MPEG VIDEO CODING

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MPEG Requirements

MPEG Coding

MPEG Frame Coding

The MPEG Coder Predictive Process
picture is known as a P frame. When coding a P frame the MPEG coder requires a reference frame on which to base its prediction. This reference frame will, in many instances, be an I frame. However once a decoder has received the data for a P frame and decoded that frame, the predicted frame may be used as the reference frame for further predictions.

The data that is transmitted for a P frame must include the motion vector data for the block being predicted, plus any small corrections for inaccurate predictions. This implies that a form of differential coding is used for the error signal. A simplified block diagram of the prediction system is shown in Figure 2.3.

Figure 2.3 - Block Diagram of Prediction System

To obtain the maximum compression the prediction system tries to predict as far forward into the future as possible. Depending on the transmission data rate in use the predicted pictures may be up to about 6 frames into the future.
If the system is able to use a current frame and measured movement to predict several frames ahead, it seems reasonable that any frames that occur between the reference and predicted frames may also be predicted on the basis of linear movement between the reference and predicted frames. This is illustrated in Figure 2.4.

Figure 2.4 - Using Linear Prediction of Position

The left hand section of Figure 2.4 shows the original position of the object as the lower left corner. The movement at the original position has been measured and used to extrapolate.

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