Help with Neural Network problem

[Haemp]

Hi guys,

I'm building an application for Android devices that requires it to recognize, by accelerometer data, the difference between walking noise and double tapping it. I'm trying to solve this problem using Neural Networks.

At the start it went pretty well, teaching it to recognize the taps from noise such as standing up/ sitting down and walking around at a slower pace. But when it came to normal walking it never seemed to learn even though I fed it with a large proportion of noise data.

My question: Are there any serious flaws in my approach? Is the problem based on lack of data?

The network
I've choosen a 25 input 1 output multi-layer perceptron, which I am training with backpropagation. The input is the changes in acceleration every 20ms and output ranges from -1 (for no-tap) to 1 (for tap). I've tried pretty much every constallation of hidden inputs there are, but had most luck with 3 - 10.

I'm using Neuroph's easyNeurons for the training and exporting to Java.

The data
My total training data is about 50 pieces double taps and about 3k noise. But I've also tried to train it with proportional amounts of noise to double taps.

The data looks like this (ranges from +10 to -10):

Sitting double taps: http://i.stack.imgur.com/a7PDT.png

Fast walking: http://i.stack.imgur.com/ljRwp.png

So to reiterate my questions: Are there any serious flaws in my approach here? Do I need more data for it to recognize the difference between walking and double tapping? Any other tips?

[Bragi]

Which data exactly are you feeding the neural net: the raw audio samples, or do you first convert them with a FFT or something similar?
Have you ever taken a look at beat recognition algorithms? It sounds like a similar operation.

[Haemp]

Thanks for the recommendation but its not sound, its accelerometer data, but I guess it looks like sound. I googled around for a Beat recognition library for Java, but couldn't find any - do you know of any?

[Bragi]

A quick google on accelerometer gave me this:  http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&ved=0CIUBEBYwBQ&url=http%3A%2F%2Fjoggers-music-app.googlecode.com%2Fsvn-history%2Fr79%2Ftrunk%2Facceleromter_related_research_papers%2F10.1.1.92.1333.pdf&ei=LK_lT-KcBMPN8QOs4NCwCg&usg=AFQjCNHUB5MQi8pGlC2YnkZuyrBFQeXTkQ

[Freddy]

That link seems to go nowhere Bragi 

[Bragi]

doesn't it open a pdf file for you? (it does for me)

[Data]

Yes it opens a PDF for me too, using IE9.

[Freddy]

No, must be a Chrome thing I guess then..

[Bragi]

I'm using firefox.  I got the link from a google search. Strange. Maybe a security thing?

[Freddy]

Maybe, don't know to be honest.

[Art]

Works for me in all save Chrome as well, Freddy.

[Data]

What can I say, the number of things Chrome can’t do seems to be growing weekly. 

[Freddy]

Haha, still a long way before it catches up with IE though