S&F Ref: 817465 AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT Name and Address NEC Infrontia Corporation, of 2-6-1, Kitamikata, Takatsu of Applicant: ku, Kawasaki-shi, Kanagawa, Japan Actual Inventor(s): Junya Katoh Address for Service: Spruson & Ferguson St Martins Tower Level 35 31 Market Street Sydney NSW 2000 (CCN 3710000177) Invention Title: Telephone transmission distortion prevention method and apparatus The following statement is a full description of this invention, including the best method of performing it known to me/us: 5845c(8791 Il) TITLE OF THE INVENTION TELEPHONE TRANSMISSION DISTORTION PREVENTION METHOD AND APPARATUS INCORPORATION BY REFERENCE This application is based upon and claims the benefit of 5 priority from Japanese patent application No. 2006-204988, filed on July 27, 2006, the disclosure of which is incorporated herein in its entirety by reference. TECHNICAL FIELD This invention relates to a telephone, more specifically to a 10 transmission distortion prevention method and apparatus in a telephone for suppressing transmission distortion when an acoustic input to a telephone transmitter is excessively large and enabling one to communicate in excellent quality. BACKGROUND ART 15 Analog telephone sets are widely used as voice or oral communication means between persons at distant locations. Such telephone set is equipped with a hand set including a receiver for receiving an audio signal from a calling person to be converted into an acoustic output and a transmitter for converting his/her own voice signal into an 20 electrical signal to be transmitted to the other person by way of a telephone line. It is general that a telephone set is also provided in integrated with or separately from the hand set with various keys for dialing telephone numbers whom he/she would like to call, a display unit for displaying information or the like relating to the 25 person to whom he/she is talking, etc. A transmission signal input that is converted into an electrical signal by the transmitter is normally inputted to a speech IC (or a communication IC) and is sent to a line (i.e., telephone network) as a transmission signal output after being normalized to a 1 predetermined magnitude (amplitude) by the speech IC. The transmission signal input to be inputted to the speech IC from the transmitter varies in magnitude depending upon differences of speakers or talkers (i.e., large or small voices), 5 the distance from the speaker's mouth to the transmitter, speaker's spiritual condition or the like. In particular, when an excited (or angry) speaker talks at a louder voice while placing his/her mouth closer to the transmitter, the transmission signal input to and the output from the speech IC tend to become excessively large. This 10 causes a broken side tone to the telephone user and a noise offensive to his/her ear. This results in difficulties to be correctly understood by the listener because of significant waveform distortion. It is preferable for the telephone users that the voice of 15 the person at the other side of the line is clearly heard and that his/her own voice is transmitted to the listening person at an appropriate volume without causing waveform distortion so that his/her voice will be heard clearly. For this end, various improvements and proposals have been made in conventional telephone 20 sets. For example, disclosed are a transmitter and a telephone set using the same that is provided with an audio input compensation function, an audio signal level detection circuit as detection means to be used for an automatic gain level control for an AGC (Automatic Gain Control) amplif ier. When the detection circuit detects an audio 25 signal of a power level in excess of the upper limit of the standard audio input range that is set in advance by the detection circuit, the gain controller changes the gain of the amplifier in counter-proportion to the power level of the audio signal on the line, thereby maintaining the power level of the signal amplified 2 by the amplifier substantially constant (for example, see Japanese patent no. 3727431, particularly pages 12-13, Fig. 3). Also disclosed is a transmitter for a telephone set in which an excessive level detection circuit is provided for detecting an 5 excessive level input to cause audio distortion in the telephone line. A predetermined level output circuit receives as its input an output from an audio band filter circuit for suppressing the excessive level input to a preset level not to cause any audio distortion (for example, see Japanese patent publication no. 10 62-31239, particularly pages 2-3, Fig. 2). Moreover, in a telephone set equipped with a transmitter for converting an audio (acoustic) signal into an electrical signal, disclosed is a telephone circuit comprising an input terminal for an external signal provided with the transmitter, an ALC circuit 15 for making a linear conversion of the external signal at a predetermined ratio when the level of the external signal that is supplied to the terminal exceeds a transmission level set to the telephone line so that distortion free electrical signal is outputted by adjusting the electrical signal level within the preset output 20 range, and an output circuit for outputting the electrical signal from the ALC circuit to the telephone line, wherein the output circuit is connected to Ll-L2 terminals that are connected to an office line by way of a rectifier bridge. When the signal supplied to the ALC circuit exceeds the output level set to the ALC circuit, a linear 25 level conversion is performed on the signal at a constant ratio so as to adjust the level within a preset output range, thereby outputting a distortion free signal (for example, see Japanese patent publication no. 63-272248, particularly pages 1-2, Fig. 1). However, prior art as described hereinabove have the following 3 problems. That is, since they detect the output from a microphone in the transmitter for controlling the gain of the amplifier or the like that is disposed at the subsequent stage, they are relatively complicated in construction and thus expensive. 