JPH05284026A - Voice input circuit provided with a/d converter - Google Patents

Abstract

PURPOSE:To improve an S/N by eliminating the output of pop noise to the next step without using any pop noise removing circuit and to perform the indication of a level meter from a much lower level. CONSTITUTION:Audio amplifiers 25 and 27 are interposed between audio signal input terminals 21 and 22 and an A/D converter IC 34. Power is normally supplied from a main power supply line 30 to the audio amplifiers 25 and 27. Power is supplied through a power source switching circuit 32 to the A/D converter IC 34. An input time constant circuit 35 is provided to lower inputs from the audio amplifiers 25 and 27 to the A/D converter IC 34 into a ground level in the manner of AC until the off set caribulation time of the A/D converter IC 34 or a little longer time passes when the power source switching circuit 32 is turned on.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is mounted on an audio device such as a DAT (digital audio tape recorder) or a DCC (digital compact cassette recorder) which converts an analog signal into a digital signal. The present invention relates to a voice input circuit with an A / D converter, and particularly to a voice input circuit with an A / D converter having an offset calibration function for removing an offset error.

[0002]

2. Description of the Related Art FIG. 2 is a circuit diagram showing a conventional audio input circuit with an A / D converter having an offset calibration function.

In the figure, 1 is a left audio signal input terminal in a jack, 2 is a right audio signal input terminal, and 3 is a ground connection terminal. A left channel input circuit 4 includes a first-stage audio amplifier 5, a volume 6 and a second-stage audio amplifier 7. Reference numeral 8 is a power supply line for the first-stage audio amplifier 5, and 9 is a power supply line for the second-stage audio amplifier 7. Both power supply lines 8 and 9 are connected to a main power supply line 11 via a power supply switching circuit 10. Reference numeral 12 is a right channel input circuit, which has a circuit configuration similar to that of the left channel input circuit 4, and therefore its circuit configuration is not shown. Reference numeral 13 is an A / D converter IC having an offset calibration function, and 14 is its power supply line.

Both the power supply lines 8 and 9 of the two audio amplifiers 5 and 7 in the left channel input circuit 4 and the power supply lines (not shown) of the two audio amplifiers in the right channel input circuit 12 are connected to the A / D converter IC 13. The power supply lines 14 are also connected to the power supply switching circuit 10 while being commonly connected to each other. The output terminal of the second-stage audio amplifier 7 in the left channel input circuit 4 is also
The output terminal of the second-stage audio amplifier (not shown) in the right channel input circuit 12 also has an A / D converter I
It is connected to the individual input terminals of C13.

Next, the offset calibration operation of the voice input circuit with an A / D converter configured as described above will be described.

It is assumed that an audio signal is input to the left audio signal input terminal 1 and the right audio signal input terminal 2 from an external sound source (not shown). In this state, when the power supply switching circuit 10 is switched from the OFF state to the ON state, the power supply line 14 of the A / D converter IC 13
Also, the power supply lines 8 and 9 of the two audio amplifiers 5 and 7 rise almost at the same time. However, due to the difference in response speed, the A / D converter IC 13 has a substantially faster power supply rise than the audio amplifiers 5 and 7. Become.

Moreover, the offset calibration operation is instantaneously performed when the power of the A / D converter IC 13 rises, and the offset calibration operation is already completed when the audio amplifiers 5 and 7 rise.

The offset calibration by the A / D converter IC 13 means the A / D converter I
This is to digitally adjust the output of C13 to the "0" level.

[0009]

In the conventional voice input circuit with the A / D converter, the A / D converter IC 13 performs the offset calibration operation, and after the offset calibration operation is completed, the power supply of the audio amplifiers 5 and 7 is completed. Was supposed to stand up. That is, the offset calibration operation has been performed and has been completed while the audio amplifiers 5 and 7 have not yet input to the A / D converter IC 13.

