Sound signal processing device and sound signal processing method

In a sound signal processing device, the level of an output signal is controlled as a function of the level of the corresponding input signal by supplying the output signal, whose level is converted by a level converter and to which a specific frequency characteristic is added, to an automatic level control section. The amplification factor of the signal is automatically controlled so as to prevent the output signal level from exceeding an allowable range. The sound signal processing device includes a control section that variably changes the gain values of two level converters and the threshold value, which is a parameter to be used by a gain computer in the automatic level control section, as a function of the peak gain that is a defined value of an equalizer as defined by the user by way of an operation section.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from Japanese Patent Application No. 2003-400177, filed in the Japanese Patent Office on Nov. 28, 2003, the entire content of which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a sound signal processing device comprising a frequency characteristic adding means for adding a desired frequency characteristic to an input sound signal and an automatic level control means for automatically controlling the signal amplification factor to prevent the output level from exceeding an allowable range. More particularly, the present invention relates to a sound signal processing device and a sound signal processing method for controlling the signal level according to the predetermined value of the frequency characteristic adding means and the value of the level conversion means of the system it belongs so as to secure a maximal dynamic range.

2. Related Background Art

Sound signal processing devices comprising an equalizer that is a frequency characteristic adding means for adding a desired frequency characteristic to a sound signal and an automatic level control means for automatically controlling the signal amplification factor so as to prevent the output level from exceeding an allowable range have been used in sound reproducing systems.

Particularly, the applicant of the present invention discloses a technology relating to an automatic level control means in Japanese Patent Application Laid-Open Publication No. 9-93063. The above patent document describes an automatic gain control circuit for equalizing the level of the input signal or limiting the level of the input signal to make its level from rising excessively.

FIG. 1of the accompanying drawings is a schematic circuit diagram of a sound signal processing device50using an automatic level control means as disclosed in the above patent document. Referring toFIG. 1, the known sound signal processing device50comprises an equalizer52adapted to add a desired frequency character to digital sound input signal Din that is input to it from input terminal51and a downstream automatic level control section53. The sound signal processing device50applies a gain computed by gain computer54that is a component of the automatic level control section53to variable gain multilayer55. Unless noted otherwise, the unit of dB (decibel) is used for all the numerical values and the signs that indicate the gain in this letter of specification. Thus, the sound signal processing device50can control the level of the output signal of the equalizer52and obtain digital sound output signal Dout of the system it belongs. A level converter57for converting the level of the digital sound input signal Din supplied to the input terminal51is arranged upstream relative to the equalizer52. A level converter58for converting the level of the output signal of the automatic level control section53is arranged downstream relative to the automatic level control section53. The system provides an effect of preventing the signal amplified by the equalizer52from going beyond an allowable range and being distorted when it becomes the output signal Dout.

FIG. 2is a schematic circuit diagram of the gain computer54in the automatic level control section53ofFIG. 1, showing the configuration thereof. The output signal to which a specific frequency characteristic is added by the equalizer52is input to the input terminal59and converted into a logarithmic value x by a log converting section60and then supplied to adder61. A threshold value th to which a negative sign is added is also supplied from threshold holding section62to the adder61. Thus, the adder61computes the difference between the output signal (value x) of the equalizer that is converted into a logarithmic value and the threshold value th. The threshold value th of this known device refers to that of the maximum output level that the automatic level control section53can output and corresponds to the threshold value th inFIG. 3that illustrates the input/output characteristics of the automatic level control section53. The difference computed by the adder61is then supplied to multiplier63that is adapted to multiply it by −1, which is a coefficient. The difference computed by the adder61is also used as the base of the switching control signal of switch65. The multilayer63multiplies the difference by −1 and supplies the product to anti-log converting section64. The anti-log converting section64reduces the value x that is multiplied by −1 to an anti-logarithmic value and supplies the obtained value to terminal to be selected65aof the switch65. The switch65has terminal to be selected65bin addition to the terminal to be selected65a. The terminal to be selected65bis connected to coefficient holding section66that holds coefficient 1. Thus, as the switch65turns movable piece65ceither to the terminal to be selected65aor the terminal to be selected65baccording to the switching control signal that is based on the difference, either the anti-logarithmic value or the coefficient 1 is selected as the output of the switch65. As pointed out above, the difference that is the difference (x−th) of the output signal (value x) of the equalizer and the threshold value (th) is output from the adder61when the difference (x−th) exceeds 0 ((x−th)>0) and hence the level of the value x is higher than the threshold value th, the difference operates as switching control signal for connecting the movable piece65cof the switch65to the terminal to be selected65aso that consequently the movable piece65cis connected to the terminal to be selected65a. As a result, the gain computer54supplies a value the suppresses the difference between the output signal of the equalizer and the threshold value th to variable gain multilayer55inFIG. 1. When, on the other hand, the difference (x−th) is not higher than 0 ((x−th)≦0) and hence the level of the value x is not higher than the threshold value th, the difference operates as switch control signal for connecting the movable piece65cof the switch65to the terminal to be selected65bso that consequently the movable piece65cis connected to the terminal to be selected65b. As a result, the gain computer54gives a one fold gain to the variable gain multilayer55inFIG. 1.

