JP2011155394A - Automatic level control circuit, digital signal processor for audio using the same, electronic equipment, and gain control method of variable gain amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ALC circuit wherein the volume can be reset in a short time after switching an input source. <P>SOLUTION: A variable gain amplifier 10 amplifies an input audio signal VI, and outputs an output audio signal VO. A gain control part 11 controls the gain of the variable gain amplifier 10 depending on the amplitude of the output audio signal VO. The gain control part 11 is constituted to decrease the gain by a predetermined amount when the amplitude of the output audio signal VO is larger than a predetermined maximum threshold Vth_max for a predetermined attack time T<SB>ATK</SB>, or increase the gain by a predetermined amount when the amplitude of the output audio signal VO is smaller than the maximum threshold Vth-max for a predetermined release time T<SB>RLS</SB>. The release time T<SB>RLS</SB>can be switched between at least two values, i.e., a first value T1 and a second value T2 which is shorter than the first value T1. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to an automatic level control circuit for adjusting a signal level of an analog audio signal.

  Devices that output audio (audio signals), such as TVs, car audio systems, home theater systems, and audio components, automatically control the volume of audio signals (ALC: Automatic) to make it easier for viewers to hear audio. Level Control). When the dynamic range of sound is very wide, such as a movie, for example, ALC can increase the level of a small sound and suppress the level of a large sound. Alternatively, when viewing a television program, ALC is effective even when the volume is greatly different between the program and the commercial.

US Pat. No. 5,631,714A

  FIGS. 1A and 1B are a circuit diagram showing the configuration of an ALC circuit studied by the present inventors and a time chart showing the operation thereof. The ALC circuit 200 monitors the level (amplitude) of the audio signal VO output from the variable gain amplifier 202 and the variable gain amplifier 202 that amplifies the audio signal VI, and controls the gain of the variable gain amplifier 202 according to the level. A gain control circuit (detect circuit) 204 is provided.

  For example, when the amplitude of the output audio signal VO exceeds a predetermined threshold Vth that exceeds the predetermined attack time (0.5 to 6 ms, for example, 1 ms), and then exceeds the threshold Vth, The gain (volume) α of the variable gain amplifier 202 is lowered by a certain amount (attack operation). On the other hand, when the output audio signal VO has an amplitude smaller than the threshold Vth for a certain release time, the gain α of the variable gain amplifier 202 is increased by a predetermined amount (referred to as a release operation or a recovery operation). If the release time is as short as the attack time, the release operation and the attack operation are alternately repeated in a short time. Therefore, the release time is sufficiently longer than the attack time, for example, about 50 to 600 ms.

  The time chart of FIG. 1B shows, in order from the top, the input audio signal VI to the ALC circuit, the gain α of the variable gain amplifier 202, and the output audio signal VO of the ALC circuit.

  A program with a certain channel is played before time t0. At this time, the audio signal of the program is input to the ALC circuit 200. The level (amplitude) of the input audio signal VI is stable, and the amplitude of the output audio signal VO is stabilized near the threshold value Vth by ALC.

  At time t0, the input source for the ALC circuit is switched according to the user's operation. “Switching input source” includes switching a TV reception channel, selecting a CD song, switching an input device, and the like. As the input source is switched, the input audio signal VI of the ALC circuit 200 is once forcibly muted, and the amplitude of the input audio signal VI becomes substantially zero. Since the ALC circuit 200 regards the muted input audio signal VI as noise at this time, the release operation for increasing the gain α is not performed, and the gain of the variable gain amplifier 202 is set to 1 time (0 dB) and maintained. The

  Thereafter, when the mute period ends, the mute for the input audio signal VI is released, and the input audio signal VI from the input source after switching is input to the ALC circuit. If the level of the input audio signal VI at this time is very small, the ALC circuit gradually increases the gain of the variable gain amplifier 202 by the release operation. However, since the release time is as long as several tens to several hundreds of ms as described above, the time until the amplitude of the output audio signal VO returns to the threshold value Vth after switching the input source becomes long, and the volume is low. Persist for a long time. As a result, a situation occurs in which it is difficult for the user to hear the voice after switching the input source.

  This problem should not be regarded as a general recognition of those skilled in the art, but has been uniquely recognized by the present inventor.

