frame. It is set to 0 during the first field of a frame, and to 1 during the second. This bit replaces the "polarity correction" bit in linear timecode. "Binary group flag" bits BGF0 and BGF2 (bits 55 and 75 for 29.97/30 frame/s, bits 35 and 55 for 25 frame/s): These indicate the format of the user bits. Both bits zero indicates no (or unspecified) format. Only BGF0 set indicates four 8-bit characters (transmitted little-endian). The combinations with BGF2 set are reserved. Bit 74 ("Binary group flag 1") was previously unassigned, but is used to indicate that the time code is synchronized to an external clock. If zero, the timecode start time is arbitrary. The checksum in bits 82–89 is a simple bytewise XOR of the previous 82 bits (including the sync bits, so bit 82 is the XOR of bits 74, 66, ..., 2), which can be described as a CRC with generator polynomial x8+1. (Preset to zero, no inversion.) The exact nature of the color frame sequence depends on the video standard being used. In the case of the three main composite video standards, PAL video has an 8-field (4 frame) color frame sequence, and NTSC and SECAM both have 4-field (2 frame) color frame sequences. Preserving the color framing sequence of video across edits and between channels in video effects was an important issue in early analog composite videotape editing systems, as cuts between different color sequences would cause jumps in subcarrier phase, and mixing two signals of different field dominance would result in color artifacts on the part of the signal that was not in sync with the output color frame sequence. To help prevent these problems, SMPTE time code contains a color framing bit, which can be used to indicate that the video material the timecode refers to follows a standard convention regarding the synchronization of video time code and the color framing sequence. If the color framing bit was set in both types of material, the editing system could then always ensure that color framing was preserved by constraining edit decisions between input sources to keep the correct relationship between the timecode sequences, and hence the color framing sequences. See also Related technologies and standards Linear timecode Burnt-in timecode CTL timecode MIDI timecode AES-EBU embedded timecode Rewritable consumer timecode References Timecodes Film and video technology