Abstract: For imparting a sostenuto effect to a series of sounds produced by key depressions successively repeated in an electronic musical instrument, a searching device searches a first memory device for a first memory channel which has already stored a note information identical with the note information produced on the basis of the repeated key depression, and, then a setting device is actuated in the presence of aforementioned first channel and operative to copy the sostenuto information indicative of the instruction of imparting the sostenuto effect from a second channel paired with aforementioned first channel having already stored the identical note information to the second channel paired with another first channel where the note information produced by the repeated key depression is newly stored, thereby imparting the sostenuto effect between tones produced by repeated key depressions.

Abstract: A key scaling apparatus includes a reference data input device which is operated by a user of an electronic musical instrument to designate at least three different key numbers together with respective key scaling data for those key numbers. When a key number is supplied from a play input device such as a keyboard in the electronic musical instrument, an interpolator computes key scaling data for that key number from the data designated by the reference data input device. The computed key scaling data serve to control a tone parameter such as an envelope level which is then used in a tone generator to develop a tone for that key number. Accordingly, a satisfactory key scaling response is obtained over the entire pitch range of keyboard.

Abstract: In a typical musical tone producing device of a full waveshape readout type, a full waveshape of a musical tone to be produced from the start to the end of sounding or a rise portion and a part of the waveshape following the rise portion of the musical tone is stored in a waveshape memory, and the musical tone is formed by reading out the waveshape from the waveform memory once, or reading out the rise portion once and thereafter reading out the partial waveshape repeatedly. A digital filter is introduced following the waveshape memory in this tone producing device. The filter characteristics are determined in accordance with a tone color change parameter such as the key touch or the tone pitch of the musical tone, thereby realizing a tone color change of the musical tone. Further, in order to realize timewise change of tone color of the musical tone, the circuit which has the tone color change parameter vary with time is proposed.

Abstract: Calculations by an envelope circuit and a waveform calculation circuit, which are principal functions of an electronic musical instrument, can be performed by the same calculation routine through use of the same general formula A=B.+-.C.times.D. Accordingly, their circuit structures are substantially simplified and the circuits can be fabricated as a one-chip large scale integrated circuit.

Abstract: An electronic musical instrument comprising a host processor (10), process/engine interface (12) and sound engine (14) the latter utilizing partial control chip modules (32, 34) and a data path chip module (36) to effect multiple partial (Fourier) synthesis, in conjunction with a sound modelling technique, to generate up to 240 independent partials and impress time-varying amplitude envelopes on them and select and apply them to sound generation, each partial being controlled by selected parameters of frequency, amplitude, phase and attack/decay rate. A modulo-sumdither and oversampling approach to noise reduction is utilized in connection with the data path chip arrangement. Log sine addition is utilized to avoid multiplication apparatus ordinarily required to combine sine wave defining parameters.

Abstract: A tone source system for a superior electronic musical instrument suitable for an LSI application in which the wave data can be provided in a time division multiplex form, or the envelope data can be provided in a time division multiplex form in a synchronous relationship with it, and the wave data to which the envelopes are attached can be provided in a time division multiplex form through multiplication of the data.

Abstract: An electronic musical instrument has a memory capable of storing musical tone waveform data therein. A read circuit reads out the musical tone waveform data from the memory and supplies the readout data to a musical tone generator. The musical tone generator generates a musical tone signal in accordance with the input musical tone waveform data, and outputs the musical tone signal. A write circuit writes new data in the memory at a timing different from the timing at which the read circuit reads out the musical tone waveform data from the memory. The write circuit partially updates the musical tone waveform data stored in the memory, and stores data of a played sound in part of the memory.

Abstract: An electronic bell-tone generating system selectively provides a plurality of bell tones having improved tonal quality includes a plurality of tone generators operated in preselected combinations by a microprocessor in response to inputs from a keyboard. Data representing characteristic bells, including a fundamental tone and associated partial tones, their initial amplitude, and decay rate, is stored within a random access memory, input periodically to the respective tone generators comprising double-buffered, digital-to-analog converters, and output simultaneously to produce the "strike" of a bell.

Abstract: A shaped waveform tone generator circuit is contained entirely in an integrated circuit and has a pair of push-pull amplifiers for driving a piezo-buzzer. The output from an RC network controls current flow in a first follower transistor connected to one voltage source and in a second follower transistor connected to a second voltage source. A pair of CMOS switches alternately connect the inputs of the amplifiers to the first and second transistors. A tone signal controls operation of the two CMOS switches.

