Suggested image preprocessing before applying threshold
In your case, I don't think a pre-processing is necessary. You may find the Hough transform useful in your case.

Since your image is big, it can be difficult to have a straight line due to the noise and blur.

This is what I got using a sobel contour detection with a smaller version of your image.

This is maybe a good start to use a line detection algorithm.



A second solution would be to use color segmentation. If you assume that your screen would always have black border you can work something in the LAb or HSV color space.