JP3929832B2 - Input signal processing circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention is preferably implemented in an apparatus that amplifies and outputs an audio signal, such as an audio device, a television receiver, and a personal computer, and realizes amplification of the audio signal using a highly efficient digital amplifier. The present invention relates to an input signal processing circuit which receives both analog signals and digital signals, selects one of them, adjusts the amplitude level, and then outputs the signal to the digital amplifier.
[0002]
[Prior art]
  In the above-mentioned audio equipment and the like, many audio signal sources (input sources) including not only analog but also digital have come to be connected by multimediaization, and a recording medium such as a DVD or a digital satellite Due to changes in transmission media such as broadcasting, the number of channels is increasing. On the other hand, the power amplifier that amplifies the audio signal has improved the fidelity of the digital amplifier (so-called class D amplifier) with the advance of semiconductor technology, and can be created at low cost. Adoption is progressing along with the advantages of efficiency.
[0003]
  FIG. 8 shows a typical conventional amplifier that functions as a preamplifier that selects either an analog signal or a digital signal as described above and adjusts and outputs the amplitude level of the digital amplifier 1 as a power amplifier. 1 is a block diagram of an audio apparatus configured with a technical input signal processing circuit 2. FIG. The configuration of FIG. 8 shows a configuration per channel. In the case of multi-channel, the same configuration is provided for the number of channels.
[0004]
  The analog input signal is output to the analog input terminal A of the digital amplifier 1 after its amplitude level is directly adjusted in the electronic volume vr1. On the other hand, the digital input signal is used for switching control of the analog switch sw. A predetermined reference DC voltage from a low voltage power source (not shown) is input to one individual contact a of the analog switch sw after being adjusted to a desired amplitude level in the electronic volume vr2. A GND potential is applied to the other individual contact b of the analog switch sw. Accordingly, the switching of the analog switch sw in response to the digital input signal outputs either the amplitude-adjusted DC voltage or the GND potential from the common contact c, and the digital input terminal of the digital amplifier 1 Input to D1.
[0005]
  The digital amplifier 1 is realized by, for example, a 1-bit amplifier using a 1-bit signal by ΔΣ modulation, and the signal input from the digital input terminal D1 is ΔΣ-modulated and predetermined in response to the signal. A power supply voltage is switched and digitally amplified, and a signal input from the analog input terminal A is converted into a 1-bit signal by the ΔΣ modulation and then used for switching the power supply voltage and digitally amplified. The digital amplifier 1 outputs, for example, a 1-bit signal that switches between the power supply voltage and the GND level. The signal passes through the low-pass filter 3 and is demodulated into an analog signal, which is audible by the speaker 4. The
[0006]
  The electronic volumes vr1 and vr2 are controlled in attenuation amount by a control circuit 5 realized by a microcomputer or the like. The digital amplifier 1 may be an amplifier using a PWM signal which is inferior in signal properties but advantageous in terms of cost.
[0007]
[Problems to be solved by the invention]
  In an audio device equipped with the input signal processing circuit 2 and the digital amplifier 1 as described above, a conventional multi-channel recording medium such as a DVD or a transmission medium such as a digital satellite broadcast is changed from the conventional two-channel stereo reproduction. In order to reproduce it, there has been a demand for multi-channel reproduction such as 5.1 channel reproduction and 7.1 channel reproduction. For this reason, there is an increasing demand for a compact and low-cost configuration for each channel. However, if the number of channels is increased as it is by using the configuration per channel as shown in FIG. 8, not only the cost is very high but also the circuit configuration becomes very complicated. For this reason, in the field of general audio, which is inexpensive and has a high penetration rate, it is not possible to employ a 1-bit amplifier having a full 1-bit configuration from the input to the output of the digital amplifier 1.
[0008]
  Specifically, since a DC voltage is input to the electronic volume vr2, in order to suppress noise jumping that occurs in the 1-bit signal system when an analog signal is input, and in the case of multi-channel control, Since 1-bit signals are combined in the volume and it is difficult to secure performance, separate electronic volumes vr1 and vr2 are provided for each channel for analog signals and for digital signal input (DC voltage). Avoid the influence of each other.
