of the equalizer 9 with equal attenuation across all frequency bands. A band filter 11 for extracting, a threshold circuit 12 for setting a threshold value, and a level adjuster 1 for comparing the output of the band filter 11 and the output of the threshold circuit 12.
0 and a comparator 13 that sends a control signal for controlling the output level of the equalizer 9. The frequency characteristics of the equalizer 9 are set so that the gain is approximately equal and flat from several tens of Hz to about 20 KHz (audible band), and the gain peaks slightly above 20 KHz, which is defined as the ultra-audible band. microphone 3, speaker 7,
Each component such as an amplifier should be sensitive and operable at least in the audible and ultra-audible bands, but its frequency characteristics are not necessarily flat in these bands, so the ultra-audible band in the equalizer 9 The magnitude of the gain is adjusted so that in the frequency characteristics of the entire system of the loudspeaker 1, the gain in the ultra-audible band is still sufficiently larger than that in the audible band. However, the gain in the ultra-audible band must not be so large that it has no relation to the possibility of howling in the audible band. The magnitude of the gain in this ultra-audible band is adjusted by an adjuster 14 installed in the equalizer 9.
Note that if the entire system inherently has an ultra-audible band due to frequency characteristics specific to the amplifier itself or other equipment, the equalizer 9 is not necessary and the control amount can be obtained from the output of the level adjuster 10, etc. good. Further, under normal conditions, the threshold value output from the threshold circuit 12 is always larger than the output value of the band filter 11, regardless of the level of the primary sound that the microphone 3 receives directly from the sound source. If howling occurs in the ultra-audible band, the value of the output of the band filter 11 is set to such a value that it quickly exceeds this threshold value. This threshold setting is adjusted by a regulator 15 installed in the threshold circuit 12. Further, if a frequency adjuster (not shown) is installed in conjunction with the equalizer 9 and the band filter 11, the frequency at which the gain peaks, which is set as the ultra-audible band, can be changed.
In the embodiment, the super-audible band is set on the high side outside the audible band, but it is theoretically possible to set the super-audible band on the low side of several tens of Hz or less.
However, although the occurrence of howling is not related to the level of the primary sound, it is difficult to obtain a sound signal with a sufficient level as a control amount in the low range, and if howling occurs in the low range, Since this may be felt by the human body as vibration, it is not preferable to set the ultra-audible band to the low frequency side.

Loudspeaker 1 of the present invention configured as described above
Explain the operation. The primary sound received by the microphone 1 and the sound fed back by the feedback rate (β) from the speaker 7 are converted into electrical sound signals, subjected to first stage amplification by the preamplifier 4, and sent to the equalizer 9 of the howling prevention device 2. Sent. The audio signal sent to the equalizer 9 has the signal level in the ultra-audible band emphasized compared to the audible band, and the level adjuster 10 and volume 5 adjust the amount of attenuation equally in all frequency bands, and then the power amplifier 6 Then, the signal is amplified in the second stage and broadcasted from the speaker 7. At this time, considering the amplification factor (μo) of the loudspeaker 1 only for the primary sound, if the gain (μoβ) of the positive feedback loop becomes greater than 1, howling will occur at that frequency.
Therefore, in the howling prevention device 2, the output of the equalizer 9 is also sent to the band filter 11, and the signal only in the ultra-audible band is extracted and sent to the comparator 13. The comparator 13 compares the level of the signal sent from the band filter 11 with a preset threshold value sent from the threshold circuit 12, and detects the signal sent from the band filter 11 due to howling. When the level of the signal exceeds the threshold, a control signal is sent to increase the amount of attenuation of the level adjuster 10, and the amplification factor (μo) is lowered so that the gain (μoβ) of the positive feedback loop is less than 1. Make it smaller to stop howling.

As explained above, the howling prevention method and device 2 of the present invention detect the occurrence of howling in the ultra-audible band and prevent howling after the fact by lowering the amplification factor (μo) of the loudspeaker 1. It is. However, since the amplification factor (μo) in the ultra-audible band is adjusted by the equalizer 9 to be larger than the amplification factor (μo) in the audio band, the gain of the positive feedback loop in the ultra-audible band (μoβ) is also the gain in the audible band (μoβ)
It is always larger, and howling is more likely to occur. For this reason, even though the howling prevention device 2 can actuate the control mechanism after the fact in response to howling that actually occurs in the ultra-audible band, the howling prevention device 2 is able to operate the control mechanism after the fact in response to howling that actually occurs in the ultra-audible band, but in the audible band that can be felt by the hearing, the howling preventive device 2 Since the gain (μoβ) can be lowered, howling can be reliably prevented for the listener in advance, and the maximum amplification volume can be obtained within the range where howling does not occur. Furthermore, since the level adjuster 10 can continuously adjust the gain according to the magnitude of the control signal from the comparator 13, there is no noise caused by amplifier switching and there is no risk of deteriorating the sound quality. .

[Brief explanation of the drawing]

FIG. 1 shows a block diagram of a loudspeaker according to the invention. 1... Public address system, 2... Howling prevention device,
3...Microphone, 7...Speaker, 9...
Equalizer, 10... Level adjuster, 12... Threshold circuit, 13... Comparator.

Claims (1)

[Claims] 1. The frequency characteristics of the loudspeaker are provided with a frequency band in which the gain is particularly larger than the audible band on the high frequency side outside the audible band, and the occurrence of howling in the frequency band with the large gain is detected and the sound is amplified. A howling prevention method for a public address system that prevents howling in the audible band from occurring in advance by uniformly attenuating the gain of the entire frequency band of the equipment's amplification system. 2. In a public address system, an equalizer is connected to the amplification system and has a frequency characteristic with a frequency band where the gain is particularly larger than the audible band on the high side outside the audible band, and only the frequency band with a large gain is extracted from the output of the equalizer. A band filter for extracting a controlled amount, a threshold value circuit for setting a threshold value, a comparator that compares the value of the controlled amount with the threshold value and issues a control signal when the value of the controlled amount exceeds the threshold value; A howling prevention device for a public address system comprising a negative feedback loop including a level adjuster that equally attenuates the gain of an amplification system over the entire frequency band using a control signal generated by a comparator.