I have a quite a few different ESCs lying around. Most ESCs have something called a "BEC" that converts the battery voltage to a steady 5V for the use by the controller, most of the radio equipment, the GPS, and by the ESC's themselves.
Some are BECs are listed as analog, some are listed as digital, some are listed as "opto". What's the difference?
The "Opto" designation was first used with ESC that had an opto-isolator on their control input. An opto isolator can allow one device to control another - even if their GND potential is different. That can be important if the primary (main power feed) wires to the ESC are long or are of too fine a gauge to handle the currents required and have a high voltage drop (> 1V). I have not seen a problem with this in quadcopters. If you happen have that much voltage drop in your power feed lines (at full throttle), you are wasting an awful lot of power and you should rewire immediately.
The ESCs that I have purchased recently that have been called OPTO do NOT have an opto-isolator on board. To that manufacturer, (and probably most others), OPTO simply means that it doesn't have a BEC at all. A quad with 4 of these ESCs needs some other 12V -> 5V converter somewhere simply to fly at all.
Some of the ESCs have an ANALOG BEC. The one I'm looking at right now claims 3Amps output. Do I believe that -NO!. A linear voltage regulator has an input current equal to the output current even though the voltages are different.
So, if an ANALOG ESC has an input voltage of 12V and an output voltage of 5V, it has a "drop" of 12V - 5V = 7V. If you take the output current times that drop, you get the power dissipation of the regulator in the ESC. If the output current is 1A, the power dissipation is 7 Watts!! That is more than those little regulators can handle.
The problem is much worse if we use a 4 cell battery (16V). Now the voltage drop is 11V and a 1A load causes a dissipation of 11Watts. This ESC will melt (or the regulator will most certainly fail.
If a digital or switching ESC is used, the regulator drops the voltage with an efficiency of around 85%, so if the output current is 1A, the output power is 1A X 5V = 5 Watts X (1- 0.85) = .75 Watts. This is a level that can be sustained. The best part is that when we increase the input voltage from 12V to 16V, the output stays the same, and the efficiency is nearly the same, so the dissipation will STILL be around 0.75W.