Assume that a flat sound input signal of 0 dB is input to the sound signal processing device50having the above described configuration and the equalizer52adds a frequency characteristic having a gain of G as peak value as shown in FIG.4(1) to the signal. Note thatFIG. 4shows spectral graphs at a given instant. In the graphs ofFIG. 4, the horizontal axis represents the frequency and the vertical axis represents the level. Assume also that the value of the gain of the level converter57ofFIG. 1is −A, whereas that of the gain of the level converter58is A (G≦A) and these values are fixed. Then, the threshold value th ofFIG. 2needs to be equal to −A in order to prevent the output signal Dout ofFIG. 1from exceeding 0 dB and being distorted if gain A is added by the level converter58.

(1), (2) and (3) inFIG. 4respectively illustrate the outputs at (1), (2) and (3) inFIG. 1when a flat signal of 0 dB is input to the system. Firstly, the flat input signal of 0 dB is attenuated by A by the level converter57ofFIG. 1and subsequently a frequency characteristic having a gain with a peak value of G is added by the equalizer52. Thereafter, the automatic level control section53shown inFIG. 1operates for level control in such a way that output signal56does not exceed 0 dB and hence is not distorted if gain A is added thereto by the level converter58. At this time, since peak level of the output of the equalizer52is (−A+G) and the threshold value th ofFIG. 2is −A, the value given from the gain computer54to the variable gain multilayer55in the automatic level control section53is equal to ((−A+G−(−A))×(−1))=−G due to the arrangement ofFIG. 2. Thus, the signal level is attenuated by G from (1) so as to become (2) inFIG. 4. Then, ultimately, the signal is amplified by A as shown i (3) ofFIG. 4by the level converter58before it is output. The gain −A of the level converter57, the gain A of the level converter58and the gain −A that is the threshold value th ofFIG. 2are constant and never vary regardless of the characteristic that is added by the equalizer52. Therefore, if G<A, it will be understood that a dynamic range loss of (A−G) arises in the signal processing operation as shown in (1) ofFIG. 4.

Assume here that a volume is added to the sound signal processing device50ofFIG. 1so as to control the sound volume of the entire system. The volume59is arranged most downstream in the system ofFIG. 1such that it can be controlled independently. Since the volume59is independent of the equalizer and the automatic level control section, if the level control operation is conducted or not depends solely on the input level of the signal. If the gain value of the volume59is −A and 0 dB is input to the system, the output at (1) and the one at (2) inFIG. 5are entirely same as those shown inFIG. 4(as shown in (1) and (2) inFIG. 6) and the output at (3) inFIG. 5is same as the one shown in (3) ofFIG. 6because it is obtained by shifting (3) inFIG. 4by the gain value −A of the volume59inFIG. 5. In this case, if the level converter58is variable and it is known that the level is raised by A at the level converter58and then lowered by −A at the volume9, 0 dB can be selected for the gain value of the level converter58and that of the volume59. If 0 dB is selected for the gain value of the level converter58and that of the volume59, the output signal Dout would not exceed 0 dB and become distorted. Therefore, it is not necessary to conduct a level control operation as shown in (2) ofFIG. 6by operating the level control section53. However, a level control operation that is supposed to be unnecessary as described above may have to be conducted since the gain −A of the level converter57, the gain A of the level converter58and the threshold value th ofFIG. 2are always fixed.

