JPH04301893A - Muting circuit - Google Patents

Abstract

PURPOSE:To prevent the quantization noide for the reduction of the sound volume level of an audio signal from being conspicuous. CONSTITUTION:A level detecting means 1 which detects shift of the envelope level of the audio signal to a prescribed level or lower and an amplitude value holding means 2 are provided, and this means 2 holds the amplitude value of the audio signal based on detection of shift of the envelope level to the prescribed level or lower in the level detecting means 1 and uses this held amplitude value as the audio output.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a muting circuit, and more specifically, the present invention relates to a muting circuit that suppresses quantization noise that becomes noticeable when the envelope level of an audio signal becomes small, or in other words, when the volume level becomes small. It concerns muting technology that makes it less noticeable.

0002

2. Description of the Related Art In recent years, it has become common practice to convert audio signals into digital signals, in other words, to approximate the waveform of an audio signal to a stepped waveform and process it in a quantized digital format.

0003

[Problems to be Solved by the Invention] However, when processing audio signals in digital format, when the envelope level becomes small and the volume level of the audio signal becomes small, the S/N ratio cannot be maintained sufficiently, and the quantum There is a problem that noise becomes noticeable.

SUMMARY OF THE INVENTION The present invention aims to solve these problems by providing a muting circuit that makes quantization noise less noticeable when the volume level of an audio signal becomes low.

[0005]

[Means for Solving the Problems] In order to achieve the above-mentioned objects, the muting circuit according to the present invention has the following features as shown in the principle block diagram of the principle configuration of FIG.
) Level detection means 1 for detecting that the envelope level of the audio signal shifts below a predetermined level; and (
b) Amplitude value holding means 2 for holding the amplitude value of the audio signal based on the detection of the transition of the envelope level to a predetermined level or less by the level detection means 1, and outputting the held amplitude value as an audio output. It is characterized by the ability to grow.

[0006]

[Operation] When the level detecting means 1 detects that the envelope level of the audio signal shifts below a predetermined level, the amplitude value of the audio signal is held by the amplitude value holding means 2 based on this detection, and the amplitude value of the audio signal is held by the amplitude value holding means 2 based on this detection. The amplitude value of the audio signal is output as audio output.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, specific embodiments of the muting circuit according to the present invention will be described with reference to the drawings. First Embodiment: A muting circuit to which the present invention is applied is shown in FIG. 2, and is used by being disposed in the audio signal path of equipment that handles digital audio signals such as digital equalizers and digital mixers. There is. In FIG. 2, a digital audio signal from a previous stage circuit is input to an envelope level detection section 11. As shown in FIG. The envelope level detection section 11 detects whether the envelope level of the audio signal is shifted to a predetermined muting level or less and is below the predetermined muting level based on the amplitude value of the input audio signal. In other words, it is detected that the quantization noise of the audio signal becomes noticeable below the predetermined muting level. Note that the envelope level, which is a change in the amplitude value of an audio signal, can be detected using a time constant circuit element that has a short rising time constant and a long falling time constant, for example, to detect the absolute value of the amplitude value of the audio signal. This is done by passing the Further, the muting level can be set to, for example, a level 70 decibels lower than the maximum level.

By the way, the envelope level detection section 11
It is generated based on the detection that the envelope level of the audio signal is below the predetermined muting level and is below the predetermined muting level. The detection signal that falls when the level is exceeded is applied to the amplitude value memory section 12 to which the audio signal is similarly input from the previous stage circuit, and further to the selection section 13. In the amplitude value memory section 12, the amplitude value of the audio signal at the rising edge of the detection signal applied when the envelope level of the audio signal becomes below a predetermined muting level is temporarily stored. Further, the amplitude value of the temporarily stored audio signal is given as an amplitude value signal from the amplitude value memory section 12 to the B terminal of the selection section 13 to which the audio signal from the previous stage circuit is inputted to the A terminal. In this selection section 13, when the envelope level of the audio signal does not become lower than the predetermined muting level, in other words, while the detection signal is not provided from the envelope level detection section 11, the audio signal input from the previous stage circuit to the A terminal is is selected as the audio output and output to a subsequent circuit such as a D/A converter. Furthermore, when the envelope level of the audio signal becomes lower than the predetermined muting level, in other words, while the detection signal is being given from the envelope level detection means 11, the constant amplitude given to the B terminal from the amplitude value memory section 12 is The amplitude value signal of the value is selected as the audio output and is output to the subsequent circuit as described above. Therefore, in this case, the audio output becomes only a DC component and cannot be heard as sound, and quantization noise is also eliminated. Further, since the amplitude value does not change suddenly when the audio output changes from the audio signal to the amplitude value signal, click noise does not occur. Note that FIG. 3 shows how the audio output changes from an audio signal to an amplitude value signal in relation to the envelope level.

