Firing angle generation is the heart of SCR based control systems. Before the advent of digital control systems, the phase angle firing decisions were determined by a complex series of analog sensing and follower circuits with descrete components. Over time, those got boiled down into a condensed set of components on a "thick film" firing board, kind of like an IC only cheaper to make in smaller quantities. The firing angle command was then sent to a "pulse generator" to provide the actual firing sequence of the gate circuit by waiting long enough to affect the RMS output. Then finally, depending on the firing method chosen for the SCRs, the signal goes through a "pulse transformer" that amplifies it to make sure there is enough energy to ignite the SCR gate. More modern controllers are now using digital firing techniques that do what the thick film and pulse generator did, but still go through the pulse transformers.
How you refer to the timing of the firing is, as you can see from the previous posts, a bit of a senmantics issue. But suffice to say that the turn-on (gate) timing is determined by first sensing where the zero-cross point is (or is supposed to be). There are several methods of accomplising that, each with their own advantages and drawbacks. Once you know where the the zero-cross point is, it's compared to the desired phase angle and then sent to the firing control system. If it's a soft starter, the determination of the desired phase angle is dynamic (changing) based on the ramp profile and/or current limit set up by the user. If the SCR control is in a rectifier, it's usually fairly static and based on the desired DC voltage level needed after rectiication.