  The present invention has been made in such a situation, and one of the exemplary purposes of an embodiment thereof is to provide an ALC circuit capable of returning the volume in a short time after switching the input source.

  One embodiment of the present invention relates to an automatic level control circuit. The automatic level control circuit includes a variable gain amplifier that amplifies an input audio signal and outputs an output audio signal, and a gain control unit that controls the gain of the variable gain amplifier according to the amplitude of the output audio signal. The gain control unit reduces the gain by a predetermined amount when the amplitude of the output audio signal is larger than the predetermined maximum threshold value and continues for a predetermined attack time, and the amplitude of the output audio signal is smaller than the maximum threshold value. If the state lasts for a predetermined release time, the gain is configured to increase by a predetermined amount. The release time can be switched between at least two of a first value and a second value shorter than the first value.

  According to this aspect, by switching the release time according to the state of the input audio signal, it is possible to suppress a situation where the volume is low and the volume is kept from increasing or decreasing in a short cycle.

The gain control unit may set the release time to the second value after the source of the input audio signal is switched.
By shortening the release time after the source of the input audio signal is switched, when the amplitude of the input audio signal related to the source after switching is small, the volume can be changed to an easily audible volume in a short time.

  The gain control unit may set the release time to the first value when the amplitude of the output audio signal reaches a maximum threshold value. Once the maximum threshold is reached, the release time can be shortened to prevent the attack operation and the release operation from being repeated alternately in a short time.

The gain control unit may invalidate the gain control based on the output audio signal and fix the gain to a predetermined value when the amplitude of the input audio signal is smaller than a predetermined minimum threshold for a predetermined time.
A signal having a level equal to or lower than the minimum threshold has a high relative ratio of noise components, or can be regarded as noise. Therefore, according to the aspect, when the input audio signal level is smaller than the minimum threshold value, it is possible to prevent the noise from being unnecessarily amplified by fixing the gain of the variable gain amplifier to a predetermined value.

  The gain control unit includes a first state in which the release time is set to a first value, a second state in which the release time is set to a second value, and a third state in which the gain is fixed to a predetermined value; May be configured to be capable of transition. In the first state, when the state in which the amplitude of the input audio signal is smaller than the minimum threshold value continues for a predetermined time, the state may transit to the third state. In the third state, when the amplitude of the input audio signal exceeds the minimum threshold value, a transition to the second state may be made. In the second state, when the amplitude of the output audio signal reaches the maximum threshold value, the transition to the first state may be made.

  Another aspect of the present invention is an audio digital signal processor. The audio digital signal processor includes the automatic level control circuit according to any one of the above-described aspects for adjusting the level of the input audio signal.

  Yet another embodiment of the present invention is an electronic device. This electronic apparatus includes the above-described audio digital signal processor.

  Still another embodiment of the present invention relates to a gain control method for a variable gain amplifier that amplifies an input audio signal and outputs an output audio signal. In this method, when a state in which the amplitude of the output audio signal is larger than a predetermined maximum threshold value continues for a predetermined attack time, the step of decreasing the gain of the variable gain amplifier by a predetermined amount and the amplitude of the output audio signal are maximized. If the state smaller than the threshold value continues for a predetermined release time, the release time is set to a first value and a first value shorter than the first value according to the step of increasing the gain by a predetermined amount and the amplitude of the input audio signal. And a step of switching between at least two of the two values.

  Note that any combination of the above-described constituent elements and the constituent elements and expressions of the present invention replaced with each other among methods, apparatuses, systems, and the like are also effective as an aspect of the present invention.

  The automatic level control circuit according to the present invention enables flexible volume control.

FIGS. 1A and 1B are a circuit diagram showing the configuration of an ALC circuit studied by the present inventors and a time chart showing the operation thereof. It is a block diagram which shows the structure of the ALC circuit which concerns on embodiment of this invention. FIG. 3 is a state transition diagram of the ALC circuit of FIG. 2. It is a time chart which shows operation | movement of the ALC circuit of FIG. FIG. 3 is a block diagram illustrating a configuration of a digital surround processing unit (referred to as an audio DSP) 2 including the ALC circuit of FIG. 2. FIGS. 6A and 6B are block diagrams showing configurations of the pseudo stereo unit and the surround unit. FIGS. 7A and 7B are block diagrams showing configurations of a 7-band parametric equalizer and a sub-output 3-band parametric equalizer, respectively.

