Editing Serial digital interface (section)

===Link numbering===
Link numbering is only an issue in multi-link interfaces. The first link (the ''primary'' link) is assigned a link number of 1, subsequent links are assigned increasing link numbers; so, the second (''secondary'') link in a dual-link system is link 2. The link number of a given interface is indicated by a VPID packet located in the vertical ancillary data space.

Note that the data layout in dual link is designed so that the primary link can be fed into a single-link interface, and still produce usable (though somewhat degraded) video. The secondary link generally contains things like additional [[Least significant bit|LSBs]] (in 12-bit formats), non-cosited samples in 4:4:4 sampled video (so that the primary link is still valid 4:2:2), and alpha or data channels. If the second link of a 1080P dual link configuration is absent, the first link still contains a valid 1080i signal.

In the case of 1080p60, 59.94, or 50&nbsp;Hz video over a dual link; each link contains a valid 1080i signal at the same field rate. The first link contains the 1st, 3rd, and 5th lines of odd fields and the 2nd, 4th, 6th, etc. lines of even fields, and the second link contains the even lines on the odd fields, and the odd lines on the even fields. When the two links are combined, the result is a progressive-scan picture at the higher frame rate.