Michel,
A 1.78 aspect ratio is a 16:9 (widescreen) ratio for HD. There's a wide range of different aspect ratio's for various formats. Some of them are as follows:
1.78 - HD
1.33 - NTSC or PAL (Regular Tv) also known as 4:3
1.85 - Letterbox
Those are just a few examples, and there's others as well, but that gives you a general idea.
I probably could have broke that down and explained it a little better. There's a lot to this subject in terms of aspect ratios for various formats. When I have more time I'll post again with more detail.
I hope this helps a little though.