However, the audio amplifiers 5 and 7
Often causes pop noise at the rising edge thereof, and in the case of the conventional example, the A / D converter I
After C13 finishes the offset calibration operation, pop noise from the audio amplifiers 5 and 7 is A /
Since it is input to the D converter IC 13, there is a problem that this pop noise is output from the A / D converter IC 13 to the next stage.

In addition, since the offset calibration is insufficient as described above, the S / N ratio is low, and the level meter provided in the equipment displays an accurate indication from the "0" level. There was also the problem of difficulty.

By the way, in order to remove such a pop noise, a conventionally considered measure is to provide a pop noise removing circuit. However, this pop noise removing circuit has a very complicated circuit configuration. Therefore, there is a drawback that the cost of the device cannot be avoided.

The present invention was devised in view of such circumstances, and improves the S / N ratio by eliminating the output of the pop noise to the next stage without using the pop noise removing circuit. The purpose is to enable marking on the level meter from a lower level.

[0014]

In a voice input circuit with an A / D converter according to the present invention, an audio amplifier is inserted between a voice signal input terminal and an A / D converter IC having an offset calibration function. Naru A / D
In a voice input circuit with a converter, power is constantly supplied to the audio amplifier, while power is supplied to the A / D converter IC via a power switching circuit. In addition, by turning on the power supply switching circuit, the A / D converter IC
When power is supplied to the A / D converter IC, the input from the audio amplifier to the A / D converter IC is dropped to the ground level in an alternating manner until the offset calibration time of the A / D converter IC or a time slightly longer than that time elapses. It is characterized in that an input time constant circuit is provided.

[0015]

[Function] When the power supply switching circuit is turned on to start the supply of power to the A / D converter IC and the offset calibration function is activated, the input time constant circuit is also turned on and the audio amplifier transfers to the A / D converter IC. The input is forcibly set to an alternating "0" level. This compulsory "0" level state continues until the offset calibration time or a little longer time elapses. Therefore, during the offset calibration operation of the A / D converter IC, the A / D converter IC The analog input becomes "0" level. Therefore, the output from the A / D converter IC is digitally brought closer to the "0" level.

When the input time constant circuit is turned off, the input from the audio amplifier to the A / D converter IC is restarted. However, since the audio amplifier is always supplied with power from before that, the offset calibration operation is not performed. There is no pop noise output after the end.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a voice input circuit with an A / D converter according to the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a circuit diagram showing a voice input circuit with an A / D converter according to an embodiment.

In the figure, 21 is a left audio signal input terminal in a jack, 22 is a right audio signal input terminal, and 23.
Is a ground connection terminal. Reference numeral 24 is a left channel input circuit, which includes a first-stage audio amplifier 25, a volume 26, and a second-stage audio amplifier 27. The input terminal of the first-stage audio amplifier 25 is connected to the left audio signal input terminal 21, and the output terminal is connected to the input terminal of the second-stage audio amplifier 27 via the volume 26. The output terminal of the second-stage audio amplifier 27 is connected to one input terminal of the A / D converter IC 34. 28 is a power supply line for the first-stage audio amplifier 25, 29 is a power supply line for the second-stage audio amplifier 27, and both power supply lines 28 and 29 are connected to each other and directly to the main power supply line 30. Connected to each other. The low-potential-side power supply terminals of both audio amplifiers 25 and 27 are commonly connected and also connected to the ground connection terminal 23.

Reference numeral 31 is a right channel input circuit, which has a circuit configuration similar to that of the left channel input circuit 24, and therefore the circuit configuration is not shown.

An A / D converter IC 34 having an offset calibration function for the main power supply line 30 via a power supply switching circuit 32 and a power supply line 33.
The power terminal of is connected.

The difference from the conventional example in the above description is that the power supply lines 28 and 29 of both audio amplifiers 25 and 27 in the left channel input circuit 24 are directly connected to the main power supply line 30 without passing through the power supply switching circuit 32. It is the point that is connected. The same applies to the right channel input circuit 31. With this configuration, if the main power source (not shown) is turned on, both audio amplifiers 25 and 27 of the left channel input circuit 24 are turned on.
Also, both audio amplifiers of the right channel input circuit 31 are constantly supplied with power.