Thus, a known sound signal processing device comprising an equalizer and an automatic level control section as principal components thereof is accompanied by a problem that a dynamic range loss may arise as pointed out earlier and a problem that a level control operation that is supposed to be unnecessary may have to be conducted. These problems give rise to degradation of the sound quality.

SUMMARY OF THE INVENTION

In view of the above identified circumstances, it is therefore an object of the present invention to provide a sound signal processing device and a sound signal processing method for conducting a level control operation, using the equalizer and automatic level control section while always securing a maximal dynamic range. Another object of the present invention is to provide a sound signal processing device and a sound signal processing method adapted to conduct a minimally required level control operation, while always securing a maximal dynamic range.

In an aspect of the present invention, the above objects are achieved by providing a sound signal processing device comprising: a first level converting means for converting the level of the input sound signal; a frequency characteristic adding means for adding a desired frequency characteristic to the output signal of the first level converting means; a level control means for controlling the level of the signal having the frequency characteristic added by the frequency characteristic adding means; a second level converting means for converting the level of the output signal of the level control means; and a control means for variably changing the parameter of the level control means and the gain of the first level converting means and that of the second level converting means.

In another aspect of the invention, in order to achieve the above objects, there is provided a sound signal processing method comprising: a first level converting step of converting the level of the input sound signal; a frequency characteristic adding step of adding a desired frequency characteristic to the sound signal having the level converted in the first level converting step; a level control step of controlling the level of the signal having the frequency characteristic added in the frequency characteristic adding step; and a second level converting step of converting the level of the signal having the level controlled in the level control step; the parameter of the level control step and the gain of the first level converting step and that of the second level converting step being made to variably change.

Thus, a sound signal processing device and a sound signal processing method respectively comprise a frequency characteristic adding means and level control means and a frequency characteristic adding step and level control step and are adapted not to fix but to variably change the parameter used by the level control means, the value of the first level converting means and that of the second level converting means and the parameter used in the level control step, the value of the first level converting step and that of the second level converting steps.

Thus, since a sound signal processing device comprising a frequency characteristic adding means and a level control means according to the invention is adapted not to fix but to variably change the parameter used by the control means and the values of the first and second level converting means, it is possible to automatically control the signal amplification factor to prevent the output level from exceeding an allowable range, while always securing a maximal dynamic range. Additionally, depending on the method for variably changing the parameter, it is possible to realize a sound signal processing device comprising a level control means adapted to conduct a minimally required level control operation. Then, as a result, it is possible to improve the problem of degradation of the sound quality by which the above described known technique is accompanied in the signal processing operation.

Thus, since a sound signal processing method comprising a frequency characteristic adding step and a level control step according to the invention is adapted not to fix but to variably change the parameter used in the control step and the gain of the first level converting step and that of the second level converting step, it is possible to automatically control the signal amplification factor to prevent the output level from exceeding an allowable range, while always securing a maximal dynamic range. Then, as a result, it is possible to improve the problem of degradation of the sound quality by which the above-described known technique is accompanied in the signal processing operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be described by referring to the accompanying drawings that illustrate preferred embodiments of the invention.FIG. 7is a schematic block diagram of the first embodiment of sound signal processing device1of the invention. The sound signal processing device1comprises an equalizer3adapted to add a desired frequency characteristic to the digital sound input signal Din coming from input terminal2and a downstream automatic level control section4. The sound signal processing device1also comprises a level converter7arranged upstream relative to the equalizer3and adapted to convert the level of the digital sound input signal Din and a level converter8arranged downstream relative to the automatic level control section4and adapted to convert the level of the output of the automatic level control section4. The sound signal processing device1further comprises a control section10for variably changing the parameter of the automatic level control section4, the value of the level converter7and that of the level converter8as a function of the set value of the equalizer3that is selected by the user by way of an operation section11. The digital sound output signal Dout whose level is converted by the level converter8is led out from the output terminal9.

The sound signal processing device1controls the output signal level as a function of the input signal level by converting the level by means of the level converter7, adding a specific frequency characteristic to the signal by means of the equalizer3and supplying the output signal of the equalizer3to the automatic level control section4. In short, the signal amplification factor is automatically controlled so as to prevent the output signal level from exceeding an allowable range.