5 SUMMARY The telephone transmission distortion prevention method and apparatus according to embodiments of the present invention employing the abovementioned unique construction exhibit the following practical advantages. That is, the use of the 10 transmission distortion prevention circuit that is connected in parallel with the speech IC enables to simply and surely prevent transmission waveform distortion in case of an excessive acoustic input. The detection of the excessive acoustic input to the transmitter based upon the transmission signal output from the 15 speech IC that outputs the transmission signal to the telephone line ensures to prevent waveform distortion of the transmission signal to the person to whom he/she is talking. Moreover, parallel connection of impedance such as a capacitor or the like at the input side of the speech IC reduces the input impedance of the speech IC, 20 thereby simply and surely attenuating the transmission signal. Furthermore, it is possible to switch the input impedances at the input side of the speech IC to a plurality of different values by using a plurality of necessary impedances at the input side of the speech IC for selectively connecting such impedances in parallel 25 at dif ferent levels of the transmission signal output from the speech IC. Other exemplary embodiments are expressed as follows: (1) In a telephone comprising a receiver for converting an audio signal that is transmitted from a person from whom he/she is 3067620-1 4 communicating into an acoustic signal and a transmitter for transmitting his/her voice onto a telephone line by way of a speech IC after converting it into an electrical signal, a transmission distortion prevention apparatus in a telephone further comprising 5 a transmission distortion prevention circuit that is connected in parallel between input and output terminals of the speech IC for suppressing the output level of the speech IC by detecting an excessive acoustic input to the transmitter based upon the transmission signal output from the speech IC, 10 wherein the transmission distortion prevention circuit comprises: a signal amplifier circuit for amplifying the transmission signal output from the speech IC at a predetermined gain, the signal amplifier circuit including: 15 first and second resistors connected in series between the output terminal of the speech IC and a ground; a first capacitor having one terminal connected to a junction of the first and second resistors; a first transistor having a base connected to the other terminal 20 of the first capacitor, and an emitter connected to the ground; a load resistor connected between a collector of the first transistor and a positive voltage source; and a f eedback resistor connected between the base and the collector of the first transistor; 25 a rectifier circuit for rectifying an output signal from the signal amplifier circuit, the rectifier circuit including: a second capacitor having one terminal connected to the collector of the first transistor; and a diode connected between the other terminal of the second 3067620-1 5 capacitor and the ground; a level detection circuit for detecting the output level of a rectifier circuit, the level detection circuit including: a second transistor having a base connected to a junction of 5 the second capacitor and the diode, and an emitter connected to the ground; and a third capacitor and a third resistor which are connected in parallel between a collector of the second transistor and the ground; and 10 a level-down circuit that operates by an output signal from the level detection circuit, the level-down circuit including: a fourth capacitor and a fourth resistor having their one terminals connected to the collector of the second transistor; a third transistor having a base connected to the other terminal 15 of the fourth resistor; a fifth capacitor and a fifth resistor having their one terminals connected to the base of the third transistor, and their other terminals connected to an emitter of the third transistor and the other terminal of the fourth capacitor, respectively; and 20 a sixth capacitor connected to a collector of the third transistor, a series circuit including an emitter-collector junction of the third transistor and the sixth capacitor being connected in parallel at an input side of the speech IC. (2) A transmission distortion prevention apparatus in a 25 telephone of the above (1) , wherein the level down by the transmission distortion prevention circuit is performed by reducing the input impedance of the speech IC. (3) A transmission distortion prevention apparatus in a telephone of the above (2), wherein the reduction of the input 3067620-1 6 impedance of the speech IC by the transmission distortion prevention circuit is implemented using the sixth capacitor connected at the input side of the speech IC. (4) A transmission distortion prevention apparatus in a 5 telephone of the above (3), wherein the sixth capacitor is connected in parallel at the input side of the speech IC by the third transistor serving as an electronic switch that is controlled by the output of the level detection circuit. 