Sustainability of Digital Formats: Planning for Library of Congress Collections

A digital, color-difference component video picture format identified by the FOURCC code UYVY. This format employs 4:2:2 chroma subsampling with each sample represented by 8 bits of data. It is essentially the same as YUY2 but with different component ordering packed within the two-pixel macropixel: Byte 0=8-bit Cb; Byte 1=8-bit Y'0; Byte 2=8-bit Cr; Byte 3=8-bit Y'1. The FOURCC YUV pixel format page provides additional information and a helpful diagram.

The structure appears to be the same as that of a format that Apple tags as 2vuy, the designation also referenced by SMPTE in the MXF standard (ST 377-1:2009, annex G). Note, however, that the same Apple developer page warns, "Despite the name, this pixel format is not a simple byte-reversal of 'yuv2' [Apple's designation for FOURCC YUY2]." Comments welcome.

The FOURCC Web page cited above states that "UYVY is probably the most popular of the various YUV 4:2:2 formats." Using their own four-character code to identify this format, the Apple developer page cited above states, "'2vuy' is the format long used by professional video tape formats, transmission protocols, processing equipment (both computer-based and not), and compression schemes. It . . . discards no information from a digital video link. Its component order also matches that transmitted over a digital video link."

UYVY encoding (like all uncompressed formats identified by FOURCC codes) allows for variation in features like picture size, aspect ratio (square or non-square pixels), component levels (i.e., levels for Y, Cb, and Cr in either video range or wide range), and a given instance may contain video from interlaced or progressive sources. (See comment about interlacing and field order in the Notes below.) In order for an application to play the video back correctly or, say, to hand it off for successful inclusion in a video production side by side with other footage, all of these facts ought to be declared in metadata embedded in the file wrapper or associated with the file. The need for such metadata (and a way to compensate if it is missing) is suggested by the "Appendix: Backwards Compatibility" section of an Apple developer page that provides "best assumptions" to support playback for 2vuy video when the metadata is absent.

Interlacing and field order. This writer's understanding is that, when the source footage is interlaced, UYVY encoding does not group together all the odd and even lines as they would be grouped in the two fields that make up a video frame on, say, a videotape. All UYVY pixel data is presented in a left-right, top-down order that could be called progressive. To put it another way, for interlaced picture data, the tape presents alternate lines in the two fields that make up a frame: line 1, line 3, line 5, etc., and then line 2, line 4, line 6, etc. In contrast, UYVY encodes the data as line 1, line 2, line 3, line 4, line 5, line 6, etc.

In fact, on a tape, which comes first (1, 3, 5 or 2, 4, 6) represents what is called field order or field cadence. There are two options for field order: upper (field 2 is dominant, so the second field is drawn first) and lower (field 1 is dominant, so the first field is drawn first). Generally, upper is used by 640 x 480 systems, while lower is most common in professional 720 x 486 and DV 720 x 480 systems.

As noted in the Description section above, even though UYVY's serialization of the data is progressive, and many players will work well without help, it may be the case that a user will wish to "restore" footage archived as UYVY in a manner that will, say, intercut with other footage. Such restoration may require the decoding system to be aware that this footage was originally interlaced and to know the original field order. That's a job for metadata.

8- and 10-bit sampling. In principle, 10-bit encoding is superior in clarity to 8-bit, due to the reduction in tonal contouring and other artifacts. Some specialists argue, however, that there is no benefit for certain classes of material. One university expert wrote, "We digitize Betacam SX tape to 8-bit UYVY but Digibeta to 10-bit V210 because these selections align with the nature of the data that is actually sent out over SDI from these tapes. . . . SDI is 10-bit data, but when I piped the SDI video data from an SX tape to a binary display I could see the 9th and 10th bits were always zero. Thus by taking only the first 8 bits I could get all meaningful data . . . . I have about 3,000 SX hours to preserve and choosing 8-bit instead of 10 saves me about 90 TB of storage" (private communication).