  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. The case where it is indirectly connected through another member that does not affect the state is also included.
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 an electrical condition. It includes the case of being indirectly connected through another member that does not affect the connection state.

  FIG. 2 is a block diagram showing a configuration of the ALC circuit 100 according to the embodiment of the present invention. For example, the ALC circuit 100 is mounted on an electronic device having an audio output function such as a television or a car audio. The ALC circuit 100 receives an analog audio signal (hereinafter referred to as an input audio signal) VI from a playback unit (not shown) provided in the preceding stage, adjusts its level (amplitude, that is, volume), and is used in a subsequent stage speaker or headphone. To an electroacoustic transducer (not shown).

  The input source for the ALC circuit 100 is switched by the user. The switching of the input source occurs, for example, by switching the receiving channel, switching the input device, or selecting a song of a CD player or silicon audio. The switching of the input source changes the amplitude level of the input audio signal VI in a discontinuous manner, and therefore suddenly becomes louder or louder. The ALC circuit 100 according to the embodiment provides a function of reducing such a change in volume. The input audio signal VI is muted for a predetermined period when the input source is switched.

  The ALC circuit 100 includes a variable gain amplifier 10 and a gain control unit 11. The variable gain amplifier 10 amplifies the input audio signal VI with a set gain α. The output audio signal VO of the variable gain amplifier 10 is given by VO = α · VI. For example, the gain α is set to any one of discrete values in the range of −∞ (0 times) to +24 dB (16 times).

  The gain control unit 11 controls the gain of the variable gain amplifier 10 according to the amplitudes of the input audio signal VI and the output audio signal VO. The gain control unit 11 includes a first control unit 12 and a second control unit 14.

  The first control unit 12 monitors the amplitude of the output audio signal VO (= α · VI) of the variable gain amplifier 10, and if it exceeds a predetermined maximum threshold value Vth_max, the gain α of the variable gain amplifier 10 is determined. Decrease the quantity (attack action). Further, the first control unit 12 increases the gain α of the variable gain amplifier 10 (release operation or recovery operation) when the amplitude of the output audio signal VO falls below the maximum threshold value Vth_max.

The first comparator CMP1 of the first controller 12 compares the amplitude of the output audio signal VO with the maximum threshold value Vth_max. A signal indicating the comparison result is input to two counters (timers) C1 and C2. When the state in which the amplitude of the output audio signal VO exceeds the maximum threshold value Vth_max is accumulated for a predetermined attack time T _ATK (can be set from 0.5 to 6 ms, for example, 1 ms), the first counter C1 performs the attack signal ATK. Is asserted.
Further, the second counter C2 generates the recovery signal REC when the state in which the amplitude of the output audio signal VO is below the maximum threshold value Vth_max continues for a predetermined release time T _RLS (can be set in the range of 50 to 600 ms, for example, 50 ms). Assert. The first counter C1 and the second counter C2 may share a single counter.

  The gain α of the variable gain amplifier 10 decreases by a predetermined step when the attack signal ATK is asserted, and increases by a predetermined step when the recovery signal REC is asserted. The first control unit 12 maintains the amplitude of the output audio signal VO of the variable gain amplifier 10 at a constant value regardless of the amplitude of the input audio signal VI. The configuration of the first control unit 12 may be a conventional ALC circuit or AGC (Automatic Gain Control) circuit technology, and is not particularly limited in the present invention.

The first control unit 12, the release time T _RLS is the first value T1, which is usually used, and a second value T2 shorter than the first value T1, between at least 2 values including, switchable Configured. That is, the release time _TRLS measured by the second counter C2 can be switched.

The first control unit 12 sets the release time _TRLS to the second value T2 after the source of the input audio signal VI is _switched . The source switching is detected by the second control unit 14. The second counter C2 of the first control unit 12, the amplitude of the output audio signal VO reaches a maximum threshold Vth_max, setting the release time _{T RLS} to a first value T1.

