Automatic Level Control Circuit

1. Field of the Invention

This invention relates to an ALC (automatic level control) circuit in a low frequency amplifier circuit.

2. Background of the Invention

When, in a low frequency amplifier, its input level is excessively high, it is necessary to decrease the level at a suitable rate to prevent the saturation of its output.

An example of a conventional ALC circuit will be described with reference to FIG. 4. In FIG. 4, an input terminal 1 is connected through a voltage dividing resistor R.sub.3 to an output terminal 12. The output produced at the terminal 12 is also supplied through an output detecting resistor R.sub.4 and an output detecting variable resistor VR.sub.5 to an amplifier section 6, where it is amplified. The output of the amplifier section 6 is applied through a time constant circuit 7 made up of an R-C (resistor and capacitor) series circuit and through a rectifying and smoothing circuit 8 to the base of a transistor TR.sub.10 Darlington-connected with a transistor TR.sub.11. The collector of the transistor TR.sub.10 is connected to a positive power source +B and its emitter is connected to the base of the transistor TR.sub.11. The collector of the transistor TR.sub.11 is connected to an output line, i.e., the output terminal 12. That is, the ALC circuit is so designed as to operate as a variable impedance to control the attenuation according to the input level.

The operation of the conventional ALC circuit thus organized will now be described. When the input level is low, a control signal produced from the output line is also low in level and therefore the transistor TR.sub.10 is maintained non-conductive (off) and the transistor TR.sub.11 is also non-conductive, i.e, the transistor TR.sub.11 has a high impedance between its emitter and collector. Accordingly, the output provided on the output line will not be decreased by the transistor TR.sub.11. That is, the output level is not decreased in this case.

When, on the other hand, the input level is increased to render the transistor TR.sub.10 conductive (on), the transistor TR.sub.11 is also rendered conductive by a current which is supplied from the positive power source according to the base-emitter current of the transistor TR.sub.10. That is, the transistor TR.sub.11 assumes a low impedance between its emitter and collector. Accordingly, the output provided on the output line is decreased by the transistor TR.sub.11. That is, the output level is decreased. FIG. 5 shows one example of frequency v. gain characteristics in this operation, in which the ordinate axis represents overall gain defined by dividing an output level by an input level. When the input level is low, the amount of control is small whereas when the input level is high, the amount of control is large, yet providing substantially flat frequency characteristic curves.

In this conventional ALC circuit, because of the circuitry described above, the level attenuation occurs over all the frequency bands concerned. That is, the operation of the ALC circuit makes a person feel the high frequency level variation which is offensive to the ear, thus causing a so-called "breezing phenomenon".

When an excessively large input is applied, especially a low frequency band must be taken into consideration because of the following reason. In general, the ALC operation is often required when a high frequency signal is superposed on a low frequency signal. Since, in terms of acoustic power, a high frequency signal is lower in level than a lower frequency signal, it is unnecessary to perform an ALC operation for all the frequency bands.

Furthermore, the ALC control carried out for each channel suffers from the following difficulty. In the case where two channels, i.e., right and left channels in a stereophonic mode, have different high frequency component attenuation from each other, high frequency components in particular, having significant directivity, adversely affect the sound image localization, thus making a person uncomfortable.

Accordingly, an object of this invention is to eliminate the above-described difficulties accompanying a conventional ALC circuit.

The foregoing object and other objects of the invention have been achieved by the provision of an ALC circuit for automatically performing an amplitude compression control operation according to the output level of a low frequency amplifier circuit. According to the invention, the ALC circuit comprises a control signal generator for providing a control signal corresponding to the output level of the low frequency amplifier circuit; an amplitude compression controller for subjecting the output level to amplitude compression according to the level of the control signal, and a differentiating circuit provided on the output side of the low frequency amplifier circuit. The differentiation time constant of the differentiating circuit is controlled according to the level of the control signal to control a cutoff frequency for a frequency band subjected to amplitude compression.

