You can decode a signal looking at one being a high level and zero being a low level.
Another way is to detect a sufficient level transition. Circuits that do this often produce a pulse which indicates a transition.
This method is used to analyze data in some experiments where transitions are more reliable than levels.
It's like taking the derivative of a signal and looking for pulses.
The results of edge detection on a noisy step edge signal. The added noise had a standard normal distribution scaled to 20% of the amplitude of the original signal.
Sorry I must have remembered the rise time levels wrong.
I saw the use of edge detectors in a presentation about new SD cards. Wasn't much detail.