Contexto de la Gestión en la Educación Superior

Motion video adds a temporaldimension to the spatial dimension of single

pictures. Another worldwide compression standard from the Moving Pic-

ture Experts Group(MPEG), was designed with this in mind. MPEG is sim- ilar to JPEG but also reduces redundancy between successive pictures of a moving sequence.

Just as your friend’s memory allows you to describe things once and then only talk about what’s changing, digital memory allows video to be com- pressed in a similar manner by first storing a single picture and then only storing the changes. For example, if the bird moves to another tree, you can tell your friend that the bird has moved without needing to describe the bird over again.

MPEG compression uses a similar technique called motion estimationor

pictures, many of which are very similar, each picture can be compared with the pictures near it. The MPEG encoding process breaks each picture into

blocks, called macroblocks,and then hunts around in neighboring pictures

for similar blocks. If a match is found, instead of storing the entire block, the

system stores a much smaller vectordescribing how far the block moved (or

did not move) between pictures. Vectors can be encoded in as little as 1 bit, so backgrounds and other elements that do not change over time are com-

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Figure 3.3

Block transforms and quantization

TEAM

FLY

pressed extremely efficiently. Large groups of blocks that move together, such as large objects or the entire picture panning sideways, are also com- pressed efficiently.

MPEG uses three kinds of picture storage methods.Intra pictures are

like JPEG pictures, in which the entire picture is compressed and stored

with DCT quantization. This creates a reference, or information, frame

from which successive pictures are built. These I framesalso allow random

access into a stream of video and in practice occur about twice a second.

Predictedpictures, or P frames,contain motion vectors describing the dif- ference from the closest previous I frame or P frame. If the block has changed slightly in intensity or color (remember, frames are separated into

three channels and compressed separately), then the difference (error) is

also encoded. If something entirely new appears that does not match any previous blocks, such as a person walking into the scene, then a new block is stored in the same way as in an I frame. If the entire scene changes, as in a cut, the encoding system is usually smart enough to make a new I frame.

The third storage method is a bidirectionalpicture, or B frame.The system

looks both forward and backward to match blocks. In this way, if something new appears in a B frame, it can be matched to a block in the next I frame or P frame. Thus P and B frames are much smaller than I frames.

Experience has shown that two B frames between each I or P frame work well. A typical second of MPEG video at 30 frames per second looks like I B B P B B P B B P B B P B B I B B P B B P B B P B B P B B (Figure 3.4). Obvi- ously, B frames are much more complex to create than P frames, requiring time-consuming searches in both the previous and subsequent I or P frame. For this reason, some real-time or low-cost MPEG encoders only create I and P frames. Likewise, I frames are easier to create than P frames, which require searches in the subsequent I or P frame. Therefore, the simplest

Figure 3.4

Typical MPEG picture sequence

encoders only create I frames. This is less efficient but may be necessary for very inexpensive real-time encoders that must process 30 or more frames a second.

MPEG-2 encoding can be done in real time (where the video stream enters and leaves the encoder at display speeds), but it is difficult to produce

quality results, especially with variable bit rate(VBR). VBR allows varying

numbers of bits to be allocated for each frame depending on the complexity. Less data is needed for simple scenes, whereas more data can be allocated for complex scenes. This results in a lower average data rate and longer playing times but provides room for data peaks to maintain quality. DVD encoding frequently is done with VBR and is usually not done in real time, so the encoder has plenty of time for macroblock matching, resulting in much better quality at lower data rates. Good encoders make one pass to analyze the video and determine the complexity of each frame, forcing I frames at scene changes and creating a compression profile for each frame. They then make a second pass to do the actual compression, varying quan- tization parameters to match the profiles. The human operator often “tweaks” minor details between the two passes. Many low-cost MPEG encoding hardware or software for personal computers uses only I frames, especially when capturing video in real time. This results in a simpler and cheaper encoder, since P and B frames require more computation and more memory to encode. Some of these systems can later reprocess the I frames to create P and B frames. MPEG also can encode still images as I frames. Still menus on a DVD, for example, are I frames.

The result of the encoding process is a set of data and instructions (Fig- ure 3.5). These are used by the decoder to recreate the video. The amount of compression (how coarse the quantizing steps are, how large a motion esti- mation error is allowed) determines how closely the reconstructed video

resembles the original. MPEG decoding is deterministic—a given set of

input data always should produce the same output data. Decoders that properly implement the complete MPEG decoding process will produce the

same numerical picture even if they are built by different manufacturers.7

This does not mean that all DVD players will produce the same video pic- ture. Far from it, since many other factors are involved, such as conversion from digital to analog, connection type, cable quality, and display quality. Advanced decoders may include extra processing steps such as block filter-

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7_{Technically, the inverse discrete cosine transform(IDCT) stage of the decoding process is not}

strictly prescribed, and is allowed to introduce small statistical variances. This should never account for more than an occasional least significant bit of discrepancy between decoders.

ing and edge enhancement. Also, many software MPEG decoders take shortcuts to achieve sufficient performance. Software decoders may skip frames and use mathematical approximations rather than the complete but time-consuming transformations. This results in lower-quality video than from a fully compliant decoder.

