JPH0272708A - Howling prevention circuit - Google Patents

Abstract

PURPOSE:To provide a simple and inexpensive circuit by constituting a howling prevention circuit of a howling frequency detection circuit consisting of a PLL circuit, and a filter. CONSTITUTION:A howling frequency detection circuit 9 consists of a phase comparator 11, a voltage controlled oscillator 12, and a frequency-voltage converter 13, and detects a howling frequency by controlling the oscillation frequency of the voltage controlled oscillator 12 setting the howling frequency of a signal inputted via a switch 8 as a reference frequency. The oscillation frequency output of the voltage controlled oscillator 12 which coincides with the howling frequency is supplied to a voltage memory circuit 10, and is preserved in the circuit. The output of the voltage memory circuit 10 is applied on first and second amplifiers 4 and 5 comprising an active filter 3, and controls the central frequency of the active filter 3 so as to coincide with the frequency of the voltage controlled oscillator 12. In such a way, it is possible to lower the gain of a system in the howling frequency, and to suppress howling.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a howling prevention circuit for suppressing howling, which is necessary for a system in which acoustic output from a speaker may echo back to an input section and cause howling.

Echo cancellers have been known as circuits that completely prevent howling. This echo canceller is configured to simply convert a signal into a digital value and perform digital processing to generate and cancel a reverse echo.

The amount that the day tries to do.

However, such a conventional cancellation method is large-scale and expensive, making it substantially difficult to apply it to consumer equipment.

The present invention has been made in view of these points, and it is an object of the present invention to provide a novel howling prevention circuit that can suppress howling with a simple configuration.

To achieve the above object, the howling prevention circuit of the present invention includes a howling frequency detection circuit composed of a PLL circuit and a howling frequency detection circuit between the signal source of the sound reproduction system and the speaker. The configuration includes a filter that is controlled by the output of the howling frequency detection circuit so as to lower the system gain.

According to this configuration, the frequency of howling due to an echo fed back from a speaker to a signal source such as a microphone is detected by a howling frequency detection circuit formed of a PLL circuit.

At this time, the howling frequency detection circuit compares the phase with the output of the voltage controlled oscillator using the howling frequency as a reference frequency, and uses the phase comparison output to control the voltage controlled oscillator to match the reference frequency (howling frequency). Thus, a control signal based on the oscillation frequency of the voltage controlled oscillator that oscillates at the howling frequency is output and applied to the filter. The gain of the filter is attenuated by its control signal. This means that the gain of the audio reproduction system is reduced at the howling frequency.

In FIG. 1, (1) is the microphone 2 (2) is the input amplifier, (3) is the first and second amplifiers (4) (5) and the capacitance (CI) (cz ) (C3)
An active filter formed by connecting as shown in the figure, (6)
is the output amplifier, (7) is the speaker, (8) is the switch,
r9) is a howling frequency detection circuit, and 00) is a voltage storage circuit.

As shown in FIG. 2, the howling frequency detection circuit (9) includes a phase comparator (11) and a voltage controlled oscillator.

and a frequency-voltage converter q3),
The howling frequency is detected by controlling the oscillation frequency of the voltage controlled oscillator 02) using the howling frequency of the signal manually input via the switch (8) as a reference frequency. That is, the output of the phase comparison H (IO) is controlled based on the frequency difference between this howling frequency and the voltage controlled oscillator 02 so that the oscillation frequency of the voltage controlled oscillator 02 matches the howling frequency. The oscillation frequency output of the voltage controlled oscillator 021 that matches the howling frequency is converted into a voltage value by the next frequency-voltage converter 03), and then placed in the voltage storage circuit 0■ and stored there. . The output of the voltage storage circuit Oω is the first one constituting the active filter (3).
．． It is applied to the second amplifiers (4) and (5), and controls the center frequency of the active filter (3) to be equal to the frequency (howling frequency in this case) of the voltage controlled oscillator 021.

This reduces the system gain at the howling frequency and suppresses howling.

It should be noted that the howling frequency detection circuit constituted by a PLL circuit has a retraction range, and outside this range, detection becomes impossible and filter control at the subsequent stage becomes impossible. Therefore, care should be taken to satisfy the following conditions.

First, the first filter in the active filter (3). The outputs of the second amplifiers (4) and (5) are made constant current, and 1 + = gm(
V+ Vo)/4tz-g+++(V v.)/
4 However, if we set g-=1-/Vt summer, = bias current of the differential input of each amplifier, the center frequency f0 of the active filter (3) is 8π I.

““5・　=8zV, ari7 shoulder becomes.

The retraction range of the howling frequency detection circuit (9) is f1
~fz, and the output of the voltage storage circuit 00) at that time is V,
, V, it can be seen that in order to match the pull-in frequency and the center frequency of the active filter (3), it is sufficient to satisfy .

The active filter controlled as above (
3), the gain at the howling frequency is reduced, so the signal passes through the output amplifier (6) and speaker (7).
Howling does not occur even if the signal is input to the microphone (1) through the .

According to the present invention, the howling prevention circuit is composed of a howling frequency detection circuit consisting of a PLL circuit and a filter, so that it is simple and inexpensive. Furthermore, since the PLL circuit is used to detect howling frequency, stable operation is possible.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a howling prevention circuit embodying the present invention, and FIG. 2 is a detailed diagram of a part thereof. (1) ---Microphone, (3) ---Active filter. (7)--Speaker, (9)--Howling frequency detection circuit.

Claims (1)

[Claims] (1) There is a PLL between the signal source of the sound reproduction system and the speaker.
1. A howling prevention circuit comprising: a howling frequency detection circuit configured from a circuit; and a filter controlled by the output of the howling frequency detection circuit so as to reduce a system gain of the howling frequency.