[0009]
  Therefore, as shown in FIG. 8, the monaural amplifier has a total of two channels of one analog channel and one digital channel, the stereo amplifier has a total of four channels of two analog channels and two digital channels, and the analog channel of the 5.1 channel amplifier has six channels. In addition, an electronic volume for a total of 12 channels including 6 digital channels is required. Further, when the digital input signal is handled as a differential signal, the monaural amplifier has a total of three channels of one analog channel and two differential digital channels, and the stereo amplifier has a total of six analog two channels and four differential digital channels. In the 5.1 channel amplifier, a total of 18 electronic volumes for 6 analog channels and 12 differential digital channels are required.
[0010]
  On the other hand, if the digital input signal is directly input to the electronic volume and the amplitude level is adjusted, it seems that the electronic volume vr1 for the analog input signal can be shared with the digital input signal. However, while the frequency band of the analog input signal is about DC to 20 kHz, the sampling frequency of the 1-bit signal is very high, such as 2.8 MHz or 5.6 MHz. For example, an inexpensive electronic volume of a resistance ladder type Then, there is a problem that the waveform of the rectangular wave collapses and cannot be used.
[0011]
  On the other hand, when the conventional analog volume is used, a gang error occurs, and a new problem arises that the level difference between the multi-channel input signals cannot be finely adjusted. For example, in surround control of Dolby Digital or the like, the level difference needs to be suppressed within a standard of 0.1 dB, and it is extremely difficult to satisfy this standard with the analog volume.
[0012]
  The gang error is, for example, a rotary analog volume. For example, when the rotation angle is essentially the same, the resistance value between the channels should be equal. Caused by a phenomenon that does not happen. As a result, the level difference between the channels becomes disjoint and the sound balance is lost. This phenomenon also occurs somewhat in electronic volume due to variations in internal elements of the integrated circuit of the electronic volume. However, in the electronic volume, the resistance value does not vary as much as the rotary type analog volume, and it can be suppressed to the standard value of the Dolby Digital.
[0013]
  Further, when the volume adjustment is performed by remote control operation, in the electronic volume, the control circuit 5 can be realized by selectively turning on a switch for selecting a ladder resistance in accordance with a remote control reception signal. In the case of an analog volume, the rotation angle of the analog volume must be adjusted by a motor, which is not realistic in consideration of cost and space.
[0014]
  SUMMARY OF THE INVENTION An object of the present invention is to select an input signal processing circuit that selects one of one or a plurality of analog inputs and one or a plurality of digital inputs, adjusts the level, and outputs the selected signal to a digital amplifier that performs power amplification. Another object of the present invention is to provide an input signal processing circuit capable of suppressing the cost for the multi-channel input.
[0015]
[Means for Solving the Problems]
The input signal processing circuit according to the present invention selects one of one or a plurality of channels of analog input and one or a plurality of channels of digital input, adjusts the signal level by the level adjusting means, and then performs power amplification. In the input signal processing circuit configured to output to the amplifier, first selection means for selecting and outputting either an analog input signal or a predetermined DC voltage, and the first selection meansInput analog input signal and predetermined DC voltageLevel adjusting means for adjusting the output level of the digital amplifier, switching means for performing switching in response to a digital input signal, the output being given to the digital input terminal of the digital amplifier, and the output from the level adjusting means for the digital The second selection means for selectively outputting to either one of the analog input terminal of the amplifier and the switching means, and the first and second selection means corresponding to the input channel to be selected. And control means for controlling in conjunction with each other.
[0016]
  According to the above configuration, in an input signal processing circuit that selects a plurality of channels and adjusts the signal level thereof, such as an audio preamplifier, and outputs the input to the amplifier, the amplifier is In a digital amplifier that performs high-efficiency power amplification with a digital signal that is ΔΣ-modulated or PWM-modulated, the analog input signal is directly adjusted by the level adjusting means by the interlock control of the first and second selecting means by the control means. After the amplitude is adjusted, it is output to the analog input of the digital amplifier and amplified. On the other hand, a digital input signal that has already been subjected to ΔΣ modulation or PWM modulation is supplied for switching control of the switching means, and the reference DC voltage to be switched is linked control of the first and second selection means by the control means. Thus, after the amplitude is adjusted by the level adjusting means, switching is performed by the switching means, and then output to the digital input of the digital amplifier for power amplification.
[0017]
  Therefore, an analog input signal and a digital reference DC voltage having no problem in frequency characteristics are input to the level adjusting means realized by a so-called electronic volume, and the level adjusting means can be shared. Costs can be suppressed for multi-channel input.
