CLIOwin 7 PCI User's Manual - Audiomatica

9.5 FFT AND RTA OPERATION
The FFT and RTA measurements (and also Multi-meter ones, see Chapter 8) differ from
MLS and Sinusoidal ones in the fact that they are interactive; the user has control over
measurement time and generated stimuli. You may also obtain answers about unknown
signals from them, without any need for generating a stimulus; or you may leave this
job to others, similar to when you measure an audio chain relying on the test signals
contained in a CD-ROM. One effect of this is that, strictly speaking, FFT measurements
may lead to less precise results if compared to other techniques; the possibility of
injecting a synchronous MLS sequence at the beginning of the same audio chain
mentioned before is surely a better approach even if, in the vast majority of cases,
unfeasible.
FFT and RTA power depends not only on the measurements settings themselves but
also on the generated signals. Please refer to chapter 7 for a detailed description of
the signal generator and its many capabilities.
When stimulating any external device with CLIO (see 4.8.2 and 4.8.3 for basic
connections) you may choose a limited bandwidth signal (like a single sinusoid) or a wide
bandwidth signal as a noise; in the first case you have the possibility of analyzing the
harmonic content of the output spectrum while in the second case you may evaluate
the frequency response of the device under test. A different stimulus, about halfway
between the two cases just mentioned, is a logarithmic chirp swept across some
octaves (like a chirp covering four octaves from 50 to 800Hz); in this case you are able
to analyze both the response plus unwanted effects like distortion and noise produced
by the D.U.T..
When using the FFT narrowband analyzer it is possible to achieve a flat response of the
analyzing chain using white noise or similar signals whose energy content varies linearly
with frequency; among these MLS, All-tones signals or linear Chirps.
When using the RTA octave bands analyzer it is possible to achieve a flat response of
the analyzing chain using a signal whose energy content varies logarithmically with
frequency; among these we find pink noises or logarithmic Chirps.
Besides the choice of the stimulus it is very important to achieve proper synchronization
between the generated signal and the acquisition; this will lead to optimum
performances avoiding the use of data windows and minimizing any spectral leakage
that may occur. Synchronization can be achieved defining the stimulus in a particular
manner or by means of proper triggering (see later internal trigger).
If you are generating a sinusoid choose a frequency that is an integer multiple of the
frequency bin (i.e. sampling frequency divided FFT size) or let CLIO calculate it setting
“FFT bin round” in the generator input form (see chapter 7). As an example we would
like to play and analyze a 1kHz sinusoid using a 64k FFT @ 48000Hz sampling; the
frequency bin associated is 0.73Hz and the nearest spectral line to 1kHz is the 1365th
one at 999.75Hz. If you simply generate a 1kHz sinusoid without rounding it to the
nearest bin you obtain the analysis of fig. where it is evident that CLIO is capable of
outputting a highly precise 1000.0Hz sinusoid but it is also evident the spectral leakage
caused by this choice.
Chapter 9 - FFT 97