Changes of fundamental frequency are normally held within these tunnels as clauses and sentences progress. The floor and ceiling of a tunnel are carefully chosen to reflect the normal speech of the person used for recording the database from which the units in the syllable inventory have been derived. The system selects the tunnel appropriate to a particular sentence from an inventory of default tunnels on the basis of sentence type as determined during the earlier parse. The micro-prosodic effects which were carefully preserved in the syllable inventory are combined with the fundamental frequency contour as determined by the abstract intonational contour. Pitch and rhythm markers fitted within a default 'tunnel'.
Vertical lines are rhythm markers, horizontal lines are pitch markers.
Hooks are inserted at this point in the prosodic process to enable these emotive overlays to be added by a separate post-processing procedure. For this reason the system has therefore to work in terms of neutral sounding prosodic contours which will consequently be in error from time to time. Especially in dialogue applications this aspect of naturalness is likely to become increasingly important, though until more understanding can be incorporated into synthesis systems errors will continue to occur. Dialogue systems however will eventually dispense with text altogether, outputting some other representation for conversion to speech. We have made provision for an input of this type, though at present there are no dialogue systems supplying it.