Abstract:
Text-to-speech conversion uses pattern-matching to predict the position of phrase boundaries in spoken output. Text input to the is analyzed to identify groups of words (known as “chunks”) which are unlikely to contain internal phrase boundaries. Both the chunks and individual words are labeled with their syntactic characteristics. Access is made to a database of sentences which also contains such syntactic labels, together with indications of where a human reader would insert minor and major phrase boundaries. The parts of the database which have the most similar syntactic characteristics are found and phrase boundaries are predicted based on the phrase boundaries found in those parts. Other characteristics may also be used in the pattern-matching process.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method and apparatus for converting text to speech. 
   2. Related Art 
   Although text-to-speech conversion apparatus has improved markedly over recent years, the sound of such apparatus reading a piece of text is still distinguishable from the sound of a human reading the same text. One reason for this is that text-to-speech converters occasionally apply phrasing that differs from that which would be applied by a human reader. This makes speech synthesised from text more onerous to listen to than speech read by a human. 
   The development of methods for predicting the phrasing for an input sentence has, thus far, largely mirrored developments in language processing. Initially, automatic language processing was not available, so early text-to-speech converters relied on punctuation for predicting phrasing. It was found that punctuation only represented the most significant boundaries between phrases, and often did not indicate how the boundary was to be conveyed acoustically. Hence, although this method was simple and reasonably effective, there was still room for improvement. Thereafter, as automatic language processing developed, lexicons which indicated the part-of-speech associated with each word in the input text were used. Associating part-of-speech tags with words in the text increased the complexity of the apparatus without offering a concomitant improvement in the prediction of phrasing. More recently, the possibility of using rules to predict phrase boundaries from the length and syntactic structure of the sentence has been discussed (Bachenko J and Fitzpatrick E: ‘A computational grammar of discourse-neutral prosodic phrasing in English’, Computational Linguistics, vol. 16, No. 3, pp 155–170 (1990)). Others have proposed deriving statistical parameters from a database of sentences which have natural prosodic phrase boundaries marked (Wang, M. and Hirschberg J: ‘Predicting intonational boundaries automatically from text: the ATIS domain’, Proc. of the DARPA Speech and Natural Language Workshop, pp 378–383 (February 1991)). These recent approaches to the prediction of phrasing still do not provide entirely satisfactory results. 
   BRIEF SUMMARY 
   According to a first aspect of the present invention, there is provided a method of converting text to speech comprising the steps of:
         receiving an input word sequence in the form of text;   comparing said input word sequence with each one of a plurality of reference word sequences provided with phrasing information;   identifying one or more reference word sequences which most closely match said input word sequence; and   predicting phrasing for a synthesised spoken version of the input text on the basis of the phrasing information included with said one or more most closely matching reference word sequences.       

   By predicting phrasing on the basis of one or more closely matching reference word sequences, sentences are given a more natural-sounding phrasing than has hitherto been the case. 
   Preferably, the method involves the matching of syntactic characteristics of words or groups of words. It could instead involve the matching of the words themselves, but that would require a large amount of storage and processing power. Alternatively, the method could compare the role of the words in the sentence—i.e. it could identify words or groups of words as the subject, verb or object of a sentence etc. and then look for one or more reference sentences with a similar pattern of subject, verb, object etc. 
   Preferably, the method further comprises the step of identifying clusters of words in the input text which are unlikely to include prosodic phrase boundaries. In this case, the reference sentences are further provided with information identifying such clusters of words within them. The comparison step then comprises a plurality of per-cluster comparisons. 
   By limiting the possible locations of phrase boundary sites to locations between clusters of words, the amount of processing required is lower than would be required were every inter-word location to be considered. Nevertheless, other embodiments are possible in which a per-word comparison is used. 
   Measures of similarity between the input clusters and reference clusters which might be used include:
     a) measures of similarity in the syntactic characteristics of the input cluster and the reference cluster;   b) measures of similarity in the syntactic characteristics of the words in the input cluster and the words in the reference cluster; and   c) measures of similarity in the number of words or syllables in the input cluster and the reference cluster;   d) measures of similarity in the role (e.g. subject, verb, object) of the input cluster and the reference cluster;   e) measures of similarity in the role of the words in the input cluster and the reference cluster;   f) measures of similarity in word grouping information, such as the start and end of sentences and paragraphs; and   g) measures of similarity in whether new or previously information is being presented in the cluster.   

