REW Beta Release What is the difference between Impulse Response and ETC curve and which one to use for Time Alignment?

[CAAudiophile]

Hi John,
1- When the IR (Impulse Response) is measured for a speaker using the MIC at listening position and then comparing it with the ETC curve noticed that the timing for each is off by a few ms ,,, which curve is better or more accurate to use for time alignment.?
2- and also noticed that the ETC doesn't show the signal polarity any idea why not?

 

[John Mulcahy]

The ETC is the envelope of the impulse response. Mathematically speaking it is the magnitude of the analytic signal derived from the IR, which is a signal whose real part is the IR itself and imaginary part is the IR phase shifted by 90 degrees, but it is probably easier to think of it as the smooth curve which, if drawn symmetrically above and below the horizontal axis, is just sufficient to contain all the extremes of the impulse response. In a sense it conveys the overall shape of the impulse response without some of the finer detail of its oscillations. Time alignment should use the IR rather than the envelope.

 

[CAAudiophile]

Thank you John.