Since HDMI version 1.4, an Audio Return Channel, or ARC for short, has extended the interface specification. Devices that support this audio return channel save an additional audio connection in certain situations. This is the case, for example, when you connect a TV with a built-in DVB-T receiver to your system so that the picture is also shown on your projector and the sound is output via the speakers.
You can easily send the video signal received by the DVB-T receiver to your projector via HDMI cable, but to transmit the sound from the TV you normally need an additional audio cable, such as an S/PDIF connection. This extra cable is no longer needed if you use the Audio Return Channel. No special HDMI cable is required for this, but the HDMI port on your TV must be ARC-capable.
However, ARC not only replaces the additional cable for the audio signal, it also makes HDMI features such as lip sync available on the Audio Return Channel.
