I'm developing a software synthesizer, and, while I do understand how ADSR envelopes work, I'm having a hard time figuring out what the x-Axis of Envelope diagrams means and what I'm controlling for example when I'm turning an ATTACK knob on a synth. Does turning the knob control the duration of the attack period in ms, is it a percentage of a pre-fixed length of a sound and if so, what lengths are used?
To illustrate what I'm saying, here the envelope settings part of NI Massive:
So, what I don't get is, in what are ADSR segments measured. When I turn the attack knob to the right, the attack line/curve gets longer towards the right of the visualization and of course the tone takes longer to reach the maximum amplitude. But what did I change? Is the attack knob controlling the attack period in a range of, say, 100 - 1000 milliseconds? Or does this visualization represent e.g. a 10 second tone and the knob controls the attack percentage of the sound from e.g. 5% - 50%? I did some timing and found that when the attack knob is to the very right, the tone takes exactly 4 seconds to reach the maximum amplitude and it is at about 50% of the envelope shown, so I thought the envelope was showing a range of 0 - 8 seconds since it took 4 seconds to get to the half point. This hypothesis was nullified pretty quickly though, as controlling the "Delay" knob, which shifts the starting point of the envelope to the right, in a way that the envelope looks like it starts at 50% again, ended in the tone starting after only 2.5 seconds. So this visualisation is not a certain time I concluded.
I found a few sentences on this here, where I read something about constant time or constant rate, although that is not really enough.
So, can I just randomly choose a duration for the ADSR on my synth and make my knobs control a percentage of the duration?
Would really like some clarification here, sorry if this is noob-ish.