Abstract: A waveshape memory stores waveshape data representing plural waveshapes each having plural periods. A reading sequence controller designates ones to be read out among said waveshape data from the waveshape memory and the readout order thereof. This order may either be a random order or a predetermined one. By performing the readout of the waveshape data, repetitive reading of the same waveshape data is basically avoided, and a periodical noise and monotonousness in the tone color which tend to arise from such repetitive reading are eliminated.

Abstract: A plurality of drum pads are each provided with a respective piezoelectric pick-up. The output of each piezoelectric pick-up is fed to a corresponding envelope-extracting circuit. The envelope signal extracted by the envelope-extracting circuit is converted by an A/D converter into a digital signal which is fed to a CPU. The CPU feeds a tone designation signal to a tone generator unit according to parameter data from a parameter data setter and the digital signals noted above, thereby causing generation of a corresponding drum sound.

Abstract: In an electronic musical instrument of the waveshape memory type including at least one waveshape memory for storing and reproducing sample values of a musical sound wave to be generated, the waveshape memory stores the sample values of the complete waveshape of a musical tone with a shaped envelope.

Abstract: A whistle synthesizer comprises a microphone, a housing, system electronics within the housing, and controls outside the housing. The player whistles into the microphone, the signals from which are processed to provide an instrument output signal suitable for communication to an external amplifier. The controls are located within easy reach of the player so that they may be manipulated all the while the player is whistling.

Abstract: A basic tone signal generator and a difference signal generator are provided. A desired tone signal is obtained by synthesizing a basic tone signal and a difference signal generated from these generators respectively. The basic tone signal has principal characteristics of a desired tone signal whose tone color (waveshape) changes subtly with time as in a tone of an acoustic musical instrument, but it is not the desired tone signal itself, including some errors, i.e., difference from the desired tone signal. The difference signal represents the difference between the desired tone signal and the basic tone signal. Since the basic tone signal is constituted by basic tone data whose data amount can be substantially reduced as compared to an amount of data required for reproducing the desired tone signal itself with a high resolution and the difference signal is constituted by difference data whose data amount can be very small.

Abstract: A tone generation control unit has four waveform read/write channels for selectively reading or writing data in a waveform memory. A plurality of waveform signals stored in the waveform memory are converted into analog signals to be subjected to timbre and tone volume control through voltage-controlled filters and voltage-controlled amplifiers before being fed to a mixing adder. An output signal of the mixing adder is converted into a digital signal which is stored in the waveform memory again through processing of the tone generation control unit.

Abstract: In a typical musical tone producing device of a full waveshape readout type, a full waveshape of a musical tone to be produced from the start to the end of sounding or a rise portion and a part of the waveshape following the rise portion of the musical tone is stored in a waveshape memory, and the musical tone is formed by reading out the waveshape from the waveform memory once, or reading out the rise portion once and thereafter reading out the partial waveshape repeatedly. A digital filter is introduced following the waveshape memory in this tone producing device. The filter characteristics is determined in accordance with a tone color change parameter such as the key touch or the tone pitch of the musical tone, thereby realizing a tone color change of the musical tone. Further, in order to realize timewise change of tone color of the musical tone, the circuit which has the tone color change parameter vary with time is proposed.

Abstract: A digital system is provided for synthesizing individual voices of musical instruments, which may then be combined into a musical composition. The system for a single voice is comprised of means for solving a system of simultaneous finite difference equations, where time is represented by real time in the computations. Musical sounds of the voice can then be produced by repetitiously solving the difference equations that model the instrument in real time, using an array of elemental means named "universal processing elements" (UPEs) interconnected by a matrix to each other and to external input and output terminals, and varying the sounds by varying the parameters. Each UPE is capable of computing Y=A+(B.times.M) from pipelined bit-serial inputs. The difference equations model a general linear filter, a second-order linear filter, a nonlinear polynomial function, and a random number (noise) generating function.

