Uncompressed vs. Compressed
Read part one of this story
Last month we looked at those who argue in favor of uncompressed high-definition mastering.
The just-announced 844/X HD codec should be added to the mildly compressed list in the last issue. At a compression ratio of 1.5:1, it is the least compressed HD mastering format.
This month we look at the other side of the debate, reasons you might choose compressed HD. We'll also examine the compressed HD codecs now available.
Those who advocate compressed HD solutions frequently speak of several benefits. There can be a major cost savings on storage — not only the amount of storage required, but also the type of storage. With uncompressed high-definition video, storage costs can be significant. But with compressed HD, you may get by with a simple four-drive striped RAID setup. If you want redundancy protection from drive failures, then the price of storage could double.
Performance is another issue often mentioned. There are major bandwidth issues to consider with a large number of client editing stations accessing HD video via a storage area network. There is also a performance issue when you need to share so many bits of information. Given the power of a specific processing technology, native HD compressed editing may yield better performance than uncompressed HD editing. For example, it may be possible to edit compressed HD on a laptop that would not be able to edit uncompressed HD because of computing power and storage requirements.
Another major benefit may be the ability to use existing infrastructure and older storage technology. Of course, compressed HD acquisition and VCR technology are probably cheaper than those available for uncompressed HD.
Finally, there is this crucial question: Can HDTV viewers actually see the difference? Let's take a look at the various compression formats.
This highly compressed HDV format was designed as a consumer format, but is being adopted for pro applications.
Editing systems supporting this format include: Applied Magic HD Cinema Workstation, Avid Technology DNA family of products, BitCentral MediaPipe, Boxx Technologies HDV Pro, Canopus Edius (HDV support has been announced for a future version of Edius), CineForm Aspect HD 2.2 for Adobe Premiere Pro, Heuris Pro Indie HD Toolkit for Final Cut Pro, KDDI MPEG Edit Studio Pro, Mac-based Lumière Media's Lumière HD, Macro-System Digital Video Casablanca Solitaire, MainConcept MPEG Pro Plug-in for Adobe Premiere Pro, MediaWare Solutions MPEG EditXpress, Pinnacle Systems Liquid family, Sony Vegas, and Ulead MediaStudio Pro 7 with the HD plug-in. Several more new solutions have yet to be announced.
The new JVC CU-VH1 HDV VCR ($1,995) and the Sony prototype camera demonstrated at IBC both support HDV format codecs.
Since CineForm transcodes video from the MPEG-2 format into a different format optimized for postproduction, I will suggest that this is another compressed HD technology.
The structure of the CineForm codec allows for realtime editing in software on multiple simultaneous streams of HDV (Aspect HD) or 1080i/p (Prospect HD), with no custom hardware required, on a high-performance desktop machine.
Not all codecs are equal from a quality standpoint.MPEG was designed as a distribution format, and DV/DVCPRO was designed as tape capture for-mat. David Taylor, CineForm CEO, says neither format is optimal from a post-production standpoint — considering performance or multi-generational PSNR characteristics. He claims Cine-Form has designed a codec built to achieve very high PSNR characteristics.
CineForm's structure supports 8 and 10 bits, not only in CineForm's codec, but also in its arithmetic engine that runs underneath Premiere. It is important to “match” a codec and a video pipeline, Taylor says. When the two are optimized in tandem, both performance and quality improvements can result, he says.
CineForm already supports HDV and has since last summer with Aspect HD. The company has been shipping support for HD-SDI since June with Prospect HD. CineForm will introduce a number of extension software products in the future, all of which will be backward-compatible with its current format. The new products will feature arithmetically lossless processing and Cinema 2K/4K resolutions. These are especially important for ensuring compatibility for high-resolution postproduction, as there are a number of different resolutions and YUV/RGB formats of source material.
Sony's HDCAM format uses a proprietary 8-bit compression technology with a bit rate of 135Mbps (1080i, 1080p24, and 720p). The luminance is down-sampled to 1440 and the chroma sub-sampling follows the 3:1:1 scheme, resulting in 480 samples per line. The price for an HDCAM studio VCR starts at $42,100 for the HDW2000 studio VTR.
HD DVCPRO HD is a 100Mbps DV-derived compressed HD format. Luminance is downsampled to 1280 samples per line, and the chroma subsampling follows the 4:2:2 scheme, resulting in 640 chroma samples per line. As a result, the frequency range goes down to two thirds of the original uncompressed image.
Prices range from $21,000 for the new Panasonic AJ-HD1200A (with optional HD and IEEE-1394 adding to the price) to $62,500 for the AJ-HD1700 VCR.
“The advantage of using the native compressed file [as recorded by the Varicam or DVCPRO HD deck], rather than an uncompressed file, is that the quality of the image being edited is the same as the original recording, but the disk storage requirement and disk throughput requirement are both greatly reduced,” says Stuart English, Panasonic's vice president of marketing.
“This leads to much lower storage costs, more layers that can be processed simultaneously, and much greater portability for the editing platform. For example, edit in HD on an Apple 1.0GHz, 1MB RAM-equipped G4 laptop. Of course, recording uncompressed direct from the camera DSP — bypassing DVCPRO HD recording section altogether — is the best quality, but typically that is not a practical consideration.
“In a competitive market, lowering the costs associated with editing HD leads to greater adoption of HD for programming, and that is a major competitive advantage for production companies wanting HD rather than SD business,” English says.
Richard Kerris, Apple's senior director of pro applications product marketing, says, “Final Cut Pro HD's support of DVCPRO HD capture and output over FireWire allows editors to use camera-native footage to produce broadcast-quality results with no generational quality loss. Final Cut Pro HD's software codec gives editors the opportunity to work with realtime native HD content on both desktop and laptop computers, which allows for the widest range of workflows.”
