Post-Production Codecs

One of the greatest gifts editors received during the year past is a second compression codec designed specifically for post production. By bringing out ProRes 422 as part of its Final Cut Studio 2 software suite, Apple has presented us with their own ability to squeeze uncompressed HD down to SD file sizes, three years after Avid introduced DNxHD in 2004.

So with the goal of enlightening rather than contrasting, here is a look at two software offerings that can make HD post feasible even on moderate laptop systems.

AVID DNxHD

“One of the main forces driving our development of the Avid DNxHD codec was to maintain the kind of workflow when using high-definition source material that we had become accustomed to with standard definition,” said Michael Phillips, senior market solutions manager at Avid Technologies. “Because true uncompressed HD files at 1.5 Gbps are so large that accessing multiple streams is not very feasible when working in a shared environment with, say, 20 workstations attached to an Avid Unity, we wanted to cut the files down to a manageable size without sacrificing visual quality.”

Avid’s answer today encompasses a variety of DNxHD alternatives, each of which gives you digital files that look indistinguishable from uncompressed HD, while providing full color information. Encoding source footage from something like D-5 through Avid DNxHD 220x gives you a 1920x1080 file in 10-bit color, while taking up 220 Mbps when running at 30 fps you get 170 Mbps at 24 fps. You can also use Avid DNxHD 220 (without the “x”) for 8-bit color sources.

For lower data rate recordings such as HDCAM or Panasonic’s DVCPRO HD, Avid DNxHD 145 provides high quality 8-bit editing and mastering pushing 145 Mbps down the pipe at 1920x1080 with 30 fps.

“You can certainly encode HDCAM in DNxHD 220, but you are not gaining a whole lot except for a more robust color space,” said Phillips, “but the full raster of DNxHD 145 provides a horizontal resolution that is 33 percent sharper. In addition, it makes the files smaller than the typical 176 Mbps of SD while preserving the HD quality.”

Two years ago, Avid brought out their latest DNxHD TR (Thin Raster) for HDV work. Although it maintains the 1440x1080 raster of HDV to spare the CPU from scaling the horizontal resolution up to 1920, it gives you better editing performance by providing independent I-frames to edit on because it is still the DNxHD codec’s format.

Then at NAB2007 Avid brought out DNxHD 36 for high-quality offline editing of HD progressive sources only. If you are following the rhythm of their logic, you can expect DNxHD 36 to clock in at just 36 Mbps while maintaining full raster visual quality at file sizes only half again larger than DV25.

“It’s rare today to see creative editorial performed by just one editor,” Phillips said. “But having mastering quality HD video with full raster and 4:2:2 color space running through infrastructure designed for SD makes team post production of HD a problem that has been solved.”

Avid’s DNxHD can output a QuickTime file that is compatible with both Mac OS and Windows platforms. Avid’s DNxHD will be the first codec compliant with SMPTE standard VC-3, an HD video compression format in development that enables open media exchange. VC-3 is progressing toward being accepted as SMPTE 2019.

APPLE PRORES 422

Apple brought out its ProRes 422 codec as part of their Final Cut Studio 2 collection of post-production software applications at NAB2007. Although Apple’s Final Cut Pro NLE software supports uncompressed HD as a real-time editing format, using ProRes 422 gets those files down to a size that is much more storage efficient.

“ProRes 422 is our next generation post-production format,” said Richard Townhill, director of pro video product marketing at Apple Inc., “using a full 1920x1080 raster in a 10-bit, 4:2:2 color space. It is designed to give you the exact same visual quality as uncompressed HD at SD file sizes. Even MacBook Pro laptops can use ProRes 422 to play back full quality 24p HD in real time.”

Since ProRes 422 invokes a variable bit-rate compression technology, it is a bit tricky to specify its space saving advantages. Its “standard” quality mode is targeted at 145 Mbps using 8-bit or 10-bit depending on the source for economy, and HQ (high quality) mode strives for 220 Mbps with 10-bit color depth. Since this is what they call “smart” variable bit-rate encoding, the actual compression depends on the content of the images.

Still, except when using shots containing unusual complexity, the actual bit-rates of ProRes 422 are typically 5 percent to 10 percent lower than each mode’s target.

ProRes 422 also enhances the editing process itself by converting even Long GOP formats like HDV into a video stream with distinct I-frames for accurate cutting. Through a new user preference in Final Cut Pro 6, editors can choose to render effects in HDV and XDCAM HD using ProRes 422 which will result in faster rendering time and a higher-quality 4:2:2 composite.

Apple has designed ProRes 422 to produce the best compressed HD images they can provide. They regularly demonstrate its output on a split screen against uncompressed HD using StEM (Standard Evaluation Material) footage provided by the American Society of Cinematographers. Using that source material, Apple claims their 10-bit ProRes 422 HQ has an average peak signal to noise ratio (PSNR) of 56.4 dB, and compare that to a PSNR for D-5 of 52.2 dB. Since the higher the PSNR the better, those are numbers worth considering.

“That’s why we say ‘uncompressed HD quality at SD file sizes,’” Townhill said. “But the proof of the pudding is in the eating. So we invite editors to make their own side-by-side comparisons on broadcast grade monitors.”

By the way, for really low data rate offline editing, Apple recommends you just invoke their native 25 Mbps HDV codec available in Final Cut Pro editing software for years. And if you start with HDV source footage and are going to deliver in HDV, you might just as well edit the whole project in that format.

So, since editors know that no technology is perfect, it’s a boon to our craft that we have a growing number of options for post-production codecs. The ultimate quality is up to the editor’s own creativity.