TABLE OF CONTENTS The standard two-channel interface was originally designed for linear PCM audio samples but in recent years there has been increasing use of data-reduced audio coding systems such as Dolby Digital (AC-3), DTS and MPEG. Because consumer systems in particular needed the ability to transfer such signals digitally, the non-audio mode of the interface has been adapted to the purpose. This is described in a relatively new IEC standard numbered 6193731. In addition to a general specification detailing the principles it also has a number of parts that describe the handling of specific data-reduced audio formats, some of which are not yet finalized at the time of writing. A similar but not identical SMPTE standard (337M)32 describes professional non-audio applications of the interface, including its use for carrying Dolby E data (see below). SMPTE 338M and 339M specify data types to be used with this standard. The SMPTE standard is also more generic than the IEC standard, designed to deal with a variety of data uses of the interface, not just low-bit rate audio. It can also carry time-stamp data in the form of SMPTE 12M timecode. The reader is referred to the standards for more precise details regarding implementation of specific formats.
In both SMPTE and IEC versions the low-bit rate audio data is carried in bursts in place of the normal linear PCM audio information, with bit 1 of channel status set to the 'other uses' or 'non-audio' state.
