  We study the magnetic excitation spectrum of the S=1 quantum Heisenberg spin
chain with Hamiltonian : H = sum_i cos(theta) S_i S_i+1 + sin(theta) (S_i
S_i+1)^2. We focus on the range -pi/4 < theta < +pi/4 where the spin chain is
in the gapped Haldane phase. The excitation spectrum and static structure
factor is studied using direct Lanczos diagonalization of small systems and
density-matrix renormalization group techniques combined with the single-mode
approximation. The magnon dispersion has a minimum at q=pi until a critical
value theta_c = 0.38 is reached at which the curvature (velocity) vanishes.
Beyond this point, which is distinct from the VBS point and the Lifshitz point,
the minimum lies at an incommensurate value that goes smoothly to 2pi/3 when
theta approaches pi/4, the Lai-Sutherland point. The mode remains isolated from
the other states: there is no evidence of spinon deconfinement before the point
theta =+pi/4. These findings explain recent observation of the magnetization
curve M approx (H -H_c)^1/4 for theta =theta_c.
