Digital Noise Reduction on DVD: Background and Examples

by Michel Hafner - (22 February 2002) (technical consultant: John G. DeGroof) (inspiration Bjoern Roy) (for a page on edge enhancement click here) (for DVD quality evaluation in general click here)


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 teleciné, comb filter artifacts in composite video sources, film speckles, dirt, scratches etc.). Why is this desirable, if at all?

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:

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 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: 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 teleciné 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.

Methodology and Examples

Following are 11 examples from 9 DVDs. Presented are a full frame jpeg of the first frame in the animation and 3 animations. All animations are direct digital transfers from the original DVD without any processing except the choice of a suitable window of interest and recompression for animation one and two. Animation one is compressed with MPEG-1, animation two with QuickTime Photojpeg. Animation 3 is uncompressed QuickTime for skeptics who think the problems are caused by recompressing the DVD and need hard evidence that it ain't so. Naturally these uncompressed versions are huge despite lossless compression with gzip. So I do not advise to use them, but if you must you can. I have made sure that all 3 versions of every example are for all noise reduction demonstration purposes the same.

It's best to watch the animations with 24bit frame buffer hardware so full DVD resolution is given, and set the playback speed to normal 24 frames/s since these are progressive frames as on film without interlace motion artifacts. For some examples it might be useful to step through the clip frame by frame as well to see in detail what's going on.

Let's have a look at the examples one by one, shall we?


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