Cryptography Reference
In-Depth Information
the motion vectors to get the predicted MB. This motion compensated MB
is subtracted from the actual P-frame to produce the motion compensated
error, which is also Intra coded. The motion vector for this MB is DPCM
coded and variable length coded, which is eventually multiplexed with the
prediction error information to form the bitstream of a P-block. As P-frame
is also used as reference frame for predictive coding of other P-frames and
B-frames, the P-frame will also be decoded and stored as previous frames.
The encoder now starts encoding the B-frames that were skipped previ-
ously when the P-frame was coded. At this time K
2
and K
5
are switched
off while the others are switched on as B-frame will not be used as reference
frame. The motion estimator operates twice and calculates forward vectors
from the previous I- or P-frame and backward vectors from the future I- or
P-frame. These are used by the motion compensator to create three predicted
frames: forward, backward, and interpolated. The input B-frame is subtracted
from these predicted frames to form the prediction error, which will be Intra
coded as before. The motion vector for this MB is, as in the P-block, also
multiplexed with the error information to form the bitstream of the B-frame.
The output buffer is used as the data meter to monitor data flow out of the
encoder. If the buffer is very full and is in danger of overflowing, the quantizer
will increase its step size to reduce the data rate. If the buffer dries up, the
quantizer will decrease its step size to increase the data rate. The quantization
parameters will also be embedded into the output bitstream so the decoder
knows which quantization parameters to choose.
To decode an I-frame, the switch K
6
and K
7
are both off. So an I-picture is
simply variable length decoded, de-quantized and Inverse DCT transformed.
This I-frame will need to be stored as the reference frames for decoding of
following P- and B-frames.
To decode a P-frame, the switch K
6
is on but K
7
is off. The motion
vector is de-multiplexed and used by the motion compensator to get motion
compensated MB in the reference frame. The incoming prediction error is
Intra decoded and added to the motion compensated MB to reconstruct the
P-block. The P-frame also is stored as the reference for decoding of other P-
and B-frames.
The decoding of the B-frame is very similar to that of P-frame. The motion
vector is de-multiplexed and used to fetch the reference MB from the I- or
P-frames to create the predicted MB. The prediction-error are decoded and
added to the predicted MB to reconstruct the B-frame.
2.5 Overview of Video Coding Standards
Video communication is greatly dependent on good standards. The presence of
standards allows for a larger volume of information exchange using a universal
language that is understood by all parties, thereby benefiting the equipment