In this embodiment, the change from an audio signal to an amplitude value signal is performed based only on the magnitude of the envelope level, but in addition to the magnitude of the envelope level, the envelope level attenuates over a predetermined time It is also good to change from the audio signal to the amplitude value signal only when there is a tendency. In addition, in this example, when changing from an amplitude value signal to an audio signal, it was done based only on the magnitude of the envelope level, but in addition to the magnitude of this envelope level, the amplitude value of the input audio signal It is also preferable that the amplitude value signal is changed to the audio signal when the difference between the amplitude value and the output amplitude value becomes within a predetermined value. In this case, changes in the amplitude value of the output audio signal become smooth.

Second Embodiment: In the first embodiment, the present invention is applied to devices that handle digital audio signals, such as digital equalizers and digital mixers, but do not generate audio signals themselves. Although the case where the muting circuit is used has been described, in this embodiment, the case where the muting circuit to which the present invention is applied is used in an electronic musical instrument that generates its own audio signal will be explained.

First, in FIG. 4 showing a schematic diagram of an electronic musical instrument, each key is pressed or released on a keyboard 15 consisting of a plurality of keys for specifying the pitch of a musical tone to be sounded. The control unit 1 comprising a microcomputer or the like determines which key is operated based on the key operation, and whether the key is pressed or released.
6. In addition, the control unit 16 similarly detects the operation state of each tone color selection switch, etc. with respect to the operator group 17, which includes tone color selection switches and the like for selecting tones of piano, harpsichord, etc., for example. Note that the muting level memory 18 from which necessary data can be read under the control of the control unit 16 stores muting level data used during muting processing for each tone range of the key operated on the keyboard 15. , are stored in association with each tone color selection switch operated by the switch of the operator group 17.

By the way, in the control section 16, based on the key operation state of each key and the switch operation state of each tone selection switch etc. detected by the control section 16, pitch, tone, and envelope are sent to the musical tone signal generation circuit 19. This musical tone signal generation circuit 19
to generate the desired musical tone. This musical tone signal generation circuit 19
When the control section 16 detects a new key press, the control section 16 generates a plurality of musical tone signal generation channels from among, for example, 30 musical tone signal generation channels constituting the musical tone signal generation circuit 19. In this embodiment, two musical tone signal generation channels are selected based on a predetermined priority order, and the control unit 1 selects these selected musical tone signal generation channels.
6 to a pitch control signal corresponding to the pressed key;
Further, this is done by providing a timbre control signal and an envelope shape control signal corresponding to the timbre selected by the timbre selection switch, as well as an envelope start signal for starting the generation of the envelope signal. These 2
The timbre control signals given to each musical tone signal generation channel are different from each other, and the envelope shape control signals are also set to values that are slightly different from each other, although the changes in the envelope level over time are almost the same. There is. The musical tone obtained in this manner is a mixture of two musical tone signals. On the other hand, when the control unit 16 detects a new key release, the control unit 16 selects two musical tone signal generation channels that are currently generating musical tones corresponding to the released key, and selects these two musical tone signal generation channels. This is done by applying an envelope end signal from the control section 16 to the selected musical tone signal generation channel to instruct the end of envelope signal generation.

The musical tone signal generated as described above is
After being converted into an analog signal by the D/A converter 20, it is amplified by the amplifier 21 and emitted as a musical tone from the speaker 22.

Next, one of the 30 musical tone signal generating channels making up the musical tone signal generating circuit 19 will be explained based on FIG.

In the address signal generation circuit 25, which is given frequency data as a pitch control signal corresponding to the pitch of the pressed key from the control section 16, the address signal generation circuit 25 sequentially accumulates this frequency data. Generates an address signal. This sequentially generated address signal is applied to a waveform memory 26 in which a plurality of musical sound waveforms corresponding to each tone selected by each tone selection switch of the operator group 17 are stored as amplitude values for each address. From the waveform memory 26, the amplitude values of musical waveforms of the timbre selected by the timbre selection switch and designated via the control section 16 are sequentially read out based on the address signal. The amplitude values of the musical waveforms read out sequentially in this way are given to the multiplier 27, and the multiplier 27 receives an envelope signal based on the envelope shape control signal given from the control section 16 based on the timbre selected by the timbre selection switch. The envelope signal is given from the envelope signal generation circuit 28 that generates it. Therefore, in the multiplier 27, the amplitude values sequentially read from the waveform memory 26 are multiplied by the envelope signal from the envelope signal generation circuit 28, and the signal whose volume is controlled by the envelope signal is converted into a musical tone signal. output as audio output.

