As part from video and audio information, the disc contains a number of signals, inserted in the non-visible lines of the blanking periods. These are in the first place Vertical Interval Reference (VIR) and Vertical Interval Test (VIT) signals for testing purposes (lines 19,20,282,283) and secondly a number of digital address signals for various purposes (lines 17, 18, 280, 281).
The address signals have the following functions:
Start code: The “read” objective is sent with normal speed forward to the program area on the disc.
The “start” code is present in all fields of the lead-in tracks.