Automatic level controlling circuit

Details Automatic level controlling circuit Automatic level controlling circuit

2001-04-16

2002-10-29

Pascal, Robert (Department: 2817)

Amplifiers

With control of power supply or bias voltage

With control of input electrode or gain control electrode bias

C330S281000

Reexamination Certificate

active

06472938

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an automatic level controlling (ALC) circuit for controlling an output signal to a stable level even when the level of an input signal such as an audio signal is high.
2. Description of the Related Art
Generally, sound in the external environment includes a faint sound and loud sound. In the processing of an audio input signal, since it is necessary to assure a wide dynamic range, when the level of the audio input signal is high, it was controlled to a stable level by an automatic level controlling circuit. In a digital video camera which has spread in recent years, making a low voltage operation has been attempted for reduction of power consumption. It has been demanded for the characteristic of the automatic level controlling circuit to be improved to assure a wide dynamic range.
FIG. 4
is a circuit diagram of a conventional automatic level controlling circuit. In
FIG. 4
, reference numeral
10
denotes an input terminal to which an input signal is supplied;
11
an amplifying circuit for amplifying the input signal;
12
a full-wave rectifying circuit for full-wave rectifying an output from the amplifying circuit;
13
a buffer amplifier; and
14
a detecting terminal to which a capacitor
15
is externally attached. The full-wave rectifying circuit
12
and the capacitor
15
connected to the detecting terminal
14
constitute a detecting circuit. Reference numeral
16
denotes a comparing circuit for comparing a detected voltage Vo appearing at the detecting terminal
14
and a reference voltage Vref from a DC power source. The comparing circuit
16
produces a controlling voltage for controlling the gain of the amplifying circuit
11
according to a comparison result, thereby controlling the level of the output signal therefrom.
FIG. 5
is an input/output characteristic graph of the automatic level controlling circuit having the configuration described above. As seen from
FIG. 5
, in the absence of the automatic level controlling circuit, the output signal exceeds a saturated level Vs when the level of the input signal exceeds V
1
, and hence is clipped. On the other hand, in the presence of the automatic level controlling circuit, it restrains the level of the output signal when the level of the input signal reaches V
2
so that clipping of the output signal can be prevented.
The characteristic of the automatic level controlling circuit is characterized by not only the above input/output characteristic but also the transient response characteristic when the level of the input signal has changed abruptly, i.e. attack/recovery characteristic.
FIGS. 6A and 6B
are waveform charts for explaining the attack characteristic. As seen from
FIG. 6A
, when the level of the input signal rises abruptly at a time t
1
, it is amplified instantaneously with a full gain. Then, as seen from
FIG. 6B
, the level of the output signal is clipped. However, the level is attenuated gradually through the automatic level controlling operation. In this case, the time taken to reach the stable level, or the time taken until the clipping of the level of the output signal is canceled is called “attack time T
A
”. The attack time is controlled by a CR time constant of the detecting circuit.
FIGS. 7A and 7B
are waveform charts for explaining the recovery characteristic. As seen from
FIG. 7A
, it is assumed that the level of the input signal has decreased abruptly at time t
3
. Then, as seen from
FIG. 7B
, it takes a time for the level of the output signal to restore to the ordinary recovery time (t
4
). This time is called “recovery time T
R
”. In this case, a current flows through a discharging current path (not shown) so that the detected voltage Vo at the detecting terminal decreases.
Where the sensitivity of the automatic level controlling circuit is high (i.e. attack/recovery time is short), the distortion of the output signal increases. On the other hand, where the sensitivity is low (i.e. attack/recovery time is long), the output signal disappears for certain period. In this way, the performance of the automatic level controlling circuit is defined by various attack/recovery characteristics.
FIG. 8
is a circuit diagram showing the circuit configuration for setting the attack time and recovery time of the automatic level controlling circuit. In
FIG. 8
, the output from the full-wave rectifying circuit
12
shown in
FIG. 4
is applied to a buffer amplifier
19
, and the output from the buffer amplifier
19
is applied to the base of an NPN-type transistor Q
1
. The one end of a resistor R is connected to the emitter of the NPN-type transistor Q
1
. The other end of the resistor R is connected to a detecting terminal
14
. The detected voltage Vo which appears at the detecting terminal
14
is applied to a buffer amplifier
21
. The output from the buffer amplifier
21
is applied to a comparing circuit
16
.
In the circuit configuration described above, the attack time is set by the charging time constant of the resistor R and capacitor
15
.
FIGS. 9A and 9B
are waveform charts showing the attack characteristic when the input signal is switched from no signal into the signal in a middle level (hereinafter referred to as a middle input signal). The attack time is set at so short a time that as seen from
FIG. 9A
, when the input signal is switched from no signal into the middle input signal at time t
1
, as seen from
FIG. 9B
, the level of the output signal is converged instantaneously. In other words, the period while the level of the output signal is clipped is set at a short time.
FIGS. 10A
to
10
C are graphs showing the attack characteristic when the input signal is switched from the middle input signal into an input signal in a high level (hereinafter referred to a high input signal). Now, as seen from
FIG. 10A
, it is assumed that the input signal has switched from the middle input signal into the high input signal at time t
2
. The waveform in the portion encircled by broken line is shown in an enlarged manner on the right side (also in FIGS.
10
B and
10
C). In this case, as seen from
FIG. 10B
, since the ability of limiting the signal level is too strong, the attack time becomes too short in the signal in which the switching between the middle input signal and the high input signal is to be repeated frequently. This gave rise to signal distortion. Particularly, in the case of a sound signal, the signal distortion led to an unpleasant phenomenon of “sound breakage”.
In order to obviate such an inconvenience, with respect to the attack characteristic of switching the input signal from the middle input signal into the high input signal, the attack time must be set as shown in
FIG. 10C
so that slight distortion component remains instantaneously, but the output signal is thereafter limited gradually.
However, in the circuit configuration shown in
FIG. 8
, since the attack time is defined by the charging time constant of the resistor R and capacitor
15
, that when the input signal is switched from the middle input signal into the high input signal could not set at a long time.
SUMMARY OF THE INVENTION
This invention has been accomplished in view of the inconvenience of the prior art described above.
An object of this invention is to provide an automatic level controlling circuit in which attack times when an input signal is switched from no signal into a middle input signal and when it is switched from the middle input signal into a high input signal can be set at optimum times, respectively, thereby solving the problem of “sound breakage” of a sound signal.
In order to attain the above object, in accordance with this invention, there is provided an automatic level controlling circuit for controlling an output from an amplifying circuit to a stable level when an input signal exceeding a prescribed level is received, comprising: a first and a second time constant circuit for setting an attack time for the automatic level controlling circuit; a comparing circuit for comparing output levels in th

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