Digital conversion
Dec 1, 2006 12:00 PM, BY ALDO CUGNINI
Digital conversion
In the digital world, the conversion between different scanning systems and resolutions is based on the same analog conversion techniques. However, the overall encoding is quite different when considering compression. A full digital standards converter thus adds the burden of conversion between different compression systems.
With MPEG-2 now a ubiquitous world standard, conversion between different compressed sources seemed to be straightforward, or even trivial. But the introduction of new standards, such as MPEG-4, keeps things interesting.
Image compression
The astute reader may have deduced that image compression may offer a shortcut to motion-compensated scan conversion, as MPEG encoding already performs motion estimation. However, this process in an encoder is aimed at lowering the energy in the frame-to-frame difference of images, and this is done on a block-by-block basis, without regard to visual objects in the image. (While certain parts of MPEG-4 actually do code visual objects within pictures, the more frequently used MPEG-4 Part 10, also called AVC or H.264, does not.)
Therefore, a motion-compensated scan converter cannot base its conversion exclusively on the MPEG motion vectors within the stream. But it can use these as a starting point to arrive more efficiently at the needed information.
DCT and quantization
In order to transcode between compressed signals, such as between MPEG-2 and MPEG-4, the brute force method is to completely decode the source and then re-encode the signal. However, this can often result in a substantial degradation of video quality, especially if the compression ratio is high.
Figure 3. Interlaced scan in a vertical-temporal image sampling scheme.
A better conversion technique is to partially decode the source, and then re-encode from this point, while paying special attention to certain coding elements, namely, Discrete Cosine Transform (DCT) and quantization. (DCT is essentially a way of converting the spatial information in a block of pixels to an array of frequency information.) After this process is performed, the resulting coefficients can be quantized, or lowered in amplitude resolution.
This step actually performs the signal compression by reducing the number of bits required to represent each block of pixels. It also creates visual artifacts and limits the number of successive encode/decode cycles that are tolerable.
For this reason, successive recoding of pictures will result in fewer artifacts if the previous information on quantization is preserved as much as possible. In fact, even if this information is not used — if all the pixel blocks line up exactly where they were in the previous encoding — a subsequent encoder will often process the images in the same manner as the previous. This will cause fewer artifacts than a completely independent recoding.
Combining these effects, a well-designed integrated format converter should always yield better resultant video than the brute force method. Some of these same considerations apply to audio. For example, when converting between different perceptual coding systems, such as Dolby and MPEG, a better result should occur when the hardware takes into account the previously applied encoding and only partially decodes the signal.
Multiple generations
Always expect artifacts to become more apparent when multiple generations of encoding and decoding are applied. A recent widely broadcast sports even unfortunately demonstrated the results of a poor concatenation of standards converters. Choosing appropriate equipment, based on knowledge of how these conversions work, can go a long way to maintaining the highest quality video and audio.
Aldo Cugnini is a consultant in the digital television industry.
Send questions and comments to: aldo.cugnini@penton.com
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