FFT and discrete values

Hi
Is there an example that shows me how I ccan get the frequency of discrete data points?
I already tried the LV6.1 example vi's but the frequency is not the right one. (I can evaluate the frequency which should be at about 200Hz. The data amount is about 2100 points).

I've attached a VI that extracts the 2nd column from your data, patches in dt (important and missing from your data set) to form a LV Waveform, and computes the Power Spectrum. You will have to enter a different value for dt which is the reciprocal of the sample rate of your data. This will affect the frequency axis of the FFT Power Spectrum, and we can go from there to compute significant components.
The first column from the data set looked like index values, not real time values so I didn't use them.
Attachments:
ComputeFrequencySpectrum.vi ‏55 KB

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FFT and Overall Values Differ btwn VI and DIAdem

Hi all,
I have an application that displays live vibration readings including the spectrum and overall vibration level. All my report generation is done in DIAdem scripts, so it's critical that any values calculated and displayed on the LabVIEW side match the values calculated and reported in DIAdem. The attached VI and Script, when fed the input from the attached TDMS file, give me significantly different values for the magnitude of the FFT and overall RMS value. Can anyone spot what I'm doing wrong?
Thanks,
Scott
CLAD
Solved!
Go to Solution.
Attachments:
FFT and RMS Script.VBS ‏5 KB
FFT and RMS VI.vi ‏39 KB
VibrationSample.tdms ‏103 KB

Hello,
Here is a great reponse from R&D. I felt it would be better to place this on the web then keep it limited to your service request.
The difference has to do with the Window-correction. The results are identical if you select rectangluar (or no) window.
Usually a Window is taking a part of the singnal away, so in the first place, the result has a different shape and is smaller compared to result with rectangular window. This can be corrected with a specific factor for each window.
There are two possible corrections. Random and periodic.
- Periodic is used if you have pure sine-wave like signals and you want to measure the peak value. A typical use case is the flattop-window which is designed for this type of calculations. The periodic correction is too large if you want to add the values to calculate the total RMS of the signal or of a the summation of a certain frequency band.
- Random is correcting the signal back to the correct overall RMS value and should be used in all other cases.
The periodic correction for the Hanning window is a factor of two. The random correction is about 1.633. DIAdem will give the same result as LabVIEW if you use the following correction type:
FFTWndCorrectTyp = "periodic"
In the world of FFT-analysis, the different corrections are very often not obvious and hidden to the user. Unfortunately this is also the case in LabVIEW. You can find a good example of an explanation here:
http://blog.prosig.com/2009/09/01/amplitude-and-energy-correction-a-brief-summary/
There are different factors for the different window functions. If you go deeper into the FFT-VI you will find the "Scaled Window VI". This has an output "window constants". With this, it is possible to get the correction values for the window functions.
I also think, that periodic is wrong in most cases when Hanning is used, because Hanning is best for the summation of RMS values in frequency bands and the results are wrong without a random correction.
Hope that helps.
Jacob R. | Applications Engineer | National Instruments

Y/t - graphic: discrete values are displayed continuously

Whenever I want to display discrete values in a y/t - graphic, they are displayed continuously (adjacent function values are connected)
See for example:
The lower graphic should show the following discrete function:
x | (fx)
---|--------
2 | 0
3 | 1.27
4 | 0
5 | 0
6 | 0.64
7 | 0
8 | 0
9 | 0.42
But as you can see, adjacent function values are connected. I couldn't find out, how I can change that. Hope someone can helb me!
thanks,
Philipp
Solved!
Go to Solution.

Excellent question and documentation! Thank you for making it very easy to answer your question.
Click on the Paintbrush icon on the Y/t chart display - this is a link to the Colors and Lines properties for the chart display. Select Bar from the drop down list for Input 0.
If you prefer a discrete symbol, choose from cross, square, circle, etc.
- cj
Measurement Computing (MCC) has free technical support. Visit www.mccdaq.com and click on the "Support" tab for all support options, including DASYLab.

Converting audio signals from mydaq into discrete values

Hi guys!
I am currently building a project that accepts audio signals from mydaq. I wanted to make the analog signals have it descrete peak values. Is this possible? I plan to use the discrete peak values and sum them up using a shift register instead of doing peak detection. I think it is more easy.
Any suggestions would help. Thank you very much!
.."in the end we all have false hopes"..
Solved!
Go to Solution.

