Video payload and blanking
The active portion of the video signal is defined to be those samples which follow an SAV packet, and precede the next EAV packet; where the corresponding EAV and SAV packets have the V bit set to zero. It is in the active portion that the actual image information is stored.
Several color encodings are possible in the serial digital interface. The default (and most common case) is 10-bit linearly sampled video data encoded as 4:2:2 YCbCr. (YCbCr is a digital representation of the YPbPr colorspace). Samples of video are stored as described above. Data words correspond to signal levels of the respective video components, as follows:
- The luma (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 hex) .
- For the chroma channels, 0 mV is assigned the code word 512 (200 hex), -350 mV is assigned a code word of 64 (40 hex), and +350 mV is assigned a code word of 960 (3C0 hex).
Note that the scaling of the luma and chroma channels is not identical. The minimum and maximum of these ranges represent the preferred signal limits, though the video payload may venture outside these ranges (providing that the reserved code words of 0 - 3 and 1020 - 1023 are never used for video payload). In addition, the corresponding analog signal may have excursions further outside of this range.
As YPbPr (and YCbCr) are both derived from the RGB colorspace, a means of converting is required. There are three colorimetries typically used with digital video:
- SD and ED applications typically use a colorimetry matrix specified in ITU-R Rec. 601.
- Most HD, dual link, and 3 Gbit/s applications use a different matrix, specified in ITU-R Rec. 709.
- The 1035-line HD standards specified by SMPTE 260M (primarily used in Japan and now largely considered obsolete), used a colorimetry matrix specified by SMPTE 240M. This colorimetry is nowadays rarely used, as the 1035-line formats have been superseded by 1080-line formats.
The dual-link and 3 Gbit/s interfaces additionally support other color encodings besides 4:2:2 YCbCr, namely:
- 4:2:2 and 4:4:4 YCbCr, with an optional alpha (used for chroma keying) or data (used for non-video payload) channel
- 4:4:4 RGB, also with an optional alpha or data channel
- 4:2:2 YCbCr, 4:4:4 YCbCr, and 4:4:4 RGB, with 12 bits of color information per sample, rather than 10. Note that the interface itself is still 10 bit; the additional 2 bits per channel are multiplexed into an additional 10-bit channel on the second link.
If an RGB encoding is used, the three primaries are all encoded in the same fashion as the Y channel; a value of 64 (40 hex) corresponds to 0 mV, and 940 (3AC hex) corresponds to 700 mV.
12-bit applications are scaled in a similar fashion to their 10-bit counterparts; the additional two bits are considered to be LSBs.
For portions of the vertical and horizontal blanking regions which are not used for ancillary data, it is recommended that the luma samples be assigned the code word 64 (40 hex), and the chroma samples be assigned 512 (200 hex); both of which correspond to 0 mV. It is permissible to encode analog vertical interval information (such as vertical interval timecode or vertical interval test signals) without breaking the interface, but such usage is nonstandard (and ancillary data is the preferred means for transmitting metadata). Conversion of analog sync and burst signals into digital, however, is not recommended—and neither is necessary in the digital interface.
Different picture formats have different requirements for digital blanking, for example all so called 1080 line HD formats have 1080 active lines, but 1125 total lines, with the remainder being vertical blanking.
The various versions of the serial digital interface support numerous video formats.
- The 270 Mbit/s interface supports 525-line, interlaced video at a 59.94 Hz field rate (29.97 Hz frame rate), and 625-line, 50 Hz interlaced video. These formats are highly compatible with NTSC and PAL-B/G/D/K/I systems respectively; and the terms NTSC and PAL are often (incorrectly) used to refer to these formats. (PAL is a composite color encoding scheme, and thus describes more than just the line-standard, while the serial digital interface— other than the obsolete 143 Mbit/s and 177 Mbit/s forms- is a component standard).
- The 360 Mbit/s interface supports 525i and 625i widescreen. It can also be used to support 525p, if 4:2:0 sampling is used.
- The various 540 Mbit/s interfaces support 525p and 625p formats.
- The nominal 1.5 Gbit/s interfaces support most high-definition video formats. Supported formats include 1080/60i, 1080/59.94i, 1080/50i, 1080/30p, 1080/29.97p, 1080/25p, 1080/24p, 1080/23.98p, 720/60p, 720/59.94p, and 720/50p. In addition, there are several 1035i formats (an obsolete Japanese television standard), half-bandwidth 720p standards such as 720/24p (used in some film conversion applications, and unusual because it has an odd number of samples per line), and various 1080psf (progressive, segmented frame) formats. Progressive Segmented frames formats appear as interlace video but contain video which is progressively scanned. This is done to support analog monitors and televisions, many of which are incapable of locking to low field rates such as 30 Hz and 24 Hz.
- The dual link HD interface supports 1080/60p, 1080/59.94p, and 1080/50p, as well as 4:4:4 encoding, greater color depth, RGB encoding, alpha channels, and nonstandard resolutions (often encountered in computer graphics or digital cinema).
- A quad link interface of HD-SDI supports UHDTV-1 resolution 2160/60p