CLIOwin 6.5 PCI User's Manual - Audiomatica Srl

9.4 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 obtain from them only answers
about unknown signals, without any care for generating a stimulus; or you may leave
to others this job, like 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.
CLIOwin adds the possibility of internal trigger (and relative delay) i.e. triggering with
respect of the generated signal thus obtaining a synchronous capture. Let's see how a
measurement presented in section 11.4 was done; please refer to figures 11.9, 11.10
and 11.11. We have an acoustical measurement of a tweeter, done stimulating it with
a 2kHz 10ms tone burst (see 5.4.2 for details about programming a bursted sinusoid);
the FFT measurement is done in internal trigger; Fig. 11.9 shows the analysis and the
captured time data that clearly show the flight time from the tweeter to the microphone;
Fig. 11.9, even if the analysis is not our final target, show the power of synchronous
acquisition which permits to display the arrival delay of sound to the microphone. To
obtain the desired result, as explained in 11.4, it is necessary to remove the flight time
plus the device settling time; this can be easily accomplished setting the internal trigger
delay, in FFT settings, to 1.5ms; the final result shown is shown in 11.11 and permits
the identification of the device harmonic distortion. To proceed further one could vary
the stimulus amplitude and test the distortion of the tweeter at different amplitudes;
using bursts prevents also the damage of the unit as the overall power delivered to it
rather low and a direct function of the duty cycle of the burst itself.
Other useful signals are noises and multitones. The predefined signal file is the
all4096.sig; you can play it pressing the FILE button from the Multi-meter panel, or
choosing it from the list of signal files. As its name suggests this multitone signal is done
adding 4096 sinusoids of the same amplitude which frequencies exactly fall inside an
FFT frequency bin; for a sampling frequency of 51200Hz a frequency bin in a 4096 points
analysis is a multiple of 12.5Hz; for a sampling frequency of 48000Hz a frequency bin
in a 4096 points analysis is a multiple of 11.71875Hz. This particular signal should be
employed, for maximum performances, when analyzing a system with a 4096 points
FFT; should you change FFT size, you should change stimulus too. Using this signal it
is possible to assess the frequency response in a single FFT acquisition without the need
for any averaging. Among noises we find the pink noise that is required when taking
RTA measurements. CLIOwin has several pseudo-random pink noises (like
pink16384.sig) that are filtered versions of an MLS sequence. Use this stimulus when
executing RTA measurements, due to its pseudo random nature permits RTA analysis
that settle in a fraction of time with respect to analysis done with true pink noise. Choose
the file with the name matching the length of the FFT used; for a 16K RTA use the
pink16384.sig signal.
The main application of RTA analysis is in assessing the quality of an audio installation
(from the placement of the speakers in a listening room to the overall sound quality of
a car stereo system). In these case it is often used the pink noise as stimulus. If you
are not using CLIO as the source of such a stimulus be sure to use a good one; you may
80 Chapter 9 - FFT