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erikthebassist
erikthebassist
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But in answer to your question,... yes, facial images are the first thing to degrade generally, especially in the case of a news castertalking head, because almost everything else in the video sequence is static except for the face of the newscaster, so that. That would be the first place you would notice compression artifacts.

But in answer to your question, yes, facial images are the first thing to degrade generally, especially in the case of a news caster because almost everything else in the video sequence is static except for the face of the newscaster, so that would be the first place you would notice compression artifacts.

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.

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erikthebassist
erikthebassist
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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.

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.

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erikthebassist
erikthebassist
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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 news caster because almost everything else in the video sequence is static except for the face of the newscaster, so 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.