Particularly, the sound signal processing device variably changes the gain values of the level converter7, that of the level converter8and the parameter that is used in the automatic level control section4by means of the control section10as a function of the peak gain G that is the set value of the equalizer3selected by the user by way of the operation section11. The expression of peak gain G as used herein refers to the highest gain of the frequency characteristic defined by the equalizer3. In other words, it refers to a point where the frequency characteristic shows the largest gain.

The parameter of the automatic level control section4, the gain values of the level converter7and that of the level converter8are made to variably change as a function of the peak gain G of the equalizer3by the sound signal processing device1instantaneously or within a certain time span at the timing defined for the equalizer3. When they are made to change instantaneously, the control section10changes the parameter, the gain value of the level converter7and that of the level converter8instantaneously by referring to the peak gain G for each sample. When they are made to change within a certain time span, on the other hand, the control section10changes the parameter and the gain values by carrying out an integral operation for every eight samples, for example, and referring to the average level of the eight samples.

As shown inFIG. 8, the automatic level control section4includes a comparator4afor comparing the output signal level of the equalizer3and the threshold value th, which will be described in greater detail hereinafter, and a variable gain multilayer4bfor multiplying the output signal of the equalizer by a variable gain according to the output of the comparator4a.

With the above-described arrangement, the sound signal processing device1can control the level of the output signal of the equalizer3and obtain the output signal Dout of the system.

As described above, in the first embodiment of sound signal processing device of the present invention, the control section10variably changes the gain values of the level converter7and the level converter8as a function of the peak gain G of the equalizer3. Additionally, the control section10variably changes the threshold value th that is the parameter to be used by the comparator4ain the automatic level control section4as a function of the peak gain G that is a set value for the equalizer3. Thus, the system formed by the automatic level control section4, the level converter7and the level converter8in the sound signal processing section1can prevent the signal Dout that is amplified by the equalizer3and output from the output terminal9from going beyond an allowable range and being distorted.

Now, the second embodiment of sound signal processing device according to the invention will be described below. This second embodiment of sound signal processing device has a configuration substantially same as the above described first embodiment of sound signal processing device. However, the sound signal processing device21of this embodiment differs from the sound signal processing device1of the first embodiment in terms of the internal configuration of the automatic level control section4.

Particularly, in the sound signal processing device21, the control section10variably changes the gain value of the level converter7, that of the level converter8and the threshold value th, which is the parameter to be used by gain computer5, which will be described in greater detail hereinafter, in the automatic level control section4, as a function of the peak gain G that is the set value of the equalizer3that is selected by the user by way of an operation section11. The expression of peak gain G as used herein refers to the highest gain of the frequency characteristic defined by the equalizer3. In other words, it refers to a point where the frequency characteristic shows the largest gain.

The automatic level control section4of the sound signal processing device21has a gain computer5for computing the variable gain on the basis of the output signal level of the equalizer3and a variable gain multilayer6for receiving the variable gain as computed by the gain computer5and multiplying the output signal of the equalizer3by the supplied variable gain.

Thus, the sound signal processing device21can obtain the output signal Dout of the system by controlling the level of the output signal of the equalizer3. The system comprising the automatic level control section4, the level converter7and the level converter8prevents the signal Dout amplified by the equalizer3and output from the output terminal9from going beyond an allowable range and being distorted. Particularly, in the sound signal processing device21, the control section10variably changes the gain value of the level converter7and that of the level converter8as a function of the peak gain G of the equalizer3. Additionally, the control section10variably changes the threshold value th, which is the parameter to be used by the gain computer5in the automatic level control section4, as a function of the peak gain G that is the set value of the equalizer3.

FIG. 10is a schematic block diagram of the gain computer5in the automatic level control section4shown inFIG. 9, illustrating the configuration thereof. The output signal, which is supplied from the input terminal12and to which a particular frequency characteristic is added by the equalizer3, is converted into a logarithmic value x by a log converting section13and then supplied to adder14. A threshold value th to which a negative sign is added is also supplied from threshold holding section15to the adder14. Thus, the adder14computes the difference (x−th) between the output signal (value x) of the equalizer that is converted into a logarithmic value and the threshold value th. As described above, the threshold value th is made to variably change as a function of the peak gain G of the equalizer3.

