indicate the start of the vertical blanking region; an EAV packet with V=1 indicates the following line (lines are deemed to start 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 and progressive 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. Other than the fact that synchronization packets occur in parallel in two datastreams (Y and C), their behavior is virtually identical to the packet types defined in CCIR 601 and SMPTE 259, the digital interface commonly used for SDTV. Line counter and CRC To provide additional robustness, the four samples immediately following the EAV packets (but not the SAV packets) contain a cyclic redundancy check field, and a line count indicator. The CRC field provides a CRC of the preceding line (CRCs are computed independently for the Y and C streams), and can be used to detect bit errors in the interface. The line count field indicates the line number of the current line. Ancillary data Like SMPTE 259, SMPTE 292 supports the SMPTE 291 standard for ancillary data. Ancillary data is provided as a standardized transport for non-video payload within a serial digital signal; it is used for things such as embedded audio, closed captions, timecode, and other sorts of metadata. Ancillary data is indicated by a 3-word packet consisting of 0, 3FF, 3FF (the opposite of the synchronization packet header), followed by a two-word identification code, a data count word (indicating 0 - 255 words of payload), the actual payload, and a one-word checksum. Other than in their use in the header, the codes prohibited to video payload are also prohibited to ancillary data payload. Video payload Within the active portion of the video, the data words correspond to signal levels of the respective video components. The luminance (Y) channel is defined such that a signal level of 0 mV is assigned the codeword 64 (40 hex), and 700 millivolts (full scale) is assigned the codeword 940 (3AC) . For the chroma channels, 0 mV is assigned the code word 512 (200 hex), -350mV is assigned a code word of 64 (0x40), and +350mV is assigned a code word of 960 (3C0). Note that the scaling