Electrical audio signal processing systems and devices – Including amplitude or volume control
Reexamination Certificate
1997-02-25
2001-11-13
Mei, Xu (Department: 2644)
Electrical audio signal processing systems and devices
Including amplitude or volume control
C381S120000, C330S257000, C330S254000
Reexamination Certificate
active
06317502
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electric volume control circuits for controlling the volume or level of a signal according to the level of a control signal and, more particularly, to electronic volume control circuits which can control the audio signal level in a natural manner according to a control signal.
2. Prior Art
Recent audio and video apparatuses include pluralities of ICs. Many of these ICs are controlled according to digital signals from microcomputers provided in apparatuses. Inside or outside such an IC, a D/A (digital/analog) converter is provided. An electronic volume control circuit effects volume or level control according to a digital signal from such a microcomputer. Such an electronic volume controls the audio signal level according to the level of a control signal.
FIG. 1
shows an electronic volume such as above. An audio input signal is provided from an input terminal
101
on a GCA (gain control amplifier)
102
, with the gain thereof varied according to a gain control signal and thereby level controlled. The level controlled signal obtained from the GCA
102
is led to an output terminal
103
. The gain of the GCA
102
is controlled according to a digital signal from a microcomputer
104
. The digital signal from the microcomputer
104
is a serial signal and is converted in a serial/parallel converter
105
into a parallel signal. The parallel signal is then converted into an analog signal in a D/A converter
106
. The analog signal is applied to a variable reference voltage supply
107
, which provides a reference voltage varying according to the control signal. According to changes in the reference voltage of the variable reference voltage supply
107
, the level of a control signal from a controller
108
is changed to change the gain of the GCA
102
.
The relation between the reference voltage of the variable reference voltage supply
107
and the gain of the GCA
102
is shown by the solid curve in FIG.
2
. Reducing reference voltage of the variable reference voltage supply
107
increases attenuation of the audio input signal. In
FIG. 2
, −100 dB is regarded to be zero audio signal level. The staircase waveform shown along the solid curve in
FIG. 2
shows steps of the digital signal obtained from the D/A converter
106
. Usually, a 5-bit control signal is applied to the D/A converter
106
, and a 32-step digital signal is provided.
In the circuit shown in
FIG. 1
, electronic volume is thus controlled according to the digital signal from the microcomputer.
In this circuit, however, the attenuation is only controlled in 32 steps over a wide range from 0 to −100 dB. In this case, when greatly attenuating the signal level, sufficient steps can not be secured in an attenuation range above about −30 dB in which the change of level is clearly understood by listeners. This means that the volume of sound in the audio apparatus is reduced largely when it is reduced by one step in a high level, and a listener feels departure from the normal sense of perception.
FIG. 3
shows the GCA
102
and the controller
108
. In the GCA
102
, the audio input signal from the terminal
101
is attenuated according to a gain control signal from terminals
208
and
209
and then led to the output terminal
103
. The control circuit
108
includes a transistor
211
, the base of which a control signal for attenuation from a terminal
212
is applied, a resistor
214
, through which a current corresponding to the emitter voltage on the transistor
211
flows, and a differential amplifier
215
, which compares a voltage at a point
0
and a reference voltage V
ref
and generates two control voltages for controlling the gain of the GCA
102
. To the control signal input terminal
212
, the output voltage of the variable reference voltage supply
107
is applied.
When the voltage of the attenuation control signal from the terminal
212
is zero, the transistor
211
is “off”, and its emitter current is zero. In this state, a current flows through the resistor
214
, providing a predetermined voltage determined by values of resistors
214
,
218
, and
219
at the point
0
. The voltage at the point
0
in this state of the circuit is sufficiently low compared to the reference voltage V
ref
.
Thus, transistors
220
and
221
are “on” and “off”, respectively, and terminals
208
and
209
are at “H” and “L” levels, respectively.
In GCA
102
, transistors
222
and
223
are “on” while transistors
224
and
225
are “off” in dependence on the “H” and “L” levels on the terminals
208
and
209
respectively.
In the meantime, currents corresponding to an audio input signal from the input terminal are generated at the collectors of transistors
226
and
227
. Transistors
222
to
225
serve as a current distributor for constant current sources
228
and
229
. When the transistors
222
and
223
are “on” while the transistors
224
and
225
are “off”, the corrector signal at the transistors
226
and
227
is provided at the same level to the collectors of the transistors
222
and
223
. The corrector signal from the transistors
222
and
223
is passed through current mirror circuits
230
to
232
for DC component removal and then led in a double end output form to the output terminal
103
. The DC component at the output terminal
103
is superimposed on the reference voltage V
ref
.
As shown above, when the control signal voltage is zero, the audio signal is not attenuated.
When the attenuation control signal voltage from the terminal
212
is increased, the transistor
211
is turned on to cause a current corresponding to its emitter current to flow through the resistor
219
.
With this emitter current, the voltage at the point
0
is increased. The voltage increase at the point
0
reduces the corrector current in a transistor
220
, causing a corrector current in a transistor
221
. As a result, the voltage level at the terminal
208
is decreased, and the voltage level at the terminal
209
is increased.
As a result, the corrector current in the transistors
222
and
223
is reduced to reduce the audio input signal transmission rate (percentage) from 100%. In this way, the input audio signal is attenuated.
In the above way, the circuit shown in
FIG. 3
can attenuate the input signal according to the control signal.
FIG. 4
shows the attenuation obtainable in the circuit shown in FIG.
3
. In the graph, the ordinate is taken for the transmission rate (inverse of attenuation), and the abscissa is taken for the control signal level (=reference voltage).
As shown, the attenuation is increased with increasing control signal level. However, when the control signal level is below V
BE
, the transistor
211
is “off”, and a certain current is caused therethrough when the level V
BE
is exceeded, that is, the attenuation is suddenly increased when the signal level V
BE
is exceeded. Therefore, when the volume is quickly reduced, a listener feels a departure from the normal sense of perception.
SUMMARY OF THE INVENTION
An object of the invention is to provide an electronic volume control which permits the attenuation of signal to be varied smoothly with volume control voltage variation.
Another object of the invention is to provide an electronic volume control which permits satisfactory volume control in a narrow attenuation range.
According to the invention, a current corresponding to an electronic volume control voltage is flowed through a resistor, and the output current from a current generator is controlled with the current through the resistor. The current from the current generator may thus only be changed according to the current received from the resistor, and can be varied smoothly. In a case where current in a transistor is controlled by applying a volume control voltage to the base of the transistor, a point arises which is subject to great current changes in dependence on the on-of operation of the transistor. According to this invention, this can be avoided.
According to this inven
Hogan & Hartson
Mei Xu
Sanyo Electric Co,. Ltd.
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