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Chapter 6: Radio-Controlled Systems and Telemetr y 135

Figure 6.5 Primary data versus DSSS spectrum distribution.

3. Transmission of start of packet (SOP) and data pseudorandom noise (PN) packets
4. Transmission of two sets of cyclic redundancy checks (CRC)
5. Transmission of the globally unique identifier (GUID)
I will briefly examine each process to provide you with some insights on how DSSS functions.

Automatic Selection of Dual Transmit Channels
When the DX-8 (or any other DSSS compliant transmitter) first transmits, it selects a pair of
channels from the 80 available. Theoretically, this would allow up to 40 DX-8’s to be operated
simultaneously in a small area without causing interference to each other.
Switching Channels for Every Data Frame Transmitted
The DX-8 will switch between the two channels for every data frame transmitted. This is a
special form of frequency hopping that will be discussed in the FHSS section. Every data
packet is transmitted twice, once on the first channel, then again on the second channel. It
takes only several microseconds to switch channels.

Transmission of SOP and PN Packets
The DX-8 system uses a set of five 72-byte PN codes. A PN code that is prepended to the
start of a data frame is called an SOP PN. A PN code that is prepended to the beginning
of the “real” data packet is also called a DATA packet. These pseudorandom-coded data
sets are so named because of how they are generated. True random data is exactly that:
totally randomized so that the next data character cannot be predicted from the previous.
Pseudorandom data, on the other hand, appears random in nature but is created by
a predefined algorithm in which all the random codes are precisely generated in a
deterministic fashion.
The receiver uses PN codes to determine whether or not to accept a particular
transmission. When a PN code is unmatched, it means that a particular data package was
not designated for that receiver and will be ignored. The SOP is 8 bytes long, while the DATA
packet is 16 bytes long. Both the SOP and DATA PN packets are extracted from one of the
five 72-byte PN sequences. Unique SOP/DATA pairs are also evenly distributed throughout
all 80 channels to further lessen any potential interference between DX-8 transmitters.
Transmission of Two Sets of Cyclic Redundancy Checks
A cyclic redundancy check (CRC) is a calculated number based on both the numerical values
contained in the whole data frame and a special manufacturing code value contained in the
transmitter chip firmware (which is explained below in the GUID section). The CRC uses