  Further, the first control unit 12 is configured to be able to switch on / off of the gain control operation of the variable gain amplifier 10, in other words, whether or not the attack signal ATK and the recovery signal REC are generated. When the control signal CNT1 input to the enable terminal EN # (# indicates logic inversion) is asserted, the first control unit 12 stops the gain control of the variable gain amplifier 10 and the control signal CNT1 is negated. Then, the gain control is executed. The control signal CNT1 is generated by the second control unit 14.

  The second control unit 14 monitors the amplitude of the input audio signal VI, and fixes the gain α of the variable gain amplifier 10 to a predetermined value α1 when the level is lower than a predetermined minimum threshold value Vth_min. For example, the predetermined value is α1 = 1 (= 0 dB). The second comparator CMP2 compares the amplitude of the input audio signal VI with the minimum threshold value Vth_min. The third counter C3 asserts the control signal CNT1 when the state where the amplitude of the input audio signal VI is below the minimum threshold value Vth_min continues for a predetermined time (settable in 1 to 5 ms). When the control signal CNT1 is asserted, the gain α of the variable gain amplifier 10 becomes 0 dB, and the gain control by the first control unit 12 is invalidated.

Further, when the amplitude of the input audio signal VI becomes larger than the minimum threshold value Vth_min, the second comparator CMP2 of the second control unit 14 detects switching of the input source and asserts a control signal CNT2 indicating the detection. When the control signal CNT2 is asserted, the second counter C2 of the first controller 12 returns the release time _TRLS from the second value T2 to the first value T1.

The above is the configuration of the ALC circuit 100. Next, the operation will be described.
In the ALC circuit 100, the following three states are realized.
(First state φ1)
State in which gain control by first control unit 12 is effective and release time _TRLS is set to the first value (second state φ2)
State in which gain control by first control unit 12 is effective and release time _TRLS is set to the second value (third state φ3)
State in which gain control by first control unit 12 is invalid and gain α is fixed to a predetermined value α1

  Then, the first state φ1 to the third state φ3 transition with the following event or condition match as a trigger. FIG. 3 is a state transition diagram of the ALC circuit 100 of FIG.

  In the first state φ1, if the state where the amplitude of the input audio signal VI is smaller than the minimum threshold value Vth_min continues for a predetermined time, it is highly likely that the input is either a silent state or noise. Therefore, in this case, the state transits to the third state φ3 (S1), the gain α is set to 0 dB, and the amplification is stopped. This can prevent noise from being amplified.

When the amplitude of the input audio signal VI exceeds the minimum threshold value Vth_min in the third state φ3, the state transitions to the second state φ2 (S2).
This means that the muting of the input audio signal VI accompanying the switching of the input source is cancelled. Therefore, the gain control unit 11 transits to the second state φ2, sets the release time _TRLS to the short second value T2, and resumes gain control.

In the second state φ2, when the amplitude of the output audio signal VO reaches the maximum threshold value Vth_max or when the gain α reaches the maximum value, the state transits to the first state φ1 (S3), and the release time _TRLS is normal. The first value T1 is set, and gain control is performed.

FIG. 4 is a time chart showing the operation of the ALC circuit 100 of FIG. The time before time t0 is the same as in FIG. Before time t0, the release time T _RLS is set to a first value, the output audio signal so that the amplitude of VO matches the maximum threshold Vth_max, gain α is controlled (first state φ1 ).

  When the input source is switched at time 0, the input audio signal VI is muted. As a result, the amplitude of the input audio signal VI becomes smaller than the minimum threshold value Vth_min, and the state transits to the third state φ3.

When the mute of the input audio signal VI is canceled at time t1, the input audio signal VI having an amplitude larger than the minimum threshold value Vth_min is input. Then, the state transits to the second state φ2, and the release time _TRLS is set short. This short release time _{T RLS,} the amplitude of the output audio signal VO reaches a very short time maximum threshold Vth_max.

When the amplitude of the output audio signal VO reaches the maximum threshold value Vth_max at time t2, the state transitions to the first state φ1, and gain control is performed based on the normal release time T _RLS (= T1).

The above is the operation of the ALC circuit 100.
According to the ALC circuit 100, when the input audio signal VI having a small amplitude is input immediately after the source switching, the release time _TRLS is set shorter than usual, so that the volume is easy to hear in a shorter time than the conventional one. Can be set to

Thereafter, after reaching a volume that is easy to hear, the attack time and release operation can be prevented from being repeated in a short cycle by returning the release time _TRLS to a normal value.

