JPH079452Y2 - Low frequency clip prevention circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a low frequency band clip prevention circuit for preventing clipping due to a low frequency signal component of an audio amplifier.

[Prior Art] A general music signal contains various frequency components such as instrument sounds and voices. In addition, due to the energy balance of the music signal, the low frequency often includes a signal having a large amplitude as compared with the middle and high frequencies, and many musical instruments such as a drum have a large volume.

Also, amplifiers such as transistor amplifiers generally
When a signal having a level exceeding a predetermined level (maximum distortion-free input level) is input, the output signal is clipped.

However, in such an amplifier, if the level of the low frequency component of the input signal is large enough to clip the amplifier,
As for this low-frequency component, even if the input signal also includes mid-high frequency components, these low-frequency components are There was a problem that it was lost by the clip of and was greatly affected by the sound.

That is, the amplifier includes three components of low, middle and high frequencies as shown in FIG. 7 (a), and particularly the level of the low frequency component is larger than the distortion-free maximum input level of the amplifier in FIG. 7 (b). If a signal having a waveform as shown in Fig. 7 is input, the output waveform of the amplifier becomes a waveform as shown in Fig. 7 (c). In this case, as shown in FIG. 7 (d), although the low frequency component is clipped, it is reproduced for the time being, while the middle and high frequency components are as shown in FIG. 7 (d). The part marked with X is missing. Then, such a defect becomes a large defect when viewed as sound.

In particular, in a music signal, the middle and high frequency components are the center of the music information in terms of hearing, so that the lack of continuity of the middle and high frequency components is musically decisive enough to cause damage. Becomes

A so-called AGC (Automatic Sound Gain Control) circuit has been conventionally known as a circuit for preventing clipping in an amplifier. FIG. 8 shows an example using such an AGC circuit. In the figure, an electronic attenuator 8 is connected in series to the input circuit of the power amplifier 1 for driving a speaker, and the control circuit 9 controls the attenuation amount in the attenuator 8 according to the level of the input signal V _i . The input signal level of is less than or equal to the clip level of the power amplifier 1.

However, when such an AGC circuit is used, the entire band is attenuated, and the sense of volume is lost.

Since the control circuit has a time constant, the sound effect may be unnatural.

In the case of stereo, even if there is a large amplitude input to only one channel, it is necessary to give the same attenuation effect to the other channel in order not to make the position sense of the sound source unstable.

There were drawbacks such as.

[Problems to be Solved by the Invention] The present invention has been made in view of the above-mentioned problems, and it is possible to prevent the amplifier from clipping due to a large-amplitude low-frequency signal component, which may lead to a failure of a music signal. It is an object of the present invention to provide a low-frequency clip prevention circuit capable of preventing loss of the middle and high frequency components, which causes significant damage.

[Means for Solving the Problem] In order to solve the above problem, in the present invention, an audio signal is divided into a low frequency component and a middle and high frequency component in a stage preceding an amplifier to be processed, and only the low frequency component is nonlinear. By smoothly and smoothly compressing the compressed low frequency component and the high and middle frequency components again, the peak value due to the low frequency component of the signal input to the amplifier is suppressed to be smaller than the clip level of the amplifier. .

[Operation] The low-frequency clip prevention circuit of the present invention is provided in a stage before the amplifier, such as a pre-stage or a pre-pre-stage of an amplifier (amplifier of interest) for which low-frequency clipping is to be prevented. Then, the low-frequency clip prevention circuit processes the input signal so that the level of the audio input signal becomes equal to or lower than the clip level (maximum distortion-free input level) of the target amplifier and supplies it to the target amplifier.

[Effect] Therefore, according to the present invention, it is possible to prevent clipping due to the low-frequency component in the target amplifier, and prevent loss of the middle-high frequency component due to excessive amplitude of the low-frequency component.

Further, in this low-frequency clip prevention circuit, only the low-frequency component is processed, so that the signal level in the mid-high range is not affected at all. Therefore, the sense of volume in the middle and high frequencies, which is the center of music information, is not impaired.

Further, the amplitude limitation of the low frequency component is performed non-linearly.
For this reason, only the peak of the low-frequency signal is suppressed, and the energy feeling of the low-frequency component is not easily impaired. Further, since it is a non-linear compression, it is not necessary to include a time constant, and an unnatural feeling as a music signal is small.

