How to read differential TTL signal from encoder using Arduino?
It looks to me like something similar to RS485 (I have a page about that here).

You could use a chip like the LTC1480 to take the A+ and A- inputs and convert them to TTL levels.

I haven't tried it with your encoder but in principle it should work. (You would need two chips if you want to handle A+/A- and B+/B-. Or three if you want to handle R+/R- as well). Still, it's about a $6 chip.