This embodiment has the following structure in addition to the above structure.

That is, a PNP type switching transistor Q1, a charging / discharging capacitor C1, a discharging resistor R1 and an NPN type grounding switching transistor Q.
An input time constant circuit 35 composed of 2 and 3 is provided. The base of the switching transistor Q1 is connected to the power supply line 33 via the base resistor R2, the emitter is connected to the main power supply line 30, and the collector is the charging / discharging capacitor C.
1 and the discharge resistor R1. The collector of the ground connection switching transistor Q2 is connected to the audio signal line 36 between the first-stage audio amplifier 25 and the second-stage audio amplifier 27, and the emitter thereof is connected to the ground connection terminal 23. The positive electrode of the charging / discharging capacitor C1 is connected to the base of the ground connecting switching transistor Q2 via the discharging resistor R1, and the negative electrode is connected to the ground connecting terminal 23.

The input time constant circuit 35 thus configured
When the offset calibration operation is started by supplying power to the A / D converter IC 34, the first stage is until the time required to complete the offset calibration operation (offset calibration time) has elapsed. The audio amplifier 25 has a function of alternatingly dropping the output to the ground level. When the output of the first-stage audio amplifier 25 is dropped to the ground level, the input to the A / D converter IC 34 from the second-stage audio amplifier 27 is eventually dropped to the ground level. Regarding the input time constant circuit 35, in order to set the time for dropping the input to the A / D converter IC 34 to the ground level in an alternating current as substantially the same as the offset calibration time of the A / D converter IC 34 as described above, The capacitance of the discharge capacitor C1 and the resistance value of the discharge resistor R1 may be set appropriately.

Although not shown, the right channel input circuit 31 is also provided with a similar input time constant circuit.

Next, the operation of the voice input circuit with an A / D converter of the present embodiment constructed as described above will be explained.

It is assumed that an audio signal is input to the left audio signal input terminal 21 and the right channel input circuit 22 from an external sound source (not shown), and the ground connection terminal 23 is grounded.

Power is constantly supplied to the first-stage audio amplifier 25 and the second-stage audio amplifier 27 from the main power supply line 30 through the power supply lines 28 and 29, and the power supply switching circuit 32. ON, OFF
These audio amplifiers 2 regardless of
5, 27 are always active.

When the power supply switching circuit 32 is OFF,
Since the base potential of the switching transistor Q1 is at "L" level, this switching transistor Q1 is in a conductive state. Therefore, if the charging / discharging capacitor C1 is charged from the main power supply line 30 through the switching transistor Q1 and the charging voltage is equal to or higher than the predetermined voltage, the ground connection switching transistor Q2 is also in the conductive state and the first stage The audio signal line 36 between the second-stage audio amplifier 25 and the second-stage audio amplifier 27 is connected to the ground connection terminal 23 and is forcibly set to an AC ground level. The input from the amplifier 25 to the second-stage audio amplifier 27, and further, the input from the second-stage audio amplifier 27 to the A / D converter IC 34 is also at an alternating “0” level.

Next, assume that the power supply switching circuit 32 is switched from the OFF state to the ON state. Then, from the main power supply line 30, the power supply switching circuit 32 and the power supply line 33.
Power is supplied to the A / D converter IC 34 via
The A / D converter IC 34 starts the offset calibration operation.

On the other hand, when power is supplied to the power supply line 33, the base voltage of the switching transistor Q1 rises, so that the switching transistor Q1 is turned off. Then, charging from the main power supply line 30 to the charging / discharging capacitor C1 is stopped, and the charging / discharging capacitor C1 is stopped.
Shifts to the discharge state.

