Editing Serial digital interface (section)

===Synchronization packets===
A synchronization packet (commonly known as the '''timing reference signal''' or '''TRS''') occurs immediately before the first active sample on every line, and immediately after the last active sample (and before the start of the [[horizontal blank]]ing region). The synchronization packet consists of four 10-bit words, the first three words are always the same—0x3FF, 0, 0; the fourth consists of 3 flag bits, along with an error correcting code. As a result, there are 8 different synchronization packets possible.

In the HD-SDI and dual link interfaces, synchronization packets ''must'' occur simultaneously in both the Y and C datastreams. (Some delay between the two cables in a dual link interface is permissible; equipment which supports dual link is expected to buffer the leading link in order to allow the other link to catch up). In SD-SDI and enhanced definition interfaces, there is only one datastream, and thus only one synchronization packet at a time. Other than the issue of how many packets appear, their format is the same in all versions of the serial-digital interface.

The flags bits found in the fourth word (commonly known as the '''XYZ word''') are known as H, F, and V. The H bit indicates the start of horizontal blank; and synchronization bits immediately preceding the horizontal blanking region must have H set to one. Such packets are commonly referred to as '''End of Active Video''', or '''EAV''' packets. Likewise, the packet appearing immediately before the start of the active video has H set to 0; this is the '''Start of Active Video''' or '''SAV''' packet.

Likewise, the V bit is used to indicate the start of the vertical blanking region; an EAV packet with V=1 indicates the following line (lines are deemed to start at EAV) is part of the [[vertical interval]], an EAV packet with V=0 indicates the following line is part of the active picture.

The F bit is used in [[Interlaced video|interlaced]] and [[Progressive segmented frame|segmented-frame]] formats to indicate whether the line comes from the first or second field (or segment). In [[progressive scan]] formats, the F bit is always set to zero.