It depends on what context you're using the term "envelope detector" in and what you're using it for. Are you asking in terms of audio electronic and circuit cost / quality? Are you speaking in terms of creating a radio? Or are you asking in terms of something more asthetic or involving a digital code base that emulates real-world analog circuitry like the signal flow and sound of a virtual instrument. You might want to be more clear and specify exactly why you want to know this information and the context you are asking it in.