In a typical 10-20 system there are 21 electrodes placed on the scalp. However, this does not mean there are 21 distinct "channels" or voltage sources.
I've heard that some of these electrodes are "reference" electrodes, while others are "active" electrodes. Sometimes electrodes can setup in a "bipolar" or "differential" fashion.
- How many actual channels (that is, distinct input sources) will there be?
- What's the difference between reference and active channels?
- How does this bipolar/differential setup work differently than a "normal" setup?
Here is my updated understanding based on @Christiaan's answer below:
time (t) F2v F3v F2-F3 F3-F2 ================================================= 1 2 1 1 -1 2 3 6 -3 3 3 5 3 2 -2
So, if my understanding is correct, then at time
t=1, the voltage of the F2 electrode might be, say, 2 (units; volts, microvolts, whatever), and the voltage of the F3 electrode might be 1. If F2 is the arbitrary active electrode, then the potential difference between F2 and F3 is 2 - 1 = 1. But, if F3 was the active electrode (and F2 the reference), then the potential difference would be the inverse (that is, 1 - 2 = -1).