We’ve spent a lot of time recently thinking about digital video issues. As mentioned in a previous blog post, the Federal Agencies Digitization Guidelines Initiative published several reports on this topic including “Creating and Archiving Born Digital Video.” Work on the “Eight Federal Case Histories” (PDF) report nudged us to add the Apple ProRes 422 family of video codecs to the Sustainability of Digital Formats website because both the American Folklife Center’s Civil Rights History Project and NOAA’s Office of Ocean Exploration and Research Okeanos Explorer make use of these codecs.
Developed and maintained by Apple, Inc, ProRes is a family of proprietary, lossy compressed, high quality intermediate codecs for digital video primarily supported by the Final Cut Pro suite of post-production and editing software programs. There are two main branches of Apple ProRes: the Apple ProRes 422 Codec Family and the Apple ProRes 4444 Codec Family.
The Apple ProRes 422 Codec Family comprises four subtypes, each of which is geared for a different end use. The 422 codecs are differentiated mainly by expected file size ranges, software version support and data rate limits. The consideration of data rate is important in three ways. First, it governs quality (the higher the rate, the better the quality); second, file size (the higher the rate, the bigger the file) and, third, it relates to the ability of a given network to carry a video signal in real time for viewing (the lower the rate, the easier to carry).
Apple ProRes 422 HQ is the highest data-rate version of the ProRes 422 codecs, applying the least compression for the best imagery and resulting in the largest files. Its high quality means that it is often selected for professional HD video production, especially in the creation of documentaries and other programs for broadcast television. One video expert states that ProRes 422 HQ provides nearly the same quality of uncompressed 10-bit 4:2:2 video but at about 1/5th the file size and throughput (the 4:2:2 ratio represents the type of chroma subsampling in each version).
Apple ProRes 422, with the second-highest data-rate of the group, frequently is used for multistream, real-time editing at a significant storage savings over uncompressed video and Apple ProRes 422 HQ.
Apple ProRes 422 LT, the third-highest data-rate version is also considered an editing codec but its smaller file sizes lend its use towards environments where storage capacity and data rate are at a premium.
Apple ProRes 422 Proxy is the lowest data-rate version of the ProRes 422 codecs and is often used in offline post-production work that requires low data rates but also a full screen picture.
The key character traits that define the ProRes 422 family are support for:
- 4:2:2 source material, as well as 4:2:1 and 4:2:0 source material if the chroma subsampling is upsampled to 4:2:2 prior to encoding.
- any frame size (including SD, HD, 2K, 4K, and 5K) at full resolution.
- 10-bit sample depth.
- intraframe (I-frame) only, and
- variable bit rate.
While ProRes is a 10-bit native codec, it can be used with either 8- or 10-bit sources. 8-bit sources (such as DVCProHD), would be upsampled to a 10-bit file.
The Apple ProRes codecs, both the 422 and 4444 families, support both interlaced and progressive scanned images and preserve the scanning method used in the source material.
Apple ProRes 4444 Codec Family comprises Apple ProRes 4444 and Apple ProRes 4444 HQ. The fourth “4” in codec family name indicates this format’s support for alpha (transparency) data, in contrast to ProRes 422. Other features include picture sizes ranging as high as 5K and 4:4:4 chroma subsampling up to 12-bits per sample. Alpha channel sampling can be as high as 16-bits. There is some use of the ProRes 4444 family in the production of advertising and in content destined for theatrical distribution.As demonstrated in the FADGI Born Digital Video case histories, the ProRes family is widely adopted in professional moving image production. The popularity of Apple’s Final Cut software suite as well as officially licensed uses in specific products and workflows has encouraged uptake of the family. Other third party implementations, including FFmpeg, have broadened the adoption beyond Apple devotees.
For preservation-minded video producers, ProRes presents a format selection dilemma, as suggested by the AFC case history for the Civil Rights History Project interviews. Using the Format Sustainability Factors website criteria, ProRes would get high marks for Adoption and Clarity and low marks for Disclosure, Documentation and Transparency.
As with any project, available skills and resources also play a part in determining deliverables. In the case of the CRHP format selection, significant factors in the decision to use ProRes 422 HQ included AFC’s existing non-linear editing system consisting of an Apple Mac with Final Cut Pro which allowed native editing within ProRes codecs, the proficiency of AFC staff in using their system and the fact that that the KiPro portable hard disk used on the project only supported Apple ProRes codecs. (It should be noted that the first two demonstrate the high marks for Adoption!) In the face of the preservation trade-offs as reflected in the Sustainability Factor scores, these considerations combined with ProRes 422 HQ’s high image quality led to the decision to use ProRes on this project.