In  1-pass  rate  control  mode,  NVENC 
estimates the required QP for the macroblock and immediately encodes the macroblock. 
In  2-pass  rate  control  modes,  NVENC  estimates  the  complexity  of  the
frame  to  be 
encoded and determines bit distribution across the frame in the first pass. In the second 
pass,   NVENC   actually   encodes   macroblocks   in   the   frame   using   the   distribution 
determined  in  the  first  pass.  As  a  result,  quality  of  the  encoded  bitstream 
is  generally 
better with 2-pass encoding, at the cost of some performance.