Abstract: The present invention provides a tone color control so that the tone color in an attack portion of a tone to be generated differs depending upon whether or not a slur effect is to be imparted. If, for example, the slur effect is not imparted, a tone color for an attack portion of a tone which is rich in variation is used, whereas if the slur effect is to be imparted, a stable tone color which is the same as the sustain portion of a tone is used. In another aspect of the invention, in the case of imparting the slur effect, a tone signal is distributed to two channels and envelopes of signals in the respective channels are separately and independently controlled by separate control signals. In one channel, a control signal of decay characteristics is used whereas in the other channel, a control signal of attack characteristics is used. The changing rate of the control signal of the attack characteristics is made greater than that of the control signal of the decay characteristics.

Abstract: An electronic musical instrument includes a first waveform memory for storing a waveform corresponding to an attack portion of a musical tone and one period of a steady waveform produced after the attack, a second waveform memory for storing one period of a waveform different from the contents of the first waveform memory, a waveform reader which reads out the first and second waveform from the first and second waveform memory, respectively, and an envelope generator which generates two separate envelope signals. The first waveform including the attack portion of the musical tone and the periodic second waveform are multiplied by the separate envelope signals, respectively, and then the products of the multiplication are added together. The sum of the products is provided as an output.

Abstract: Waveshapes of plural periods for plural channels having characteristics different from each other are stored in a waveshape memory and read out from it when a musical tone is to be produced. Read out waveshapes are respectively weighted by weighting data supplied from a weighting data generator and thereafter a desired tone waveshape is obtained by electrically or acoustically combining these weighted waveshape. As an example, the waveshapes are composed of plural attack waveshapes equal in number to channels and only one sustain waveshape. In this case, the attack portion of the musical tone is formed by the combined one of the attack waveshapes and the sustain portion is formed by the sustain waveshape. This enables the memory capacity of the waveshape memory to be reduced and facilitates the complex tone color control in the attack portion. In another case, each of the waveshapes is composed of an attack portion and a sustain portion, sustain waveshapes forming the sustain portions being matched in phase.

Abstract: An electronic musical instrument comprises a keyboard for generating tone data or key codes each representative of a tone to be generated and a tone generating circuit having a plurality of tone generating channels. The electronic musical instrument also comprises a RAM having a plurality of storing positions for storing the key codes each of which is assigned under the control of a central processing unit (CPU) to a respective one of the tone generating channels to thereby generate a tone determined by the assigned key code. When a key is newly depressed on the keyboard, each of the key codes stored in the RAM is read therefrom and supplied to an envelope generator which in turn generates data representative of an amplitude of each of the envelopes given to the tones. The key codes read from the RAM are also supplied to a ROM to read therefrom weighting data.

Abstract: In an apparatus for generating an aperiodic waveform having a gradually decaying envelope, a sequence of digital samples representing the magnitude of the waveform at sample points is stored in a memory. The stored digital samples are grouped into at least first and second continuous portions, the second portion including that portion of the sequence in which none of the digital samples has a magnitude exceeding 1/n the magnitude of the largest of the digital samples, where n is an integer greater than unity, each of the digital samples of the second portion being scaled by a factor of n. The memory further stores a code indicating the beginning of the second portion. An address counter develops an address signal for sequentially addressing the stored digital samples and code from the memory in response to clock pulses. When the stored code is addressed, an additional clock pulse is supplied from a code detector to the address counter.

Abstract: A touch sensitive electronic musical or sound generating instrument is disclosed. The invention utilizes digital techniques to sense how hard any one of a plurality of push buttons on a keyboard is depressed or actuated. The instrument generates a desired or particular sound (such as a musical note) at a desired parameter such as volume, corresponding to how hard a specific button was depressed.

Abstract: To enhance low and high frequency components in a sound signal, low frequency components are used to generate new yet lower frequencies (subharmonics), and high frequency components are used to generate new yet higher frequencies (harmonics), the new frequencies added to the original signal thereby increasing the original signal bandwidth.

Abstract: An acoustic output device for a personal computer includes an envelope setting circuit for setting elements of a tone including an envelope pattern, a first storage circuit for storing the element data of the tone, a music note setting circuit for setting note information of a music, a second storage circuit for storing the note information, and an acoustic output circuit for reading-out the note information and providing a tone with the element data.

Abstract: A CPU reads out sampled waveform data stored in a waveform memory and detects a peak value of the read-out data for each predetermined block using working registers, thereby obtaining an interpolated envelope curve. The CPU then divides the sampled waveform data by the interpolated envelope curve to obtain a normalized sound waveform without any envelope in an original sound signal.

