Spread Spectrum technology uses different digital formats to
transmit and receive radio signals on the 2.4GHz band. The band actually
spans from 2.400 to 2.4835 making eighty channels available with a
band width of around 1 MHz. it has been I use for some time by government
agencies all over the word and has proven over time to be very, very
reliable. It is also controlled by a set of rules agreed to by agencies
and governments all over the world.
DX7 features DSM2™, which is second generation DSM.
DSM2 provides all the benefits of DSM but with the added bonus of
doing it faster. By processing the signal faster than any PCM radio
system available today and at 1024 bit resolution the link that connects
pilot and plane is sharpened making him feel more in contact and
control of his plane.
This is a system of transmitting a radio signal to a receiver
where rather than being fixed onto one channel it hops from one channel
to another constantly, using a random sequence. In short the transmitter
sends a stream of control data at the end of which there is a code
that tells the receiver which frequency to hop to next. It does this
hundreds of times a second and so the chances of any type of interference
are virtually nil; if two devices did end up during their sequences
on the same frequency at one time it would only be for milliseconds
so you simply wouldn’t detect it. Unfortunately there are two
big drawbacks for FHSS with regard to using it for models. Firstly
the latency (response time). Ideally we want it to be zero but practically
it tends to be around five milliseconds for most conventional radio
systems. Some a quicker, some slower but FHSS at best has proven
to be around twenty five milliseconds but more generally around fifty.
Secondly is its response to interference. Should there be a break
in signal it can take from two to four seconds for the transmitter
and receiver to resynchronise and this clearly when controlling a
plane, helicopter or car travelling at speed is unacceptable.This is the system used buy Spektrum® (with a K). DSSS
is unlike any other system we have had before in that it is aware
of what is going on around it. When turned on a transmitter scans
to find one of the eighty channels that is not being used, locks
on and occupies it and then starts to transmit it’s GUID code.
While this has been happening the receiver had been scanning the
bands also looking for it’s transmitter GUID. When it detects
it the two lock together to form a solid RF link. Once functioning
the data transmitted is actually spread across the complete bandwidth
using a random noise code which has the effect of digitally increasing
the range of the equipment. The receiver is aware of its transmitters
spreading code and can distinguish its intended signal.
A unique code that is programmed into every Spektrum transmitter.
During the binding process the receiver assimilates the code and
from that point in time will only respond to that transmitter.
There is no limit to the number of receivers that can be bound
to one transmitter so it can be used for several models. The GUID
remains with that receiver until it is bound to a different GUID
(transmitter). The GUID is actually a multi-digit code that makes
possible 4.2 billion different individual ID’s so it’s
going to be a while before they all get allocated.
The process whereby a receiver assimilated the GUID code of a transmitter.
Once done the code remains with the receiver until it is bound
with another transmitter. Once a receiver is bound to a transmitter
there is no need to at any point re-bind it unless new failsafe
settings need to be adopted.
SmartSafe deals subtly differently with two differing safety critical
scenarios, one of which is especially important for electric powered
planes. Fail-safe settings are established during the binding process
at the positions of the sticks, usually low throttle and neutral
on other channels.The first scenario is that the receiver is turned
on without a signal from the transmitter. In this event all of the
channels go to the fail-safe positions set during the binding process
but receiver doesn’t
send any signals out on the throttle channel, so the speed controller
cannot arm and start the motor.Once the transmitter is turned on
and an RF link is established the receiver will send out a low throttle
signal and arm the speed controller once the stick is positioned
at low throttle. From that point it functions, as do the other channels,
as normal.
The second case scenario is that the signal is lost whilst
both transmitter and receiver have an established RF link. In this
event, as unlikely as it is, SmartSafe will take the throttle channel
to its setting programmed during the binding process, while all other
channels stay at last position held.
With a surface radio system the vehicles, be it cars or boats, move
in two dimensions, the third one being the fixed surface they are
moving on. Because of this their aerials are for the most part
pointing upwards so that orientation to the transmitter remains
constant. Planes are different in that they manoeuvre in three
dimensions and as such are prone to what is called ‘fade’.
This has the effect of momentarily reducing the output of the transmitter,
which can obviously have disastrous consequences. Obviously it
depends of the orientation of the receivers aerial too but it can
usually be provoked by pointing the aerial of a transmitter directly
at the model.
When a plane flies and performs aerobatic manoeuvres its aerial’s
orientation to the transmitter is constantly changing and because
of this it can at any time find itself in a sub-optimal position
and subject to fade.
Spektrum radios feature DualLink, a system that is specifically designed
to resolve this problem. Every DualLink receiver is in actual fact
two receivers, each with its own aerial, the two of which are oriented
at ninety degrees to each other. When the transmitter if turned on
it scans for and occupies two free channels and then starts to transmit
the same data simultaneously on both. Because there are two paths
(the term for the relationship of the position of the transmitting
aerial relative to the receiving aerial) should one be lost due the
other will continue to supply the signal.
A new feature on DX7 that re-sequences the bits of data based on
the type of mixing selected. So if dual elevators or 3-serco CCPM
has been set-up ServoSync will see to it that those servos are
performance optimised by receiving their impulses together. This
results in a more positive feel to control input. Not featured
in DX6.
It’s not uncommon to forget to check that the model that is
about to take off is the one that’s profiled in the transmitter
display. The consequences can be disastrous though. Once again Spektrum
redefines flight safety with ModelMatch by making this kind of error
a thing of the past. With DX7 a profile code is embedded into the
receiver and if it doesn’t match the one embedded in the signal,
indicated by the model name on the display, the model’s controls
simply won’t respond. Not featured in DX6.
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