Further, as described above, since the loss of energy feeling due to the compression of the low frequency signal is small, it is possible to perform the signal processing independently for each channel even in the case of stereo or the like. In addition to the fact that the mid-high frequency level is not manipulated, the loss of mid-high frequency components due to signal processing does not occur.

Furthermore, since the low-frequency components are smoothly compressed to suppress the generation of higher-order distortion components due to this compression, it is possible to reduce the sense of distortion that accompanies the compression of the low-frequency components.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a basic configuration of a low frequency band clip prevention circuit according to an embodiment of the present invention. Low frequency clip prevention circuit 2 in the same figure
Is for preventing low-frequency clipping of the power amplifier 1 that drives the speaker 10, and includes a low-pass filter (LPF) 3 and a high-pass filter (HP
F) 4, a phase shift circuit 5, a soft clip circuit 6 for limiting the amplitude of low-frequency signal components, and an adder 7 for band synthesis. The phase shift circuit 5 is for correcting the phase relationship between the low frequency component and the middle and high frequency components so that band synthesis is preferably performed, and is provided as necessary. When the phase shift circuit 5 is provided, the circuit 5 can be connected to the front stage of the LPF 3 or the rear stage of the soft clip circuit 6, or the front stage or the rear stage of the HPF 4 on the mid-high range component side, in addition to the position shown in the figure. .

FIG. 2 shows an operation conceptual diagram for explaining the operation of the low frequency band clip prevention circuit of FIG. Next, the operation of the circuit of FIG. 1 will be described with reference to the operation conceptual diagram of FIG.

In the low-frequency clip prevention circuit 2 of FIG. 1, the audio input signal V _i is first passed through HPF4 and LPF3 to become a high-frequency component (FIG. 2) and a low-frequency component (FIG. 2). The band is divided. Then, the low-frequency component passes through the phase shift circuit 5 so as to match the phase characteristic of the HPF4, and then passes through the soft clip circuit 6 so that the peak at the time of a large amplitude signal is soft clipped (FIG. 2). . This soft clip circuit 6 outputs the peak of the low frequency component to the power amplifier 1
The clip is limited to a level lower than the clip (hard clip) level that depends on the power supply ± Vcc, and is made a soft clip so that the higher-order distortion due to the amplitude limitation is not heard. This low-frequency component whose amplitude is limited is
The mid and high frequency components that have passed through F4 but have not been processed are combined in the adder 7 to form a single signal (FIG. 2), which is input to the power amplifier 1.

FIG. 3 shows a specific circuit example of the basic configuration shown in FIG. In the figure, as LPF3 and HPF4, a secondary filter with the same cutoff frequency f _{0 and the} same Q value Q = 0.5 is used so that the gain and the phase match when these outputs are mixed and combined in the latter stage. Is forming. The signal (low-frequency component) that has passed through LPF3 is input to the inverting amplifier 61. Diodes 62 and 63 connected in anti-parallel are connected to the negative feedback system of the inverting amplifier 61, and form a logarithmic amplifier as a whole. As a result, the peak gain when a large signal comes in is suppressed, and a soft clipped waveform is obtained. In this case, depending on the element to be clipped (clip level) and the gain and maximum output of the subsequent power amplifier 1, the gain of the inverting amplifier 61 and the mixing (addition) for preventing the output of the power amplifier 1 from clipping. The values of the resistors 71 and 72 may be set. For example, when the maximum output of the power amplifier 1 is small, the gain of the inverting amplifier 61 is set to be large so that the low-frequency clipping circuit clips quickly, and the f characteristic (frequency characteristic) at the time of a small signal is flat (flat). The resistance value of the resistor 71 is set to be larger than that of the resistor 72.

In the circuit of FIG. 3, the characteristics of LPF3 are However, s = jω ω is an angular frequency The characteristics of HPF4 are: Therefore, it can be seen that the output T _LP (s) of LPF3 and the output T _HP (s) of HPF4 are 180 ° out of phase with each other.

Therefore, if these outputs are combined with a mixing ratio of 1: 1, the combined output S _MIX is Here, because Q = 0.5 That is, in the circuit of FIG. 3, the output of LPF3 and HPF4
It can be seen that the gain becomes constant regardless of the frequency if the output of is mixed with the output of so that the mixing ratio becomes 1: 1.