Encoders, on the other hand, can and do vary widely. The encoding process has the greatest effect on the final video quality. The MPEG stan-

dard prescribes a syntaxdefining what instructions can be included with

the encoded data and how they are applied. This syntax is quite flexible, and leaves much room for variation. The quality of the decoded video depends very much on how thoroughly the encoder examines the video and how clever it is about applying the functions of MPEG to compress it. In a

Figure 3.5

MPEG video

sense, MPEG is still in its infancy, and much remains to be learned about efficient encoding. DVD video quality steadily improves as encoding tech- niques and equipment get better. The decoder chip in the player will not change—it doesn’t need to be changed—but the improvements in the encoded data will provide a better result. This can be likened to reading aloud from a book. The letters of the alphabet are like data organized according to the syntax of language. The person reading aloud from the book is similar to the decoder—the reader knows every letter and is famil- iar with the rules of pronunciation. The author is similar to the encoder— the writer applies the rules of spelling and usage to encode thoughts as written language. The better the author, the better the results. A poorly written book will come out sounding bad no matter who reads it, but a well-

written book will produce eloquent spoken language.8

It should be recognized that random artifacts in video playback (aberra- tions that appear in different places or at different times when the same video is played over again) are not MPEG encoding artifacts. They may indicate a faulty decoder, errors in the signal, or something else indepen- dent of the MPEG encode-decode process. It is impossible for fully compli- ant, properly functioning MPEG decoders to produce visually different results from the same encoded data stream.

MPEG (and most other compression techniques) are asymmetric,mean-

ing that the encoding process does not take the same amount of time as the decoding process. It is more effective and efficient to use a complex and time-consuming encoding process because video generally is encoded only once before being decoded hundreds or millions of times. High-quality MPEG encoding systems can cost hundreds of thousands dollars, but since most of the work is done during encoding, decoder chips cost less than $20, and decoding can even be done in software.

Some analyses indicate that a typical video signal contains over 95 per- cent redundant information. By encoding the changes between frames, rather than reencoding each frame, MPEG achieves amazing compression ratios. The difference from the original generally is imperceptible even when compressed by a factor of 10 to 15. DVD-Video data typically is com- pressed to approximately one-thirtieth of the original size (Table 3.1).

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8_{Obviously, it would sound better if read by James Earl Jones than by Ross Perot. But the anal-}

ogy holds if you consider the vocal characteristics to be independent of the translation of words to sound. The brain of the reader is the decoder, the diction of the reader is the post-MPEG video processing, and the voice of the reader is the television.

Native Native Compressed

data rate (kbps) Compression Rate (kbps)* Ratio Percent

720 480 99,533 MPEG-2 3,500 28:1 96 12 bits 24 fps 720 480 99,533 MPEG-2 6,000 17:1 94 12 bits 24 fps 720 576 119,439 MPEG-2 3,500 34:1 97 12 bits 24 fps 720 576 119,439 MPEG-2 6,000 20:1 95 12 bits 24 fps 720 480 124,416 MPEG-2 3,500 36:1 97 12 bits 30 fps 720 480 124,416 MPEG-2 6,000 21:1 95 12 bits 30 fps 352 240 24,330 MPEG-1 1,150 21:1 95 12 fps 24 bits 352 288 29,196 MPEG-1 1,150 25:1 96 12 fps 24 bits 352 240 30,413 MPEG-1 1,150 26:1 96 12 fps 30 bits 2 ch 48 kHz 1,536 Dolby Digital 2.0 192 8:1 87 16 bits 6 ch 48 kHz 4,608 Dolby Digital 5.1 384 12:1 92 16 bits 6 ch 48 kHz 4,608 Dolby Digital 5.1 448 10:1 90 16 bits 6 ch 48 kHz 4,608 DTS 5.1 768 6:1 83 16 bits 6 ch 48 kHz 4,608 DTS 5.1 1,536 3:1 67 16 bits 6 ch 96 kHz 11,520 MLP 5,400 2:1 53 20 bits 6 ch 96 kHz 13,824 MLP 7,600 2:1 45 24 bits

*MPEG-2 and MLP compressed data rates are an average of a typical variable bit rate

TABLE 3.1 Compression Ratios

Birds Revisited: Understanding