[0018]
  Furthermore, the input signal processing circuit of the present invention further includes a time constant circuit having a time constant longer than the maximum repetition time constant of “1” of the digital input signal between the level adjusting means and the switching means. It is characterized by.
[0019]
  According to the above configuration, the switching reference DC voltage whose amplitude is adjusted by the level adjusting means is input to the switching means via the time constant circuit realized by an RC integrating circuit or the like. The time constant circuit is set to a time constant longer than the maximum repetition time constant of “1” of the digital input signal.
[0020]
  Therefore, even if “1” continues to the digital input signal, the amplitude of the signal input to the digital amplifier can be maintained at the DC voltage adjusted by the level adjusting means, and the digital waveform distortion is reduced. Occurrence can be suppressed. In particular, when a differential circuit configuration is used, the stability can be dramatically improved. In addition, when the input signal is an audio signal, it is possible to prevent abnormal noise during the step response of the level adjusting means.
[0021]
  In the input signal processing circuit of the present invention, the switching means switches between the DC voltage adjusted by the level adjusting means and the GND potential in response to the digital input signal. , And held at the GND potential.
[0022]
  According to the above configuration, the switching means is forcibly switched to the GND potential at the time of analog input, thereby reducing noise such as a digital input signal entering the analog input signal line and a high frequency component of the clock, and the analog of the digital amplifier. It is possible to greatly improve the performance at the time of input, particularly the SN ratio.
[0023]
  Furthermore, the input signal processing circuit of the present invention is further characterized by further comprising short-circuit means for setting the analog input terminal of the digital amplifier to the GND potential at the time of digital input.
[0024]
  According to the above configuration, at the time of digital input, the analog input terminal of the digital amplifier is forcibly switched to the GND potential by the short-circuit means, so that the digital input signal enters the analog input signal line and is coupled within the digital amplifier. In addition, noise generated near the maximum volume value can be removed.
[0025]
  Furthermore, in the input signal processing circuit of the present invention, the switching means performs switching in response to the digital input signal, and the output thereof is an in-phase input with respect to the digital input signal. A first switching means applied to the digital input terminal, and performs switching in response to the digital input signal, the output of which is a reverse-phase input to the digital input signal, and the second digital input terminal of the digital amplifier And a second switching means.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
  The following describes the first embodiment of the present invention with reference to FIGS.
[0027]
  FIG. 1 is a block diagram of an audio apparatus including an input signal processing circuit 11 according to the first embodiment of this invention. In this audio apparatus, the input signal processing circuit 11 of the present invention that functions as a preamplifier generally selects either an analog signal or a 1-bit digital signal, adjusts the amplitude level thereof, and uses a power amplifier. After being output to a certain digital amplifier 12 and amplified by the digital amplifier 12, it is given to a speaker 14 through a low-pass filter 13 for sounding. The configuration of FIG. 1 shows a configuration per channel. In the case of multi-channel, the same configuration is provided for the number of channels. The input signal itself (acoustic signal source) may be provided with a plurality of channels at least one of the analog and digital.
[0028]
  The analog input signal is input to the electronic volume VR via the analog switch SW1, and after the amplitude level is directly adjusted, the analog input signal is output to the analog input terminal A of the digital amplifier 12 via the analog switch SW2. On the other hand, the digital input signal is used for switching control of the analog switch SW3.
[0029]
  In the analog switch SW1, a sufficiently stable reference DC voltage from a low voltage power supply circuit (not shown) is input to one individual contact a1, and the analog input signal is input to the other individual contact b1. The output from the common contact c1 is input to the electronic volume VR. The output from the electronic volume VR is input to the common contact c2 of the analog switch SW2. In the analog switch SW2, the output from one individual contact a2 is input to one individual contact a3 of the analog switch SW3. The output from the other individual contact b2 is input to the analog input terminal A of the digital amplifier 12.
[0030]
  Thus, even if the electronic volume VR is shared by the reference DC voltage and the analog input signal, low-frequency noise such as a mole current superimposed on the DC component and a ripple component of the power source is not detected in the analog mode. There is no input to the input terminal A. The analog signal circuit such as the electronic volume VR, the low-voltage power supply circuit (not shown), and the 1-bit signal circuit such as the analog switch SW3 are isolated from each other and formed so that the patterns are not parallel to each other. Has been.