   One or a weighted combination of the above measures might be used. Other possible inter-cluster similarity measures will occur to those skilled in the art. 
   In some embodiments, the comparison comprises measuring the similarity in the positions of prosodic boundaries previously predicted for the input sentence and the positions of the prosodic boundaries in the reference sequences. In a preferred embodiment a weighted combination of all the above measures is used. 
   According to a second aspect of the present invention, there is provided a text to speech conversion apparatus comprising:
         a word sequence store storing a plurality of reference word sequences which are provided with prosodic boundary information;   a program store storing a program;   a processor in communication with said program store and the word sequence store;   means for receiving an input word sequence in the form of text;   wherein said program controls said processor to:   compare said input word sequence with each one of a plurality of said reference word sequences;   identify one or more reference word sequences which most closely match said input word sequence; and   derive prosodic boundary information for the input text on the basis of the prosodic boundary information included with said one or more most closely matching reference word sequences.       

   According to a third aspect of the present invention, there is provided a program storage device readable by a computer, said device embodying computer readable code executable by the computer to perform a method according to the first aspect of the present invention. 
   According to a fourth aspect of the present invention, there is provided a signal embodying computer executable code for loading into a computer for the performance of the method according to the first aspect of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     There now follows, by way of example only, a description of specific embodiments of the present invention. The description is given with reference to the accompanying drawings in which: 
       FIG. 1  shows the hardware used in providing a first embodiment of the present invention; 
       FIGS. 2A and 2B  show the top-level design of a text-to-speech conversion program which controls the operation of the hardware shown in  FIG. 1 ; 
       FIGS. 3A &amp; 3B  show the text analysis process of  FIG. 2A  in more detail; 
       FIG. 4  is a diagram showing part of a syntactic classification of words; and 
       FIG. 5  is a flow chart illustrating the prosodic structure assignment process of  FIG. 2B . 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     FIG. 1  shows a hardware configuration of a personal computer operable to provide a first embodiment of the present invention. The computer has a central processing unit  10  which is connected by data lines to a Random Access Memory (RAM)  12 , a hard disc  14 , a CD-ROM drive  16 , input/output peripherals  18 , 20 , 22  and two interface cards  24 , 28 . The input/output peripherals include a visual display unit  18 , a keyboard  20  and a mouse  22 . The interface cards comprise a sound card  24  which connects the computer to a loudspeaker  26  and a network card  28  which connects the computer to the Internet  30 . 
   The computer is controlled by conventional operating system software which is transferred from the hard disc  14  to the RAM  12  when the computer is switched on. A CD-ROM  32  carries:
     a) software which the computer can execute to provide the user with a text-to-speech facility; and   b) five databases used in the text-to-speech conversion process.   

