Cure for concatenation
Mar 1, 2009 12:00 PM
Use precision filter technology to preserve 4:2:0 chroma integrity in concatenated operations.
Precision filter technology
One solution now available uses a precise set of filters that process the 4:2:2 chroma component and downsamples it with a low-pass filter and 2:1 vertical sampler to create a 4:2:0 output. At the receive end, the 4:2:0 chroma component is accurately upsampled to recreate a 4:2:2 signal, as shown in Figure 4, that can be passed to the next concatenated link over an HD/SD-SDI interconnect.
This process allows an encoded 4:2:0 stream to be carried efficiently over any digital communications link.
Figure 5. 4:2:2 downsampling/4:2:0 upsampling process
Click to enlarge
To maintain non-degraded 4:2:0 chroma resolution throughout the concatenated chain, it is necessary for all encoders and decoders in the transmission link to use identical filter sets. However, the 4:2:0 stream may be decoded with degradation, but without undue errors or color displacement by decoders not fitted with the precision filter technology. (See Figure 5.)
The precision filter technology has been proven in tests demonstrating its ability to protect the integrity of the 4:2:0 chroma component in up to 16 concatenated encode and decode operations. Now, let's examine the protection that such precision filter design provides.
In a series of tests, a comparison between the original chroma PSNR was made with that at the output of the second, fourth, eighth and 16th concatenation of the 4:2:2/4:2:0 conversions. Test results shown in Figure 6 demonstrate chroma degradation in conventional concatenated operations and the level of protection provided by the non-degraded 4:2:0 precise filter solution that maintained PSNR close to that of the original 4:2:2 source.
Figure 6. Chroma PSNR of conventional and non-degraded technology
Click to enlarge
It is generally recognized that downsampling and upsampling is more difficult for interlace than for progressive video largely because of nonlinear phase characteristics. The generation of unwanted artifacts is also more pervasive. Even so, the precise filtering discussed here properly handles both video formats.
It appears that such a solution can enable longer-distance encoded video communications links with less degradation. As such, many of the problems of multiple concatenations could become a thing of the past.
Author's note: The original research and material of Akira Nakagawa of Fujitsu Laboratories contributed to this article.
Keith Dunford is currently a consultant for the video solutions group of Fujitsu Computer Products of America as well as managing partner of The Exam Group.
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