[0031]
  In the analog switch SW3, a GND potential is applied to the other individual contact b3, and an output from the common contact c3 is input to the digital input terminal D1 of the digital amplifier 12. Therefore, by switching of the analog switch SW3 in response to the digital input signal, either the amplitude-adjusted DC voltage or the GND potential is output from the common contact c3, and the digital input terminal of the digital amplifier 1 is output. Input to D1.
[0032]
  The amount of attenuation of the electronic volume VR is controlled by a control circuit 15 realized by a microcomputer or the like. The analog switches SW1 and SW2 are controlled in conjunction with each other by the control circuit 15, and the common contacts c1 and c2 are connected to the individual contacts b1 and b2 when the analog mode is set, and the digital mode is set. When set, they are connected to the individual contacts a1 and a2.
[0033]
  The digital amplifier 12 is realized by a 1-bit amplifier using a 1-bit signal by ΔΣ modulation, and a signal input from the single-ended digital input terminal D1 is ΔΣ-modulated and then responds to the signal, A predetermined power supply voltage is switched and digitally amplified, and a signal input from the single-ended analog input terminal A is converted into a 1-bit signal by the ΔΣ modulation and then used for switching the power supply voltage. Digitally amplified. The digital amplifier 12 outputs, for example, a 1-bit signal Oa that switches between the power supply voltage and the GND level. The signal Oa passes through the low-pass filter 13 and is demodulated into an analog signal. It becomes. Thus, sound reproduction can be performed at a desired volume using the digital amplifier 12.
[0034]
  The digital amplifier 12 may be an amplifier using a PWM signal that is advantageous in terms of cost, although the signal characteristics are slightly inferior. That is, a PWM signal generation circuit with a switching frequency on the order of several hundreds of kHz is cheaper than a 1-bit signal generation circuit with an order of several MHz, and the cost of the product can be reduced. This PWM signal is suitable for the field of general audio.
[0035]
  FIG. 2 is an electric circuit diagram showing an example of the configuration of the electronic volume VR. This electronic volume VR is a resistance ladder type electronic volume realized by, for example, M62447SP, which is a sound controller LSI for volume control manufactured by Mitsubishi Electric Corporation, and has resistance elements r1, r2,. rn and analog switches s1, s2,..., sn, sn + 1 for taking out the voltages of the respective terminals. The analog input signal or reference DC voltage selected by the analog switch SW1 is input to the first-stage resistor element r1, and any one of the analog switches s1, s2,..., Sn + 1 is selectively turned on by the control circuit 15. To derive an output with an adjusted amplitude level.
[0036]
  Here, as the electronic volume VR, for example, as shown in FIG. 3, it is conceivable to use a feedback amount variable type amplifier that adjusts the feedback amount of the operational amplifier to a desired amplitude level. In order to adjust the reference DC voltage, the resistance ladder type is particularly preferable. When applied to an audio signal as described above, in the present invention, the electronic volume VR does not directly adjust the amplitude level of the rectangular wave with the high-speed sampling frequency of 2.8 MHz or 5.6 MHz, but such a high frequency. Since this is separated from the high frequency digital input signal pattern by the analog switch SW3, the electronic volume VR only needs to have a pass band of DC to 20 kHz or more, and is inexpensive with the resistor ladder type electronic volume. Can be realized.
[0037]
  As described above, the input signal processing circuit 11 selects either the analog input signal or the digital input signal, adjusts the amplitude level thereof, and then outputs the digital input signal to the digital amplifier 12. Is applied to the analog switch SW3, the reference DC voltage switched by the analog switch SW3 and the analog input signal are switched by the analog switches SW1 and SW2, and the amplitude is adjusted by the electronic volume VR. The electronic volume VR can be shared with a digital reference DC voltage having no problem in frequency characteristics. Therefore, conventionally, a monaural amplifier has a total of 2 channels of 1 analog and 1 digital channel, a stereo amplifier has a total of 4 channels of 2 analog and 2 channels, and a 5.1 channel amplifier has 6 analog and 6 digital channels. In this input signal processing circuit 11, the electronic volume for a total of 12 channels was required, such as 1 channel for a monaural amplifier, 2 channels for a stereo amplifier, and 6 channels for a 5.1 channel amplifier. The number of channels is sufficient, and the cost can be suppressed for the multi-channel input.
