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Chapter 6: Radio-Controlled Systems and Telemetr y 133

An R/C transmitter has no way of discriminating which signal it should respond to, so it
will attempt to respond to them all, if possible. What happens, as you may imagine, is chaos
where uncommanded actions may happen or the receiver simply does not respond at all.
There is really no easy way to counteract cross-channel interference other than by shutting
down all the interfering transmitters or relocating to a quiet location where you are operating
the only transmitter.
The nature of PM interference is almost identical to that of FM interference, as both are
quite similar in how they respond to interference.

Direct-Sequence Spread Spectrum
Direct-sequence spread spectrum (DSSS) is the modulation technique used in the Spektrum
DX-8 transmitter in my Elev-8 project. Before I start my discussion, I would like to point out
that Spektrum calls their modulation technique DSM2, which is simply a marketing term for
the DSSS standard. As far as I can determine, DSM2 is not different from DSSS; however,
Spektrum is not at all forthcoming regarding any additions they may have added to the
DSSS standard protocol. Therefore, I am assuming they are one and the same.
Further proof that DSM2 is DSSS is revealed by the transmitter module used in the
DX-8, which may be seen at the top, center of Figure 6.3. The shiny silver case contains
the Cypress Semiconductor CYRF6936 Wireless USB transceiver chip. The Cypress chip is
fully compliant with the DSSS standard, which means that the Spektrum DSM2 must also
be fully compliant.

Figure 6.3 Interior of Spektrum DX-8 transmitter.