Abstract: An electronic musical instrument includes a memory in which digital samples of an aperiodic waveform are stored. Digital samples stored in a first portion of the memory represent a rapidly rising portion of the waveform and those stored in a second portion of the memory represent a rapidly declining portion of the waveform whose amplitude and spectral energy distributions are equalized. The first memory portion is addressed in forward scan and subsequently the second memory portion is addressed recyclically in forward and rearward scans to generate an output waveform having a first part corresponding to the rising waveform section and a second part corresponding to a series of the recyclically addressed versions of the equalized waveform section. After delivery of the first part of the output waveform, a monotonically declining envelope is impressed upon the amplitudes and the spectral energy distributions of the second part.

Abstract: A keyboard operated musical instrument is disclosed in which the musical tones are generated by reading out a sequence of data values stored in memories. A first memory stores the musical waveshape for the attack and decay phases of the musical tone. One of a plurality of memories, each storing a period of a different waveshape, are selected by means of a touch response signal generated by the manner with which a keyswitch is actuated. At the end of the decay phase the waveshape selected by the touch response signal is read out sequentially and repetitively and is substituted for the data stored in the first memory. A means is provided to minimize the transient created in the transition between data read out of the first memory and the selected second memory. Provision is also made for a touch response loudness effect.

Abstract: In a musical instrument in which a plurality of data words corresponding to the amplitudes of a corresponding number of evenly spaced points defining a cycle of an audible musical waveform are transformed at an average rate corresponding to the fundamental frequency of the tone being generated, a frequency generator is provided using a single clock source for selectively producing the entire frequency range of selected musical notes. A non-integer frequency generator is implemented which periodically adds a frequency number, corresponding to an actuated keyswitch, to itself in an accumulator. The integer portion of the accumulator content is used to address out waveshape values stored in a memory. The decimal portion of the accumulator content is used to select the time at which either the current read out waveshape data value or a previously read out waveshape data value is converted into an analog signal by means of a digital-to-analog converter.

Abstract: An electronic musical instrument which has a keyboard and creates a musical tone by striking keys of the keyboard, is provided with an operation control circuit and an adder both for effecting keyboard switch chattering prevent operation processing and touch response select operation processing, a timing control circuit for controlling the switching between the chattering preventing operation and the touch response detecting operation and the scanning of the keyboard switches to detect their status, a musical tone generator for performing a musical tone generating operation asynchronously with the keyboard switch scanning operation, the keyboard switch chattering preventing operation and the touch response detecting operation, and a transfer circuit for transferring touch response information to the musical tone generator.

Abstract: An electronic musical instrument is provided with a first programmable counter for generating a reference clock signal corresponding to a musical frequency and a plurality of cascade-connected programmable counters which are each triggered by the preceding programmable counter. A multi-level signal which assumes one of a plurality of levels for each period specified by one of the cascade-connected programmable counters is output as a primary waveform. The electronic musical instrument is capable of setting various variations of the primary waveform abundantly containing harmonics and is satisfactory in the tone quality and in the degree of freedom in setting tones, and hence is of high musicality.

Abstract: An electronic musical instrument includes a first memory in which audio samples of lower frequency components of an aperiodic waveform are stored and a second memory in which audio samples of a higher frequency components of the waveform are stored. Digital samples stored in a first portion of the second memory represent a rapidly rising portion of the higher frequency waveform and those stored in a second portion of the memory represent a rapidly declining portion of the higher frequency waveform whose amplitude and spectral energy distribution profiles are preferably equalized. The first memory is addressed throughout in forward scan to generate a first output waveform. The second memory is addressed in an initial forward scan throughout its first and second portions and the direction of scan is reversed at the end of the second portion to recyclically address it in rearward and forward directions to generate a second output waveform, which is combined with the first output waveform.

Abstract: Two waveshapes corresponding to an attack portion and a part of portion succeeding the attack portion of a tone to be produced are stored in a waveshape memory. The waveshape of the attack portion is read out once and thereafter the other waveshape is repetitively read out. In order to connect smoothly the repetitively read out waveshape, an interpolation operation is performed in specified sections at the end and at the beginning of the waveshape. A tone waveshape to be finally formed is constituted by the other waveshape itself in other section than the specified sections and by a new waveshape formed by the interpolation operation in the specified sections in the other waveshape, thereby realizing the smooth connection of the repetitive read out waveshapes.