I asked if there were any decompress/recompress steps for effects or graphics composites or if there were any conditions in a typical edit session that might require decompress/recompress steps. An Apple spokesman replied: “When effects are applied, Final Cut Pro HD uncompresses the DVCPRO HD video and promotes it to a 4:4:4 color space. The uncompressed frames are stored in memory, and the effects are added (with either 8-bit or 32-bit float precision, depending on the filters and the user's sequence settings). The video is then put back into a 4:2:2 space and compressed into the DVCPRO HD codec again.
“Apple's position is that when you uncompress the video and promote it to a 4:4:4 color space for effects work, the process yields the highest quality results, outweighing the concerns around compression.”
Canopus' Edius HD NLE system just introduced at NAB offers two HD codecs, the Canopus HD codec and the scalable HQ codec. The Canopus HD codec processes HD information the same way as the DVCPRO HD codec. (But it is not file-compatible with DVCPRO HD files.)
Canopus' HQ codec keeps the chroma subsampling at 4:2:2 and downsamples the luma resolution to 1440. This means the luminance resolution is equal to that of HDCAM, and the chroma resolution is higher than those of both DVCPRO HD and HDCAM.
Another important characteristic of the Canopus HQ codec is variable bit-rate support. Bit rates may be varied to allocate the maximum amount of data to the images that require it, such as very complex scenes, and a minimum amount of data to images with minimal complexity. Variable bit-rate encoding is an efficient way to be sure that each frame gets the appropriate amount of data required to produce high-quality HD video.
Edius HD is also designed to support various subsampling methods to ensure scalability. In the future, image quality and performance may increase on systems with higher specifications and faster performance. This means that it will eventually be possible to compress the video image without having to downsample the original resolution.
Pinnacle Systems has introduced the HD Elite codec, a long-GOP MPEG-2 compressed HD mastering technology. HD Elite is a variable compression technology, meaning you can select the amount of compression. Pinnacle believes there is a “sweet spot” at around 50Mbps that will provide high-quality HD postproduction (equivalent to 150Mbps “I” frame MPEG).
At this level of compression, many facilities can use existing infrastructures and storage technologies. Such compression offers compatibility with less expensive storage solutions and fewer bandwidth constraints, and will work with lower-cost editing systems. Also, since the signal stays MPEG, there are fewer recompression concatenation concerns.
At NAB, Avid introduced DNxHD, which consists of three codecs: 220Mbps for both 10-bit and 8-bit video and an 8-bit 145Mbps codec. Each is contained within an MXF wrapper. These DNxHD codecs support: 720p at 23.976fps, 29.97fps, or 59.94fps; 1080i at 50fps or 59.94fps; and 1080p at 23.976fps. In comparison, uncompressed 10-bit SD media requires approximately 200Mbps, and uncompressed HD media can require up to 1.2Gbps.
It also should be mentioned that this technology is scalable. With DNxHD it's possible to add different formats, resolutions, and data rates. In other words, if there is a demand to move to a higher or lower bit rate, Avid promises to provide a DNxHD resolution for that need.
The DNxHD source code will be licensable and free from the Avid website for anyone who wants to compile it on any platform. Avid President David Krall says that since the intention from the start was to make this product licensable and free, there were two teams working on the development: Avid engineers and a team of lawyers making sure that there were no proprietary intellectual property infringements with this product. The primary reason for the license is to ensure that nothing will be modified that might cause incompatibility with other products using DNxHD files.
The benefits of Avid DNxHD include improved HD workflow (many more workstations can share HD on an Avid Unity), smaller storage requirements, high performance, and high-quality images. Players for all operating systems can be developed using the down-loadable source code, so users will always be able to play back archived video.
Users can also edit single-stream HD on a laptop using the low-cost NewsCutter XP application. Because DNxHD has low bandwidth requirements, single editing systems can work in HD with a simple four- or eight-way drive stripe set. Up to three realtime streams of HD multi-camera functionality are possible with the Avid DNxHD 145 setting. With lower bandwidth and storage requirements allowing lower-cost workstation editing, you can work in the final mastering resolution and eliminate offline-to-online workflow — a major source of operator error problems.
The DNxHD codec set will be available across the entire line of Avid editing products, starting with DS Nitris 7.5 this summer. This will be followed by the “dot releases” of Media Composer Adrenaline, NewsCutter Adrenaline FX, News Cutter XP, and Xpress Pro, expected in the fourth quarter. Ikegami and NL Technology plan to offer an Editcam using this technology as well.
Krall says unlike other compressed HD formats, such as HDCAM and DVCPRO HD, Avid DNxHD maintains the full raster of the active video, sampling every available pixel in the image.
Krall says this concept of a free compression technology was inspired by broadcasters' frustrations with new formats that make it difficult to access archived material. Avid will provide free players for all computer operating systems so media owners will always be able to access archived DNxHD material.
The DNxHD web page at http://www.avid.com/DNxHD/index.asp offers more information, including charts comparing DNxHD to DVCPRO HD and HDCAM.
There are strong arguments to be made for and against compressed HD. As Discreet's Maurice Patel said last month, much depends on your needs.
I also feel consideration should be given to what the viewer sees. If the viewer can see no difference between expensively produced uncompressed HD and less-expensive compressed HD, then cost savings are clearly justified. Fortunately, many of today's editing applications will work with both compressed and uncompressed video. Just remember that what cannot be seen today may be easily seen at some future point. After all, Avid's AVR 26 codec was once considered an acceptable online resolution.
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