  Next, a digital surround processing unit (audio DSP) including the ALC circuit 100 of FIG. 1 will be described.

  FIG. 5 is a block diagram showing a configuration of a digital surround processing unit (referred to as an audio DSP) 2 including the ALC circuit 100 of FIG. The digital surround processing unit 2 is mounted on, for example, a thin television and performs various signal processing on the audio signal. The audio DSP 2 includes the following blocks.

1. Prescaler 20
The level of the digital signal input to the digital surround processing unit 2 may be full scale, and if surround or equalizer processing is performed at this time, it overflows. The prescaler 20 adjusts the input gain for the input audio signal. Adjustment is possible from +24 dB to -103 dB at 0.5 dB.

2. HPF22
This filter removes the DC offset component of the digital signal input to the audio DSP 2. The HPF 22 is a primary filter having a cutoff frequency fc of 1 Hz, for example.

3. Channel mixer 24
Mixing setting of the sound of the left channel L and the right channel R of the digital signal input to the audio DSP 2 is performed. The channel mixer 24 converts the stereo signal to monaural.

4). P ² Volume (Perfect Pure Volume) 100
This is a unit corresponding to the ALC circuit 100 shown in FIG.

5. Tone control unit 26
5.1 BASS
A peaking filter or a low shelf filter can be used. When the frequency f0, the Q value, and the gain Gain are given, the digital filter coefficients are calculated in the IC and transferred to a coefficient RAM that holds them. A soft transition function prevents switching shock noise during setting.

5.2 MIDDLE
A peaking filter is available. When the frequency f, Q value, and gain Gain are given, the coefficients of the digital filter are calculated in the IC and transferred to a coefficient RAM that holds them. A soft transition function prevents switching shock noise during setting.

5.3 TREBLE
A peaking filter or a low shelf filter can be used. When the frequency f, Q value, and gain Gain are given, the coefficients of the digital filter are calculated in the IC and transferred to a coefficient RAM that holds them. A soft transition function prevents switching shock noise during setting.

6). First scaler 28
In order to prevent overflow in the audio DSP 2, the gain is adjusted by the first scaler 28. Adjustment is possible from +24 dB to -103 dB at 0.5 dB.

7). Pseudo stereo surround processing unit 30
7.1 Pseudo stereo unit Performs signal processing on monaural signals to reproduce the stereo effect. The configuration is shown in FIG.

7.2 Surround unit (Matrix Surround 3D)
A large sweet spot and a surround space with little feeling of fatigue even when viewed for a long time are provided. Reproduces the natural expanse of sound in middle and high sounds, and realizes a sound field that does not impair the sense of localization of vocals. The configuration is shown in FIG. By turning on the loop, the number of stages of the phase shifter can be increased in a pseudo manner.

8). Heavy bass equalizer (P ² Bass: Perfect Pure Bass) 32
This equalizer is used to reproduce powerful deep bass and real sound close to the original sound even on flat-screen TVs with limited speaker enclosures. A clear heavy bass with a thick and less distorted feeling is realized. Even if the bass is boosted, it does not interfere with the vocal band, so a rich and natural deep bass can be achieved.

9. Medium and high tone equalizer (P ² Treble: Perfect Pure Treble) 34
It is provided to achieve clarity, sound growth, and crispness. In the set in which the speaker is provided on the lower side of the display, it is possible to realize an effect that the sound is raised and heard.

10. Second scaler 36
In order to prevent overflow in the audio DSP 2, the gain is adjusted by the second scaler 36. Adjustment is possible from +24 dB to -103 dB at 0.5 dB.

11. 7 band parametric equalizer 38
As the 7-band parametric equalizer 38, a peaking filter, a low shelf filter, or a high shelf filter can be used. When the frequency f0, the Q value, and the gain Gain are given, the digital filter coefficients are calculated in the IC and transferred to a coefficient RAM that holds them.

12 Main output electronic volume 40
12.1 Main output EVR
The volume can be selected from +24 dB to −103 dB at 0.5 dB. When the volume is switched, a soft transition is performed. The soft transition time is about 22 ms from 0 dB.

