Sub-channel below the perceptual threshold in audio

This paper explores the concept and possible ways of making use of an audio channel´s capacity below the perceptual threshold as a hidden sub-channel. Such an audio channel can be a one with conventional telephony or with voice-over-IP. Since the sub-channel is hidden, not detectable by the human ear, a communications system equipped with this technology will be compatible with the existing system in terms of audio signal transmission. Two potential applications of the technology are concurrent services and the extension of audio bandwidth. The latter application provides the listener with an improved audio quality.