[0038]
  As a result, even in the field of general audio, it is possible to employ a 1-bit amplifier having a full 1-bit configuration from input to 1-bit amplifier output. Further, as described above, the frequency characteristic of the electronic volume VR can be the low-cost resistance ladder type electronic volume having a pass band from DC to about 20 kHz. Control with a remote controller can also be expected, and the level difference between each channel in multi-channel can be suppressed within 0.1 dB, which is a Dolby Digital standard, for example.
[0039]
  The following describes the second embodiment of the present invention with reference to FIG.
[0040]
  FIG. 4 is a block diagram of an audio apparatus including the input signal processing circuit 21 according to the second embodiment of this invention. The input signal processing circuit 21 is similar to the input signal processing circuit 11 described above, and corresponding portions are denoted by the same reference numerals and description thereof is omitted. It should be noted that this input signal processing circuit 21 is provided with an analog switch SW4 that holds the switching input at the GND potential at the time of analog input in the line of the digital input signal to the analog switch SW3. The line to the input terminal A is provided with an analog switch SW5 that holds the analog input terminal A at the GND potential at the time of digital input. The reference DC voltage line to the analog switch SW3 includes a resistor R and a capacitor C. The time constant is longer than the maximum repetition time constant of “1” of the digital input signal, that is,
    RC time constant> Maximum pulse length of input signal
The time constant circuit T having the relationship is provided.
[0041]
  In the analog switch SW4, the digital input signal is given to one individual contact a4, the GND potential is given to the other individual contact b4, and the output from the common contact c4 is used for switching of the analog switch SW3. Is done. In the analog switch SW5, the GND potential is given to one individual contact a5, the output from the individual contact b2 of the analog switch SW2 is given to the other individual contact b5, and the output from the common contact c5 is The signal is output to the analog input terminal A of the digital amplifier 12. These analog switches SW4 and SW5 are also controlled in conjunction with each other by the control circuit 15 together with the analog switches SW1 to SW3.
[0042]
  Since the ΔΣ modulation 1-bit signal or PWM signal is a rectangular wave and includes a harmonic component, when the digital amplifier 32 is driven in the analog mode, the harmonic component enters the analog line as noise. When the control circuit 15 selects the analog mode while the S / N ratio on the analog line is affected, the digital input terminal D1 of the digital amplifier 32 is connected to the GND by the analog switch SW4. It is possible to prevent such a malfunction. In particular, the SN ratio can be greatly improved.
[0043]
  Further, in the digital mode, the analog input terminal A of the digital amplifier 32 is connected to the GND by the analog switch SW5, so that the harmonic component enters and is combined in the digital amplifier 12, and near the maximum value of the volume. Generation of noise can be prevented.
[0044]
  Furthermore, even if “1” continues to the digital input signal by the time constant circuit T, the amplitude of the signal input to the digital amplifier 12 is changed to a DC voltage whose amplitude is adjusted by the electronic volume VR. It can be maintained, and the occurrence of digital waveform distortion can be suppressed. Further, when the digital input signal is an audio signal, even if the volume value of the electronic volume VR changes suddenly stepwise, a sudden change in DC voltage can be eliminated and the occurrence of click noise can be prevented. .
[0045]
  The following describes the third embodiment of the present invention with reference to FIG. 5 and FIG.
[0046]
  FIG. 5 is a block diagram of an audio apparatus including the input signal processing circuit 31 according to the third embodiment of this invention. The input signal processing circuit 31 is similar to the input signal processing circuits 11 and 21 described above, and corresponding portions are denoted by the same reference numerals and description thereof is omitted. It should be noted that in this input signal processing circuit 31, a buffer B1 and an inverter B2 are provided in the digital input signal line, and the digital input signal is converted into a differential signal. Correspondingly, the analog switch SW3 is composed of a pair of analog switches SW31 and SW32, respectively provided with time constant circuits T1 and T2, and the digital amplifier 32 uses a 1-bit signal by ΔΣ modulation. The analog input terminal A having a single end and differential 1-bit digital input terminals D1 and D2 are provided.
