Binary onset and offset maps (transientMapProc.m)

The information about sudden intensity changes, as represented by onsets or offsets, can be combined in order to organise and group the acoustic input according to individual auditory events. The required processing is similar for both onsets and offsets, and is summarised by the term transient detection. To apply this transient detection based on the onset strength or offset strength, the user should use the request name ’onset_map’ or ’offset_map’, respectively. Based on the transient strength which is derived from the corresponding onset strength and offset strength processor (described in Onset strength (onsetProc.m) and Offset strength (offsetProc.m), a binary decision about transient activity is formed, where only the most salient information is retained. To achieve this, temporal and across-frequency constraints are imposed for the transient information. Motivated by the observation that two sounds are perceived as separated auditory events when the difference in terms of their onset time is in the range of 20 ms – 40 ms [Turgeon2002], transients are fused if they appear within a pre-defined time context. If two transients appear within this time context, only the stronger one will be considered. This time context can be adjusted by trm_fuseWithinSec. Moreover, the minimum across-frequency context can be controlled by the parameters trm_minSpread. To allow for this selection, individual transients which are connected across multiple TF units are extracted using Matlab’s image labelling tool bwlabel . The binary transient map will only retain those transients which consists of at least trm_minSpread connected TF units. The salience of the cue can be specified by the detection thresholds trm_minStrengthdB. Whereas this thresholds control the required relative change, a global threshold excludes transient activity if the corresponding rate-map level is below a pre-defined threshold, as determined by trm_minValuedB. A summary of all parameters is given in Table 28.

Table 28 List of parameters related to 'onset_map' and 'offset_map'.
Parameter Default Description
trm_fuseWithinSec 30E-3 Time constant below which transients are fused
trm_minSpread 5 Minimum number of connected TF units
trm_minStrengthdB 3 Minimum onset strength in dB
trm_minValuedB -80 Minimum rate-map level in dB

To illustrate the benefit of selecting onset and offset information, a rate-map representation is shown in Fig. 31 (left panel), where the corresponding onsets and offsets detected by the transientMapProc, through two individual requests ’onset_map’ and ’offset_map’, and without applying any temporal or across-frequency constraints are overlaid (respectively in black and white). It can be seen that the onset and offset information is quite noisy. When only retaining the most salient onsets and offsets by applying temporal and across-frequency constraints (right panel), the remaining onsets and offsets can be used as temporal markers, which clearly mark the beginning and the end of individual auditory events.


Fig. 31 Detected onsets and offsets indicated by the black and white vertical bars. The left panels shows all onset and offset events, whereas the right panel applies temporal and across-frequency constraints in order to retain the most salient onset and offset events.

[Turgeon2002]Turgeon, M., Bregman, A. S., and Ahad, P. A. (2002), “Rhythmic masking release: Contribution of cues for perceptual organization to the cross-spectral fusion of concurrent narrow-band noises,” Journal of the Acoustical Society of America 111(4), pp. 1819–1831.