Incidentally, the generation of the envelope signal is started by the envelope start signal given from the control section 16, and the generation of the envelope signal is similarly terminated by the control section 16.
The generated envelope signal is also provided to the control section 16 in addition to the multiplier 27. The typical waveform shape of the envelope signal generated in the envelope signal generation circuit 28 is shown in FIG. Correspondingly, the level attenuation speed and the like of the envelope signal are made different.

Next, muting processing will be explained. This muting process is performed by the control unit 16 at predetermined time intervals, for example, at 10 m se
This is accomplished by performing processing every c. The muting level corresponding to the pitch of the musical tone being generated in the musical tone signal generation channel in which the envelope signal is in an attenuated state, in other words, the range of the key for the musical tone being generated, and the currently selected tone selection switch. , read from the muting level memory 18. Next, this read muting level is compared with the envelope signal level of the musical tone signal generation channel,
When this envelope signal level becomes lower than the read muting level, another musical tone signal generation channel that is currently generating a musical tone is searched for in response to the same key press as that of the musical tone signal generation channel, and both musical tone signal generation channels are "0" is given as frequency data to each address signal generation circuit 25 in the signal generation channel. This address signal generation circuit 25 is
Because it accumulates given frequency data,
When "0" is given as frequency data, an address signal of the same value is repeatedly generated as an address signal. Therefore, the amplitude values of the same value are repeatedly read out from each waveform memory 26 of both musical tone signal generation channels by the address signals of the same value, and the generated first and second musical tone signals are simply It becomes a direct current component and cannot be heard as sound, and quantization noise also disappears.

In short, any one of the two musical tone signals generated in response to a sound generation instruction based on the same key press.
When the envelope signal level of each musical tone signal moves below the muting level, all musical tone signals caused by generation instructions based on the same key depression are muted.

The muting level data stored in the muting level memory 18 is, for example, a value about 70 decibels lower than the maximum level of the envelope signal level, and is selected again as the sound range becomes higher. The value is set to increase as the attenuation speed of the envelope signal level generated corresponding to the timbre decreases. For example, the muting level corresponding to a tone whose envelope signal level decay rate is a standard value is set to be 70 decibels lower than the maximum level in the center range, and the range is one octave below this value. 2 as it gets higher each time
The value increases in decibel increments, and decreases in 2 decibel increments as the range gets lower by one octave. Furthermore, the muting level corresponding to a tone whose envelope signal level decay rate is half the standard rate is a value 65 dB lower than the maximum level in the center range, and the change in the tone range with respect to this value is It is set up the same way. Furthermore, the muting level corresponding to a tone in which the envelope signal level decay rate is twice the standard rate is set to a value 75 decibels lower than the maximum level in the center range, and the changes in the range with respect to this value are as described above. The settings are the same as the tone ones. The reason for this is that quantization noise tends to be more noticeable the higher the range of the musical tone being generated and the slower the attenuation rate of the envelope signal level. This is to set the level. By setting in this manner, muting is performed that is appropriate for the characteristics of each musical tone. In addition, since muting processing is performed simultaneously on the two first and second musical tone signals generated in response to instructions for sound generation based on the same key depression, each musical tone as described below The unnaturalness that occurs when muting processing is performed at separate timings for each signal does not occur.

As shown in FIG. 7, the envelope signal levels of the first and second musical tone signals generated in response to instructions for producing sound based on the same key depression both exceed the muting level. In this case, the desired tone is synthesized. However, although the first musical tone signal is muted at point A when its envelope signal level falls below the muting level, the second musical tone signal is not muted yet, so the synthesized tone may not be the desired one. On the other hand, the timbre changes rapidly, resulting in an unnatural musical sound.

Note that even if the same amplitude value is repeatedly read out from the waveform memory 26 using the same address signal, the finally obtained musical tone signal is added to the amplitude value read out from the waveform memory 26 by a multiplier. 27 by the envelope signal from the envelope signal generation circuit 28. Therefore, the amplitude value after multiplication changes over time as the envelope signal level of the envelope signal changes, even if the same amplitude value is read out repeatedly from the waveform memory 26, for example. The sound generated by this is almost never unpleasant to the ears. However, if this sound is harsh,
At the same time as giving "0" as frequency data to the waveform memory 26, the envelope signal generation circuit 28 is controlled so as to keep the envelope signal level of the envelope signal constant, so that the amplitude value of the musical tone signal output from the multiplier 27 is completely It is also good to have a constant value. In addition, in the second embodiment, the frequency data is set to "0" when the envelope signal level becomes below the muting level. It is also good to set it to a value other than "0" which corresponds to a low frequency.

