CLIOwin 7 PCI User's Manual - Audiomatica

10.7 MLS Vs. LOG CHIRP
As anticipated in the introduction, some advise are given to help choosing between MLS
and LOG CHIRP stimuli. Both approach are valid and bring to equivalent results. In both
cases the device we want to measure is assumed to be Linear and time Invariant. This
assumption while reasonably true in general cases in never met in absolute terms. There
is always a certain degree of non linearity and, in less degree, a time variance. The two
approach shows different sensitivity to these facts. Advantage of MLS are that is that
much less computing requiring, historically much more widespread and known and has
a flat frequency energy distribution. The latter feature can be an advantage or not
depending on situation/DUT.
Figure 10.39
In the above figures the spectrum of both signals is showed. As usually signal to noise
ratio get worse at lower frequency, the higher signal energy that LOG CHIRP has here
is an advantage for S/N ratio. If the DUT does not appreciate low frequency high energy
content, i.e. a tweeter, the advantage become a disadvantage.
Also, there is an important difference on how non linearity affects the two procedure.
In MLS distortion transforms itself in noise spread over the whole impulse response while
with LOG CHIRP concentrate itself in single impulses (one for each harmonic) in the tail
of the impulse and can easily be manually removed.
128 Chapter 10 - MLS