FalseHope wrote:
Well actually, i wanted to make the acquired signal have its discrete values because i will use the number of peaks of the sound to determine which sound is louder (my assumption is that, the louder the sound or the nearer the sound to the microphone. The more sound peaks can be acquired. While lesser peaks are acquired the least louder the sound).
That is wrong (unless you are dealing with Doppler Shifts, but they implies the object is moving). Closer peaks mean that you are dealing with a higher frequency (or pitch). Louder sounds just have higher amplitudes (larger peaks).
FalseHope wrote:
I will acquire sound signal from the mydaq, then detect the peaks and add those peaks while it is entering the acquisition. I wanted to make it discrete so that i will be able to add the numbers and the addition will stop once the sound is no longer present. The sound im talking about is a sound burst. Or a single instant sound and is not continuous.
Just use the peak detection function. You can then add whatever peaks it finds from the current acquisition.
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Avoiding null and duplicate values using model clause

Hi,
I am trying to use model clause to get comma seperated list of data : following is the scenario:
testuser>select * from test1;
ID VALUE
1 Value1
2 Value2
3 Value3
4 Value4
5 Value4
6
7 value5
8
8 rows selected.
the query I have is:
testuser>with src as (
2 select distinct id,value
3 from test1
4 ),
5 t as (
6 select distinct substr(value,2) value
7 from src
8 model
9 ignore nav
10 dimension by (id)
11 measures (cast(value as varchar2(100)) value)
12 rules
13 (
14 value[any] order by id =
15 value[cv()-1] || ',' || value[cv()]
16 )
17 )
18 select max(value) oneline
19 from t;
ONELINE
Value1,Value2,Value3,Value4,Value4,,value5,
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thanks,
Edited by: orausern on Feb 19, 2010 5:05 AM

Hi,
Try this code.
with
t as ( select substr(value,2)value,ind
        from test1
        model
        ignore nav
        dimension by (id)
        measures (cast(value as varchar2(100)) value, 0 ind)
        rules
        ( ind[any]= instr(value[cv()-1],value[cv()]),
        value[any] order by id = value[cv()-1] || CASE WHEN value[cv()] IS NOT NULL
                                           and ind[cv()]=0     THEN ',' || value[cv()] END
select max(value) oneline
from t;
SQL> select * from test1;
        ID VALUE
         1 Value1
         2 Value2
         3 Value3
         4 Value4
         5 Value4
         6
         7 value5
         8
8 ligne(s) sélectionnée(s).
SQL> with
2   t as ( select substr(value,2)value,ind
3          from test1
4          model
5          ignore nav
6          dimension by (id)
7          measures (cast(value as varchar2(100)) value, 0 ind)
8          rules
9          ( ind[any]= instr(value[cv()-1],value[cv()]),
10          value[any] order by id = value[cv()-1] || CASE WHEN value[cv()] IS NOT NULL
11                                             and ind[cv()]=0     THEN ',' || value[cv()] END
12          )
13        )
14   select max(value) oneline
15   from t;
ONELINE
Value1,Value2,Value3,Value4,value5
SQL>

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I like to know, is it NULL and Space value is same in ABAP, if it is not how to check null value.
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Senthil

everything is correct though some answers are not correct.
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How to differentiate the EMPTY Records and Null Values in DSO

Hello....how is everyone here?? Ehehehe!
I try to load some data from the flat file which contains some EMPTY data and Null Values for the records. The data type for the InfoObjects of the fields "Quantity" is "number". The sample data from the flat file (CSV) are as below:
Food              Quantity
Hamburger -       12
Cheese        -       0
Vegetable      -               (Empty)
When I try to load the above sample data to the DSO, I get the results of the data as follow:
Food              Quantity
Hamburger     - 12.000
Cheese           - 0.000
Vegetable         - 0.000
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Hi Fluffy,
It depends on the initial values of the data type
The inital values For quantity/Currency/ Numbers it takes spaces as 0
for char it is SPACE
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IF you have to force this then define quantity as char and load the data. we will not have units and aggregation in this case.
Hope this helps.
PV

From two given tables, how do you fetch the values from two columns using values from one column(get values from col.A if col.A is not null and get values from col.B if col.A is null)?

From two given tables, how do you fetch the values from two columns using values from one column(get values from col.A if col.A is not null and get values from col.B if col.A is null)?

Hi,
Use NVL or COALESCE:
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I hope this answers your question.
If not, post a little sample data (CREATE TABLE and INSERT statements, relevant columns only) for all tables involved, and also post the results you want from that data.
Explain, using specific examples, how you get those results from that data.
Always say which version of Oracle you're using (e.g., 11.2.0.2.0).
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i want to union two tables, first one with 3 fields and second one with 4 fields (diffrent count of fields).
I can add null value at end of first select but it does not work with float values in second table. Value form second table is convert to integer.
For example:
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As workaround i can type:
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Is any better/proper way to do this? Can I somehow set float field type in first select?
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Lukasz.
WIN XP, MAXDB 7.6.03

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do one thing ,
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GL~1001~157747~FEB-13~CREDIT~A~N~USD~NULL~
FEB
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str
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PART
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GL~1001~157747~JAN-13~CREDIT~A~N~USD~NULL~
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Message was edited by: chris227 year included

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FFT and discrete values

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