12.2 Main Output Balance Unit The balance unit can attenuate the volume in 1 dB steps from the volume setting value. A soft transition is executed when switching. The soft transition time is about 22 ms.

13. Main output postscaler 42
13.1 Main Output Postscaler This unit adjusts the level when outputting data calculated by a 32-bit wide DSP in a 24-bit width. The adjustment can be selected from +24 dB to -103 dB at 0.5 dB.

13.2 Main Output Clipper When measuring the rated output (practical maximum output) of a television, a level with a total distortion rate (THD + N) of 10% is used. By using the clipper function, an audio signal can be clipped with an arbitrary output amplitude. For example, a rated output of 10 W or 5 W can be realized using an amplifier of 15 W output.

14 Sub input data selection unit 44
This unit selects sub inputs (sub woofer processing, etc.). The data after the processing of the first scaler 28, the data after the P ² Bass processing, or the digital signal (input 2) from another path is received and selected. When data after P ² Bass processing is selected, sub-woofer output linked with P ² Bass gain setting can be realized.

15. Sub output channel mixer 46
Mixing settings for the sound of the left channel L and right channel R of the sub output digital signal input to the audio DSP 2 are performed. The sub output channel mixer 46 converts the stereo signal into monaural.

16. Sub output LPF48
This is a crossover filter for sub-woofer output.

17. Sub output 3-band parametric equalizer 50
The sub output 3-band parametric equalizer 50 can use a peaking filter, a low shelf filter, or a high shelf filter. When the frequency f0, the Q value, and the gain Gain are given, the digital filter coefficients are calculated in the IC and transferred to a coefficient RAM that holds them.

18. Sub output electronic volume 52
18.1 Sub output EVR
The sub output volume can be selected from +24 dB to -103 dB at 0.5 dB. When the volume is switched, a soft transition is performed. The soft transition time is about 22 ms from 0 dB.

18.2 Sub-output balance unit The balance unit can attenuate the volume in 1 dB steps from the volume setting value. A soft transition is executed when switching. The soft transition time is about 22 ms.

19. Sub output postscaler 54
19.1 Sub-Output Postscaler This is a unit that adjusts the level when data calculated by a 32-bit width DSP is output in a 24-bit width. The adjustment can be selected from +24 dB to -103 dB at 0.5 dB.

19.2 Sub-Output Clipper When measuring the rated output (maximum practical output) of a television, a level with a total distortion rate (THD + N) of 10% is used. By using the clipper function, an audio signal can be clipped with an arbitrary output amplitude. For example, a rated output of 10 W or 5 W can be realized using an amplifier of 15 W output.

  The above is the configuration of the audio DSP 2. Next, the configuration of each unit will be described.

  FIGS. 6A and 6B are block diagrams showing configurations of the pseudo stereo unit and the surround unit. The pseudo stereo unit 30a shown in FIG. 4A has a phase shifter 60 that changes the phase of the signal for each channel, an amplifier 62 that amplifies the output audio signal of the phase shifter, and an output of the amplifier 62 for the input audio signal. A selector 64 for selecting either one is provided.

  Reference is made to the surround unit 30b of FIG. The subtracter 70 subtracts the right channel signal Rch from the left channel signal Lch. The switch 72 is provided between the subtracter 70 and the adder 74. The phase shifter 76 changes the phase of the output of the adder 74. The amplifier 78 amplifies the output of the phase shifter 76. The LPF 80 removes high frequency components from the output of the amplifier 78. The output of the LPF 80 is input to the adder 74 via the switch 82. The adder 90L adds the left channel signal Lch and the output of the LPF 80. Similarly, the adder 90R adds the right channel signal Rch and the output of the LPF 80.

  FIGS. 7A and 7B are block diagrams showing configurations of the 7-band parametric equalizer 38 and the sub-output 3-band parametric equalizer 50, respectively. Each of the 7-band parametric equalizer 38 and the sub-output 3-band parametric equalizer 50 is a second-order IIR digital filter (Bi-quad Filter). The five coefficients b0, b1, b2, a1, and a2 of this filter are configured to be directly settable from the outside. By using this function, filter types other than peaking, low shelf, and high shelf, frequency setting, Q value setting, and gain setting can be freely performed.