The difference computed by the adder14is then supplied to multiplier16that is adapted to multiply it by −1, which is a coefficient. The difference computed by the adder14is also used as the base of the switching control signal of switch18. The multilayer16multiplies the difference by −1 and supplies the product to anti-log converting section17. The anti-log converting section17reduces the value x that is multiplied by −1 to an anti-logarithmic value and supplies the obtained value to terminal to be selected18aof the switch18. The switch18has terminal to be selected18bin addition to the terminal to be selected18a. The terminal to be selected18bis connected to coefficient holding section19that holds coefficient 1. Thus, as the switch18turns movable piece18ceither to the terminal to be selected18aor the terminal to be selected18baccording to the switching control signal that is based on the difference, either the anti-logarithmic value or the coefficient 1 is selected as the output of the switch18and led out from the output terminal20.

The switching operation of the switch18proceeds in a manner as described below. The difference between the output signal (value x) of the equalizer and the threshold value (th), or (x−th) is output from the adder14. When the difference (x−th) exceeds 0 ((x−th)>0) and hence the level of the value x is higher than the threshold value th, the difference operates as basis for the switching control signal so that consequently the movable piece18cof the switch18is connected to the terminal to be selected18a. As a result, the gain computer5supplies a value the suppresses the difference between the output signal of the equalizer and the threshold value th to variable gain multilayer6inFIG. 9. When, on the other hand, the difference (x−th) is not higher than 0 ((x−th)≦0) and hence the level of the value x is not higher than the threshold value th, the difference operates as basis for the switch control signal so that consequently the movable piece18cof the switch18is connected to the terminal to be selected18b. As a result, the gain computer5gives a one fold gain to the variable gain multilayer6inFIG. 9.

Now, the operation of the sound signal processing device21when it receives a flat sound input signal of 0 dB and the peak gain of the equalizer3as illustrated inFIG. 9is G from a reference level will be described below by referring toFIG. 11.FIG. 11shows the spectral graphs of an instant. In the graphs ofFIG. 11, the horizontal axis represents the frequency and the vertical axis represents the signal level.

In the sound signal processing device21, the control section10variably changes the gain value of the level converter7, that of the level converter8and the threshold value th, which is the parameter to be used by gain computer5in the automatic level control section4, as a function of the peak gain G of the equalizer3. Therefore, when a frequency characteristic that provides a gain of G for the peak value is added by the equalizer3shown inFIG. 9, the gain value of the level converter7inFIG. 9becomes equal to −G whereas that of the level converter8becomes equal to G under the control of the control section10. Additionally, the threshold value th that is used by the gain computer5becomes equal to −G.

Then, (1), (2) and (3) inFIG. 11respectively illustrate the outputs at (1), (2) and (3) inFIG. 9when a flat signal of 0 dB is input to the system. Firstly, the flat input signal of 0 dB is attenuated by gain G by the level converter7and subsequently a frequency characteristic having a gain with a peak value of G is added by the equalizer3((1) inFIG. 11). Thereafter, the automatic level control section4operates for level control, using −G ((−G+G−(−G))×(−1)), or −G, so that the output signal Dout does not exceed 0 dB and become distorted when a gain of G is added at the level converter8(or it is amplified by the gain G at the level converter8from (2) ofFIG. 11and output ((3) inFIG. 11)). Thus, while the ultimate output level of (3) inFIG. 11is same as that of the system ofFIG. 1, it will be seen by comparingFIG. 11andFIG. 4, which is used for describing the prior art, that (1) inFIG. 4shows a dynamic range loss of (A−G) but (1) inFIG. 11does not show such a dynamic range loss and hence this embodiment provides an improvement for the dynamic range loss in the signal processing operation.

In this way, as a result of variably changing the gain value of the level converter7, that of the level converter8and the threshold value th of the gain computer5inFIG. 4as a function of the peak gain G of the equalizer3, it is now possible to realize a level control operation and, at the same time, secure a maximal dynamic range.

