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136 Bu il d Y o ur O w n Q u a d c o p t e r

an error-correcting algorithm to create a two-byte number, using the frame data and the
embedded code value. This number is then appended to the data frame and subsequently
transmitted. The receiver uses the received CRC value and compares it to a recalculated
value based on the received data frame. The receiver already knows the special embedded
code because of the binding (pairing) process that is described in another section. A
mismatch in the values indicates that a transmission error has occurred and the data frame
must be rejected.
A second CRC is also created by exclusive ORing the data frame containing the first CRC
with the hexadecimal value 0xFFFF. This just adds an additional error-checking capability to
DSSS for further redundancy.
Transmission of the GUID
The globally unique identifier (GUID) is a two-byte value (for DSSS purposes) that is generated
from a manufacturing code contained within the transmitter-chip firmware. The GUID for
the DX-8 is based on the very unique manufacturing code created when the Cypress
CYRF6936 chip was produced. This is very similar to how network adapter cards create
media access codes (MAC) that uniquely identify a computer to the network to which it is
attached. The MAC value is essentially the GUID for a networked computer.
The transmitter GUID is loaded into the R/C receiver during the binding process, which
is why a DSSS transmitter-receiver pair will not function without doing this binding process.
In addition, at least for the DX-8, all the positions and settings of the transmitter’s controls
at binding time are also stored in the receiver’s memory. These are the fail-safe positions that
will automatically be selected if the receiver loses connection with the transmitter.
The five DSSS processes practically guarantee that interference is eliminated and only
the paired transmitter will function with its receiver. This is a big confidence booster that has
promoted the DSSS standard among R/C enthusiasts. All currently available 2.4-GHz
systems are extremely reliable because they use either DSSS or FHSS. The latter technology
is discussed next.

Frequency-Hopping Spread Spectrum

Frequency-hopping spread spectrum (FHSS) adheres closest to the original spread spectrum (SS)
concept that was invented and patented during WWII. Actress and inventor Hedy Lamarr,
shown in Figure 6.6, originated the concept to help the Allies war effort.
Her patent envisioned the remote control of a torpedo with a radio carrier wave that
hopped or skipped over 88 frequencies, which incidentally is the number of keys on a
standard piano. She thought that the enemy would not be able to easily intercept or jam
radio-controlled signals that were hopping about the spectrum. She was absolutely correct
in her reasoning. It turned out that the U.S. government was not really interested in her
invention and never adopted it for use in the war. Years later, it was widely adopted when
researchers realized how robust SS was in minimizing corruption of communications from
interception and interference.
A transmitter-receiver system using FHSS needs to be bound or paired in the same
manner as a DSSS system. Most readers will be familiar with the Bluetooth (BT), which is
designed to be a close-range personal area network (PAN). BT uses the FHSS modulation
scheme to minimize interference, since many BT-equipped devices are often used in close
proximity to each other. Of course, the power levels are much higher when FHSS is used for
R/C purposes than when it is used for BT to couple your cell phone with a remote microphone/
earpiece.