http://en.wikipedia.org/wiki/Statistical_multiplexing
Multiplexing is a common compression technique. Basically, when a video sequence is somewhat static, the bandwidth to that image decreases to allow bandwidth to increase on other channels on the same frequency with more dynamic video sequences.
Cable companies pump anywhere between 3 and 15 channels down each 6mhz frequency band. More channels per frequency = more multiplexing.
As the static to dynamic ratio changes bandwidth is allocated back and forth between channels, which can cause all kinds of artifacts, the most common being pixelization. You might notice during sporting events that as the camera sweeps from one end of the field to another the resolution drastically decreases for a brief moment. This is a direct result of multiplexing.
But in answer to your question... yes, facial images are the first thing to degrade generally, especially in the case of a talking head, because almost everything else in the video sequence is static except for the face of the newscaster. That would be the first place you would notice compression artifacts.
http://www.ciscopress.com/articles/article.asp?p=106971&seqNum=3
MPEG-2 is a lossy video compression
method based on motion vector
estimation, discrete cosine
transforms, quantization, and Huffman
encoding. (Lossy means that data is
lost, or thrown away, during
compression, so quality after decoding
is less than the original picture.)
Taking these techniques in order:
Motion vector estimation is used to
capture much of the change between
video frames, in the form of best
approximations of each part of a frame
as a translation (generally due to
motion) of a similar-sized piece of
another video frame. Essentially,
there is a lot of temporal redundancy
in video, which can be discarded. (The
term temporal redundancy is applied to
information that is repeated from one
frame to another.)
MPEG-2 Artifacts
What are MPEG artifacts? In practice,
all lossy encoders generate artifacts,
or areas of unfaithful visual
reproduction, all the time; if the
encoder is well designed, all these
artifacts will be invisible to the
human eye. However, the best laid
plans sometimes fail; the following
are some of the more common MPEG-2
artifacts:
If the compression ratio is too high,
there are sometimes simply not enough
bits to encode the video signal
without significant loss. The better
encoders will progressively soften the
picture (by discarding some picture
detail); however, poorer encoders
sometimes break down and overflow an
internal buffer. When this happens,
all kinds of visual symptoms—from
bright green blocks to dropped
frames—can result. After such a
breakdown, the encoder will usually
recover for a short period until once
again the information rate gets too
high to code into the available number
of bits.
Another common visible artifact is
sometimes visible in dark scenes or in
close-ups of the face and is sometimes
called contouring. As the name
suggests, the image looks a little
like a contour map drawn with a
limited set of shades rather than a
continuously varying palette. This
artifact sometimes reveals the
macro-block boundaries (which is
sometimes called tiling). When this
happens, it is usually because the
encoder allocates too few quantization
levels to the scene.
NOTE
Macro-blocks are areas of 16-by-16
pixels that are used by MPEG for DCT
and motion-estimation purposes. See
Chapter 3 of Modern Cable Television
Technology; Video, Voice, and Data
Communications by Walter Ciciora and
others, for more details.
High-frequency mosquito noise will
sometimes be apparent in the
background. Mosquito noise is often
apparent in surfaces, such as wood,
plaster, and wool, that contain an
almost limitless amount of detail due
to their natural texture. The encoder
can be overtaxed by so much detail and
creates a visual effect that looks as
if the walls are crawling with ants.
There are many more artifacts
associated with MPEG encoding and
decoding; however, a well-designed
system should rarely, if ever, produce
annoying visible artifacts.
and further down in this link:
Statistical Multiplexing
Statistical multiplexing is a
technique commonly used in data
communications to extract the maximum
efficiency from a CBR link. A number
of uncorrelated, bursty traffic
sources are multiplexed together so
that the sum of their peak rates
exceed the link capacity. Because the
sources are uncorrelated, there is a
low probability that the sum of their
transmit rates will exceed the link
capacity. However, although the
multiplex can be engineered so that
periods of link oversubscription are
rare, they will occur. (See Murphy’s
law!) In data communications networks,
periods of oversubscription are
accommodated by packet buffering and,
in extreme cases, packet discard. (The
Internet is a prime example of an
oversubscribed, statistically
multiplexed network where packet delay
and loss may be high during busy
periods.)
Video material has a naturally varying
information rate—when the scene
suddenly changes from an actor sitting
at a table to an explosion, the
information rate skyrockets. Although
MPEG-2 is designed to compensate by
encoding more or less detail according
to the amount of motion, the encoded
bit rate may vary by a ratio of 5 to 1
during a program.
