Telephone.

BACKGROUND OF THE INVENTION

0001The present invention relates to a telephone and, more particularly, to a transmitting circuit of the telephone.

0002In a conventional telephone, a transmitter receives an input sound to convert it into an input electrical signal, and an amplifier amplifies the input electrical signal. The amplified signal is transmitted to a telephone line. A recent telephone uses an extremely sensitive transmitter, such as an electret microphone type transmitter and a ceramic microphone type transmitter. The transmitter can receive not only speaker's speech but also a small noise. Thus the input electrical signal according to a small noise is also amplified and transmitted to a telephone line. This results in the problem that the signal according to the small noise disturbs speaker's speech and is annoying to the listener.

SUMMARY OF THE INVENTION

0003An object of the present invention is to provide a telephone which can transmit only the signal according to speaker's speech and have a high speech quality.

0004To achieve this object, the present invention provides a telephone comprising: means for converting an input sound into an input electrical signal; means for detecting a voltage level of said input signal; means for comparing said voltage level of the input signal with a predetermined reference voltage level to output a result of the comparison; means for transforming said input signal at a first transformation factor when said result of the comparison represents that said input signal level is lower than said predetermined reference voltage level, and at a second transformation factor which is larger than said first transformation factor when said result of the comparison represents that said input signal level is not lower than said predetermined reference voltage level; and means for transmitting a signal transformed by said transforming means to a telephone line.

BRIEF DESCRIPTION OF THE DRAWINGS

0005In the accompanying drawings: <ul id="ul0001" list-style="none"><li>FIG. 1 is a block diagram showing a transmitting circuit of a telephone according to a first embodiment of the present invention;</li><li>FIG. 2 is a graph showing a characteristic of the transmitting circuit;</li><li>FIG. 3 is a graph showing a characteristic of a conventional transmitting circuit;</li><li>FIG. 4 is a block diagram showing a transmitting circuit of a telephone according to a second embodiment of the present invention;</li><li>FIG. 5 is a block diagram showing a transmitting circuit of a telephone according to a third embodiment of the present invention;</li><li>FIG. 6 is a block diagram showing a transmitting circuit of a telephone according to a fourth embodiment of the present invention;</li><li>FIG. 7 is a circuit diagram showing a main part of a transmitting circuit of a telephone according to a fifth embodiment of the present invention; and</li><li>FIG. 8 is a graph showing a characteristic of the transmitting circuit.</li></ul>

DETAILED DESCRIPTION OF THE INVENTION

0006FIG. 1 shows a transmitting circuit of a telephone according to a first embodiment of the present invention. A transmitter 1 converts an input sound to an electrical signal. When the transmitter 1 receives an input sound, such as speaker's speech and a noise, the transmitter 1 outputs the input signal according to the input sound. The input signal is inputted to a detector 2 and an amplifier 3. The detector 2 detects a voltage level of the input signal to output the voltage level signal to a comparator 4. The comparator 4 compares the voltage level with a predetermined reference voltage level to output a result of the comparison to an amplifier 3. It is desirable that the predetermined reference voltage level is lower than the minimum level of speaker's speech and higher than the level of a noise which the transmitter 1 may receive.

0007The amplifier 3 amplifies the input signal at an amplification factor which varies depending on the result of the comparison. Specifically, when the voltage level of the input signal is lower than the reference voltage level, the amplifier 3 amplifies the input signal at a low amplification factor. When the voltage level of the input signal is not lower than the reference voltage level, the amplifier 3 amplifies the input signal at a high amplification factor. Therefore, the amplifier 3 has a relationship between an input signal level and an output signal level as shown in FIG. 2. For comparison, FIG. 3 shows a relationship between an input signal level and an output signal level of an amplifier used in a conventional transmitting circuit. In FIGS. 2 and 3, these signals are shown on a log scale. A line driver 5 transmits the output signal of the amplifier 3 to telephone lines 6a, 6b.

0008As stated above, the present embodiment makes it possible to improve a speech quality of the telephone. This is because a small input sound like a noise is amplified at the low amplification factor not to be transmitted to the telephone line and only the large input sound like speaker's speech is amplified at the high amplification factor to be transmitted to the telephone line.

0009FIG. 5 shows a transmitting circuit of a telephone according to a second embodiment of the present invention. The transmitting circuit uses an attenuator 7 instead of the amplifier 3 and further comprises an amplifier 8 connected to the transmitter 1. The attenuator 7 attenuates an output signal of the amplifier 8. The attenuator 7 attenuates the output signal at an attenuation factor which varies depending on the result of the comparison of the comparator 4. Specifically, when the voltage level of the output signal of the amplifier 8 is lower than the reference voltage level, the attenuator 7 attenuates the output signal at a high attenuation factor. When the voltage level of the output signal is not lower than the reference voltage level, the attenuator 7 attenuates the output signal at a lower attenuation factor or does not attenuate the output signal. Therefore, the attenuator 7 has a relationship between an input signal level and an output signal level as shown in FIG. 2.

