Video encoding technology
May 1, 2009 12:00 PM, By John Watkinson
Today's electronic imaging technology has come a long way from the first, pre-WWII, monochrome TV services that began the long-time competition with the cinema.
The invention of pulse-code modulation allowed analog signals to be expressed by binary number, which inevitably and irrevocably forged a link between computing and audiovisual information. Once audio and pixel information are expressed by binary numbers, the resulting data are distinguished from other types of data, such as text, only by the fact that they need to be reproduced with the original time base. Computing, which we now call information technology (IT), is adept at processing, storing and networking data. With the advent of error-correcting techniques like Reed-Solomon coding, such data could be preserved to arbitrary accuracy, although the result of the use of error correction in digital television broadcasts is that the compression artifacts are delivered accurately.
The spectacular growth of IT led to computers shrinking from the size of a house down to the size of a match head, along with a comparable reduction in price. As a result, computers today are essentially consumer products. One of the unfortunate consequences is the ubiquity of consumer-grade software that is quite unsuitable for anything important. Another consequence is that the television and cinema industries found the computer to be a double-edged sword because it helped them produce material more quickly and efficiently while at the same time presenting their audiences with an alternative medium in the shape of the Internet.
Compression techniques
Electronic images have always required a lot of bandwidth, and compression techniques have been used since the earliest days of television. The use of gamma allows the same perceived quality to be obtained at a lower signal-to-noise ratio. Color difference signals need less bandwidth than RGB. Interlace is a compression technique that results in well-known artifacts. Composite video, such as NTSC, allows color in the same bandwidth as monochrome.
Information theory tells us that the greater the compression factor, the more complex the processing. While composite video and interlace are easily performed in the analog domain, the adoption of digital techniques allows greater complexity at lower cost. While the IT industry has lossless codecs that deliver bit-accurate pixels, the possible compression factors are not considered high enough for television. As a result, TV codecs are lossy. The decoded signal is not as good as the original. Compression also increases the characteristic time span of the signal. The four-field sequence of NTSC and the group of pictures in MPEG are direct parallels.
Compression can take place within individual pictures by identifying plain areas like sky or repetitive patterning. This is called intracoding or spatial coding. Compression can also take place between successive pictures, and this is even more successful when combined with compensation for object motion. This is known as intercoding or temporal coding. Groups start with an anchor picture and alter it to move forward. Some of the pictures are recreated by taking parts of earlier or later pictures, moving them across the screen to compensate for motion and only using new information for filling in the gaps. It could be likened to making a meal out of kitchen scraps.
Temporal coding is the more powerful of the two techniques, which is why delivery codecs run with long picture groups. Of course, long group coding makes production difficult. In order to perform any production step, the material first has to be decoded and then re-encoded. The problem occurs when a former “kitchen scraps” picture is encoded as an anchor. The generation loss is breathtaking.
So while long group compression is ideal for final delivery of video to the consumer, the generation loss due to temporal coding means you would only suggest it for production purposes if you had a serious conflict of interest. For questionability, it's right up there with using interlace for HD.
Moving image compression
It is disappointing that HDTV appears to be the same juddery old thing but with more pixels. The greatest technical shortcoming in television has always been the inadequate frame rates and the poor motion portrayal that results. The most tangible improvement in television comes not from increasing the static resolution, but from improving the dynamic resolution by increasing the frame rate. In a compressed delivery environment, given that temporal coding is more efficient than spatial, increasing the frame rate doesn't increase the bit rate much, whereas increasing static resolution drives the bit rate up dramatically without a corresponding quality increase.
At the time of writing this article, moving image compression seems to have settled into a number of basic applications. Digital cinema requires high pixel counts, and the contrast ratio possible in the cinema demands a greater number of bits in the pixel. On the other hand, digital cinema does not have a bandwidth problem. Cinemas can use fiber-optics networks or download data in non-real time to local file servers. Digital cinema exploits that freedom to use relatively mild compression techniques that produce pictures that are substantially free from compression artifacts. For production purposes, digital cinema recorders may use lossless or mild spatial coding.
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