Macroblocking and Pixelation: Video Artifacts

What are all those squares and jagged edges I sometimes see on my TV screen?

Macroblocking error caused in transmission error. Simulated by removing a random amount of data from a video file.

Likaki Photos / Wikimedia Commons / CC BY SA 3.0

When we watch a program or movie on a TV or video projection screen, we want to see smooth clean images without disruption, and without artifacts. Unfortunately, there are times where that does not occur. Two undesirable, but common, artifacts you might see on your TV or projection screen in the course of viewing are macroblocking and pixelation.

What Is Macroblocking?

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 these services sometimes employ excessive video compression in order to squeeze more channels within their bandwidth infrastructure. Said another way, the TV can't handle the amount of data it's being asked to process, so it blocks the image together in a less data-intensive form.

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 this requires more video data to be transferred at a 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 composed of squares and horizontal or vertical bars.

Macroblocking can also be the result of poor video processing or upscaling by the playback or display device. For example, if you have an upscaling DVD player that cannot process and upscale video from standard to HD resolution fast enough, then you may see some intermittent macroblocking. This will most likely occur during scenes with lots of motion or panning backgrounds. Macroblocking might also be noticeable on TV, cable, or satellite broadcasts with very fast motion. Also, if your internet speed isn't fast enough, it could also cause macroblocking issues with streaming content.

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. It 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. 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's monitor or laptop screen. 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 you might encounter macroblocking or pixelation is on homemade DVD recordings. If your DVD recorder (or PC-DVD writer) does not have an adequate disc writing speed or if you select the 4, 6, or 8 record modes (which increase the amount of compression used) to fit more video time on the disc, then 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 or macroblocking effects are actually recorded onto the disc, then no additional video processing built into a DVD player or TV can remove them.

Compression Is Often the Cause

Macroblocking and pixelation are artifacts that can occur while viewing video content from a variety of sources. Since macroblocking and pixelation can be the result of any one of several factors, no matter what TV you have, you may experience their effects on occasion.

However, improved video compression codecs (such as Mpeg4 and H264) and more refined video processors and upscalers have reduced instances of macroblocking and pixelation. These improvements affects all media, including broadcast, cable, and streaming services, but signal interruption is sometimes unavoidable.

Also, it must be noted that macroblocking and pixelation can sometimes be generated on purpose by content creators or broadcasters, such as when peoples' faces, car license plates, private body parts, or identifying information is purposely obscured by the content provider.

This is sometimes done in TV newscasts, reality TV shows, and some sporting events where people might not have given permission to use their image. It is also used to protect suspects from being identified during an arrest or blocking out brand names affixed to tee-shirts or hats.

However, apart from purposeful use, macroblocking and pixelation are definitely undesirable artifacts you don't want to see on your TV screen.