ChnFFT1 Issue

[guigui27]

Hello,

I have a question about the ChnFFT1 function in Diadem.

I have to process a signal with 134217728 values and a sampling frequency of 4 MHz.

The FFT parameters for my analysis are :

- Output : Frequency & RMS amplitude.

- PeakMax average.

- Length of the individual intervals : 2048.

- Overlap : 75%

I calculated that this corresponds to (4*134217728/2048)-4=262140 intervals.

I tried the following Diadem parameters but the result does not match with what I expect :
FFTIndexChn = 0
FFTIntervUser = "NumberStartOverl"
FFTIntervPara(1) = 262140
FFTIntervPara(3) = 1
FFTIntervOverl = 75
FFTNoV = 0
FFTWndFct = "Hanning"
FFTWndCorrectTyp = "Random"
FFTAverageType = "PeakMax"
FFTAmplFirst = "Amplitude"
FFTAmpl = 1
FFTAmplType = "Ampl.RMS"
FFTCalc = 0
FFTAmplExt = "No"
FFTPhase = 0
FFTCepstrum = 0

I tried also the following Diadem parameters but that does not work :
FFTIndexChn = 0
FFTIntervUser = "LengthStartOverl"
FFTIntervPara(2) = 2048
FFTIntervPara(3) = 1
FFTIntervOverl = 75
FFTNoV = 0
FFTWndFct = "Hanning"
FFTWndCorrectTyp = "Random"
FFTAverageType = "PeakMax"
FFTAmplFirst = "Amplitude"
FFTAmpl = 1
FFTAmplType = "Ampl.RMS"
FFTCalc = 0
FFTAmplExt = "No"
FFTPhase = 0
FFTCepstrum = 0

Finally, I obtain the good result when I split my signal into packets with a maximum of 50k intervals.

I do FFT calculation for each sub-signals and take the maximum values at the end. 

Is there a limitation of the ChnFFT1 in term of number of intervals ?

Or, am I not using the function as it should be ?

Thank you in advance

[Walter_Rick]

Hi guigui27,

With your information above is the number of intervals this here:

|(134217728 - 2048 * 0.75) / (2048 * 0.25)| = 262141

This means that 1 frequency channel and 262141 RMS channels. These are more channels than DIAdem supports. The max number is (currently) 150000.

Theoretically the interval length of 4096 would result in 131069 RMS channels, but I’m not sure whether this huge number of channels is really what you would like to have.

Greetings

Walter

[guigui27]

Hello,

Thank you for your response and your correction about the number of intervals.

In fact, I do the peak max average so that results in 1 frequency channel and 1 RMS Channel.

I work with Diadem 15.0 and I identified that the problem occur when the number of intervals is greater than 2^16.

I found no information about this limitation in the help menu.

Cordially

[Walter_Rick]

Hi Cordially,

Yes, DIAdem versions before DIAdem 2019 SP1 sopport up to 65535 channels. Since DIAdem 2019 SP1 is the limit at 150000

Greetings

Walter

[guigui27]

Ok,

But the FFT calculation issue occurs in PeakMax average too which generates only 2 channels.

Why the channel number limitation interfere in this calculation ?

Guillaume

[Walter_Rick]

Hi Guillaume,

It is possible, that for performance reasons all FFTs are calculated first and after that the max peaks.

Greetings

Walter