lsim warning for TF systems #624

Dear all,
I have just installed control 0.9.0 and when using lsim (or forced_response) on a transfer function system I got the following warning: "return_x specified for a transfer function system. Internal conversion to state space used; results may meaningless."

Can I skip this warning? Is there another function for simulating transfer function systems that does not require a conversion to state space?
I cannot figure out if the warning is something I should worry about or not.

Thanks,

Marco from SIPPY teams

The example tested is the following:

from future import division
from past.utils import old_div
from sippy import functionset as fset
from sippy import *
import numpy as np
import control.matlab as cnt

sampling_time = 1. # [s]
end_time = 400 # [s]
npts = int(old_div(end_time, sampling_time)) + 1
Time = np.linspace(0, end_time, npts)

switch_probability = 0.08 # [0..1]
[Usim,,] = fset.GBN_seq(npts, switch_probability, Range = [-1, 1])

white_noise_variance = [0.01]
e_t = fset.white_noise_var(Usim.size, white_noise_variance)[0]

DEN = [1., -2.21, 1.7494, -0.584256, 0.0684029, 0., 0., 0., 0., 0., 0., 0., 0., 0., 0.]
NUM = [1.5, -2.07, 1.3146]

g_sample = cnt.tf(NUM, DEN, sampling_time)

Y1, Time, Xsim = cnt.lsim(g_sample, Usim, Time)