Transmission System and Relay Device

1. A relay device adapted to be positioned in a transmission system for transmitting a video signal from a source device to a sink device, the relay device comprising: a video signal receiving unit that receives the video signal from the source device; a video signal transmitting unit that transmits the video signal to the sink device; a first communication unit that receives a first message regarding a power supply capability of the sink device; a second communication unit that transmits a second message regarding a power supply capability of the relay device to the source device; a first power circuit that receives power supplied by the sink device; and a second power circuit that supplies power to the source device; wherein an amount of sum of the power supply capability of the relay device and power consumed at the relay device does not exceed an amount of power supply capability of the sink device, and wherein an amount of power to be supplied from the second power circuit to the source device is arranged to be difference between an amount having been supplied from the sink device to the first power circuit and another amount consumed at the relay device.

2. The relay device according to claim 1 , wherein the amount of power having been consumed at the relay device comprises amounts of power consumed at both the video signal receiving unit and the video signal transmitting unit; and wherein the first power circuit is arranged to transmit, to the source device, information on power supply capability of the relay device having been changed in response to other information on change of power supply capability of the sink device.

3. A method for transmitting a video signal from a source device to a sink device using a relay device, the method comprising: receiving, at the relay device, the video signal from the source device; transmitting, by the relay device, the video signal to the sink device; receiving, at the relay device, a first message regarding a power supply capability of the sink device; transmitting, by the relay device, a second message regarding a power supply capability of the relay device to the source device; receiving, at the relay device, power supplied by the sink device; and transmitting, by the relay device, power to the source device; wherein an amount of sum of the power supply capability of the relay device and power consumed at the relay device does not exceed an amount of power supply capability of the sink device, and wherein an amount of power to be supplied from the second power circuit to the source device is arranged to be difference between an amount having been supplied from the sink device to the first power circuit and another amount consumed at the relay device.

4. The method according to claim 3 , wherein the amount of power having been consumed at the relay device comprises amounts of power consumed at both the video signal receiving unit and the video signal transmitting unit; and wherein the first power circuit is arranged to transmit, to the source device, information on power supply capability of the relay device having been changed in response to other information on change of power supply capability of the sink device.