[0047]
  The time constant circuits T1 and T2 include resistors R1 and R2 and capacitors C1 and C2, and have a time constant longer than the maximum repetition time constant of “1” of the digital input signal. A DC voltage from the electronic volume VR output from the individual contact a2 of the analog switch SW2 is commonly applied to the time constant circuits T1 and T2. Similar to the analog switch SW3, the DC voltage via the time constant circuits T1 and T2 is applied to one individual contact a31, a32 of the analog switches SW31, SW32, and the other individual contact b31 of these analog switches SW31, SW32, The GND potential is applied to b32, and the outputs from the common contacts c31 and c32 are input to the digital input terminals D1 and D2 of the digital amplifier 32, respectively.
[0048]
  The analog switches SW31 and SW32 perform a reciprocal operation by the buffer B1 and the inverter B2, and when the digital input signal is at a high level, the digital input terminal D1 becomes a DC voltage that is voltage-adjusted by the electronic volume VR. When the digital input terminal D2 is at the GND potential and the digital input signal is at a low level, the digital input terminal D2 is at the DC voltage, the digital input terminal D1 is at the GND potential, and the digital input terminal D1 is in phase with the digital input signal. And the digital input terminal D2 is an input with reversed phase. The differential 1-bit signals Oa and Ob from the digital amplifier 32 are demodulated into analog signals by passing through the low-pass filter 33 and are made acoustic by the speaker 4.
[0049]
  FIG. 6 is a waveform diagram for explaining the operation at the time of digital input of the audio apparatus configured as described above. The reference DC voltage of the voltage V from the low voltage power supply circuit is attenuated to V1 in the electronic volume VR. This DC voltage is switched by the analog switches SW31 and SW32 in response to a digital input signal input in synchronization with the clock signal, so that an input to the digital input terminals D1 and D2 of the digital amplifier 32 is obtained. The voltage is amplified, and output 1-bit signals Oa and Ob are obtained.
[0050]
  Thus, in the case of the differential circuit configuration, the stability can be drastically improved by providing the time constant circuits T1 and T2 in the differential analog switches SW31 and SW32 with respect to the reference DC voltage. . Further, in the case of a differential circuit configuration, conventionally, a monaural amplifier has a total of 3 channels of analog 1 channel and digital 2 channels, a stereo amplifier has a total of 6 channels of analog 2 channels and a digital 4 channel, and a 5.1 channel amplifier has analog channels The electronic volume for 18 channels in total of 6 channels and 12 digital channels was necessary. Even in this input signal processing circuit 31, one channel is used for monaural amplifiers, two channels are used for stereo amplifiers, and 5.1 channel amplifiers are used. As with six channels, the number of channels is sufficient, and the effect of reducing the electronic volume is even greater.
[0051]
  The following describes the fourth embodiment of the present invention with reference to FIG.
[0052]
  FIG. 7 is a block diagram of an audio apparatus including the input signal processing circuit 41 according to the fourth embodiment of this invention. The input signal processing circuit 41 is similar to the above-described input signal processing circuits 21 and 31, and corresponding portions are denoted by the same reference numerals and description thereof is omitted. It should be noted that the input signal processing circuit 41 is provided with analog switches SW4 and SW5 of the input signal processing circuit 21 in addition to the configuration of the input signal processing circuit 31 having the above-described differential circuit configuration. .
[0053]
  The present invention can be applied to a configuration using a digital amplifier, and is not limited to an audio signal, and can be implemented by, for example, a multi-channel measuring instrument amplifier.
[0054]
  The input signal processing circuit according to the present invention selects one of one or more analog inputs and one or more digital inputs, adjusts the signal level by the level adjusting means, and then performs power amplification. In the input signal processing circuit configured to output to the digital amplifier to be performed, the analog input signal is output to the digital amplifier via the level adjustment unit at the time of the analog input, and to the level adjustment unit at the time of the digital input. May receive a predetermined DC voltage, and output the DC voltage adjusted by the level adjusting means to the digital amplifier via switching means that performs switching in response to the digital input signal.
