tv_delta_double
offset = tv_delta_double(&rtv, <v);
time_axis = tv_delta_double(&info->lin_tv, ltv);
uncorrected_offset = offset - tv_delta_double(&info->lin_btv, lbtv);
double tv_delta_double(struct timeval *tv1, struct timeval *tv2);
tv_add_micro(rtvp, (long)(tv_delta_double(&tv1, ltvp) * 1000000.0) / 2);