10 BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings, Fig. 1 is a block diagram to show the construction of a preferred embodiment of the telephone transmission distortion prevention apparatus according to the present invention; 15 Fig. 2 is an electrical circuit diagram to show exemplified circuit portions that constitute transmission distortion prevention circuit that is shown in Fig. 1; Fig. 3 is a flowchart for describing a preferred embodiment of the telephone transmission distortion prevention method 20 according to the present invention; and 3067620-1 7 Fig. 4 is a block diagram to show an example of a speech IC that is used in the present invention. EXEMPLARY EMBODIMENT Now, the construction and operation of exemplary embodiment 5 of the telephone transmission distortion prevention method and apparatus according to the present invention will be described in detail with reference to the accompanying drawings. Firstly, reference is made to Fig. 1 that is a simplif ied block diagram to show the construction of an exemplary embodiment of the 10 telephone transmission distortion prevention apparatus of the present invention. The telephone transmission distortion prevention apparatus 10 comprises a transmitter 20, a speech IC (or a communication IC) 30 connected at the output side of the transmitter 20 and a transmission distortion prevention circuit 40 connected 15 in parallel between input and output sides of the speech IC 30. It is to be noted herein that the transmitter 20 is, for example, a commercially available microphone provided in a handset of a telephone (not shown) . The speech IC 30 is constructed to receive a transmission signal input that is outputted from the transmitter 20 20 and send a transmission voice signal output onto the line (telephone line) 35. The speech IC 30 is a semiconductor device or an integrated circuit (semiconductor integrated circuit) and is preferably, for example, the TEAll10AT (low voltage versatile telephone transmission circuit with dialer interface) that is 25 commercially available from Phillips Semiconductor Corporation. For the detail of such IC, reference should be made to the data sheet for the IC model TEAll10A dated February 15, 2000 and published from the company. A block diagram of such speech IC 30 is shown in Fig. 4. The 8 speech IC 30 is a DIP (Dual Inline Package) type IC having 14 pins. As best shown in Fig. 4, pin #1 is a positive line terminal LN, pin #2 is a slope (DC resistance) adjustment SLPE, pin #3 is decoupling fora linevoltage regulatorREG, pin#5 is adual tone multi - frequency 5 input DTMF, pin #6 is a mute (silent) input, pin #7 is an input to a receiving amplifier IR, pin #8 is an automatic gain control AGC, pin #9 is an inverted input for a microphone MIC-, pin #10 is a non-inverted input for microphone MIC+, pin #11 is a negative line terminal VEE, pin #12 is an output of an earpiece amplifier QR, pin 10 #13 is a gain adjustment for the earpiece amplifier GAR and pin #14 is a power supply voltage terminal VCC for internal circuitry. And pin # 4 is not in use. The transmission distortion prevention circuit 40 isacircuit designed to prevent distortion in the transmission signal output 15 waveform by reducing or suppressing the transmission signal output level from the speech IC in case when the voice input from the user (or talker) of the transmitter 20 is large and the transmission signal output that is outputted to the telephone line 35 from the speech IC 30 becomes such an excessive level to cause waveform distortion. 20 The transmission distortion prevention circuit 40 comprises a signal amplifier circuit 41, a rectifier circuit 42, a level detection circuit 43 and a level-down circuit 44 sequentially connected from the output side to the input side of the speech IC 30. Now, exemplified electrical circuits of the signal amplifier 25 circuit 41, the rectifier circuit 42, the level detection circuit 43 and the level-down circuit 44 that constitute the transmission distortion prevention circuit 40 in Fig. 1 are shown in Fig. 2. The circuits 41-44 constituting the transmission distortion prevention circuit 40 as shown in Fig. 2 will be described in detail hereunder. 9 The signal amplifier circuit 41 comprises resistors 411, 412 connected in series between the output terminal of the speech IC 30 and ground, a capacitor 413 having one terminal connected to the junction of the resistors 411, 412 and the other terminal connected 5 to the base of a transistor (NPN type transistor) 415 having its emitter connected to ground, a load resistor 416 connected between the collector of the transistor 415 and a positive voltage source Vdd and a feedback resistor 414 connected between the base and the collector of the transistor 415. 