TABLE OF CONTENTS The U bit of each subframe has a multiplicity of uses, many of which have remained hidden from the user of commercial equipment, such as the carrying of text, subcode, and other non-audio data. It is most widely used in consumer equipment and there is now a rather complicated AES standard for its use in professional applications (AES18, see below). There is also a Philips method for inserting data into the user channel, called ITTS (Interactive Text Transmission System), on which the CD system relies for the transferring of subcode and other non-audio data over the consumer interface. The U bit is only a single bit in each subframe, potentially allowing a user channel to accompany each audio channel, and its definition in the various standards is normally 'for any other information'. The user bits are not normally aggregated over the same block length as channel status data (192 subframes), although they may be, but are often aggregated over different block lengths depending on the application, or may simply be used as individual flags. Many devices, especially professional ones, do not use them at all, although this may change in the future.
In the following sections a number of the most common applications for user data are outlined, although the standards do not really prohibit users or manufacturers using this capacity for alternative purposes. AES3-1992 signals the use of the user bits in byte 1, bits 4 7 of channel status as shown in Table 4.1.
