The Art of the Helicopter John Watkinson - Karatunov.net

294 The Art of the Helicopter
wear occurs. A digital system can transfer whole numbers from one end of the airframe
to the other without any loss of accuracy at all and will not be affected by wear.
Essentially, digital signalling carries the original control parameter numerically. The
number of the sample is an analog of time, and the magnitude of the sample is an analog
of the value of the parameter. As both axes of the digitally represented waveform are
discrete, the variations in the parameter can accurately be restored from numbers as
if they were being drawn on graph paper. If greater accuracy is required, paper with
smaller squares will be needed. Clearly more numbers are then required and each one
could change over a larger range.
Humans expect numbers expressed to the base of ten, having evolved with that number
of digits. Other number bases exist; most people are familiar with the duodecimal
system using the dozen and the gross. The most minimal system is binary, which has
only two digits, 0 and 1. BInary digiTS are universally contracted to bits. These are
readily conveyed in switching circuits by an on state and an off state or in optical fibres
by the two states of a light source. With only two states, there is little chance of error.
In decimal systems, the digits in a number (counting from the right, or least significant
end) represent ones, tens, hundreds and thousands etc. Figure 7.29 shows that in binary,
the bits represent one, two, four, eight, 16 etc. A multi-digit binary number is commonly
called a word, and the number of bits in the word is called the word length. The righthand
bit is called the least significant bit (LSB) whereas the bit on the left-hand end of
the word is called the most significant bit (MSB). Clearly more digits are required in
binary than in decimal, but they are more easily handled.
The word length limits the range of a binary number. The range is found by raising
two to the power of the word length. Thus a four-bit word has 16 combinations, and
could set a control to only 16 positions. Clearly this would not be good enough for a
flight control. However, a ten-bit word has 1024 combinations, which is close to 1000.
A mechanical flight control that could be positioned to one part in a thousand would
be considered remarkable.
In a digital signalling system, the whole number representing the value of the sample
is expressed in binary. The signals sent have two states, and change at predetermined
times according to some stable clock. Figure 7.30 shows the consequences of this form
of transmission. If the binary signal is degraded by noise, this will be rejected by the
Fig. 7.29 Binary coding principles. See text for details.