22-05-2012, 12:15 PM
A Practical Guide to Video and Audio Compression
A_Practical_Guide_To_Video_And_Audio_Compression__2005_.pdf (Size: 10.18 MB / Downloads: 280)
Introduction to Video Compression
We (that is, you and I) are going to explore video compression together. It is a journey of
discovery and surprise. Compression might seem daunting at this point, but like the old
Chinese proverb says, “Even the longest journey starts with a single step.” Let’s head into
that unknown territory together, taking it carefully, one step at a time until we reach our
destination.
Video Compression Is Like . . .
It really is like trying to get a grand piano through a mailbox slot or an elephant through
the eye of a needle. In fact, we thought the elephant was such an appropriate description,
my friend Russell Merryman created a cartoon to illustrate the concept:
Video compression is all about trade-offs. Ask yourself what constitutes the best
video experience for your customers. That is what determines where you are going to
compromise. Which of these are the dominant factors for you?
● Image quality
● Sound quality
● Frame rate
● Saving disk space
● Moving content around our network more quickly
● Saving bandwidth
● Reducing the playback overhead for older processors
● Portability across platforms
● Portability across players
● Open standards
● Licensing costs for the tools
● Licensing costs for use of content
● Revenue streams from customers to you
● Access control and rights management
● Reduced labor costs in production
You will need to weigh these factors against each other. Some of them are mutually exclusive.
You cannot deliver high quality from a cheap system that is fed with low-quality
source material that was recorded on a secondhand VHS tape. Software algorithms are
getting very sophisticated, but the old adage, “Garbage in, garbage out” was never truer
than it is for video compression.
It’s Not Just About Compressing the Video
The practicalities of video compression are not just about how to set the switches in the
encoder but also involve consideration of the context—the context in which the video is
arriving as well as the context where it is going to be deployed once it has been processed.
Together, we will explore a lot of background and supporting knowledge that you
need to have in order to make the best decisions about how to compress the video. The
actual compression process itself is almost trivial in comparison to the contextual setting
and the preprocessing activity.
2 A Practical Guide to Video and Audio Compression
Figure 1-1 How hard can it be?
What Is a Video Compressor?
All video compressors share common characteristics. I will outline them here and by the
end of the book you should understand what all of these terms mean. In fact, these terms
describe the step-by-step process of compressing video:
● Frame difference
● Motion estimation
● Discrete cosine transformation
● Entropy coding
Wow! Right now you may be thinking that this is probably going to be too hard.
Refrain from putting the book back on the shelf just yet though. Compression is less
complicated than you think. If we take it apart piece by piece and work through it one
item at a time, you will see how easy it is. Soon, you will be saying things like, “I am
going to entropy code the rest of my day,” when what you actually mean is you are
going home early because there is nothing to do this afternoon. You can have a secret
guffaw at your colleagues’ expense because you know all about video compression
and they don’t.
The Informed Choice Is Yours
Despite all the arguments about the best technology to use, in the end your decisions
may be forced by your marketing department arguing about reaching larger audiences.
Those decisions should be backed up by solid research and statistics. On the
other hand, they might be based just on hearsay. The consequences of those decisions
will restrict your choice of codecs to only those that your selected platform supports.
However, you will still have some freedom to innovate in building the production
system.