   To use the software, the user loads the CD-ROM  32  into the CD-ROM drive  16  and then, using the keyboard  20  and the mouse  22 , causes the computer to copy the software and databases from the CD-ROM  32  to the hard disc  14 . The user can then select a text-representing file (such as an e-mail loaded into the computer from the Internet  30 ) and run the text-to-speech program to cause the computer to produce a spoken version of the e-mail via the loudspeaker  26 . On running the text-to-speech program both the program itself and the databases are loaded into the RAM  12 . 
   The text-to-speech program then controls the computer to carry out the functions illustrated in  FIGS. 2A and 2B . As will be described in more detail below, the computer first carries out text analysis process  42  on the e-mail (shown as text  40 ) which the user has indicated he wishes to be converted to speech. The text analysis process  42  uses a lexicon  44  (the first of the five databases stored on the CD-ROM  32 ) to generate word grouping data  46 , syntactic information  48  and phonetic transcription data  49  concerning the text-file  40 . The output data  46 , 48 , 49  is stored in the RAM  12 . 
   After completion of the text analysis program  42 , the program controls the computer to carry out the prosodic structure prediction process  50 . The process  50  operates on the syntactic data  48  and word grouping data  46  stored in RAM  12  to produce phrase boundary data  54 . The phrase boundary data  54  is also stored in RAM  12 . The prosodic structure prediction process  50  uses the prosodic structure corpus  52  (which is the second of the five databases stored on the CD-ROM  32 ). The process will be described in more detail (with reference to  FIGS. 4 and 5 ) below. 
   Once the phrase boundary data  54  has been generated, the program controls the computer to carry out prosody prediction process ( FIG. 2B ,  56 ) to generate performance data  58  which includes data on the pitch, amplitude and duration of phonemes to be used in generating the output speech  72 . A description of the prosody prediction process  56  is given in Edgington M et al: ‘Overview of current text-to-speech techniques part 2—prosody and speech synthesis’, BT Technology Journal, Volume 14, No. 1, pp 84–99 (January 1996). The disclosure of that paper (hereinafter referred to as part 2 of the BTTJ article) is hereby incorporated herein by reference. 
   Thereafter, the computer performs a speech sound generation process  62  to convert the phonetic transcription data  49  to a raw speech waveform  66 . The process  62  involves the concatenation of segments of speech waveforms stored in a speech waveform database  64  (the speech waveform database is the third of the five databases stored on the CD-ROM  32 ). Suitable methods for carrying out the speech sound generation process  62  are disclosed in the applicant&#39;s European patent no. 0 712 529 and European patent application no. 95302474.9. Further details of such methods can be found in part 2 of the BTTJ article. 
   Thereafter, the computer carries out a prosody and speech combination process  70  to manipulate the raw speech waveform data  66  in accordance with the performance data  58  to produce speech data  72 . Again, those skilled in the art will be able to write suitable software to carry out combination process  70 . Part  2  of the BTTJ article describes the process  70  in more detail. The program then controls the computer to forward the speech data  72  to the sound card  24  where it is converted to an analogue electrical signal which is used to drive loudspeaker  26  to produce a spoken version of the text file  40 . 
   The text analysis process  42  is illustrated in more detail in  FIGS. 3A and 3B . The program first controls the computer to execute a segmentation and normalisation process ( FIG. 3A ,  80 ). The normalisation aspect of the process  80  involves the expansion of numerals, abbreviations, and amounts of money into the form of words, thereby generating an expanded text file  88 . For example, ‘£100’ in the text file  40  is expanded to ‘one hundred pounds’ in the expanded text file  88 . These operations are done with the aid of an abbreviations database  82 , which is the fourth of the five databases stored on the CD-ROM  32 . The segmentation aspect of the process  80  involves the addition of start-of-sentence, end-of-sentence, start-of-paragraph and end-of-paragraph markers to the text, thereby producing the word grouping data ( FIG. 2A :  46 ) which comprises sentence markers  86  and paragraph markers  87 . The segmentation and normalisation process  80  is conventional, a fuller description of it can be found in Edgington M et al: ‘Overview of current text-to-speech techniques part 1—‘text and linguistic analysis’, BT Technology Journal, Volume 14, No. 1, pp 68–83 (January 1996). The disclosure of that paper (hereinafter referred to as part 1 of the BTTJ article) is hereby incorporated herein by reference. 
   The computer is then controlled by the program to run a pronunciation and tagging process  90  which converts the expanded text file  88  to an unresolved phonetic transcription file  92  and adds tags  93  to words indicating their syntactic characteristics (or a plurality of possible syntactic characteristics). The process  90  makes use of the lexicon  44  which outputs possible word tags  93  and corresponding phonetic transcriptions of input words. The phonetic transcription  92  is unresolved to the extent that some words (e.g. ‘live’) are pronounced differently when playing different roles in a sentence. Again, the pronunciation process is conventional—more details are to be found in part 1 of the BTTJ article. 
   The program then causes the computer to run a conventional parsing process  94 . A more detailed description of the parsing process can be found in part 1 of the BTTJ article. 
   The parsing process  94  begins with a stochastic tagging procedure which resolves the syntactic characteristic associated with each one of the words for which the pronunciation and tagging process  90  has given a plurality of possible syntactic characteristics. The unresolved word tags data  93  is thereby turned into word tags data  95 . Once that has been done, the correct pronunciation of the word is identified to form phonetic transcription data  97 . In a conventional manner, the parsing process  94  then assigns syntactic labels  96  to groups of words. 
   To give an example, if the sentence ‘Similarly Britain became popular after a rumour got about that Mrs Thatcher had declared open house.’ were to be input to the text-to-speech synthesiser, then the output from the parsing process  94  would be: 
   SENTSTART &lt;ADV Similarly — RR ADV&gt;, — , (NR Britain — NP1 NR) [VG became — VVD VG] &lt;ADJ popular — JJ ADJ&gt; [pp after — ICS (NR a — AT1 rumour — NN1 NR) pp] [VG got — VVD about — RP VG] that — CST (NR Mrs — NNSB1 Thatcher — NP1 NR) [VG had — VHD declared — VVN VG] (NR open — JJ house — NNL1 NR) SENTEND . — . 
   Where SENTSTART and SENTEND represent the sentence markers  86 ,  — RR,  — NP1 etc. represent the word tag data  95 , and &lt;ADV . . . . . . . . . . . . ADV&gt;, (NR . . . . . . . . . . . . NR) etc. represent the syntactic groups  96 . The meanings of the word tags used in this description will be understood by those skilled in the art—a subset of the word tags used is given in Table 1 below, a full list can be found in Garside, R., Leech, G. and Sampson, G. eds ‘The Computation Analysis of English: A Corpus based Approach’, Longman (1987). 
   