[0055]
  According to the above configuration, in an input signal processing circuit that selects a plurality of channels and adjusts the signal level thereof, such as an audio preamplifier, and outputs the input to the amplifier, the amplifier is In the case of a digital amplifier that performs high-efficiency power amplification with a digital signal that is ΔΣ modulated or PWM modulated, the analog input signal is output to the analog input of the digital amplifier after amplitude adjustment is performed by the level adjusting means, Further, after being converted into a digital signal after being subjected to the ΔΣ modulation or PWM modulation, the power is amplified. On the other hand, for the digital input signal that has already been ΔΣ modulation or PWM modulated, the digital input signal is switched. The DC voltage supplied to the switching means and switched by the switching means is adjusted in amplitude by the level adjusting means, and The output to the digital amplifier of the digital input after being etching, is power-amplified.
[0056]
  Here, a digital input signal having a high frequency cannot be directly subjected to amplitude adjustment from the frequency characteristic of the level adjusting means realized by a so-called resistance ladder type electronic volume, and is desired by the level adjusting means as described above. The amplitude adjustment is realized by switching the DC voltage adjusted to the amplitude level in response to the digital input signal. Conventionally, separate level adjusting means are provided for the analog input signal and the digital input signal, whereas in the present invention, the analog input signal and the digital reference DC that has no problem in frequency characteristics are provided. The level adjusting means is shared with the voltage.
[0057]
  Therefore, for example, for audio signals, conventionally, a monaural amplifier has a total of 3 channels of analog 1 channel and differential 2 digital channels, a stereo amplifier has a total of 6 channels of analog 2 channels and 4 digital channels, and a 5.1 channel amplifier. Electronic volumes for a total of 18 channels (6 analog channels and 12 digital channels) were necessary. In the present invention, as described above, both analog and digital channels are used. In the 5.1 channel amplifier, 6 channels are sufficient, and the cost can be suppressed against the multi-channel input.
[0058]
  As a result, even in the field of general audio, it is possible to employ a 1-bit amplifier having a full 1-bit configuration from input to 1-bit amplifier output. As the frequency characteristic of the level adjusting means, for example, in the audio signal, the low-cost electronic volume having a pass band from DC to about 20 kHz can be used. In addition, the level difference between the channels in the multi-channel mode can be suppressed, for example, within the Dolby Digital standard.
[0059]
【The invention's effect】
  As described above, the input signal processing circuit of the present invention selects an input of a plurality of channels and adjusts the signal level of the input signal, such as an audio preamplifier, and then outputs the input signal to the amplifier. In the processing circuit, when the amplifier is a digital amplifier that performs power amplification with high efficiency by using a digital signal that is ΔΣ modulated or PWM modulated, the analog input signal is directly amplitude-adjusted by a level adjusting unit, and then the digital amplifier In response to this, the digital input signal is supplied to the switching means, the DC voltage switched by the switching means is adjusted by the level adjusting means, and the switched DC voltage is converted to the digital of the digital amplifier. Output to input.
[0060]
  Therefore, the level adjustment means can be used for the analog input signal and the digital reference DC voltage having no problem in frequency characteristics, so that the cost can be suppressed for the multi-channel input. As a result, even in the field of general audio, it is possible to employ a 1-bit amplifier having a full 1-bit configuration from input to 1-bit amplifier output. As the frequency characteristic of the level adjusting means, for example, in the audio signal, the low-cost electronic volume having a pass band from DC to about 20 kHz can be used. In addition, the level difference between the channels in the multi-channel mode can be suppressed within the Dolby Digital standard, for example.
[0061]
  In addition, as described above, the input signal processing circuit of the present invention selects an input of a plurality of channels and adjusts the signal level and outputs it to the amplifier, such as an audio preamplifier. In the input signal processing circuit, when the amplifier is a digital amplifier, the amplitude of the analog input signal is directly adjusted by the level adjusting means by the interlock control of the first and second selecting means by the control means, and then the digital amplifier In contrast, the digital input signal is supplied to the switching means, the DC voltage switched by the switching means is adjusted by the level adjusting means, and the switched DC voltage is converted to the digital of the digital amplifier. Output to input.
[0062]
  Therefore, an analog input signal and a digital reference DC voltage having no problem in frequency characteristics are input to the level adjusting means realized by a so-called electronic volume, and the level adjusting means can be shared. Therefore, the cost can be suppressed for the multi-channel input.
[0063]
  Further, as described above, the input signal processing circuit of the present invention realizes the switching reference DC voltage amplitude-adjusted by the level adjusting means by the RC integrating circuit or the like, and the digital input signal "1" The signal is input to the switching means through a time constant circuit set to a time constant longer than the maximum repetition time constant.
