MPEG video compression - The MPEG - page 201

However, in many, perhaps most, applications of MPEG, the bit rate is not decided by the encoder, but by some
other decision process which may be economically biased. All MPEG coders contain an output buffer, and if the
allocated bit rate is not adequate, this will start to fill up. To prevent data loss and decoder crashes, the encoder
must reduce its bit rate.
It cannot make any economy in the lossless coding. Reducing the losslessly coded information would damage the
prediction mechanism and increase the residual. Reducing the accuracy of vectors, for example, is
counterproductive. Consequently all that can be done is to reduce the bit rate of the residual. This must be done
where the DCT coefficients are quantized because this is the only variable mechanism at the encoder's disposal.
When in an unsympathetic environment, the coder has to reduce the bit rate the best it can.
The result is that the output pictures are visually different from the input pictures. The difference between the two is
classified as coding noise . This author has great difficulty with this term, because true noise should be decorrelated
from the message whereas the errors of MPEG coders are signal dependent and the term coding distortion is more
appropriate. This is supported by the fact that the subjective visual impact of coding errors is far greater than would
be caused by the same level of true noise. Put another way, quantizing distortion is irritating and pretending that it
is noise gives no assistance in determining how bad it will look.
In motion-compensated systems such as MPEG, the use of periodic intra-fields means that the coding error may
vary from picture to picture and this may be visible as noise pumping . The designer of the coder is in a difficult
position as the user reduces the bit rate. If the I data are excessive, the P and B data will have to be heavily
quantized resulting in errors. However, if the data representing the I pictures/objects is reduced too much, P and B
pictures may look better than the I pictures. Consequently it is necessary to balance the requantizing in the I , P and
B pictures/objects so that the level of visible artifacts in each remains roughly the same in each.
Noise pumping may also be visible where the amount of motion changes. If a pan is observed, as the pan speed
increases the motion vectors may become less accurate and reduce the quality of the prediction processes. The
prediction errors will get larger and will have to be more coarsely quantized. Thus the picture gets noisier as the
pan accelerates and the noise reduces as the pan slows down. The same result may be apparent at the edges of a
picture during zooming. The problem is worse if the picture contains fine detail. Panning on grass or trees waving in
the wind taxes most coders severely. Camera shake from a hand-held camera also increases the motion vector
data and results in more noise, as does film weave.
Input video noise or film grain degrades inter-coding as there is less redundancy between pictures and the
difference data become larger, requiring coarse quantizing and adding to the existing noise.
Where a codec is really fighting the quantizing may become very coarse and as a result the video level at the edge
of one DCT block may not match that of its neighbour. As a result the DCT block structure becomes visible as a
mosaicing or tiling effect. MPEG-4 introduces some decoding techniques to filter out blocking effects and in
principle these could be applied to MPEG-1 and MPEG-2 decoders.
Coarse quantizing also causes some coefficients to be rounded up and appear larger than they should be. High-
frequency coefficients may be eliminated by heavy quantizing and this forces the DCT to act as a steep- cut low-
pass filter. This causes fringeing or ringing around sharp edges and extra shadowy edges which were not in the
original and is most noticeable on text.
Excess compression may also result in colour bleed where fringeing has taken place in the chroma or where high-
frequency chroma coefficients have been discarded. Graduated colour areas may reveal banding or posterizing as
the colour range is restricted by requantizing. These artifacts are almost impossible to measure with conventional
test gear.
Neither noise pumping nor blocking are visible on analog video recorders and so it is nonsense to liken the
performance of a codec to the quality of a VCR. In fact noise pumping is extremely objectionable because, unlike
steady noise, it attracts attention in peripheral vision and may result in viewing fatigue.
In addition to highly detailed pictures with complex motion, certain types of video signal are difficult for MPEG to
handle and will usually result in a higher level of artifacts than usual. Noise has already been mentioned as a
source of problems. Timebase error from, for example, VCRs is undesirable because this puts succesive lines in
different horizontal positions. A straight vertical line becomes jagged and this results in high spatial frequencies in
the DCT process. Spurious coefficients are created which need to be coded.
Much archive video is in composite form and MPEG can only handle this after it has been decoded to components.
Unfortunately many general-purpose composite decoders have a high level of residual subcarrier in the outputs.
This is normally not a problem because the subcarrier is designed to be invisible to the naked eye. Figure 5.92
shows that in PAL and NTSC the subcarrier frequency is selected so that a phase reversal is achieved between
successive lines and frames.