Page 280 - Anatomy of a Robot

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COMMUNICATIONS 265
Compression
Often, the bandwidth available for digital communication is limited. This may occur for
several different reasons:
Regulated spectrum The government may regulate access to the spectrum and
make everyone share it.
Cost It is often too expensive to purchase rights to the needed spectrum.
Energy As we discussed before, sending bits across a wireless channel literally
requires a sufficiently high Eb/No. In satellite transmission, this fact literally
comes home as satellite batteries and solar panels struggle to provide energy to
each and every bit. Robots in remote locations are often up against this very prob-
lem. Don’t forget one thing though. It takes energy to compress the data in the first
place. The compression process may have to be very energy efficient and the
entire process will have to be analyzed.
Whatever the reason, there is little point in sending useless data across the channel.
Most digital communications can be compressed to a smaller amount of data. Shannon
toyed with this at some length. To test this assertion, pick a few different types of files
on a computer and try to compress them with WinZip.exe, a trademarked program from
™
WinZip Computing, Inc. It’s presently available for trial use at www.winzip.com/
ddchomea.htm.
The following compression rations can often be achieved:

Standard text files A factor of 6 to 10.
Program files A factor of 2.
Video or graphic files A factor of 1.1.

™
Try compressing these types of files with the WinZip utility to see what can be
achieved. If the robot’s digital communications must be compressed prior to transmis-
™
sion, several options are available. WinZip may not be usable on the robot’s computer.
It’s likely that the robot’s operating system software library (or freeware) may already
have compression utilities that can be used. Two basic types of compression are com-
monly used by these programs. These techniques are used in standard compression pro-
grams and can be rewritten to suit the needs of the robot.

Fourier Transforms

Graphics and video transmissions are routinely compressed using Fourier transforms.
In MPEG video compression, the pictures are converted to a series of coefficients that