[0064]
  Therefore, even if “1” continues to the digital input signal, the amplitude of the signal input to the digital amplifier can be maintained at the DC voltage adjusted by the level adjusting means, and the digital waveform distortion can be maintained. Can be suppressed. In particular, when a differential circuit configuration is used, the stability can be dramatically improved. In addition, when the input signal is an audio signal, it is possible to prevent abnormal noise during the step response of the level adjusting means.
[0065]
  The input signal processing circuit of the present invention forcibly switches the switching means to the GND potential during analog input as described above.
[0066]
  Therefore, it is possible to reduce noise such as a digital input signal and a high-frequency component of a clock that enter the analog input signal line, and to greatly improve the performance of the digital amplifier at the time of analog input, particularly the SN ratio.
[0067]
  Furthermore, as described above, the input signal processing circuit of the present invention forcibly switches the analog input terminal of the digital amplifier to the GND potential by the short-circuit means at the time of digital input.
[0068]
  Therefore, a digital input signal enters the analog input signal line and is combined in the digital amplifier, and noise generated near the maximum volume value can be removed.
[Brief description of the drawings]
FIG. 1 is a block diagram of an audio apparatus including an input signal processing circuit according to a first embodiment of this invention.
FIG. 2 is an electric circuit diagram showing a configuration example of an electronic volume in the input signal processing circuit shown in FIG.
FIG. 3 is a block diagram showing another example of an electronic volume.
FIG. 4 is a block diagram of an audio apparatus including an input signal processing circuit according to a second embodiment of the present invention.
FIG. 5 is a block diagram of an audio apparatus including an input signal processing circuit according to a third embodiment of the present invention.
6 is a waveform diagram for explaining an operation at the time of digital input of the audio apparatus shown in FIG.
FIG. 7 is a block diagram of an audio apparatus including an input signal processing circuit according to a fourth embodiment of the present invention.
FIG. 8 is a block diagram of an audio apparatus including a conventional input signal processing circuit.
[Explanation of symbols]
11, 21, 31, 41 Input signal processing circuit
12, 32 Digital amplifier
13,33 Low pass filter
14 Speaker
15 Control circuit (control means)
B1 buffer
B2 inverter
C; C1, C2 capacitor
R; R1, R2 resistance
r1, r2, ..., rn resistance elements
s1, s2, ..., sn, sn + 1 Analog switch
SW1 Analog switch (first selection means)
SW2 Analog switch (second selection means)
SW3; SW31, SW32 Analog switch (switching means, First switching means, second switching means)
SW4 Analog switch
SW5 Analog switch (short-circuit means)
T; T1, T2 Time constant circuit
VR Electronic volume (level adjustment means)

Claims (5)

An input signal that selects one of one or more analog inputs and one or more digital inputs, adjusts the signal level by level adjusting means, and then outputs the signal to a digital amplifier that performs power amplification In the processing circuit,
First selection means for selecting and outputting either an analog input signal or a predetermined DC voltage;
Level adjusting means for adjusting the output level of the analog input signal input from the first selecting means and a predetermined DC voltage ;
Switching means for performing switching in response to a digital input signal, the output of which is applied to the digital input terminal of the digital amplifier;
Second selection means for selectively outputting the output from the level adjustment means to either the analog input terminal of the digital amplifier or the switching means;
An input signal processing circuit comprising: control means for controlling the first and second selection means in conjunction with each other in accordance with an input channel to be selected. 2. The input signal processing according to claim 1 , further comprising a time constant circuit having a time constant longer than a maximum repetition time constant of “1” of the digital input signal between the level adjusting means and the switching means. circuit. The switching means switches between the DC voltage adjusted by the level adjusting means and the GND potential in response to the digital input signal, and is held at the GND potential at the time of the analog input. The input signal processing circuit according to claim 1 . When the digital input, the input signal processing circuit according to claim 1, further comprising a short-circuiting means to GND potential analog input of the digital amplifier. The switching means includes
Switching in response to a digital input signal, the first switching means whose output is given to the first digital input terminal of the digital amplifier as an input in phase with the digital input signal;
Switching in response to a digital input signal, and having a second switching means whose output is applied to a second digital input terminal of the digital amplifier as an input having a phase opposite to that of the digital input signal. The input signal processing circuit according to any one of claims 1 to 4 .

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