igm - is a gain mode index 0,1,2 for Normal-00, Med-01, Low-11
        gbits = raw>>14 # 00/01/11 - gain bits for mode 0,1,2
        fg0, fg1, fg2 = gbits==0, gbits==1, gbits==3
        bad = (np.logical_not(fg0),\
               np.logical_not(fg1),\
               np.logical_not(fg2))[igm]