10 The rectifier circuit 42 comprises a capacitor 421 having one terminal connected to the collector of the transistor 415 in the signal amplifier circuit 41 and a diode 422 connected between the other terminal of the capacitor 421 and ground. The level detection circuit 43 comprises a transistor (NPN 15 type transistor) 431 having the base connected to the junction of the capacitor 421 and the diode 422 in the rectifier circuit 42 and a capacitor 432 and a resistor 433 connected in parallel between the collector of the transistor 431 and ground. The level-down circuit 44 comprises a capacitor 441 and a 20 resistor 442 having their one terminals connected to the collector of the transistor 431 in the level-down circuit 43, an electronic switch or a transistor (PNP type transistor) 445 having the base connected to the other terminal of the resistor 442, a capacitor 443 and a resistor 444 having their one terminals connected to the 25 collector of the transistor 445 and their other terminals connected to the emitter of the transistor 445 and a capacitor 446 connected to the collector of the transistor 445. And a series circuit of the emitter-collector junction of the transistor 445 and the capacitor 446 is connected in parallel at the input side of the speech IC 30. 10 Now, the operation of the transmission distortion prevention circuit 40 will be described with reference to the circuit diagram in Fig. 2. The signal amplifier circuit 41 detects the high impedance transmission signal output from the speech IC 30 and amplifies it 5 at a preset gain. The gain of the signal amplifier circuit 41 is a constant value that is essentially determined by the resistance ratio of the input resistor 411 and the feedback resistor 414. The capacitor 413 is a DC blocking capacitor and is used so that only the transmission signal (AC signal) outputted from the speech IC 10 30 is amplified. The rectifier circuit 42 performs a half-wave rectification of the transmission signal output that is amplified by the signal amplifier circuit 41 at a constant gain and is outputted from the output end of the capacitor 421 (i.e., the junction of the capacitor 15 421 and the diode 422) . The capacitor 421 is charged to the amplified and rectified transmission signal output. The level detection circuit 43 detects whether or not the output from the rectifier circuit 42 exceeds the preset (or threshold) value, i.e., whether the transmission signal output from the speech 20 IC 30 is excessive or not. When the level detection circuit 43 detects excessive acoustic input (i.e., when the output voltage of the rectifier circuit 42 exceeds, for example, about 0.5 volt that is required for forward biasing the base and emitter junction of the transistor 431), the transistor 431 in the level detection circuit 25 43 is turned on (or becomes conductive) . As a result, the level detection circuit 43 detects whether or not the output voltage from the rectifier circuit 42 exceeds such particular level. A result of level detection by the level detection circuit 43 is inputted to the level-down circuit 44. If the transistor 431 11 in the level detection circuit 43 is turned on, the collector current flows through the resistors 442, 444, thereby decreasing the base voltage of the transistor 445 in the level-down circuit 44 lower than the emitter voltage by such a voltage to turn on the transistor 5 445 (i. e. , making the transistor active) . As a result, the transistor 445 in the level-down circuit 44 acts as an electronic switch. As described hereinabove, the switching transistor 445 and the capacitor 446 in the level-down circuit 44 are connected in parallel at the input side of the speech IC 30. Accordingly, if the 10 transistor 445 in the level-down circuit 44 is turned on, the capacitor 446 is connected in parallel at the input side of the speech IC 30, thereby decreasing the transmission input impedance and attenuating the transmission level. As a result, even if the transmission voice input may be excessively large, the transmission 15 signal will be sent out onto the telephone line 35 without causing distortion in the transmission waveform. Now, the telephone transmission distortion prevention method of the present invention will be described with reference to the flowchart in Fig. 3. Firstly, the transmission signal level is 20 detected at the output side of the speech IC 30 in Fig. 1 (step Sl) Then, judgment is made if there is an excessive acoustic input to the speech IC 30 (step S2). If there is no excessive acoustic input (i.e., step S2 is N), the transmission distortion prevention operation will be terminated (step S5). 25 On the other hand, in case of an excessive acoustic input (i.e., step S2 is Y), the input impedance of the speech IC 30 or the transmission input impedance is reduced (step S3) . The transmission output level that is outputted from the speech IC 30 is, then, suppressed (step S4) before terminating the transmission voice 12 distortion prevention operation (step S5). The exemplary embodiments of the telephone transmission distortion prevention method and apparatus have been described in both construction and operation. However, it is to be noted that 5 such embodiments are nothing but examples of the present invention and that a person having an ordinary skill in the art can easily make various changes and modifications without departing from the scope and spirit of the present invention. For example, only one capacitor is connected to the input side 10 of the speech IC by way of a switch in the above embodiment, it is possible to provide a plurality of series connections of capacitors and switches so that they are selectively connected in parallel by closing the switches at different levels of the transmission signal output, thereby switching the input impedance to a plurality of 15 different values. It is also possible to replace the capacitor with any other impedance element such a resistor or the like. Although all steps of implementing the steps of the transmission distortion prevention method are made by hardware, it is possible to implement a part of the steps by software. 20 13