Introduction

Digital noise reduction (DNR) in the context of DVD mastering and film to digital video transfers (including High Definition) is a process which uses a digital filtering algorithm on the digital image data to reduce the amount of random noise (like film grain, electronic noise of the telecin�, comb filter artifacts in composite video sources, film speckles, dirt, scratches etc.). Why is this desirable, if at all?

To reduce the visibility of the random noise during playback of the video. Obvious film grain, scratches, dirt particles and the like are usually considered unwanted for aesthetic reasons if not annoying, hence the wish to get rid of it. Pleasing customers and avoiding complaints is the purpose here.
To reduce entropy (information content) in the images which allows for more efficient compression at a lower bit rate without unwanted and distracting compression artifacts. Here cost and bit saving is the desired goal. (Entropy can also be removed with a low pass filter that removes all fine detail beyond a certain frequency. This creates no noise reduction artifacts but also removes no noise, just blurrs it like the rest of the image.)
To restore a film on video to a state closer to its original state on film before degradation happened. Film restoration is the goal.
Obviously these goals are not necessarily compatible or only to some extent.

How is it done? Since random noise is random it's generally not possible to perfectly remove it and unveil what has been poluted or erased by the noise. But using the high degree of redundancy present within and in consecutive film/video frames and using knowledge about the nature of the noise (such as its expected value and standard deviation) mathematically derived/optimised filters have been suggested and are used to deal with different kinds of noise. These filters come in different flavours:

Hardware implemented real time filters versus computationally very costly non real time filters
Motion adaptive filters versus non motion adaptive filters
Motion compensated filters versus non motion compensated filters
Filters optimised for scratches, speckles or general random noise
People working with digital noise reduction in the video/film industry usually distinguish between noise reduction, which reduces film grain, electronic noise from sensors and other types of general random noise, and specialised algorithms that go for specific kinds of noise, such as scratch concealment going after scratches and dust busters going after dust particles, speckles and the like. Mathematically speaking it's all digital noise reduction/removal, though. And that's how the term is used on this page.

Motion is very important since the data redundancy comes either from within the same frame or consecutive frames in time. If the redundancy of consecutive frames is to be used one must pay attention to the fact that corresponding parts of the image are not generally at the same place in consecutive images since the represented objects do move in time and space and therefore also on the images. Finding the corresponding image parts by calculating the motion from frame to frame is in general a very difficult and computationally intensive task. Ignoring the motion or getting it not right inevitably creates incorrect filtering results when several frames are combined in a filtering operation. These filtering errors may be visible to the human eye or not, but they usually are, so care is required or random noise is traded in for new random noise generated by the filtering which may be more objectionable than the original noise. Naturally real time filters are limited in their precision and filters ignoring or miscalculating motion will create artifacts on moving image parts. So the best results can be expected from non real time filters with motion compensation. In DVD mastering facilities real time filters dominate that can not estimate motion correctly and therefore have to be carefully tuned to leave moving image parts alone or do not filter them much compared to non moving parts.

Noise Reduction Artifacts on DVDs

The purpose of this page is to document noise reduction artifacts on some DVDs to make a couple of points:

- Noise reduction is widely applied to DVDs, like edge enhancement. Affected are DVDs (almost) everyone knows or owns.
Noise reduction adds artifacts to many DVDs that are as objectionable as edge enhancement if not more so in some cases.
- Noise reduction can ruin the film look and give a DVD an unnatural digital video look that can be outright ugly.
- Noise reduction can make a DVD look worse than it would look without any noise reduction (and compression at a proper bit rate suitable for the amount of noise present).
- Noise reduction is visible even on simple regular TVs if you know what to look for. It affects everybody with an eye for quality, not just videophiles in their 'high end ivory towers'.

Noise reduction artifacts are pretty much artifacts caused by incorrect or missing motion estimation or misjudging image details for what they really are (treating window bars or highlights as scratches/speckles, for example). By combining image parts from several frames that do not belong together the following artifacts are usually created:
- Image detail in textures that should move naturally starts flickering when moving
- Image detail in textures that should move naturally gets smeared and blurred when moving (well beyond motion blur of film frames)
Image detail in textures that should move naturally disappears completely and reappears when motion stops.
- Objects with a high contrast to the background create multiple echoes of their edges on this background when in (fast) motion.
- Solid objects when moving get suddenly holes, break apart or are eaten into from the edges (such as letters in a text).
- Textures look strangely noisy but differently than normal noise from film grain or sensors.
Image parts that belong naturally together (like sky and earth) move independently in different directions. The effect can be a subtle floating or a surreal effect with massive movement of the parts against each other.

Scratch removal attacking real image detail causes flickering as well and removal of parts or all of tiny objects like highlights or window bars. The type of filter used and the parametrisation define which artifacts are created. Median and other order statistic filters tend to create flickering, linear filters tend to smear and create echoes.
Noise reduction can be done during telecin� or before MPEG compression. The latter is far better since the transfer itself is not affected by noise reduction artifacts and the compressionist can use as much DNR as he needs given his bit budget (he might need none). As time goes by and he gets better hardware he can do a better job in subsequent versions of a DVD. Once the damage is done in the transfer there is no way back. You have to retransfer to fix the problem.

Conclusion

Conclusion
I hope I have demonstrated a couple of points with this page:

Noise reduction and its artifacts can be found on many DVDs from all labels. It's widespread like edge enhancement.
Noise reduction artifacts go from almost invisible to easily visible.
Noise reduction artifacts come in various forms.
Noise reduction artifacts can become very distracting and the obvious ones have no place on a high quality DVD.
Sloppy noise reduction does not improve image quality, it degrades it, destroys the film look and replaces familiar film artifacts with less familiar digital processing artifacts. A very dubious trade.
The proper way to deal with a bad film master is to search for a better film master. If that's not possible noise reduction should be done with the best algorithms available to minimise artifacts. Simple film grain as authentic attribute of film should not be reduced at the cost of adding artifacts, but compressed with a suitably high bit rate

- Quoted from Michael Hafner's courtesy

Mods, can you make this topic 'Sticky'?