Macroblocking and Pixelation - Video Artifacts

Macroblocking error caused in transmission error. Simulated by removing a random amount of data from a video file.
Macroblocking error caused in transmission error. Simulated by removing a random amount of data from a video file. Likaki Photos/Wikimedia Commons

What Macroblocking Is

Macroblocking is a video artifact in which objects or areas of a video image appear to be made up of small squares, rather than proper detail and smooth edges. The blocks may appear throughout the image, or just in portions of the image. The causes of macroblocking are related to one or more of the following factors: video compression, data transfer speed, signal interruption and video processing performance.

When Macroblocking Is Most Noticeable

Macroblocking is most noticeable on cable, satellite, and internet streaming services as those services sometimes employ excessive video compression in order to squeeze more channels within their bandwidth infrastructure.

Macroblocking can also occur, to a lesser degree, during over-the-air TV broadcasts. Its effects are more visible in program segments with lots of motion (football is a common example) as that requires more video data to be transferred at any given moment.

Another factor that can cause macroblocking is the intermittent interruption of the broadcast, cable or streaming signal. If this occurs, you may see a momentary still image displayed on your TV or projection screen that is composed of squares and horizontal or vertical bars.

Macroblocking can also be the result of poor video processing and upscaling by the playback or display device.

 For instance, if you have an upscaling DVD player that cannot process and upscale video from standard to HD resolution fast enough, you may see some intermittent instances of macroblocking, again, most likely during scenes with lots of motion or panning backgrounds.

Pixelation

Macroblocking is also sometimes referred to as pixelation, and although they are similar, pixelation is a less dramatic, more stair-step type of effect that is sometimes visible along edges of objects in relation to a background, or interior object edges, such as hair on a head or body.

Pixelation gives objects a rough appearance, and depending on the resolution of the image, the size of the screen or how close or far you sit from the screen, the effect of pixelation may be more or less noticeable.

The best way to understand pixelation is to take a photo using a digital camera or phone and view it on your PC. Then zoom in or blow up the size of the image. The more you zoom in or blow up the image, the rougher the image will look, and you will begin to see jagged edges and loss of detail. Eventually, you will begin to notice that small objects and the edges of large objects begin to look like a series of small blocks.

Macroblocking and Pixelation on Recorded DVDs

Another way that you might encounter macroblocking and/or pixelation is on homemade DVD recordings. If your DVD recorder (or PC-DVD writer) does not have an adequate disc writing speed or you select the 4, 6, or 8 record modes (which increase the amount of compression used) in order to fit more video time on the disc, the DVD recorder may not be able to accept the amount of incoming video information. As a result, you may end up with both intermittent dropped frames, pixelation and even periodic macroblocking effects.

In this case, since the dropped frames and pixelation and/or macroblocking effects are actually recorded onto the disc, no additional video processing built into a DVD player or TV can remove them.

Final Take

On the positive side on these issues, improved video compression codecs (such as Mpeg4 and H264) and more refined video processors and upscalers have reduced instances of macroblocking and pixelation across the board from broadcast, cable and streaming services, but signal interruption is sometimes unavoidable.

One interesting thing about macroblocking and pixelation is that sometimes they are generated on purpose, such as when peoples' faces, car license plates, private body parts or other identifying information is purposely obscured by the content provider from being visible by TV viewers.