Even when the charging / discharging capacitor C1 is discharged, the ground connection switching transistor Q2 remains conductive until the voltage across the capacitor C1 drops to the cutoff voltage of the ground connection switching transistor Q2. During this period, the A / D converter IC 34 performs the offset calibration operation. During the offset calibration operation, the switching transistor Q2 for ground connection is in the conductive state, and the input audio signal is dropped to the AC "0" level. Therefore, the output from the A / D converter IC 34 is also digital. Results in a level close to "0". In such a state, the offset calibration operation of the A / D converter IC 34 ends.

By the time constant of the input time constant circuit 35, substantially simultaneously with the end of the offset calibration operation, the switching transistor Q for ground connection.
2 is turned off, and both audio amplifiers 25 and 27
The audio signal line 36 between them is disconnected from the ground level. Therefore, the input of the audio signal from the first-stage audio amplifier 25 to the second-stage audio amplifier 27 and the input of the audio signal from the second-stage audio amplifier 27 to the A / D converter IC 34 are started.

Therefore, although the input audio signal is A / D converted by the A / D converter IC 34 whose offset calibration has been completed, both audio amplifiers 2
Power is already supplied to 5, 27 from the main power supply line 30 via the power supply lines 28, 29, so that pop noise caused by the rising of the audio amplifiers 25, 27 is not generated. Therefore, an expensive pop noise removing circuit having a complicated circuit configuration is not required. Further, the S / N ratio is improved, and the marking on the level meter can be performed from a lower level.

The reason why the signal line of the audio signal is not set to the DC ground level but is set to the AC ground level is that pop noise may be generated if the DC ground level is used.

As the input time constant circuit 35, the capacitance of the charging / discharging capacitor C1 and the discharging capacitor are used so that the input to the A / D converter IC 34 is AC-grounded for a time slightly longer than the offset calibration time. The time constant determined by the resistance value of the resistor R1 may be adjusted.

The ground connection switching transistor Q2 may be inserted between the output terminal of the second-stage audio amplifier 27 and the ground connection terminal 23. 1
The second-stage audio amplifier 25 and the second-stage audio amplifier 27 may be integrated into one.

[0039]

As described above, according to the present invention, during the offset calibration operation by the input time constant circuit, the input from the audio amplifier to the A / D converter IC is forcibly set to the AC "0" level. Therefore, the output from the A / D converter IC can be made digitally closer to the "0" level, and the S / N ratio can be improved. Also, the marking on the level meter can be performed from a lower level. Further, since pop noise is not output after the offset calibration operation is completed, it is not necessary to provide a pop noise removing circuit as in the conventional example, and the cost can be reduced accordingly.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an audio input circuit with an A / D converter having an offset calibration function according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a conventional audio input circuit with an A / D converter having an offset calibration function.

[Explanation of symbols]

 21 Left audio signal input terminal 22 Right audio signal input terminal 23 Ground connection terminal 24 Left channel input circuit 25 1st stage audio amplifier 26 Volume 27 2nd stage audio amplifier 28,29 Power supply line 30 Main power supply line 31 Right channel input Circuit 32 Power supply switching circuit 33 Power supply line 34 A / D converter IC 35 Input time constant circuit 36 Audio signal line Q1 Switching transistor Q2 Ground connection switching transistor C1 Charge / discharge capacitor R1 Discharge resistor

Claims (1)

[Claims] 1. An audio input circuit with an A / D converter in which an audio amplifier is inserted between an audio signal input terminal and an A / D converter IC having an offset calibration function. The A / D converter IC is configured to be constantly supplied with power, while the A / D converter IC is configured to be supplied with power via a power supply switching circuit, and the power supply switching circuit is turned on. When power is supplied to the A / D converter IC, the input from the audio amplifier to the A / D converter IC is AC-converted until the offset calibration time of the A / D converter IC or a time slightly longer than the time elapses. Sound with an A / D converter characterized by being provided with an input time constant circuit for dropping to the ground level Input circuit.