The density of states of a lead doped sillenite crystal Bi12TiO20 (BTO:Pb) was investigated by means of the modulated photocurrent (MPC) and transient photocurrent (TPC) techniques. The modulated photocurrent signal as well as the TPC transients are limited by the bandwidth of the measurement systems. The MPC signal can be easily corrected by the measurement of the phase shift and modulus modification introduced by the measurement system. We propose a method to correct the TPC transient limitation and illustrate it by means of a simulation. Further, this method is applied to experimental data and we compare the density of states extracted from both methods applied to the same BTO: Pb crystal.