User`s manual

Another factor of maximum importance in order to obtain the best results is to
properly set input sensitivity for both input channels separately; the two peak
meters of CLIO desktop should help you in this task; set input sensitivity so that
both readings average in their respective green areas. If you are taking as
reference the line level signal fed to the sound reinforcement system and are
measuring from a microphone it is quite common the situation in the following
figure where the two input channels have sensitivity which differ of 40dB or more.
When the measurement is started you will see the A/B magnitude frequency
response which is averaging on the screen and fast refreshing; sometimes you may
notice, in dependance of moments when sound is particularly low or absent, that
the measurement freezes and the Level Threshold display becomes red:
You may freely choose the most appropriate level threshold so that your
measurement accumulates only when signal is present; take into consideration that
the threshold relates to peak level vs. full input scale so it is not an absolute value
but can be directly compared with the CLIO desktop peak meters.
Another important indication about how good is your measurement is given by the
coherence graph.
The coherence function tells us in which frequency zones the energy content of the
measured signal relates, i.e. is coherent, with the reference signal; the more the
signals are coherent the more the coherence function approximates unity (or
100%); the less they are coherent the function tends to zero. In figure above we
may say that both in the highest frequencies region (above 15kHz) and in the mid-
bass region there are zones of low correlation (<50%) where the measured
response should be considered with attention.
112 9 FFT, RTA AND “LIVE” TRANSFER FUNCTION