Noise Filters

The role of noise filters in preprocessing in a codec is a double edged sword, some encoders attempt to use heavy filtering to reduce noise and detail because this reduction can result in pictures that are encoded easily with less compression artifacts. H.264 (MPEG4-AVC) includes the use of an in-loop deblocking filter which explicitly will target this, however, while reducing the blocky artifacts, the result is often video that looks washed out and lacking in detail. This has to be used carefully and should be done only with extremely low bitrate video at low resolutions. It should be used minimally for DVD movie quality media as the loss of detail from too heavy a filtering can be unacceptable.

Among filtering options, Xcode II provides an advanced edge-detection filter that is intended to reduce noise, but prevent blending of edges. This preserves detail and reduces random speckle noise.

Inverse Telecine.

NTSC is 59.94 fields/s interlaced. While this is roughly treated as 30 frames/s, the pictures are arriving a field at a time. However, movies are generally shot on film at 24fps. When the movie is deliverd on an analog video signal via NTSC, fields are repeated in a 3:2 pattern to use up all the (roughly) 60 fields. Because some of these fields are repeated, an advanced video capture digital preprocessor subsystem can detect these duplicate fields and not encode them. Such detection known as inverse-telecine can gives us two advantages :

. Less pixel content – by going from 60 fields (30 frames) to 24 frames, we are encoding 20% less information, this results in more efficient and better quality compression at the same bitrate.
. DeInterlacing. By detection that the material is progressive, this will aid in the motion search to proceed using frame motion instead of field motion and result in better motion estimation.

If Inverse Telecine is to be used, it has to be done with all captured fields as changing between commercials and film can happen anytime on TV inputs.

Summary of the ViXS XCODE-II

The ViXS XCODE family of chips were designed to extend the limits of what is possible in an MPEG2 codec solution. XCODE-II is the latest offering in that family. Some of the decision points in the algorithms are more complex and hence we chose the hybrid programmable Processor to solve the complex decisions and handle exceptions. However as the logic and arithmetic operations required are quite large, parallel vector processors are required to deliver enough horsepower to complete the tasks. We have worked on several key inventions that we believe are at the cutting edge of what is known and not known about MPEG2, and thus we believe we are pushing the envelope where in practical terms, we are delivering the Purest Video possible at the most cost effective value for the multimedia AV system designer.

Conclusion

In this paper, I have discussed some of the background and issues one must confront in building silicon for a digital video processor codec application. We briefly covered the technical challenges and the proposed solution from algorithmic and IC concerns, and it is appropriate to conclude with some simple observations to help in picking the video codec to use.

. There are rich areas of new Digital Video Applications that can be enabled with a video processor capable of codec as well as transrating and transcoding at high speeds and at high quality. Choosing a codec IC with transrating capability and enabling these applications will result in richer end-user experience and greater marketability of a product.

. Whether dealing with transcoding or encoding, there is a large difference in the CODEC chips offered today. The true challenge of an efficient encoder solution is one that can perform well at capture and encoding of 720x480i (interlaced) materials at 1.5 to 2Mbps today, regardless of actual codec standard used.

. When the video source is analog, video acquisition plays a large role in ensuring video quality.

. When there is an analog tuner, temporal noise reduction plays a large role in ensuring video quality

. The best AVC encoder will outperform the best MPEG2 encoder at . the bitrate.

. The best MPEG2 encoder can still match or even outperform a mediocre AVC encoder

References

ISO/IEC 14496-10:2004, Advanced Video Coding (2nd Ed), Mar 2004.
ISO/IEC 13818-2 (MPEG-2) Generic coding of moving pictures and associated audio
information – Part 2 : Video, Nov 1994

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