In the second embodiment, regarding detecting the transition of the envelope signal level to a predetermined level or lower, in other words, the muting level or lower,
The muting level is changed based on the characteristics of the musical tone, but the muting level is kept constant and the envelope signal level of the envelope signal obtained from the envelope signal generation circuit 28 is changed based on the characteristics of the musical tone. It goes without saying that the same effect can be obtained by comparing with the current level. For example, when the pitch of the musical tone being generated is high, the envelope signal level of the envelope signal obtained from the envelope signal generation circuit 28 is changed to be small, and when the pitch of the musical tone being generated is low, the envelope signal level of the envelope signal obtained from the envelope signal generation circuit 28 is changed. After changing the envelope signal level of the signal to increase it, it is compared with the muting level.

In the second embodiment, when determining whether or not to mute a musical tone signal, the range of the musical tone and the attenuation speed of the envelope signal level are taken into account as the characteristics of the musical tone. However, it is not limited to these sound ranges and attenuation speeds. For example, other factors may be taken into consideration, such as the brightness of the timbre and whether the musical tone has a constant period or is aperiodic noise-like. Furthermore, when controlling the characteristics of musical tones according to factors such as key pressing speed, it is good to mute directly based on the characteristics of the musical tones, but It is also a good idea to perform muting based on the factors.

In the second embodiment, the envelope signal level of the envelope signal generated in the envelope signal generation circuit 28 is used to obtain the envelope level of the musical tone signal being generated. It is also possible to arrange a level detector on the output side of the multiplier 27 and obtain the signal using this envelope level detector.

In the first and second embodiments, the audio output obtained when the envelope level of the audio signal falls below the muting level contains a DC component, but the level of this DC component is extremely low. Normally this is not a problem as it is low. If this DC component becomes a problem, a high-pass filter or the like that removes the DC component may be placed on the output side. Alternatively, if muting processing is performed at the first zero-crossing point of the amplitude value of the audio output after the envelope level of the audio signal becomes less than the muting level, no DC component is generated. Note that in the case of the first embodiment, the amplitude value memory section 12
Since the amplitude value held in is predetermined to be "0", it is sufficient to always give "0" to the B terminal of the selection section 13.
The amplitude value memory section 12 becomes unnecessary.

[0026]

[Effects of the Invention] As explained above, according to the present invention, the envelope level of the audio signal is reduced,
When the audio output becomes small and quantization noise becomes noticeable, a signal with a constant amplitude value is output as the audio output instead of an audio signal, and only the DC component is output. Therefore, it is no longer audible as sound, and the quantization noise is also reduced, making it less noticeable. In addition, in this case, in order to maintain the amplitude value of the audio signal at the time when the transition of the envelope level of the audio signal to a predetermined level or below is detected and output it as audio, the amplitude value of the audio output should not change suddenly. There is no click noise.

[Brief explanation of drawings]

FIG. 1 is a principle block diagram for explaining the principle configuration of a muting circuit according to the present invention.

FIG. 2 is a block circuit diagram of a first embodiment of a muting circuit according to the invention.

FIG. 3 is a waveform diagram during the muting process described in FIG. 2;

FIG. 4 is a schematic block diagram of a second embodiment of a muting circuit according to the invention;

FIG. 5 is a block circuit diagram of the musical tone signal generation channel explained in FIG. 4;

FIG. 6 is a waveform diagram of a typical envelope signal generated as described in FIG. 5;

7 is an explanatory diagram showing temporal changes in envelope signals of first and second musical tone signals generated in response to a sound generation instruction based on the same key depression described in FIG. 5; FIG.

[Explanation of symbols]

1 Level detection means 2 Amplitude value holding means 11 Envelope level detection section 12 Amplitude value memory section 13 Selection section 15 Keyboard 16 Control section 17 Operator group 18 Muting level memory 19
Musical tone signal generation circuit 20 D/A converter 21 Amplifier 22 Speaker 25 Address signal generation circuit 26 Waveform memory 27 Multiplier

Claims (1)

[Claims] 1. (a) Level detection means for detecting that the envelope level of the audio signal shifts below a predetermined level; and (b) Based on the detection of the shift of the envelope level below the predetermined level by the level detection means, A muting circuit comprising amplitude value holding means for holding an amplitude value of an audio signal and outputting the held amplitude value as an audio output.