The nature, utility and principle of the invention will be more clearly understood from the following detailed description when read in conjunction with accompanying drawings.

In the accompanying drawings:

FIG. 1 is a circuit diagram showing a first example of an ALC circuit according to this invention;

FIG. 2 is a characteristic diagram for a description of the operation of the ALC circuit shown in FIG. 1;

FIG. 3 is a circuit diagram showing a second example of the ALC circuit according to the invention;

FIG. 4 is a circuit diagram showing an example of a conventional ALC circuit; and

FIG. 5 is a characteristic diagram for a description of the operation of the conventional ALC circuit shown in FIG. 4.

A first example of an ALC circuit according to the invention will be described with reference to FIG. 1, in which those circuit elements which have been previously described with reference to FIG. 4 are therefore designated by the same reference numerals or characters.

In FIG. 1, an input buffer circuit 13 is connected between the input terminal 1 and a capacitor 14 connected in series with the output line. The capacitor 14 and the collector-emitter resistance of the Darlington-connected transistor TR.sub.11 form a voltage dividing circuit, the voltage division output of which is applied to the output terminal 12. Similarly as in the conventional ALC circuit described before, the output provided at the output terminal is supplied through the amplifier circuit 6, the time constant circuit 7 and the rectifying and smoothing circuit 8 to the base of the transistor TR.sub.10. That is, the transistor TR.sub.10 is driven according to the output level.

In the ALC circuit thus organized, the capacitor 14 and the collector-emitter resistance of the transistor TR.sub.11 further form a high pass filtering circuit. In the high pass filtering circuit, since the collector-emitter resistance of the transistor TR.sub.11 changes with the control signal level the filtering RC time constant is changed. In this way, the frequency band subjected to amplitude compression and the compressed level in the frequency band are changed. More specifically, if the filtering time constant is small, the cutoff frequency is increased and frequencies lower than the cutoff frequency are attenuated. This filtering circuit acts as a differentiating circuit at low frequencies. Namely, the transfer function is proportional to the frequency at frequencies well below the cutoff frequency.

Accordingly, as the input signal level increases, the collector-emitter resistance Of the transistor TR.sub.11 is decreased, the filter time constant is decreased, and the cutoff frequency is increased so that attenuation is carried out to high frequencies. That is, the range of level attenuation is extended to higher frequencies, but will not be extended so high as to adversely affect high frequency components. When, on the other hand, the input signal level is low, the collector-emitter resistance of the transistor TR.sub.11 is high, the filtering time constant is large, and the cutoff frequency is low, so that no attenuation is effected even in the low frequency range.

FIG. 2 shows the results of experiments performed with the above-described circuit. FIG. 2 shows the case where a high level input (indicated by the solid line) is higher by 10 dB than a low level reference level (indicated by the dotted line). When these signals are applied to the ALC circuit of the invention, the cutoff frequency is 50 Hz for the input reference level but is increased to 315 Hz for the input signal which is higher by 10 dB in level.

In the above-described ALC circuit, the voltage dividing circuit made up of the series-connected capacitor 14 and the collector-emitter resistance of the transistor TR.sub.11 serves also as a high pass filter circuit whose cutoff frequency is variable. However, the ALC circuit may be so modified that, as shown in FIG. 3, the capacitor 14 is shunted by resistor so that an upper limit is given to the attenuation in the low frequency range.

As was described above, in the ALC circuit of the invention, the high pass filtering circuit made up of the series-connected capacitor and the collector-emitter of the resistance of the transistor which changes with the input level can change the cutoff frequency of a frequency band subjected to amplitude compression. As a result, no amplitude compression is given to a frequency band higher than the cutoff frequency. Therefore, the breezing due to the variation in level of high frequency components or the fluctuation in sound image localization due to the difference in level between channels is never caused. Due to the nature of the low frequency component, the sound image localization does not tend to occur. Hence, the ALC circuit of the invention can perform ALC control satisfactorily at all times.