Monday, 13 April 1987

dual FFT analyses

part 2: Getting to know dual FFT analysers


Now we know that we need both magnitude and phase plots to describe the behaviour of our system under test.
So wouldn't it be nice to have some tool to measure these?
For some 15 years now our PC's are powerful enough to do the multiple, fast calculations we need to make use of mathematics developed by Jean Baptiste Fourier.
This clever guy proved that (within certain restrictions we will talk about later) you can also describe the behaviour of your system by looking at how it responses to a unity impulse (Dirac).
So we actually have two methods for analyses: one in the frequency domain (that's your bode plots), one in the time domain (that will be this impulse response).
And the mathematical system to go between these two domains is hence known as FastFourierTransform (FFT and iFFT for the reverse).

Enough of this math stuff which I also hardly understand, why dual FFT?

It's perfectly possible to do build a RTA (real time analyser, you know, these dancing light bars you find on every crappy audio device nowadays) with a single FFT.

I remember (back in the '70's!) staring at the dancing lights of that precious Klark Teknik DN60 while trying to do system tuning and thinking f... that, I can hear the EQ changes I apply before I see them on that thing.
Sorry friends no serious audio use for these toys!

Now for the dual FFT
Remember phase being a time related phenomena?
So if you want time information you need a reference point!
We will feed our dual FFT's machines with a reference signal as well as the measurement signal.
By mathematics (or magic if you are more inclined to the esoteric) the software will give you pictures like this:

This as a magnitude (above) and phase (lower) plot measured with Easera Systune.
But I am not biased, you may also prefer to use Smaart or Wavecapture or even Meyer SIM3 if you really want to fork out money. (I have been using all of these, I even used to own the very last working copy of MACFoH r.i.p. ;) )
There are also a zillion shareware utils out there ARTA or Holm being the more serious candidates. Linux people (like me) are not very lucky though..

Dual FFT performs a 'substrative' measurement, I will not go into the depth's of measurement techniques now but do realize that your measurement is relative to your reference signal.

Which can be anything!

So please stop the white noise terror: use your favourite music and everybody will be more happy.
Keep an eye on the coherence trace (the blue line in the picture above) as long as there is enough spectral content in your music source ye're just fine.

Now how to set up a measurement?

I have seen people in (club) venues putting their measurement mic at some 20 meters away from a full blasting PA system (in stereo) pumping white noise into a reverberant room. What do you expect to measure?

Now surely, it is possible to exclude some reflections from your measurement by carefully trimming your measurement window.
But if you really want to make some scientifically viable measurements you will need to restrict the variables.
Think!
What's the wavelength of the lowest frequency you are investigating, how close is the closest reflecting wall/floor/ceiling, what part of the sound system am I investigating etc etc.

OK, your just set up fine now.
Let's assume you are trying to tune a loudspeaker X-over (hell, this was this tutorial about..)
At this instance I am developing filters for this ancient cabinet (Axys T89)

This is a full range measurement (with Smaart for a change) from the 12' horn loaded part of this cabinet.
For the moment I have left the measurement compensation delay snapped to the maximum of the IR (here you can read more about this)





This is the same measurement (of course I adjusted the gain setting) a bit further away. See the difference?


So practice, practice, practice in measuring as long as is needed to acquire pictures which will help you in doing:

next part 3: X-over adjustment