Originally Posted by SirJW
I'm fairly new to the site and I want to make sure I'm on the same page as everyone.
What exactly is macro blocking and how is it caused?
Is it from the source or a result of the monitor?
Video has more data than can be transmitted directly in a practical manner, so compression is used. MPEG 2 is the most common, but an improved version called MPEG 4 is being phased in. DVDs use MPEG2, as well as the Over The Air (OTA) digital channels. Digital cable also uses MPEG 2. Satellite companies have been using MPEG 2, but they are moving to MPEG 4 because it offers higher amounts of compression for the same picture quality. MPEG4 is also expected to be used in the new optical disc formats.
More motion in the picture requires more data. If the speed of the motion requires more data than can be sent by a channel, the image will start to degrade into mosaic blocks. The higher the amount of data (bitrate) that a channel sends, the lower the chance that macroblocking will occur.
The big problem is that there is only so much data that cable, satellite and broadcasters can send. Broadcasters have been adding extra channels (multicasting subchannels), which reduces the amount that can be sent on for the HD channel. Cable and satellite companies want to send more channels, so they reduce bitrate they get from the channel (HD-Lite). All things being the same, lowering the bitrate will increase the chance of macroblocking.
The receiver is not at fault for this kind of problem.
MPEG 2 encoding is a lossy compression based on Discrete Cosine Transformation (DCT). It breaks the image into small rectangular areas called macroblocks. Within these blocks the grid of picture elements (pixels) are encoded to represent their horizontal and vertical video frequencies. It does this so that when it has to throw out some information, it starts with the higher frequencies (finer detail) and works its way down.
Motion vectors is another compression technique that MPEG 2 uses to take advantage of redundant frame-to-frame information. The use of motion vectors allows the amount of DCT compression to be decreased. If there is so much motion that the encoder cannot keep up, it can no longer effectively use the motion vectors, and the amount of compression is increased. Fine detail is eliminated in each of the blocks, and what's left is more of an average. Since each block probably has a different average, it makes a mosaic looking set of squares on the screen. When things slow down, and the encoder can decrease the amount of compression the detail will return.Longer Story:
research DCT and MPEG 2.
Welcome to the same page.