Physically, the disk of an encoder with 2000 counts/rev has 2000 lines for channel A and 2000 lines for channel B.
There are 3 encoding modes :
1) X1 encoding --> only rising edges of one channel --> 2000 counts/rev
2) X2 encoding --> rising and falling edges of one channel --> 4000 counts/rev
3) X4 encoding --> rising and falling edges of both channels --> 8000 counts/rev
See also pages 3-18 to 3-20 in the
user manual of the PCI-660X.