     
       
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Word Tag 
               Definition 
             
             
                 
             
           
           
             
               ( ) , - .  . . . 
               Punctuation 
             
             
               : ; ? 
             
             
               AT1 
               singular article: a, every 
             
             
               CST 
               that as conjunction 
             
             
               DA1 
               singular after-determiner: little, much 
             
             
               DDQ 
               ‘wh-’ determiner without ‘-ever’: what, which 
             
             
               ICS 
               preposition-conjunction of time: after, before, since 
             
             
               IO 
               of as preposition 
             
             
               JJ 
               general adjective 
             
             
               NN1 
               singular common noun: book, girl 
             
             
               NNL1 
               singular locative noun: island, Street 
             
             
               NNS1 
               singular titular noun: Mrs, President 
             
             
               NP1 
               singular proper noun: London, Frederick 
             
             
               PPH1 
               it 
             
             
               RP 
               prepositional adverb which is also particle 
             
             
               RR 
               general adverb 
             
             
               RRQ 
               non-degree ‘wh-adverb’ without ‘-ever’: where, when, why 
             
             
               TO 
               infinitive marker to 
             
             
               UH 
               interjection: hello, no 
             
             
               VBO 
               base form be 
             
             
               VBDR 
               imperfective indicative were 
             
             
               VBDZ 
               was 
             
             
               VBG 
               being 
             
             
               VBM 
               am, &#39;m 
             
             
               VBN 
               been 
             
             
               VBR 
               are, &#39;re 
             
             
               VBZ 
               is, &#39;s 
             
             
               VDO 
               base form do 
             
             
               VDD 
               did 
             
             
               VDG 
               doing 
             
             
               VDN 
               done 
             
             
               VDZ 
               does 
             
             
               VHO 
               base form have 
             
             
               VHD 
               had, &#39;d (preterite) 
             
             
               VVD 
               lexical verb, preterite: ate, requested 
             
             
               VVG 
               ‘-ing’ present participle of lexical verb: giving 
             
             
               VVN 
               past participle of lexical verb: given 
             
             
                 
             
           
        
       
     
   
   Next, in chunking process  98 , the program controls the computer to label ‘chunks’ in the input sentence. In the present embodiment, the syntactic groups shown in Table 2 below are identified as chunks. 
   
