FO and pitch

In this book we have used the terms FO (fundamental frequen( ) and pitch interchangeably, as there are few cases where the difference matters. Strictly speaking, pitch is what is perceived, such that some errors or non-linearities of perception may lead this to be slightly different to fundamental frequency. Fundamental frequency is a little harder to define in a true periodic signal this is simply defined as the reciprocal of the period, but as speech is never purely periodic this definition itself does not suffice. An alternative definition is that it is the input or driving, frequency of the vocal folds. 

In prosody, FO is seen as the direct expression of intonation and often intonation is defined as the linguistic use of FO. The relationship between the two is a little more subtle than this though as it is clear that listeners do not perceive the FO contour directly, but rather a processed version of this. The exact mechanism is not known, but it is as if the listener interpolates the contour through the imvoiced regions so as to produce a continuous, unbroken contour. 

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One ofthe most widely used techniques is the STRAIGHT set of analysis tools [ ].This system is operates as high quality speech analysis-modification-synthesis method implemented as a channel vocoder and has separate components for instantaneous-ffequency-based FO extraction and pitch-adaptive spectral smoothing. STRAIGHT attempts to obtain a more accurate spectral estimation and a use more sophisticated soiuce model than simple impulses. A comparison of STRAIGHT and standard cepstral analysis is showen in Figure 15.17. 

Figure 6.1 Example of FO contour with pitch accents and boundary tones maiked.

Intonation is sometimes used as a synon for prosody, and as such includes prominence and phrasing as just described. Here we use a more narrow definition in which intonation is the systematic use of pitch for communication. Often we describe intonation with reference to the pitch contour or the FO contour, which is the pattern of pitch/FO variation over an utterance" ... 

Of all the prosodic phenomena we have examined intonational tune is the most heavily related to augmentative and particularly affective content. In situations where these effects are absent, we can say to a first approximation that all utterances have in fact the same intonational tune the only differences occur as to where the pitch accents and boundary tones which make up this tune are positioned. Hence we can almost argue that for discourse neutral synthesis, there simply isn t any intonational tune prediction to be done. In other words, the real task is to predict a suitable FO contour that e q)resses the prominence and phrasing patterns and encodes the suprasegmental, rather than true prosodic patterns of the utteranee. 

In practice standard regression algorithms are difficult to use directly for FO prediction, because of a serious mismatch in the nature of the input and output representations. In general the input representation is a representation of intonational form as just discusses (for example, pitch accent types and positions) whereas the output is a continuous list of real valued numbers. In particular, the feature combination needs to generate not one, but a sequence of FO values, which is further complicated because the number of values in this sequence can vary from case to case ... 

Much of the discussion on the subject of tune centres around how to deseribe pitch accents. A pitch accent is commonly manifested in the FO contour as a (relatively) sudden excursion from the previous contour values. This is where association comes in, as pitch accents only occur in conjunction with prominent syllables, and in doing so attract attention to that syllable. Pitch accents can only occur in association with prominent syllables (see Section 6.3 on prominence), but need not occur on all prominent syllables. 

The other main area of interest in tune description concerns what happens at the ends of intonation phrases. Often FO is low at a phrase boundary, but in many circumstances FO is high. For instance, if another phrase directly follows the current one, a continuation rise may be present. If the tune is that of a yes/no question, the final pitch may also be high. The British school deals with these effects by using different nuclear accent and tail configurations. The AM model makes use of high and low boundary tones which distinguish the different types of contour. 

FO/pitch and timing are the two main acoustic representations of prosody, but this is largely because these are quite easy to identify and measure, other aspects of the signal, most notably voice quality are heavily influenced also. 

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