JP5908679B2 - Audio signal processing circuit and audio apparatus using the same - Google Patents

Description

  The present invention relates to an audio signal processing technique.

  In an audio device, an analog audio signal is once converted into a digital audio signal by an A / D converter, a volume is adjusted by digital signal processing, and a frequency characteristic is changed (equalized), and then a D / A converter is converted. May reconvert to an analog audio signal.

  FIG. 1 is a block diagram showing the configuration of an audio apparatus examined by the present inventors. The audio device 1r includes a sound source 2, an electroacoustic conversion element (hereinafter simply referred to as a speaker) 4 such as a speaker or a headphone, a first audio signal processing circuit 200r, and a second audio signal processing circuit 100.

The sound source 2 outputs an analog audio signal S1. The sound source 2 is, for example, a CD player, a DVD player, or a silicon audio player having an analog output. The first audio signal processing circuit 200r has a plurality of input ports Pi _{1 to} Pi ₃ and receives the audio signal S1 from the sound source 2 respectively.

  The selector 50 selects the audio signal of the input port Pi selected by the user. The A / D converter 54 converts the analog audio signal S2 from the selector 50 into a digital audio signal S3.

  The amplitude, that is, the volume of the analog audio signal input to each input port varies depending on the sound source. Therefore, the digital gain control circuit 70 multiplies the digital audio signal S3 input to each input port by a gain (coefficient) set for the input port in order to equalize the volume for each sound source. To do. The second audio signal processing circuit 100 at the subsequent stage performs various digital signal processing on the digital audio signal S4 and then converts it to an analog audio signal S5. The analog audio signal S5 is input to the speaker 4 at the subsequent stage of the second audio signal processing circuit 100 through an amplifier and a filter (not shown).

JP 2000-151309 A JP-A-9-205482 JP 2007-325057 A

  As described above, the level of the analog audio signal S1 varies for each sound source 2. When the level of the audio signal S1 is small, since it is converted into a digital audio signal using a very small part of the input voltage range of the A / D converter 54, there is a problem that the S / N ratio deteriorates.

  The present invention has been made in view of these problems, and one of the exemplary purposes of an aspect thereof is to improve the S / N ratio of an audio apparatus.

  One embodiment of the present invention relates to an audio signal processing circuit. The audio signal processing circuit includes a selector for selecting an analog audio signal input to one input port designated by a user from among analog audio signals input to a plurality of input ports, and an analog audio signal from the selector. An analog gain control circuit that amplifies at a gain set for each input port, and is configured to gradually change the gain when switching the gain, and an output of the analog gain control circuit And an A / D converter that converts the signal into a digital audio signal, and is integrated on a single semiconductor substrate. Each of the analog gain control circuit and the A / D converter is configured to operate based on clock signals having an integer multiple of each other and originating from a common master clock signal.

In recent years, with the reduction in power consumption of sound sources connected to each input port, the amplitude of an analog audio signal input to the input port tends to decrease. According to this aspect, an analog gain control circuit having a so-called soft transition switching function is provided in the previous stage of the A / D converter, and by integrating them, the A / D converter is optimal for its input range. Therefore, it is possible to supply an analog audio signal having a proper signal level, to maximize the performance of the A / D converter, and to improve the S / N ratio.
In addition, the analog gain control circuit and the A / D converter are built in a common IC (Integrated Circuit) and operated in synchronization with a clock signal originating from a common master clock signal. Beat noise generated when the / D converter is asynchronous can be reduced.

  “Integrated integration” includes the case where all the circuit components are formed on a semiconductor substrate and the case where the main components of the circuit are integrated, and is used for adjusting circuit constants. These resistors and capacitors may be provided outside the semiconductor substrate. “Amplification” includes not only the case where the gain is larger than 1, but also the case where the gain is smaller than 1, that is, “attenuation”.

When the level of the digital audio signal is smaller than a predetermined threshold, the gain of the analog gain control circuit may be set to a minimum value.
Thereby, the S / N ratio when there is no input or no signal can be greatly improved.

Prior to switching the selector, the gain of the analog gain control circuit may be set to a minimum value, and after the switching of the selector is completed, the gain of the analog gain control circuit may be set to a value corresponding to the input port after selection.
Thereby, it is possible to suppress the occurrence of noise when the selector is switched.

