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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