Now, the third embodiment of the present invention will be described. The third embodiment of the present invention is a sound signal processing device22having a configuration as shown inFIG. 12. With this embodiment, the level of the input signal is converted by the level converter7and the output signal level is controlled as a function of the input signal level by supplying the output signal obtained by adding thereto a specific frequency characteristic by the equalizer3to the automatic level control section4. In other words, the sound signal processing device22is adapted to automatically control the amplification factor of the signal so as to prevent the output signal from going beyond an allowable range. InFIG. 12, the component blocks same as those of the sound signal processing device1ofFIG. 9are denoted respectively by the same reference symbols and will not be described further.FIG. 13illustrates the gain computer5of this embodiment, showing the configuration thereof.

This sound signal processing device22differs from the sound signal processing device21ofFIG. 9in that the level converter8is controlled independently from the level converter7. In other words, the control section23variably changes the threshold value th of the gain computer5ofFIG. 13as a function of the value of the level converter8. The control section23variably changes the value of the level converter7as a function of the peak gain G of the equalizer3.

When the peak gain of the equalizer3inFIG. 12is G from a reference level, the gain value of the level converter7is made equal to −G as in the case of the sound signal processing device21ofFIG. 9as the control section23makes it variably change as a function of the peak gain.

Assume that the level converter8converts the level by V (0 dB<V<G) for example. Then, (1), (2) and (3) inFIG. 14respectively illustrate the outputs at (1), (2) and (3) inFIG. 12when a flat signal of 0 dB is input to the system. Firstly, the input signal is attenuated by gain G by the level converter7and subsequently a frequency characteristic having a gain with a peak value of G is added by the equalizer3((1) inFIG. 14). It is necessary to select −V (as obtained from (−G+G−(−V))×(−1) inFIG. 13) for the threshold value th in order to prevent the output signal Dout from exceeding 0 dB and being distorted when a gain of V is added thereto by the level converter8. Therefore, a level control operation needs to be conducted for a gain of −V from (1) inFIG. 14(to (2) inFIG. 14). Then, the output signal is amplified by gain V by the level converter8to obtain a signal as shown by (3) inFIG. 14.

Now, (1), (2) and (3) inFIG. 15respectively illustrate the outputs at (1), (2) and (3) inFIG. 12when a gain value of V′ (V′<0 dB) is used by the level converter8for a flat signal of 0 dB is input to the system. The input signal is attenuated by gain G by the level converter7and subsequently a frequency characteristic having a gain with a peak value of G is added by the equalizer3((1) inFIG. 15). Since the gain value of the level converter8is V′ (V′<0 dB) and hence the output signal Dout does not exceed 0 dB to become distorted if the gain is added, it is not necessary for the level control section4to suppress the signal level shown in (1) ofFIG. 15. Therefore, a signal as shown in (2) ofFIG. 15is obtained when 0 dB is selected for the threshold value th and ultimately a signal as shown in (3) ofFIG. 15is obtained from the signal of (2) ofFIG. 15after passing through the level converter8. This indicates that a level control operation may or may not be conducted depending on the gain value of the level converter8if the level of the input signal remains same.

In the case of the system of the prior art illustrated inFIG. 5, if a level control operation is conducted or not solely depends on the level of the input signal and hence an unnecessary level control operation may be conducted. To the contrary, in the case of the system ofFIG. 12, if a level control operation is conducted or not depends not only on the level of the input signal but also on the gain value of the level converter8. In other words, only a minimally necessary level control operation is conducted in the case of the system ofFIG. 12.

Thus, in the sound signal processing device22of the third embodiment, no dynamic range loss takes place in the process of obtaining an output signal as shown in (3) ofFIG. 14or (3) ofFIG. 15due to the selected parameter value unlike the case of the prior art ofFIG. 4nor an unnecessary level control operation is conducted unlike the case of the system of the prior art ofFIG. 5. Therefore, it is possible to improve the problem of degradation of the sound quality in the signal processing operation.

As described above, in the sound signal processing device22, the control section23variably changes the gain value of the level converter7ofFIG. 12as a function of the peak gain G of the equalizer3and can independently control the level converter8so as to make the threshold value th ofFIG. 13variably change as a function of the gain value of the level converter8. Thus, with the sound signal processing device22of the third embodiment, only a minimally necessary level control operation is conducted while securing a maximal dynamic range.

