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August 17, 2011
News - Crowdsourcing Algorithms
about 4 years ago
You can't do regression since you don't have the hits labelled. You can watch the video and hand-label, but it is hard to align the video to the accelerometer data.
This task is very challenging (and therefor interesting from the data-analysis point of view). I don't have time to participate, but I will share some of my insights which might help others.
First, I took one of the files (the 77 hits one) and I watched the video many (many) times. I also ripped the audio track, and I loaded it to my analysis software (Matlab) alongside with the accelerometer data. I tried to align the audio and acc data (not trivial) and I watched and listened. I listened in the original speed and in X2 and X4 times slower rate. I am telling you all this because you might want to do it as well. The things I learned (some are facts, some hypotheses):
1. A typical hit is followed by 3 "after-hits" of the bag on the wooden top. That is, after the hit the bag goes forward, hits the top, bounces backward, hits the top, bounces forward, hits the top, and then gets hit again by the boxer.
2. The main impact is therefor not from the hit itself, but from the after-hits, since the accelerometer is mounted on the top. You can clearly see this by the fact that the z-data has the strongest signal (I am NOT talking about the constant DC due to gravity, as I subtracted the average of the first 1000 samples from each channel).
3. Therefor, I do not advise you to look at the norm of the vector (root of sum of squares), because this way you loose important information about the sign (positive or negative) of the z-component.
4. The fact that much of the signal is not from the hit we need to count, but from the after-hits, makes the task challenging.
- don't look at the norm, use all 3 channels
- if you do use the norm (as well as the 3 channels), don't subtract the gravity from it (like was suggested in some of the comment). This is mathematically wrong, that's not the way to add/subtract vectors. You add/subtract component-wise, only then take the norm.
- Use the typical hit (hit followed by 3 after-hits) as the basic event to look for.
- watch the videos, listen to the audio, slow it down. Look at the data before you try and solve the problem.
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