FIG. 4 shows a second specific example of the circuit of FIG. The circuit of FIG. 4 is obtained by removing the buffer amplifiers 31 and 41 of LPF3 and HPF4 from the circuit of FIG. 3 and adding a buffer amplifier 73 for output, thereby omitting one operational amplifier. Is. Further, from a different point of view, the circuit of FIG. 4 can be considered to be the buffer amplifier 31 of the LPF3 further provided with the functions of phase inversion and soft clipping as compared with the circuit of FIG.

FIG. 5 shows a third specific example of the circuit of FIG. In the circuits of FIGS. 3 and 4, limiter diodes 62 and 63 are connected to the negative feedback system of the inverting amplifier 61, but these are connected in series to the output circuit of the inverting amplifier 61 as shown in FIG. By connecting between the sliding terminal of the variable resistor 64 and the ground, the magnitude of the limiter effect, that is, the soft clip level can be varied.

As described above, by using the low band clipping prevention circuit of the present invention, the allowable input of the low band signal to the amplifier can be increased. That is, even if the low frequency band of the input signal to the amplifier is boosted in advance, there is little or no adverse effect on the mid-high frequency band signal due to the low frequency band clipping. Therefore, the present invention can be suitably applied to a device that uses a small speaker and boosts low frequencies to eliminate insufficient bass, such as a speaker circuit of a radio-cassette.

[Modification of Embodiment] The present invention is not limited to the above-described embodiment, and can be implemented by being modified appropriately. For example, LPF3
The HPF4 may be any one as long as it divides the audible wave signal to be processed into a low-pass component and a mid-high pass component, and one or both of them is used as another low-pass means and a mid-high pass means such as a bandpass filter. It can be replaced.

FIG. 9 is a diagram in which HPF4 in the configuration of FIG. 1 is replaced with another middle- and high-pass means, that is, a circuit for removing low-pass components passing through LPF3 from the input signal V _i . In FIG. 9, a differential amplifier 41 including an operational amplifier or the like is supplied with an audio frequency input signal V _i at a non-inverting input terminal (+ terminal) and a low-voltage input signal (-terminal) at a low level as shown in FIG. Is supplied with a range component. As a result, as the output of the differential amplifier 41, a signal of (audio frequency input signal V _i ) − (output of LPF3), that is, a middle-high range component as shown in FIG. 2 is obtained.

Further, in the above description, the case where the present invention is mainly applied to the low frequency clipping prevention of the power amplifier has been described, but the present invention is applied to the low frequency clipping prevention of the voltage or current amplifier such as a so-called line amplifier. It is also possible to apply. In particular, an amplifier using a low-voltage power source such as a 3V battery has a clip level as low as about 1V, and thus the present invention can be effectively applied.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic configuration of a low frequency anti-climb circuit according to an embodiment of the present invention, FIG. 2 is an explanatory diagram of an operation concept of the circuit of FIG. 1, and FIG. 3 is FIG. FIG. 4 is a circuit diagram showing a first concrete example of the circuit of FIG. 4, FIG. 4 is a circuit diagram showing a second concrete example of the circuit of FIG. 1, and FIG. 5 is a modification of the circuits of FIGS. 3 and 4. FIG. 6 is a frequency characteristic diagram for explaining the modified examples of FIGS. 3 to 5, FIG. 7 is a waveform diagram for explaining a low-frequency clip of a conventional amplifier, and FIG. FIG. 8 is a circuit diagram showing an example of a conventional clip prevention circuit using an AGC circuit, and FIG. 9 is a circuit diagram showing a modified example of the circuit of FIG. 1: Power amplifier 2: Low-pass clipping prevention circuit 3: Low-pass means (low-pass filter) 4: High-pass means (high-pass filter) 5: Phase correction circuit 6: Soft clip circuit 7: Adder circuit

Claims (1)

[Scope of utility model registration request] 1. A circuit which is connected in front of an audio amplifier and prevents clipping of an output signal of the amplifier due to a low-frequency signal component, which divides an audio-frequency input signal into a low-frequency component and a mid-high frequency component. A band dividing circuit, an amplitude limiting circuit that non-linearly and smoothly compresses the amplitude of the low-frequency component so that the input level of the amplifier is equal to or less than the distortion-free maximum input level, the low-frequency component whose amplitude is limited, and the middle and high frequencies. And a band synthesizing circuit for synthesizing a component and a low-frequency clipping prevention circuit.