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MillimeterHDV@Work
Jan 23, 1997 4:48 PM
Just What is 1080i? Part 1
Steve Mullen
If you asked many prosumers, and very likely many of those in the video and film business, “What is 1080i?”, you would likely receive a response that includes a number. For example, you might hear “2 million pixels” or “four to six times more pixels than NTSC.”
A supposedly more knowledgeable response would be “1920 by 1080 pixels.” These two numbers receive confirmation from our ATSC standard that defines 1080i to indeed have 1920x1080 pixels and use interlace scanning.
However, at the recent CES 2006 show, consumers were told that for maximum 1080i picture quality they really needed a “1080p display.” This new display marketing campaign overlaps the marketing of 1080i camcorders from Canon, Panasonic, and Sony. As we will learn in this series, every marketing department is working with its own definition of 1080 and, naturally, each definition is favorable to the equipment that each company's engineers have developed.
To start on the right track we need to understand that 1080 resolutions vary whether we are talking about cameras, codecs (recording systems), transmission/storage technology, or displays.
Looking first at the transmission/storage of 1080i, the reference standard is our ATSC standard of 1920x1080. While this standard applies to over-the-air, it does not apply to satellite, cable, or optical discs. DirecTV, for example, transmits 1080i as a 1280x1080 image, as do some cable systems. In fact, cable and satellite systems are free to use different specifications for different classes of programming. Pay-for-TV movies, for example, could be allowed a full 1920x1080 resolution while network TV could be reduced to 1280x1080. (The latter only if an OTA re-transmission contract permits the reduction. It is rumored that CBS does not allow a reduction of resolution.)
Note that different classes of programming may also receive different amounts of compression. Nothing other than a re-transmission contract prevents an OTA channel from being decoded and recoded at a lower bit rate. (The conversion step itself is unavoidable for OTA channels from DirecTV because of its use of H.264/AVC MPEG-4.)
Depending which cable or satellite system you subscribe to, and what channels you watch, you may find far more quality variability with 1080i than with 720p video. The reason is simple. When a 720p60 signal is placed within a 19.2Mbps channel, it is possible to squeeze in a second low-bit-rate SD channel without an increase in MPEG-2 artifacts. However, any attempt to carry a second channel with 1080i results in reduced picture quality from MPEG-2 motion artifacts introduced by lowering the HD bit rate. This is why PBS 1080i HD often looks so terrible whenever sub-channels are turned on. (When stations sell their so-called “spare DTV bits” to USDTV for its wireless cable subscription service, the quality of 1080i is also very likely reduced.)
The bottom line is that 1080i very often suffers from both a loss in resolution and an increase in MPEG motion artifacts whenever it is “transmitted.”
Although we expect Blu-ray and HD-DVD discs to carry movies with 1920x1080 resolution, I’m not aware that there is any requirement to do so. In fact, when a season of a 1080i TV program is released on HD optical disc, it may be encoded at 1280x1080 resolution as this will allow more content to be included.
The use of different resolutions for different types of programming comes from real differences in the origin of HD material. For example, telecine machines used to transfer film to tape have three image sensors — each with 2Kx2K or 4Kx4K pixels. The output is recorded to a Panasonic D-5 deck or a Sony HDCAM SR VTR that can record 1920x1080 pixels when in 1080i mode.
Studio HD cameras use a wide range of pixel-count sensors. And HD camcorders use both a wide range of pixel-count sensors and a wide range of recording codecs. So wide that it makes sense to create six descriptive categories, composed of three subcategories within two primary categories. The two primary categories are “format resolution” and “codec resolution.” Two of the three subcategories are “oversampled” and “undersampled.” The third sub-category is neither over nor undersampled. I will call it “equal-sampled.”
Format resolution is simply the maximum horizontal and vertical resolution that is specified by the ATSC specification for a format. For 720p, this is 1280x720, and for 1080i it is 1920x1080. The other primary category is the maximum horizontal and vertical resolution that is specified for a specific codec. Examples of codecs include HDCAM and DVCPRO HD.
When a codec matches a format, it is neither oversampling nor undersampling the format. For example, the HDCAM SR (H.264/AVC) MPEG-4 codec equal-samples the 1080i (1920x1080) format.
When image sensors match a format, they are neither oversampling nor undersampling the format. The CCDs used in HDCAM cameras have 1920x1080 elements and so can be considered to equal-sample the image with respect to 1080i format.
When image sensors match a codec, they are neither over-sampling nor under-sampling the codec. The CCDs used in Sony’s HDC-1000 studio camera have 1920x1080 pixels, which match the HDCAM SR MPEG-4 codec. (They can capture 1920x1080 progressive video at up to 60Hz.)
Codecs for both 1080i HDV (MPEG-2, MainProfile@1440) and HDCAM support a resolution of 1440x1080. The DVCPRO HD codec supports two resolutions: 1280x1080 (for 1080i) and 960x720 (for 720p). These codecs all undersample their formats.
Currently, there are no codecs that oversample a format — although theoretically MPEG-2, MP@H1440 oversamples the 720p format. Likewise, only a few exotic cameras such as the Thomson Viper (1920x4320) and Dalsa (4Kx2K) employ CCDs that oversample the 1080i (1920x1080) format.
When CCDs oversample a codec, image resolution is increased and picture quality may be enhanced. For example, HDCAM camcorders employ 1920x1080 pixel CCDs that oversample an image with respect to their 1440x1080 codec. The Sony HDC-1000 studio camera oversamples both codec and format when outputting 720p.
