Macroblock

 The four 8 by 8 blocks of luminance data and the two (for 4:2:0 chroma format), four (for 4:2:2 chroma format) or eight (for 4:4:4 chroma format) corresponding 8 by 8 blocks of chrominance data coming from a 16 by 16 section of the luminance component of the picture. Macroblock is sometimes used to refer to the pel data and sometimes to the coded representation of the pel values and other data elements defined in the macroblock header. The usage should be clear from the context.

Model-Based Coder

 Communicating a higher-level model of the image than pixels is an active area of research. The idea is to have the transmitter and receiver agree on the basic model for the image; the transmitter then sends parameters to manipulate this model in lieu of picture elements themselves. Model-based decoders are similar to computer graphics rendering programs.

The model-based coder trades generality for extreme efficiency in its restricted domain. Better rendering and extending of the domain are research themes.

Modem (Modulator/demodulator)

An electronic device for converting between serial data (typically RS-232) from a computer and an audio signal suitable for transmission over telephone lines. The audio signal is usually composed of silence (no data) or one of two frequencies representing 0 and 1. Modems are distinguished primarily by the baud rates they support which can range from 75 baud up to 19200 and beyond.

Data to the computer is sometimes at a lower rate than data from the computer on the assumption that the user cannot type more than a few characters per second. Various data compression and error algorithms are required to support the highest speeds. Other optional features are auto-dial (auto-call) and auto-answer which allow the computer to initiate and accept calls without human intervention.

Motion compensation

The use of motion vectors to improve the efficiency of the prediction of pel values. The prediction uses motion vectors to provide offsets into the past and/or future reference pictures containing previously decoded pel values that are used to form the prediction error signal.

The book Motion analysis for Image Sequence Coding by G.Tziritas and C.Labit documents the technical advances made through the years in dealing with motion in image sequences.

Motion estimation

The process of estimating motion vectors during the encoding process.

Motion vector

 A two-dimensional vector used for motion compensation that provides an offset from the coordinate position in the current picture to the coordinates in a reference picture.