| ||
In component standards, only the active line is transmitted and this leaves a good deal of spare capacity. The two line standards differ on how this capacity is used. In 625 lines, only the active line period may be used on lines 20 to 22 and 333 to 335 5 . Lines 20 and 333 are reserved for equipment self-testing .
In 525 lines there is considerably more freedom and ancillary data may be inserted anywhere where there is no active video, either during horizontal blanking where it is known as HANC, vertical blanking where it is known as VANC, or both 4 . The spare capacity allows many channels of digital audio and considerably simplifies switching.
The all zeros and all ones codes are reserved for synchronizing, and cannot be allowed to appear in ancillary data. In practice only seven bits of the eight-bit word can be used as data; the eighth bit is redundant and gives the byte odd parity. As all ones and all zeros are even parity, the sync pattern cannot then be generated accidentally .
Ancillary data is always prefaced by a different four-symbol TRS which is the inverse of the video TRS in that it starts with all zeros and then has two symbols of all ones followed by the information symbol. See section 8.7 for treatment of embedded audio in SDI.
| ||