Likewise, when CCDs undersample a codec, image resolution is decreased and picture quality may be impaired. Until last year, we have not had to be concerned with undersampling because almost every three-chip camera had CCDs that equal-sampled both its codec and its format. This changed with the introduction of Sony’s FX1 and Z1 HDV camcorders that have 960x1080 CCDs. These CCDS undersample the 1080i format and, in theory, the MPEG-2 (High-1440) codec.
However, both Sony camcorders employ green-shift technology. By offsetting the Green imager 1/2-pixel from the Red and Blue imagers, additional luminance information is obtained. The theoretical increase is 150 percent. Unfortunately, this is a best-case number that can be obtained only when the image is static. In the real world of motion video, the luminance increment is far less - perhaps less than 125 percent.
Until last year, while cameras have had CCDs that equal- or oversample their codecs, displays have always undersampled the video being fed to them. Last year, however, we saw the introduction of progressive displays that featured 1920x1080 pixels.
Progressive displays have always been specified with a “p” after their line count. EDTVs typically were 480p or 540p displays. HDTVs were typically 720p, 768p, or 1024p displays. Obviously, the new displays with 1080 lines are simply 1080p displays. They feature nothing new, except additional rows of pixels. Yet manufactures are proclaiming a major advance called “1080p” displays.
In the coming HDV@WORK Newsletters, we’ll look more deeply at these new 1080p displays, and at both single-chip 1080i HDV camcorders and the newest low-cost HD camcorders.
By the end of the series, we will have a far more accurate understanding of 1080i.
Correction
In the last HDV@Work newsletter, a missing header (“1080i DVCPRO HD”) made a paragraph less than clear. Here is the corrected paragraph.
For 1080i DVCPRO HD, the 1280-pixel luminance signal is DV-compressed and recorded to 1/4in. tape. The Cb and Cr color difference signals are filtered to provide 640x1080 sampling for each chroma component. The result is recorded at 100Mbps. With a source that provides 1920x1080 pixels, the 1920 horizontal pixels are pre-filtered to yield 1280 horizontal samples."
I also stated that Time Warner movies were only to be released on HD-DVD. TW recently added Blu-ray. Paramount is an example of an HD-DVD-only studio.
Edgewise Media answers questions via newsletter, new book on HDV
Andrew Lock, a respected industry advisor who has many years of production experience, has a newly released book, How to Shoot, Edit and Distribute HDV. It will benefit many industry professionals with its direct approach and focus on practical tips and techniques - the same style Lock uses in answering questions for newsletter subscribers.
Readers of the company's free weekly newsletter are invited to submit their questions via email for an informed response by Lock. Questions can be related to audio, video, tape stock, lighting, postproduction - any aspect of production. Edgewise also has the ability to draw on other industry experts in order to provide helpful, practical solutions to the industries it serves.
To see a sample of the newsletter and benefit from the weekly tips, techniques, and ideas, visit the Edgewise Media homepage (www.edgewise-media.com), and enter your email address in the box on the right hand side of the page.
OpenHD Alliance expands to include 10 companies
Adobe Systems Incorporated announced the expansion of the Adobe OpenHD alliance - originally started with Dell, HP, Intel, and Microsoft - to include 10 additional partners delivering capture cards, graphics and video accelerators, audio tools, and data storage products. These new Adobe OpenHD alliance partners, each of which will be included in one or more certified turnkey solutions, include AJA, AMD, Blackmagic, Bluefish, CineForm, Focusrite, Huge/Ciprico, Matrox, NVIDIA, and Rorke. This expansion, along with the introduction of Adobe Production Studio, has enabled the creation of seven new Adobe OpenHD turnkey solutions supporting HDV to realtime compressed and uncompressed HD, which will be available starting in February.
Adobe has worked closely with the OpenHD alliance partners, testing the Adobe Production Studio with each of the specific capture cards and hardware configurations, to help ensure the highest performance and reliability. The combination of Adobe Production Studio and Adobe OpenHD certified systems means that creative professionals will now have access to the scalability and reliability they need to support their HDV, SD, and HD workflows.
“We’re excited to add such an impressive list of companies to the Adobe OpenHD alliance, expanding our ability to provide the most comprehensive desktop HD offerings on the market today,” said Jim Guerard, vice president of web and video products for Adobe. “The new turnkey HD systems provide solutions to address the needs of creatives and videographers up to the largest postproduction houses. With each milestone we reach and each new partner that comes aboard, the Adobe OpenHD alliance is making HD technology more affordable and accessible to a wider audience of creative professionals.”
Adobe OpenHD was started in April 2005 by Adobe, Dell, HP, Intel and Microsoft to deliver a line of integrated, certified, Windows-based HD solutions. Through a joint certification program, these companies help ensure compatibility between hardware and software components, simplifying the purchasing process and giving customers the confidence that they are buying a validated turnkey system.
“Adobe's OpenHD technology standard will drive mainstream HD productions with greater plug-and-play options,” said Paul Sidlo, president and founder of REZN8 Productions. “With Adobe aligning and certifying its software with leading technology partners to deliver powerful end-to-end solutions, it is empowering production houses such as REZN8 to tap into the true creative potential of leading-edge high-definition design.”
Continue the discussion on “Crosstalk” the Millimeter Forum.
