Image Processing Reference
transmission interlaces alternate lines of a picture. Don't worry if the concept of interlac-
ing is unfamiliar to you at this stage. It is fully explained in Chapter 5.
Interlacing separates the odd and even lines and transmits them separately. It allows
the overall frame rate to be half what it would need to be if the whole display were
delivered progressively. Thus, it reduces the bandwidth required to 50% and is there-
fore a form of compression.
Interlacing is actually a pretty harsh kind of compression given the artifacts that it intro-
duces and the amount of processing complexity involved when trying to eliminate the
Harsh compression is a common result of squashing the video as much as possible,
which often leads to some compromises on the viewing quality. The artifacts you can
see are the visible signs of that compression.
Got Your Ears On?
It's been a long time since audiences were prepared to put up with silent movies. Chapter
7 looks at how to encode the audio we are going to use with our video. Because the sam-
pling and compression of audio and video are essentially the same, artifacts that affect one
will affect the other. They just present themselves differently to your ears and eyes.
Checking the Map
In Chapters 8 to 14, we investigate how a video encoder actually works. If you drive a
car, you may not know how the right fuel and air mixture is achieved by adjusting the
carburetor. But everyone who drives a car will know that you press the accelerator
pedal to go and the brake pedal to stop. Likewise, it is not necessary to use mathemat-
ical theory to understand compression. Pictures are helpful; trying it out for yourself is
Working Out the Best Route
Chapter 15 is about live encoding. This is content that is delivered to you as a continuous
series of pictures and your system has to keep up. There is little opportunity to pause or
buffer things to be dealt with later. Your system has to process the video as it arrives. It is
often a critical part of a much larger streaming service that is delivering the encoded video
to many thousands or even millions of subscribers. It has to work reliably all the time,
every time. That ability will be compromised if you make suboptimum choices early on.
Changing your mind about foundational systems you have already deployed can be dif-
ficult or impossible.