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HelicopterMain Discussion › Remote recording of headspeed...
10-20-2004 08:49 AM  13 years agoPost 21
oyvindk

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Haugesund, Norway

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I'm not going to release the code, but as I said, I might release a free version of the software.

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10-20-2004 10:30 AM  13 years agoPost 22
RCfun

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Could you tell us a bit more about what frequency analysis method you are using? If you are just interested in the dominant frequencies over the time interval measured FFT should be quite suitable unless you want to measure how the RPM changes over time. A reference to the analysis method would be nice if you can give it away.

By the way I think this is facinating having worked a lot with wavelets during my studies (for data compression - a somewhat different context).

/RCfun

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10-20-2004 11:06 AM  13 years agoPost 23
oyvindk

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It's a bit complicated to explain, but it works by searching for target frequencies in the sound data. The results of the search is displayed as a grayscale image. The resulting image looks a lot like what you'd get from a regular FFT.

There are two parameters to adjust for the analysis, one is the search window length, the other is gain. By tweaking these two parameters, it is usually possible to easily find the target frequency.

Here's a screenshot of the analysis of Alan Szabo piroflipping in one spot (from the video that Conrod referred to earlier):


The status line shows the data under the cursor. This data updates continuously as you move the cursor with the mouse.

You can see that Alan's governor is doing a good job in keeping the RPM quite steady...

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10-20-2004 12:08 PM  13 years agoPost 24
RCfun

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Thanks for the image. Looks very much like the output of a windowed fourier transform.

Would be cool to make a program for a PDA with a microphone input that would show headspeed on the screen at the field in real time.

/RCfun

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10-20-2004 12:15 PM  13 years agoPost 25
oyvindk

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Yes, that would be cool (the PDA-idea). Now, if I just had a PDA... :-)

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10-20-2004 02:17 PM  13 years agoPost 26
frasenci

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Chile

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this is SO interesting !!!!!

what about a little description of the procedures to folow !!!!

i already have an video of me flying the heli , the PC and audio software like Wavelab III....

i do not know how to look for the frecuencies .....

Thank you

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10-21-2004 04:08 AM  13 years agoPost 27
Love2Crash

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Spring Grove, PA

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frasenci,

The method discussed is FFT or Fast Fourier Transform which is an efficient implementation of the Discrete Fourier Transform. These are mathematical methods used extensively in a wide range of digital signal processing applications. Implimenting them is quite easy (in relative terms) due to there popularity. Software packages like MathCAD, MATLAB and Octave include several variations of FFT's functions. There are also free binaries availible if your interested in C++.

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10-21-2004 07:54 AM  13 years agoPost 28
oyvindk

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Haugesund, Norway

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Yes, there are many software packages that has FFT implemented. My software uses a different method, but the end result is similar. The other difference is that my program is tailored to show the frequency as RPM.

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10-22-2004 07:09 PM  13 years agoPost 29
RCfun

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Denmark

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Might I ask how you captured the audio in that video file and how you capture it in general. This topic as aroused my interest I want to make a small experiment myself if you dont mind.


Thanks
RCfun

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10-22-2004 08:00 PM  13 years agoPost 30
oyvindk

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Haugesund, Norway

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I actually did it in the simplest way possible. I just played the video file in Media player, and at the same time I started a recording session within the audio editor (I'm using an outdated version of GoldWave).

Depending on your hardware, this may or may not work. I'm using an nForce-based mainboard, and just set the recording options to "Stereo mix" from the nForce control panel (accessed from the system tray). YMMV.

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HelicopterMain Discussion › Remote recording of headspeed...
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