Addressing System In The Information Area
The address of a Section of an Information Track on the disk is given as the elapsed time from the start of the User Data Area to that Section. This time is recorded in the Control bytes of each Section, and is called absolute time. It is given with a resolution of 1/75 of a second. The time is given for a data rate from the disk of 4,321 8 x 10^6 Channel bits per second. This amounts to exactly 75 Sections per second.
The address of a Sector is recorded in the Sector Header, also as an absolute time. It has no prescribed relation to the addresses of the Sections, because the mapping of a Sector on the Sections during recording is implementation-dependent due to the freedom left in clause 16. Therefore, the address of a Sector is filled in just before the Sector enters the CIRC encoder.
The nominal value of the absolute time in the Header of a Sector shall be equal to the absolute time recorded in the Control bytes of that Section which is being processed by the 8-to-14 encoder at the instant that the Sync of the Sector enters the CIRC encoder. This prescription assumes that the CIRC encoder is the only delaying element in the recording electronics.
The tolerance on the nominal time in the address of the Header of a Sector shall be ± 1s. This tolerance is large compared with the recording time of a Section (1/75 s) and of a Sector, in order to accommodate the freedom this ECMA Standard leaves for the implementation.
Each Sector has a unique address. The address of the first Sector with User Data of an Information Track is written in the table of contents of the disk. Thus, the table of contents points to the start of an Information Track on the disk in terms of the absolute time in the Control bytes with an accuracy of ± 1 s.