When the level of the digital audio signal is clipped, the gain of the analog gain control circuit may be lowered.
By implementing this function, the gain for each input port can be set higher in advance, and when an analog audio signal with a small amplitude is input, an audio signal with a larger amplitude is To the A / D converter. As a result, the S / N ratio when reproducing the audio signal can be improved.

The frequency component contained in the digital audio signal may be detected, and the gain transition time of the analog gain control circuit may be shortened as the contained frequency increases.
If the gain is changed in a short time with respect to the audio signal in which the low frequency component is dominant, audible noise is generated. On the other hand, in an audio signal in which a high-frequency component is dominant, audible noise hardly occurs even if the gain is changed in a short time. According to this aspect, the gain transition time can be optimized according to the frequency component of the audio signal.

As the amplitude of the digital audio signal is smaller, the gain transition time of the analog gain control circuit may be shortened.
If the gain is changed in a short time for an audio signal having a large amplitude, audible noise is generated. On the other hand, with an audio signal having a small amplitude, audible noise hardly occurs even if the gain is changed in a short time. According to this aspect, the gain transition time can be optimized according to the amplitude of the audio signal.

  Another aspect of the present invention relates to an audio device. The audio apparatus includes the audio signal processing circuit according to any one of the above aspects.

  Note that any combination of the above-described components, or a conversion of the expression of the present invention between methods, apparatuses, systems, and the like is also effective as an aspect of the present invention.

  According to an aspect of the present invention, the S / N ratio of an audio device can be improved.

It is a block diagram which shows the structure of the audio apparatus which this inventor examined. It is a block diagram which shows the structure of the audio apparatus which concerns on embodiment.

  The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and processes shown in the drawings are denoted by the same reference numerals, and repeated descriptions are omitted as appropriate. The embodiments do not limit the invention but are exemplifications, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

  In this specification, “the state in which the member A is connected to the member B” means that the member A and the member B are physically directly connected, or the member A and the member B are electrically connected to each other. Including the case of being indirectly connected through other members that do not substantially affect the state of connection, or do not impair the functions and effects achieved by the combination thereof. Similarly, “the state in which the member C is provided between the member A and the member B” refers to the case where the member A and the member C or the member B and the member C are directly connected, as well as their electric It includes cases where the connection is indirectly made through other members that do not substantially affect the general connection state, or that do not impair the functions and effects achieved by their combination.

FIG. 2 is a block diagram illustrating a configuration of the audio apparatus 1 according to the embodiment.
The audio apparatus 1 mainly includes a sound source 2, a first audio signal processing circuit 200, a second audio signal processing circuit 100, a speaker 4, and a microcomputer 6.

  A digital audio signal S1 is output from the sound source 2. The sound source 2 is, for example, a CD player, an DVD player, or a silicon audio player having an analog output, or a DSP that converts a digital audio signal into an analog audio signal.

  The microcomputer 6 is a unit that controls the audio device 1 as a whole. The microcomputer 6 outputs volume data VOL indicating the volume set by the user to the first audio signal processing circuit 200.

The first audio signal processing circuit 200 includes three input ports Pi _{1 to} Pi ₃ . The number of input ports is arbitrary. Analog audio signals S1 _{1 to} S1 ₃ from the sound source 2 connected to the input ports Pi _{1 to} Pi ₃ are input to the input ports Pi _{1 to} Pi ₃ , respectively.

  The first audio signal processing circuit 200 includes a selector 50, an analog gain control circuit 52, an A / D converter 54, an oscillator 56, a control unit 58, and a digital volume circuit 60, and is a functional IC integrated on a single semiconductor substrate. is there.

The selector 50, an analog audio signal _S1 1 _{~S1 3} input to a plurality of input ports _Pi 1 _{~Pi 3} is input. Further, the microcomputer 6 receives selection data SEL indicating the input port Pi _k (k = 1, 2, 3) designated by the user. The selector 50 is controlled by the control unit 58 selects the analog audio signal S1 _k input to the input port Pi _k one specified by the user.