0010As stated above, the present embodiment makes it possible to improve a speech quality of the telephone. This is because the small input sound like a noise is attenuated at the high attenuation factor not to be transmitted to the telephone line and only the large input sound like speaker's speech is attenuated at the low attenuation factor to be transmitted to the telephone line.

0011FIG. 5 shows a transmitting circuit of a telephone according to a third embodiment of the present invention. A transmitter 11 converts an input sound to an input electrical signal. The converted input signal is amplified by an amplifier 12 and outputted to a detector 13 and a switch 14. The detector 13 detects a voltage level of the input signal to generate a direct- current voltage signal which is proportional to an amplitude of the amplified input signal. The direct- current voltage signal is inputted to one terminal of a comparator 17. The other terminal of the comparator 17 is connected to a reference voltage supply 18. The comparator 17 compares the output signal of the detector 13 with the reference voltage of the supply 18 to output the result of the comparison to the switch 14.

0012When the result of the comparison represents that the output signal of the detector 13 is larger than the reference voltage, an input terminal of the switch 14 is connected to an output terminal a. Thus the output signal of the amplifier 12 is amplified by a high gain amplifier 15 having a high amplification factor. The amplified signal is transmitted to telephone lines 20a and 20b by a line driver 19.

0013When the result of the comparison represents that the output signal of the detector 13 is not larger than the reference voltage, the input terminal of the switch 14 is connected to an output terminal b. Thus the output signal of the amplifier 12 is amplified by a low gain amplifier 16 having a low amplification factor. The amplified signal is transmitted to telephone lines 20a and 20b by a line driver 19.

0014As stated above, the transmitting circuit of the telephone according to the present embodiment has the characteristic as shown in FIG. 2 by switching selectively the high gain amplifier 15 or the low gain amplifier 16 on the basis of the result of the comparison.

0015FIG. 6 shows a transmitting circuit of a telephone according to a fourth embodiment of the present invention. The transmitting circuit uses an attenuator 25 instead of the high gain amplifier 15 and an attenuator 26 instead of the low gain amplifier 16. The attenuator 25 has a low attenuation factor, and the attenuator 26 has a high attenuation factor. The transmitting circuit of the telephone according to the present embodiment has the characteristic as shown in FIG. 2 by switching selectively the attenuator 25 with the low attenuation factor or the attenuator 26 with the high attenuation factor on the basis of the result of the comparison.

0016FIG. 7 shows a main part of a transmitting circuit of a telephone according to a fifth embodiment of the present invention. The transmitting circuit uses a high reference voltage supply 18a, a low reference voltage supply 18b, and a switch 18c instead of the reference voltage supply 18. The high reference voltage supply 18a produces a high reference voltage V_refl and the low reference voltage supply 18b produces a low reference voltage V_ref2' The switch 18c has one input terminal connected to the supply 18a, the other input terminal connected to the supply 18b, and an output terminal connected to the comparator 17. The switch 18c selectively connects the output terminal to the one input terminal or the other input terminal in accordance with the result of the comparison of the comparator 17.

0017The transmitting circuit according to the present embodiment has a "hysteresis characteristic" by which the reference voltage level varies in accordance with the direction of the change of the output signal of the detector 13, as shown in FIG. 8. The "hysteresis characteristic will be discussed more specifically as below. When the output signal of the detector 13 is zero or very small, the switch 18c connects the output terminal to the one input terminal connected to the supply 18a. Thus the high reference voltage V _refl is supplied to the comparator 17. When the output signal increases to exceed the reference voltage V_refl, the switch 18c connects the output terminal to the other input terminal connected to the supply 18b. Thus the low reference voltage V_ref2is supplied to the comparator 17. Now the comparator 17 compares the output signal with the low reference voltage V_ref2. Therefore, even if the output signal decreases to be lower than the high reference voltage V_refl, the result of the comparison does not change. When the output signal further decreases to be lower than the low reference voltage V_ref2, the switch 18c connects the output terminal to the one input terminal connected to the supply 18b.

0018As stated above, the present embodiment makes it possible to improve a speech quality of the telephone. When the input sound is very small like a small noise, the reference voltage level is high. Thus, a small noise is not transmitted to the telephone line. On the contrary, when a large input sound like speaker's speech is supplied, the reference voltage level becomes low.

0019Thus, even if speaker's voice becomes small, the speaker's speech is still amplified at a high amplification factor to be transmitted to the telephone line.

0020The present invention may be practiced or embodied in still other ways without departing from the spirit or essential character thereof. For instance, while in FIGs. 2 and 8, the relationship between the input signal and the output signal of the amplifier is represented by a direct line, the relationship may alternately be represented by a curved line. Also the transmitter can be replaced by another type transmitter, such as an electret microphone type transmitter, a ceramic microphone type transmitter, and a carbon microphone type transmitter.

0021As stated above, the present invention makes it possible to transmit only the signal according to speaker's speech and have a high speech quality.