that's their signal
( that ( 's ( their signal ) ) )
(ROOT (SBAR (WHNP (WDT that)) (S (VP (VBZ 's) (NP (PRP$ their) (NN signal))))))
That isn't their signal. 
( That ( ( ( is n't ) ( their signal ) ) . ) )
(ROOT (S (NP (DT That)) (VP (VBZ is) (RB n't) (NP (PRP$ their) (NN signal))) (. .)))