The analog gain control circuit 52 receives the analog audio signal S2 selected by the selector 50. The analog gain control circuit 52 amplifies the analog audio signal S2 with the gain g set by the control unit 58. The gain g is determined for each of the input ports Pi _{1 to} Pi ₃ . For example, the control unit 58 has a table indicating the relationship between the input ports Pi _{1 to} Pi ₃ and the gains g ₁ to g ₃ to be set for the input ports Pi _{1 to} Pi ₃ . Control unit 58, the selection data SEL is when indicating the k-th input port Pi _k, sets the gain _{g k} in the analog gain control circuit 52.

  The analog gain control circuit 52 has a soft switching function and is configured to gently change the gain g when switching the gain g. The configuration of the analog gain control circuit 52 is not particularly limited, and a known technique may be used.

  The A / D converter 54 converts the output signal S3 of the analog gain control circuit 52 into a digital audio signal S4.

  The digital volume circuit 60 includes a multiplier that multiplies the digital audio signal S4 by a coefficient corresponding to a volume value designated by the user. The digital volume circuit 60 may be omitted.

  The digital audio signal S5 whose volume is controlled by the digital volume circuit 60 is output from the output port Po to the second audio signal processing circuit 100 in the subsequent stage. The second audio signal processing circuit 100 performs various digital signal processing on the digital audio signal S5 and then converts the digital audio signal S5 into an analog audio signal S6. The analog audio signal S6 is input to the speaker 4 at the subsequent stage of the second audio signal processing circuit 100 through an amplifier and a filter (not shown). The speaker 4 may be another electroacoustic transducer such as a headphone.

  The oscillator 56 generates a master clock signal MCLK having a predetermined frequency. The analog gain control circuit 52 divides the master clock signal MCLK by the first division ratio to generate a transition clock signal. The analog gain control circuit 52 changes the gain g from the initial value to the target value stepwise in synchronization with the transition clock signal when the gain g is changed.

  The A / D converter 54 divides the master clock signal MCLK by the second frequency division ratio to generate a conversion clock signal having a predetermined sampling frequency. The A / D converter 54 converts the analog audio signal S3 into a digital audio signal S4 using the conversion clock signal.

  That is, each of the analog gain control circuit 52 and the A / D converter 54 is configured to operate based on clock signals that have an integer multiple of each other and originate from a common master clock signal MCLK.

The above is the basic configuration of the first audio signal processing circuit 200.
Recently, the sound source 2, in particular due to the low power consumption of the portable audio player, the amplitude of the input ports _Pi 1 analog audio signal _S1 input to the ~Pi ₃ 1 ~S1 ₃ which is connected to each input port _Pi 1 _{~Pi 3} Tend to be smaller.
According to the first audio signal processing circuit 200 of FIG. 2, the analog audio signal S having an optimum signal level for the input range can be supplied to the A / D converter 54, and the A / D converter 54 can be supplied. Can maximize its performance. Thereby, the S / N ratio can be improved.

  The analog gain control circuit 52 and the A / D converter 54 are built in a common IC and operated in synchronization with a clock signal originating from the common master clock signal MCLK. Beat noise generated when the / D converter 54 is asynchronous can be reduced.

Next, further features of the first audio signal processing circuit 200 will be described.
The gain g of the analog gain control circuit 52 is set to a minimum value that is substantially zero when the level of the digital audio signal S2 is smaller than a predetermined threshold value TH. This is called a mute state.

  In this way, in the no-signal or no-input state where the level of the digital audio signal S4 is lower than the threshold value TH, the gain g of the analog gain control circuit 52 is made substantially zero, so The S / N ratio can be greatly improved.

  Further, the control unit 58 sets the gain g of the analog gain control circuit 52 to the minimum value before switching the port of the selector 50. After the switching of the selector 50 is completed, the control unit 58 sets the gain g of the analog gain control circuit 52 after the selection. A value corresponding to the input port may be set. Thereby, it is possible to suppress the generation of noise when the selector 50 is switched.

  Further, when the level of the digital audio signal S4 is clipped in the non-mute state (normal state), the gain g of the analog gain control circuit 52 is reduced to avoid clipping. Specifically, the control unit 58 monitors the level of the digital audio signal S4. When the level of the digital audio signal S4 continues the maximum value for a predetermined time, the control unit 58 determines that the clip is in a clip state, and sets the gain g of the analog gain control circuit 52. Reduce by a predetermined amount. This is called a clip avoidance function.

