Dear Professor Phil:
What causes “tiling” in video, particularly when watching HDTV?
Jose, Tampa, Fl
To understand what causes tiling, we must first understand certain aspects of MPEG compression. This is the compression method used by nearly all modern video systems.
The camera typically records 30 frames per second. In a frame, MPEG compression breaks the image into square macroblocks. In each macroblock, the luminance (brightness) value is measured at 16 points in each direction, horizontally and vertically. This yields 256 data values. The compression algorithm uses the average of these 256 values as the basis of a mathematical process called the DCT (discrete cosine transform). Color representation is done in a similar manner but uses fewer data points.
When the compressor analyzes the various blocks from the frame, it may find that there is a high level of spatial redundancy. This means that some blocks contain result in data that is almost identical to other blocks. If this happens, the compressor may decide to use the average from block as a reference point. Then it can calculate the difference of the nearly identical blocks. This results in much smaller data sets. If reference block is on the left of a group and the other blocks are to its right, it creates what is called a slice.
Now, suppose a block is incorrectly decoded by a receiver (TV) because part of the data was lost in transit. This will cause a distorted block on the television. If the block was the left-most block in a slice, the entire slice will be distorted. Carefully studying the tiles often will reveal that they are black or a uniform color that is represented somewhere nearby in the scene. The first instance is usually the result of an incorrect calculation in the brightness value. The second occurs because the decoder decides to use the last value it calculated or an overall average value for a slice.
Phil Hippensteel is a professor of information systems at Penn State Harrisburg. If you have a question for Professor Phil, email us at AVTIntern@nbmedia.com.