Coarse summary of various methods for detecting when a signal 'starts' My question is basically in regards to how to detect when (in time) a signal 'starts', and it might also be called 'transient detection', or, as I have read in some papers regarding music, the 'onset' of a signal. 

The motivation here is ultimately a Time-Delay-Of-Arrival (TDOA) estimate between two signals, but suppose you do not have the luxury of doing a cross-correlation because the signals are either way too long, and/or there is so much multi-path corruption that the cross-correlation method doesnt work. In this case my belief is to look for where a signal 'starts' and go from there.

I am looking for more breadth than depth here, as in, a survey of possible/common methods used in doing such a thing. 

Thanks!
If both signals are audio signals from the same source (just different paths to get there) you can do 


Calculate the envelope (various methods for that, depends on the signals and the accuracy you want)
Down sample
Do a running cross correlation (at the down sampled rate)