Maybe you've wanted tv to be that way - but it looks too grey-ish and faded to me.
The trick is - the scanlines can't be too dense. You can't use scanlines that are designed for shader overlay - because you'll be getting too dark or too pale image. This is because your snap preview is relatively small.
Belive me - I've spent couple of hours of experimenting to come to that conclusion.
---
Below zelda screenshot with my scanlines overlayed on top of it.
This is
- zelda image 640x480
- my scanlines fit to zelda size without any cropping
I'm attaching my overlay png also - this is the same as in robospin thread
Maybe the effect looks little exaggerated - but remeber, that preview image will be likely 2x smaller and it will look ok