Now, the fourth embodiment of the present invention will be described below. The fourth embodiment of the present invention is a sound signal processing device31having a configuration as shown inFIG. 16. With this embodiment, the level of the input signal is converted by the level converter7and the output signal level is controlled as a function of the input signal level by supplying the output signal obtained by adding thereto a specific frequency characteristic by the equalizer3to the automatic level control section4in such a way that the amplification factor of the signal is automatically controlled so that the output signal level may not exceed an allowable range. Additionally, the sound signal processing device31is adapted to regulate the sound volume in the most downstream step. InFIG. 16, the component blocks same as those of the sound signal processing devices21,22ofFIGS. 9,12are denoted respectively by the same reference symbols and will not be described further.

This sound signal processing device31differs from the sound signal processing device22ofFIG. 12in that a D/A converter33and a volume34are arranged downstream relative to the level converter8. The output of the level converter8is subjected to D/A conversion by the D/A converter33and delivered to the volume34that is electronically controllable.

The gain computer5illustrated inFIG. 17variably changes the threshold value th as a function of the gain value of the level converter8under the control of the control section32. The gain value of the level converter8is also made to variably change as a function of the value selected for the volume34. The volume34is defined by the user who uses operation section11that is connected to the control section32. Thus, the control section32variably changes the gain value of the level converter8as a function of the value selected for the volume34and the threshold value th of the gain computer5as a function of the gain value of the level converter8. The control section32also variably changes the gain value of the gain converter7as a function of the peak gain G of the equalizer3.

With the above described arrangement, if the signal level of the output (digital output signal) Dout of the level converter8inFIG. 16can exceed 0 dB, the overall sound volume is regulated by way of the combination of the gain value of the level converter8and the threshold value th of the gain computer5. If the signal level of the output Dout cannot exceed 0 dB as described above by referring toFIG. 15, it is possible to regulate the sound volume by means of a minimal necessary level control operation of the volume34, while securing a maximal dynamic range for the digital data.

Note that, inFIG. 16, the D/A converter33and the electronically controllable volume34may be realized as a digital amplifier12adapted to control the output level by the input from the control terminal as will be described hereinafter. As shown inFIG. 18, the digital amplifier12includes a control section35ahaving a terminal to which supply voltage Vcc is applied and a terminal to which control signal CNT is supplied and an amplifier35bhaving a terminal to which input signal Vin is supplied, a terminal for producing output signal Vout and a terminal connected to the ground GND. The amplifier35bamplifies the input signal Vin under the control of the control section35ato produce the output signal Vout. The control section35achanges the value of the amplitude level Vp-p of the pulse wave as shown inFIG. 19by controlling the supply voltage to the digital amplifier. Thus, the digital amplifier35can variably change the output level.

Now, the fifth embodiment of the present invention will be described below. The fifth embodiment of the present invention is a sound signal processing device41having a configuration as shown inFIG. 20. This embodiment is adapted to feedback type control such that the control section10detects the output level of the automatic level control section4and variably changes the parameter of the automatic level control section4, the gain value of the level converter7and that of the level converter8as a function of the detected output level. Thus, the control operation of this embodiment differs from that of the sound signal processing device1ofFIG. 7, in which the parameter of the automatic level control section4, the gain value of the level converter7and that of the level converter8are made to variably change as a function of the selected value of the equalizer3.

Thus, in the sound signal processing device41, the system formed by the automatic level control section4, the level converter7and the level converter8can prevent the signal Dout amplified by the equalizer3and output from the output terminal9from exceeding an allowable range and being distorted.

Likewise, in the sound signal processing device42(the sixth embodiment) as shown inFIG. 21and corresponding to the sound signal processing device21of the second embodiment, the control section10is adapted to detect the output level of the automatic level control section4and variably change the parameter of the automatic level control section4, the gain value of the level converter7and that of the level converter8as a function of the detected level.

The arrangement that the control section detects the output level of the automatic level control section4and variably changes the gain value of the level converter7and that of the level converter8as a function of the detected level can be applied to the sound signal processing device22of the third embodiment and also to the sound signal processing device31of the fourth embodiment.