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Section II - the Encoder
The previous section discussed the basic servo operation, which stated that the sole purpose of the transmitters stick was to control the "length" of a pulse sent to the servo. This section will address how that is performed within the basic transmitter.
For simplicity sake let's look at the operation of a basic Pulse Positional Modulated (PPM) 4 channel radio (Refer to Figure 1). Here we have two control sticks, each controlling two channels. For now we will just call these channel 1 through 4. The four controls provide input to an "Encoder" which will produce an output suitable to be modulated onto a Radio Frequency carrier.
For now let us concentrate on the input and encoder (Refer to figure 2).
Each axis (channel) of the control stick is connected to a potentiometer (pot for short) which controls the length of a pulse generated by a pulse generator.
As discussed under servo operation, the length of the pulse is varied from 1.0 ms to 2.0 ms with 1.5 ms being the pulse length when the stick is in its center position. For simplicity let us say that both sticks are centered and all four channels are
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generating a 1.5 ms pulse.
Since all four pulses can not be sent at the same time, they must be "encoded" into a format suitable for transmission and decoding. To accomplish this the pulses are encoded into a serial string. This creates the problem that if the pulses were just serially sequenced, they would become one long pulse and we could not detect their individual width. To resolve this what the encoder generates is a string of markers defining the start and end of each pulse.
If both sticks were centered, the string would begin with a "start pulse", which defines the start of the channel 1 pulse, it would be followed 1.5 ms later with a pulse defining the end of the channel 1 pulse and the start of the channel 2 pulse, 1.5 ms later with a pulse defining the end of the channel 2 pulse and the start of the channel 3 pulse, 1.5 ms later with a pulse defining the end of the channel 3 pulse and the start of the channel 4 pulse, and 1.5 ms later with a pulse defining the end of the channel 4 pulse.
We now see, that should you have a radio with more than 4 channels, that the pulse string could continue indefinitely.
After the last pulse we have a reset period, which is
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