The present invention relates to a human body communication system and a communication device that communicate data using a human body as a transmission path, and particularly to a human body communication system and a communication device that communicate data via an electric field formed by the intervention of a human body.
More particularly, the present invention relates to a human body communication system and a communication device that transmit a relatively large amount of data such as audio via a human body, and particularly to a human body communication system and a communication device that transmit data via a human body with secrecy, with low power consumption, and without interfering with another communication system in a neighborhood.
In related art, listening to music with headphones connected to a portable type music reproducing device typified by a “WALKMAN (Registered Trademark of Sony Corporation)” is widely performed. The reproducing device proper can be put in a bag or a pocket, or worn on a body by a belt. However, when headphones are used, a headphone cable often becomes a hindrance and restricts the movement of a user when the cable winds around the body of the user.
As one method of achieving cable-less communication between the portable type music reproducing device and the headphones, the use of infrared communications such as IrDA and the like is considered. However, considering outdoor use, satisfactory communication quality may not be obtained due to effects of natural light. In addition, since infrared communications have a high directivity, positional relation between a transmitting device and a receiving device is limited considerably (for example a condition in which the transmitting device and the receiving device are within a line-of-sight range and there is no obstruction between the devices needs to be secured). Further, the light emission of an LED on the transmitter side increases power consumption. Infrared communications are thus considered to be unsuitable for practical use in battery-driven portable devices.
Bluetooth communication has spread as a radio communication technology using weak radio waves. Using the communication system for music data transmission between a portable type music reproducing device and headphones is considered, and the communication system is partly put to practical use. However, the Bluetooth communication uses a 2.4-GHz band as an ISM band (Industrial, Scientific and Medical frequency band) that does not require a license, interference of other radio waves of a microwave oven within a house, a wireless LAN and the like which radio waves use the same frequency band is inevitable. It is thus hard to say that the Bluetooth communication is a satisfactory transmission system from viewpoints of communication quality and secrecy.
Human body communication technology that incorporates a human body in a transmission path has recently started to draw attention. For example, a communication system (for personal authentication, payment, settlement of accounts or the like) is proposed in which a human body side communication device worn in contact with the skin of a human body is provided and an apparatus side communication device is provided in the vicinity of the human body, and by applying an alternating voltage to the human body via an electrode on the side of the human body communication device, simple information data is transmitted and received on the basis of the action of a capacitor with the human body interposed as a medium between the electrodes of the human body communication device and the apparatus side communication device, using a phenomenon of electrostatic induction occurring at the electrode of the apparatus side communication device (see for example “Human body communication already partly put to practical use, 10-Mbits/second prototype” (NIKKEI BYTE 2004 December)).
The present inventor et al. consider that it is possible to establish a connection between a portable music reproducing device and headphones by human body communication and thereby transmit audio data in a wireless manner. It is known for example that a communication between two devices that have an electrode, are independent of each other, and transmit weak radio waves having a level insufficient for mutual communication as they are is enabled by strengthening the radio waves with a human body as a medium (see for example, Japanese Patent Laid-open No. Hei 7-170215 and U.S. Pat. No. 5,914,701).
A human body has properties as a conductor and properties as a dielectric. A human body communication system using the former properties is referred to as a current type. In this case, the transmitting and receiving electrodes of a portable music reproducing device and headphones need to be in contact with a human body at all times. The human body communication system therefore has a problem in usability because the electrodes give an unpleasant feeling or a feeling of discomfort, for example.
A human body communication system using the properties of the human body as a dielectric is referred to as an electric field type. In this case, one of a radiation field propagating through space, a dielectric electromagnetic field, and a quasi-electrostatic field (not propagated) using the charging of a human body is formed between a transmitting electrode and a receiving electrode which electrodes are disposed such that the human body is interposed therebetween. A signal can be transmitted on the basis of a potential difference acting in such an electric field. While several human body communication systems of the electric field type are proposed, it has not been fully elucidated which of these electric fields is used.
For example, in a case where a portable type music reproducing device and headphones are connected to each other by a human body communication of the electric field type and an electric field formed by the interposition of a human body is a radiation field rather than a quasi-electrostatic field, even when excellent audio data transmission is achieved, the radiation field becomes a wave interfering with a neighboring radio system using a same frequency band, or another radio system interferes with the human body communication instead. In this case, the human body communication can be intercepted, and is therefore unable to maintain secrecy. Further, since an extra electric field (stronger than necessary) is radiated to the outside of the human body, power consumption is increased, which represents a critical problem especially for battery-driven portable devices.
The human body communication system in related art is predicated on the action of a capacitor with a human body interposed as a medium between the transmitting electrode and the receiving electrode. In such a case, a desired information communication speed cannot be obtained unless an electrode area is made considerably large, and it is therefore difficult to reduce the size of the device side communication device. When the electrode area is made considerably large, susceptibility to noise is increased instead, and therefore communication quality is lowered.