I'm going to cover 3 filters here, but there are a lot more that are either built into AviSynth or available as plugins. For more information you can always check out my AviSynth guide here at AfterDawn, the AviSynth homepage (www.avisynth.org) or Doom9's AviSynth forums. The 3 filters I use regularly are Undot, Deen, and Greyscale.
Just like the name suggests, Undot removes small dots from the video. Although many people use it for TV captures, it's also good for filtering DVD sources. Its biggest strength is that it usually doesn't make major changes to the source. That also means it doesn't always have a great deal of impact on compression, but it's probably the most useful general purpose noise removal filter available.
It's great for removing the grain and age related noise from film sources, both of which use bits that could be better spent on the actual movie.
Deen is a 3D denoiser. In English this means that it makes changes based not only on the surrounding pixels of the current frame, but also the content of surrounding frames. This is important for denoising since it's basically just a selective smoothing of details. I don't recommend using this filter unless you have to compress the DVD a lot with Rebuilder or have a poor quality source to begin with. Smoothing is very helpful to CCE because sharp edges and fine details are some of the biggest challenges for lower bitrate encoding.
This filter changes everything that isn't already black, white, or grey to a shade of grey. This won't necessarily improve quality, but since pure black and white video requires fewer bits than color, it will reduce the bits wasted on noise, thereby increasing the bitrate available to the actual movie.
Written by: Rich Fiscus