Image Processing Reference
Encoded Output Delivered
as a Bit Stream
The Next Step
All codecs eventually produce a bit stream as their output. The bit stream is either multi-
plexed and transmitted live or packaged and stored in a file for later use. This is the end
product of a coding operation, but just knowing that doesn't mean we will produce an
optimum bit stream.
Producing the Output Bit Stream
A challenging aspect of producing the outgoing bit stream is to even out the flow of data
so the available bit rate is used fully. The ideal situation is to use every last available bit
but not run out of capacity. To do this, you have to develop a kind of “living-on-the-edge”
Taking the simplest case where I-frames are encoded to produce a bit stream, the
amount of data in each frame will vary according to the amount of detail in the image.
This is because even without coding in the differences and motion compensation from
frame to frame, the entropy coding will yield frames of different sizes after each frame
The encoding complexity will vary across the area of an image. Some parts of the
image may code very easily while others may require substantially more information to
describe the fine detail. Large areas of the same color such as sky or space scenes tend to
encode very well indeed. Figure 14-1 shows a screenshot from the Tektronix MTS4EA
video analyzer that you can install on a Windows PC. This example shows one of the sta-
tistical measurement displays that help you diagnose image-compression problems.
This topic crops up again in Chapter 20 in the discussion about streaming and again
in Chapter 27 regarding digital TV systems.
The output is an elementary stream and requires further processing to be able to
deliver it to an end point.