Apple Orchard 1980 Fall v1n2 reduced

PAGE 32
THE APPLE ORCHARD
FALL 1980
RFI: The F.C.C. and Your Apple
INTRODUCTION
What Interference Is
Radio and TV sets operate by
" receiving" electro-magnetic
waves. They all have antenna's
which convert the electromagnetic
wave into voltage which is in turn
converted to sound, picture or
both. The antenna may be built into
the set or be remotely connected.
(Very remote in the case of cable TV
hook-ups.)
Interference, in the sense we'll
use here, is a voltage appearing to
the receiving set as though it were
from the antenna (as it may well be)
which is not intended to be sent by
the broadcaster. Interference is
always present to some extent.
Interference from outer space,
sunspots, random collisions of
electrons, etc. is called "random
noise" and on a TV set appears as
"snow''. Snow is a result of
receiving electromagnetic waves
which are not at all related to each
other in time and wavelength.
" Noise" is used to describe
interference which is substantially
" snow-like''.
Noise from automotive ignition
systems is another kind of
interference. It is transmitted as an
electromagnetic wave packet of
short but high intensity, wherein an
enormous number of wavelengths
are represented. Because of this, it
will be received everywhere in the
spectrum, appearing as bright dots
on almost all channels. This form of
wide band interference was very
quickly recognized when the TV set
was introduced and subsequently
automatic ignition systems were
designed to reduce the radiated
energy to acceptable levels.
Interference generated by
computers is similar to both
automotive ignition noise and
snow in the sense of being very
wide band. It is made up of short
intense packets. The packets are
not nearly as intense as ignition
noise, but they occur much more
frequently and the rates at which
the packets are generated are
related to each other. That is to say,
there is a pattern in time to the
packets. Consequently, the visual
effect of computer generated
interference is almost snow with
squiggles and bars moving about.
In general, the existence of
patterns in interference makes it
more objectionable than the
purely random snow and for the
same levels of interference,
computer generated interference
is much more obnoxious than
snow.
General Comments re FCC
Briefly (and consequently at the
risk of being inaccurate), we will
comment on the role of the Federal
Communication Commission. The
FCC has among its many duties that
of keeping the air waves pure. Like
any environmental protection
agency, it is asked to mediate
conflicts between public interest
groups when they arise: such as the
conflict between computer users
and TV watchers. Clearly, these
groups have both rights and
responsibilities. And despite the
enormous technical and political
complexities, the Commission is
dealing with the problem.
The Commission has decided
that some pollution from
computers is unavoidable. In
keeping with their tradition they
believe that, since the computer is
the new kid on the block, most of
the burden shall fall upon the
manufacturer and user of
computers. In office, commercial
and industrial environments,
substantial pollution will be
allowed since the TV set is not likely
to be used and then only at
substantial distances from the
computer itself. In residential
environments, the TV set is
ubiquitous and likely to be closer to
the computer and consequently,
the allowable radiated pollution
will be markedly lower.
The Commission also has taken
the attitude that the residential
computer user may very well
interface with his own TV set. The
user has the choice of turning off
either the computer or the TV set.
The possible conflicts between
spouse or between grandpa and
the kids is not the FCC's concern.
But the computer user must not
interfere with his neighbor's TV. In
fact, the Commission puts the full
b ur d e n o n t h e r e s i d e n t i a I
computer user to avoid such
"harmful" interference" right up
to the point of ceasing to operate
computer. This is so- regardless of
whether or not the manufacturer's
equipment complies with the
applicable rules and regulations.
If you are interfering with your
own TV set, it is possible that your
neighbor is having problems as
well. You should check. (the
distance from the computer to your
neighbor's antenna is a key
parameter. Buying a troubled
neighbor a new TV antenna may be
a more palatable solution than
restricting your computing times
to the wee hours of the morning.
Whence Cometh the Radio
Frequency Interference
Computers today operate at
radio frequencies and the bursts of
voltages and currents that take
place when information is latched
in a memory IC (for example)
have significant energies in the
frequency range from 30 MHz to
several hundred MHz. Since the
voltages are relatively low, we'll be
talking mostly about the currents,
but this is for convenience and you
should remember that both are
inevitably present and both create
a field and that field will radiate a
wave.