  The coefficients b0, b1, b2, a1, and a2 are stored in the coefficient RAM. The voice DSP 2 has a coefficient RAM automatic update function. By using this function, the coefficient RAM can be automatically overwritten and updated.

  Although the present invention has been described using specific words and phrases based on the embodiments, the embodiments are merely illustrative of the principles and applications of the present invention, and the embodiments are defined in the claims. Many modifications and arrangements can be made without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 100 ... ALC circuit, 10 ... Variable gain amplifier, 11 ... Gain control part, 12 ... 1st control part, 14 ... 2nd control part, VI ... Input audio signal, VO ... Output audio signal, C1 ... 1st counter, C2 2nd counter, C3 ... 3rd counter, CMP1 ... 1st comparator, CMP2 ... 2nd comparator, 2 ... Audio DSP, 20 ... Prescaler, 22 ... HPF, 24 ... Channel mixer, 26 ... Tone controller, 28 ... Scaler 30 ... Pseudo stereo surround processing unit, 32 ... Deep bass equalizer, 34 ... Medium / high tone equalizer, 36 ... Scaler, 38 ... 7 band parametric equalizer, 40 ... Main output electronic volume, 42 ... Main output postscaler, 44 ... Sub Input data selection unit, 46 ... sub output channel mixer, 48 ... sub output LP , 50 ... sub output 3-band parametric equalizer, 52 ... sub output electronic regulator, 54 ... sub output post scaler.

Claims (11)

A variable gain amplifier that amplifies an input audio signal and outputs an output audio signal;
A gain control unit for controlling the gain of the variable gain amplifier according to the amplitude of the output audio signal;
With
The gain control unit reduces the gain by a predetermined amount when the state in which the amplitude of the output audio signal is larger than a predetermined maximum threshold value continues for a predetermined attack time, and the amplitude of the output audio signal is maximized. Configured to increase the gain by a predetermined amount when a state smaller than the threshold value lasts for a predetermined release time;
The automatic level control circuit is characterized in that the release time can be switched between at least two of a first value and a second value shorter than the first value.   2. The automatic level control circuit according to claim 1, wherein the gain control unit sets the release time to the second value after the source of the input audio signal is switched.   3. The automatic level control according to claim 1, wherein the gain control unit sets the release time to the first value when an amplitude of the output audio signal reaches the maximum threshold value. circuit.   The gain control unit invalidates the gain control based on the output audio signal and fixes the gain to a predetermined value when the amplitude of the input audio signal is smaller than a predetermined minimum threshold value for a predetermined time. The automatic level control circuit according to any one of claims 1 to 3, wherein The gain controller is
A first state in which the release time is set to the first value;
A second state in which the release time is set to the second value;
A third state in which the gain is fixed to the predetermined value;
Is configured to be transitionable,
In the first state, when the state where the amplitude of the input audio signal is smaller than the minimum threshold value continues for a predetermined time, the state transitions to the third state,
In the third state, when the amplitude of the input audio signal exceeds the minimum threshold value, transition to the second state,
5. The automatic level control circuit according to claim 4, wherein in the second state, when the amplitude of the output audio signal reaches the maximum threshold value, the automatic level control circuit transitions to the first state.   6. An audio digital signal processor comprising the automatic level control circuit according to claim 1, which adjusts the level of an input audio signal.   An electronic apparatus comprising the audio digital signal processor according to claim 6. A gain control method for a variable gain amplifier that amplifies an input audio signal and outputs an output audio signal,
Reducing the gain of the variable gain amplifier by a predetermined amount when a state in which the amplitude of the output audio signal is larger than a predetermined maximum threshold value continues for a predetermined attack time;
Increasing the gain by a predetermined amount when a state in which the amplitude of the output audio signal is smaller than the maximum threshold lasts for a predetermined release time;
Switching the release time between at least two of a first value and a second value shorter than the first value, depending on the amplitude of the input audio signal;
A method comprising the steps of:   9. The method of claim 8, wherein the release time is set to the second value after the source of the input audio signal is switched.   The method according to claim 8 or 9, wherein when the amplitude of the output audio signal reaches the maximum threshold, the release time is set to the first value.   11. The method according to claim 8, further comprising the step of fixing the gain of the variable gain amplifier to a predetermined value when the level of the input audio signal is lower than a predetermined minimum threshold value. .

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