1. Field of the Invention
The present invention relates to a communication system for enabling communication through a human body or the like and a receiver used in the communication system.
2. Description of the Related Art
A communication device which communicates through tissue of a living body such as a human body is known. For example, a technique is known in which data can be exchanged by a user merely holding a hand over a receiver while a portable electronic device such as a portable phone on which a transmitter is mounted is placed in a pocket of clothing of the user, or while the portable electronic device is hung around the neck.
For example, as shown in FIGS. 22A and 22B, a transmitter 100 comprises an encoder 10, a transmission amplifier 12, an environment-side electrode 14, and a living body-side electrode 16, and a receiver 102 comprises a decoder 18, a reception amplifier 20, an environment-side electrode 22, and a living body-side electrode 24. The transmitter 100 is mounted on a portable electronic device or the like which is carried by the user. The receiver 102 is placed on a ticket barrier of a station, a vending machine, a shop, etc.
FIG. 23 shows a relationship between the transmitter 100, the receiver 102, and the human body or the like during the communication. FIG. 24 shows an equivalent circuit of the relationship.
The transmitter 100 capacitively couples with the receiver 102 through tissue of a living body such as human body or the like (hereinafter simply referred to as “human body or the like”). The environment-side electrode 14 of the transmitter 100 forms a capacitive coupling A with an external environmental ground potential, a capacitive coupling B with the human body or the like, and a capacitive coupling D with an external environment. Similarly, the environment-side electrode 22 of the receiver 102 forms a capacitive coupling H with the external environmental ground potential and a capacitive coupling G with the external environment. As described, the environment-side electrodes 14 and 22 are electrodes which form capacitive couplings with the external environment during the communication.
The living body-side electrode 16 of the transmitter 100 forms a capacitive coupling C with the human body or the like. The living body-side electrode 24 of the receiver 102 forms a capacitive coupling F with the human body or the like. Moreover, a capacitive coupling E is formed between the human body or the like and the external environment. As described, the living body-side electrodes 16 and 24 are electrodes which form capacitive couplings with the human body or the like during the communication.
The transmission amplifier 12 of the transmitter 100 receives information encoded by the encoder 10 and outputs as a potential difference between the environment-side electrode 14 and the living body-side electrode 16. When the transmitter 100 and the receiver 102 are electrically coupled through the human body or the like as described above, the potential difference between the environment-side electrode 14 and the living body-side electrode 16 of the transmitter 100 causes a change in a potential difference between the environment-side electrode 22 and the living-body side electrode 24 of the receiver 102. The reception amplifier 20 of the receiver 102 amplifies the potential difference between the environment-side electrode 22 and the living body-side electrode 24 and outputs the amplified signal. The output of the reception amplifier 20 is decoded by the decoder 18. In this manner, the communication is established.
For example, communication is enabled by a user who carries the transmitter 100 holding a hand over (or contacting with a hand) the living body-side electrode 24 of the receiver 102 placed on a ticket barrier of a station.
In the above-described communication device, as shown in FIG. 24, the output of the reception amplifier 20 is determined based on the relationships between the capacitive coupling F and the other capacitive couplings A-E and G-H. Therefore, for the communication, it is desirable that the capacitive couplings A-E and G-H are stable.
For example, if the capacitive coupling D between the environment-side electrode 14 of the transmitter 100 and the external environment fluctuates, the communication becomes unstable. However, the capacitive coupling D between the environment-side electrode 14 of the transmitter 100 and the external environment would change depending on how the user carries the transmitter 100 such as, for example, holding in the hand, placed in a pocket of clothing, placed in a bag, etc., which may result in unstable communication.
In addition, as shown in FIG. 25, a noise source such as a personal computer and a digital television may exist between the environment-side electrode 14 and the grounding point of the external environment, or between the environment-side electrode 22 and the grounding point of the external environment. FIG. 26 shows an equivalent circuit of the communication system when a noise source exists. When a noise source exists, the capacitive coupling D formed between the environment-side electrode 14 and the grounding point of the external environment and the capacitive coupling G formed between the environment-side electrode 22 and the grounding point of the external environment are affected, and a problem may be caused in which the communication between the transmitter 100 and the receiver 102 cannot be realized.