Abstract: Disclosed is a sound source apparatus which is composed of a plurality of sound component pattern generators each capable of providing a predetermined pattern, and a sound component picking circuit for sweeping the plurality of sound component pattern generators repeatedly in such a rapid sequence that the sound component thus provided and taken may constitute a musical sound at a desired pitch.

Abstract: The realism by which the sound produced by various musical instruments may be electronically simulated is enhanced by the storage of digital data commensurate with scanning values of several periods of a note including the release portion thereof. Command words are also memorized and, in response to the reading of a command word and the state of the input command to the instrument by the player, indicative of whether the note is to be sustained or terminated, the reading of a command word may cause the addressing of the waveform memory to be jumped to the release portion where the read-out will be continued from a scanning value approximately equal to the value read immediately prior to the reading of the command word. The waveform memory may contain a segment of the sustain portion of the note which will be repetitively read so long as the player's input command indicates that the note is to be sustained, the repetition also being in response to memorized command words.

Abstract: A touch response apparatus for an electronic musical instrument wherein a nonlinear analog-to-digital conversion is effected using a single A/D converter in correspondence with a plurality of keys, and wherein touch data are proportional to the period of time and amplitude value of an envelope waveform.

Abstract: A musical tone generation device of waveshape memory type stores, in its memory, waveshape data of an entire part of a rising portion of a musical tone waveshape and waveshape data of only a part of a remainder portion succeeding to the rising portion. The rising portion data is read out once while the remainder portion data is repetitively read out from the memory thereby to reproduce the entire musical tone waveshape. By changing a rate for the repetitive readout operation, a pitch of the musical tone waveshape at the portion succeeding to the rising portion can be changed with passage of time.

Abstract: The sound generator applies an envelope to the note signal. Current to the sound producing element is controlled by a series transistor whose base is driven with a signal having a frequency corresponding to the musical note. The driving voltage applied to the base has a voltage amplitude corresponding to the voltage stored in a capacitor connected through a resistor and discharging through said resistor and the base of the transistor controlling the current to the sound producer.

Abstract: A waveform step counter receives an output clock from a tone clock generator and an envelope status signal from an envelope counter. The waveform step counter generates an address signal for performing predetermined address skip, and a waveform memory is accessed in accordance with this address signal. Since the address signal performs the predetermined address skip, waveform information read out from the waveform memory indicates a timbre different from the original information stored in the memory.

Abstract: Analog output signals of a key touch sensor and tonal effect setting manual operators are respectively time division multiplexed and thereafter are converted to digital signals by using an analog to digital converter on a time shared basis. The key touch sensor capable of detecting a key touch of a depressed key can also be used for obtaining an initial touch detection signal by additionally providing a device for holding a peak value of an output signal of the sensor at the beginning of depression of the key. Control factors of various tonal effects are controlled in response to these converted digital signals. Control factors of an attack pitch control effect are controlled in response to the key touch. In imparting a delay vibrato effect, the attack pitch control is automatically applied before the delay vibrato is initiated. In imparting a vibrato effect, the attack pitch control is automatically applied and the vibrato is applied thereafter if a key has been depressed in a staccato form.

Abstract: An electronic music note generator having a volume control circuit using an RC network to control sound amplitude. Signals are formed by the discharge characteristics of an RC network and the voltage level at charging determines the volume of the sound produced. Reverberation characteristics are provided by a persistence in the signal between notes as a result of the RC discharge characteristic.

Abstract: The differentiation data of a basic waveform is read out from a differentiated value waveform memory at a clock timing corresponding to a musical scale of an operated play key, and it is multiplied by an envelope value of a musical sound waveform to form a musical sound waveform differentiation value. In parallel therewith, the differentiation data is accumulated at the clock timing to form a compensative value. The compensative value is added with the musical sound waveform differentiation value at the point of time at which the envelope value is changed to cancel a DC component which would otherwise develop in a produced musical sound waveform. The musical sound waveform differentiation values thus compensated are accumulated, and are subjected to digital-to-analog conversion, so that the musical sound waveform free from the DC component is produced.

Abstract: Under the presence of an internally generated mandolin clock (MDN) or an externally generated mandolin clock (MDN'), an envelope clock (ENV-CLK) obtained in accordance with a lower bit output having three-bit signals (including the LSB) is supplied through AND gates to an envelope counter. The counter is set in an up count state in response to an output from a flip-flop set in response to a key on pulse. When a carry signal is generated by the envelope counter, the counter is set in a down count state. In the latter case, the fourth-bit signal from the LSB of the output from the binary counter is gated through the AND gate. The envelope counter counts down the envelope clock (ENV-CLK) having a period twice that of the envelope clock (ENV-CLK) in the up count state.

