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{{see also|Trellis modulation}}
A convolutional encoder is a [[Finite-state machine|finite state machine]]. An encoder with ''n'' binary cells will have 2<sup>''n''</sup> states.
Imagine that the encoder (shown on Img.1, above) has '1' in the left memory cell (''m''<sub>0</sub>), and '0' in the right one (''m''<sub>-1</sub>). (''m''<sub>1</sub> is not really a memory cell because it represents a current value). We will designate such a state as "10". According to an input bit the encoder at the next turn can convert either to the "01" state or the "11" state. One can see that not all transitions are possible for (e.g., a decoder can't convert from "10" state to "00" or even stay in "10" state).
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