By implementing a clip avoidance, it is possible to set a higher gain g ₁ to g ₃ for each input port Pi ₁ ~Pi ₃ as compared with the case not implementing. When an analog audio signal S1 having a small amplitude is input, an audio signal S2 having a larger amplitude than when the clip avoidance function is not provided can be input to the A / D converter 54 at the subsequent stage. As a result, the S / N ratio when reproducing the audio signal can be improved. When an analog audio signal S1 having a somewhat large amplitude is input, signal distortion can be suppressed by the clip avoidance function.

  Those skilled in the art will understand that the above-described embodiment is an exemplification, and that various modifications can be made to the combination of each component and each processing process, and such modifications are within the scope of the present invention. . Hereinafter, a modification is shown.

The shorter the transition time τ _T of the gain g, the better. However, when the gain g is changed in a short time with respect to the audio signal S1 in which the low frequency component is dominant, audible noise is generated. On the other hand, in the case of the audio signal S1 in which the high frequency component is dominant, audible noise hardly occurs even if the gain g is changed in a short time.

Therefore, the control unit 58 may detect the frequency component included in the digital audio signal S4, and may shorten the transition time τ _T of the gain g of the analog gain control circuit 52 as the included frequency is higher. Thereby, the transition time τ _T of the gain g can be optimized according to the frequency component of the audio signal S1.

  Further, if the gain g is changed in a short time with respect to an audio signal having a large amplitude, audible noise is generated. On the other hand, in an audio signal having a small amplitude, audible noise hardly occurs even if the gain g is changed in a short time.

Therefore, the control unit 58 may shorten the transition time τ _T of the gain g as the amplitude of the digital audio signal S4 is smaller. Thereby, the transition time τ _T of the gain g can be optimized according to the amplitude of the audio signal S1.

  Although the present invention has been described using specific terms based on the embodiments, the embodiments only illustrate the principles and applications of the present invention, and the embodiments are defined in the claims. Many variations and modifications of the arrangement are permitted without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 1 ... Audio device, S1, S2, S3, S4 ... Audio signal, Pi ... Input port, Po ... Output port, 2 ... Sound source, 4 ... Speaker, 6 ... Microcomputer, 50 ... Selector, 52 ... Analog gain control circuit, 54 ... A / D converter, 56 ... oscillator, 58 ... control unit, 60 ... digital volume circuit, 200 ... first audio signal processing circuit, 100 ... second audio signal processing circuit.

Claims (6)

A selector that has a plurality of input ports to which analog audio signals are input, and that selects an analog audio signal of a designated input port that is one designated by the user;
An analog gain control circuit that amplifies the analog audio signal from the selector with a gain set for the designated input port, and at the time of switching the gain, the gain is switched from an initial value before switching. An analog gain control circuit having a soft transition switching function for gradually changing to a target value;
An A / D converter that converts an output signal of the analog gain control circuit into a digital audio signal;
Integrated into a single semiconductor substrate,
When the level of the digital audio signal is clipped, the gain of the analog gain control circuit is gently reduced by a predetermined amount,
(I) a gradual change in the gain from the initial value to the target value in the analog gain control circuit; and (ii) a transition clock signal for gradual decrease in the gain by a predetermined amount; An audio signal processing circuit characterized in that conversion clock signals for conversion operation in the D converter originate from a common master clock signal and have an integer multiple relationship with each other.   2. The audio signal processing circuit according to claim 1, wherein when the level of the digital audio signal is smaller than a predetermined threshold value, the gain of the analog gain control circuit is set to a minimum value. When the user is instructed to change the designated input port, prior to switching the selector, the gain of the analog gain control circuit is set to a minimum value,
The analog gain control circuit is configured to gradually change the gain from the initial value which is the minimum value to the target value which is the gain set in the designated input port after the change after the selector switching is completed. The audio signal processing circuit according to claim 1, wherein the audio signal processing circuit is provided. It said digital audio signal by detecting a frequency component included in the audio signal according to any one of claims 1 to 3, characterized in that the frequency to shorten the transition time of the gain of the higher the analog gain control circuit included Processing circuit.   5. The audio signal processing circuit according to claim 1, wherein the transition time of the gain of the analog gain control circuit is shortened as the amplitude of the digital audio signal is smaller. Audio device, characterized in that it comprises an audio signal processing circuit according to any one of claims 1 to 5.

Images