Part of the S2DFS of observation 2005a of PSR B1839−04, showing spectral power as a function of pattern repetition frequency in component II and time (defined as the start pulse number of the 128 pulses long sequence of pulses used to determine the spectral power). Transitions from the Q mode, during which there is a strong P/P₃ = 0.08 spectral feature, to the B mode, during which low-frequency modulation dominates, are evident.