Image Edge Detection of Colour Image. Please help

Hi,

I have the following image (<http://i.imgur.com/bZCK1Te.jpg>) and I'd like to be able to produce a black and white image just showing the wave form at the bottom.

Any help would be greatly appreciated. I've got more images like this to process so help with the code would be great.

I've tried the edge detection tutorial on the website but can't get it to work.

Many thanks.