Four experiments measured sensitivity (d') to differences in fundamental frequency (FO) between two simultaneously presented groups of frequency-modulated harmonics. Each group was passed through a bandpass filter in either a LOW (125-625 Hz), MID (1375-1875 Hz), or HIGH (3900-5400 Hz) frequency region. In the first two experiments, a dynamic FO difference (Delta F0) was created by introducing a 180 degrees disparity between the frequency modulations imposed on the two groups. Experiment 1 measured sensitivity to such Delta F0's between a MID group with a baseline FO of 125 Hi and all components summed in sine phase, and a HIGH group, in four conditions. When the baseline FO of the HIGH group was also 125 Hz, performance was good when its components were summed in sine phase and bad when they were in alternating phase. Conversely, when the HIGH FO was 62.5 Hz, performance was better for alternating phase than for sine phase, consistent with alternating phase doubling the internal representation of HIGH group's FO. Similar results were obtained for a comparison between the LOW and MID groups. Experiment 2 measured sensitivity to Delta F0's between either the LOW and MID groups or between the MID and HIGH groups, for baseline F0s of 88 and 250 Hz. Sensitivity was best when the combination of frequency region and FO was such that both groups were resolved or both unresolved by the peripheral auditory system, and worst when the groups differed in ''resolvability.'' Experiment 3 replicated experiment 2 using a different paradigm, in which the two groups were always modulated coherently, and in which the Delta F0 was constant throughout the signal. Experiments 2 and 3 also measured sensitivity to differences in FO between successively presented tokens of the same group. Experiment 4 showed that the high sensitivity to (simultaneous) Delta F0's when both groups were unresolved could be attributed to listeners detecting the pitch pulse asynchronies that inevitably arise from Delta F0's. Finally, a method for predicting sensitivity to simultaneous, across-frequency Delta F0's from that to successive within-channel differences, on the basis of optimum combination of information, was applied to the results of experiment 3. The method succeeded in predicting sensitivity to Delta F0's between two groups of resolved harmonics, but over-estimated performance when one group was resolved and the other unresolved. The results suggest that the F0's of resolved and unresolved harmonics are processed by separate mechanisms, in contrast to the predictions of ''autocorrelation'' models of FO encoding.