Abstract: A touch response apparatus for an electronic musical instrument wherein a nonlinear analog-to-digital conversion is effected using a single A/D converter in correspondence with a plurality of keys, and wherein touch data are proportional to the period of time and amplitude value of an envelope waveform.

Abstract: An electronic signal X can have its envelope shaped by an electronic control signal Y. For music purposes, signal X can be an electronically generated note and control signal Y can be derived from the musician's voice. The signal shape controller exponentially modifies the control signal Y and multiplies signal X by the exponentially modified control signal Y to form an output signal having an envelope shape XY.sup.n. The exponent n has at least one value less than 1 applied to larger values of control signal Y and at least one value greater than 1 applied to lower levels of control signal Y. The controller also preferably includes an attack enhancement circuit to emphasize an attack portion of a control signal and a variable time integrator circuit that integrates the control signal over a short time interval when it is changing rapidly and over a longer time interval when it is changing slowly.

Abstract: A wave reading apparatus includes a wave generator for generating a plurality of wave signals, a read-out frequency generator for generating a plurality of read-out frequencies, a controller for controlling calculation, writing and reading of wave samples, a writing device, a plurality of buffer memories and a plurality of read-out devices. The controller informs requests of wave samples calculation to the wave generator in accordance with the read-out frequencies. The calculated wave samples are written through the writing device to the buffer memories and read out by the read-out device in accordance with the read-out frequencies, at least one of which is different in frequency from the remainder.

Abstract: A musical tone forming system includes a waveform storage device for storing data representative of an attack portion of a musical tone waveform and data representative of part of the other portion of the musical tone waveform following the attack portion. An address device addresses the waveform storage device to first read the data representative of the attack portion and to then repeatedly read the part of the waveform from the waveform storage device to thereby form a musical tone signal. There is provided a device for generating data varying with time. The address device feeds an address data to the waveform storage device in accordance with the time-varying data to randomly designate as a starting address one of those addresses of the waveform storage device storing the data of the part of the waveform, thereby realizing a noisy nature characteristic of a percussion instrument.

Abstract: A channel processor capable of assigning a key code provided by a key coder and representing making (or breaking) of a key switch to one of a plurality of channels for storage therein and subsequently detecting breaking (or making) of the same key switch on the side of the channel processor. The assignment of the key code is implemented by holding the key code provided by the key coder during a predetermined period of time, detecting whether conditions for the key code assignment have been satisfied or not in a former half of the holding period and, if such conditions have been satisfied, causing the key code to be stored in an empty channel of a main memory device in a latter half of the holding period. Detection of breaking of the made key switch (or vice versa) is made by once clearing a memory storing the assigned channels by means of a start code generated by the key coder and subsequently finding that a channel among the cleared channels is not stored in the memory again, i.e.

Abstract: In a electronic musical instrument wherein a portamento is played by supplying the pitch voltage corresponding to a subsequently depressed key to a capacitor holding the pitch voltage corresponding to a previously depressed key, the charge and discharge currents of the capacitor corresponding to the difference between the two pitch voltages are controlled to vary exponentially thus changing exponentially the capacitor terminal voltage. The terminal voltage of the capacitor is applied to drive a voltage controlled oscillator to vary its oscillation frequency. To vary exponentially the terminal voltage of the capacitor, a mutual conductance converter is connected between the capacitor and a keyboard section and the output current from the mutual conductance converter is controlled by a control signal corresponding to the terminal voltage of the capacitor.

Abstract: An envelope generator is provided with a count circuit the count value of which is varied through addition or subtraction or combination thereof, and a conversion circuit which operates to convert the count value into amplitude data, so as to generate an envelope having a shape corresponding to variations with time of the count value. According to one aspect of the invention, the count circuit is a circuit which carries out computation for exponentially varying the count value through polygonal line approximation, so as to form an envelope of exponential characteristic. According to another aspect of the invention, the conversion circuit is a memory which has stored amplitude data corresponding to count values in advance so as to convert count values in the last linear region of the envelope obtained by the polygonal line approximation into amplitude data in exponential relation and to convert count values contained in the remaining polygonal line regions into amplitude data in linear relation.