     
       
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               TAG 
               Description 
               Example 
             
             
                 
             
           
           
             
               IVG 
               Infinite verb group 
               [IVG to — TO be — VBO IVG] 
             
             
               VG 
               (non infinite) verb group 
               [VG was — VBDZ beaten — VVN 
             
             
                 
                 
               VG] 
             
             
               com 
               comment phrase 
               &lt;com Well — UH corn&gt; 
             
             
               vpp 
               verb with preposistional 
               [vpp of — IO | — | [VG 
             
             
                 
               particle 
               handling — VVG VG] 
             
             
                 
                 
               vpp] 
             
             
               pp 
               preposistional phrase 
               [pp in — II (NR practice — NN1 
             
             
                 
                 
               NR) pp] 
             
             
               NR 
               noun phrase (non referent) 
               (NR Dinamo — NP1 Kiev — NP1 
             
             
                 
                 
               NR) 
             
             
               R 
               noun phrase (referent) 
               (R it — PPH1 R) 
             
             
               WH 
               wh-word phrase 
               (WH which — DDQ WH) 
             
             
               QNT 
               quantifier phrase 
               &lt;QNT much — DA1 QNT&gt; 
             
             
               ADV 
               adverb phrase 
               &lt;ADV still — RR ADV&gt; 
             
             
               WHADV 
               wh-adverb phrase 
               &lt;WHADV when — RRQ 
             
             
                 
                 
               WHADV&gt; 
             
             
               ADJ 
               adjective phrase 
               &lt;ADJ prone — JJ ADJ&gt; 
             
             
                 
             
           
        
       
     
   
   The process then divides the input sentence into elements. Chunks are regarded as elements, as are sentence markers, paragraph markers, punctuation marks and words which do not fall inside chunks. Each chunk has a marker applied to it which identifies it as a chunk. These markers constitute chunk markers  99 . 
   The output from the chunking process for the above example sentence is shown in Table 3 below, each line of that table representing an element, and ‘phrasetag’ representing a chunk marker. 
   
     
       
             
             
           
         
             
                 
               TABLE 3 
             
             
                 
                 
             
           
           
             
                 
               SENTSTART 
             
             
                 
               phrasetag(&lt;ADV) Similarly — RR 
             
             
                 
               ’ — ’ 
             
             
                 
               phrasetag((NR) Britain — NP1 
             
             
                 
               phrasetag([VG) became — VVD 
             
             
                 
               phrasetag(&lt;ADJ) popular — JJ 
             
             
                 
               phrasetag[pp after — ICS (NR a — AT1 rumour — NN1 NR) pp] 
             
             
                 
               phrasetag[VG got — VVD about — RP VG] 
             
             
                 
               that — CST 
             
             
                 
               phrasetag(NR Mrs — NNSB1 Thatcher — NP1 NR) 
             
             
                 
               phrasetag[VG had — VHD declared — VVN VG] 
             
             
                 
               phrasetag(NR open — JJ house — NNL1 NR) 
             
             
                 
               SENTEND 
             
             
                 
               . — . 
             
             
                 
                 
             
           
        
       
     
   
   The computer then carries out classification process  100  under control of the program. The classification process  100  uses a classification of words and pronunciation database  100 A. The classification database  100 A is the fifth of the five databases stored on the CD-ROM  32 . 
   The classification database is divided into classes which broadly correspond to parts-of-speech. For example, verbs, adverbs and adjectives are classes of words. Punctuation is also treated as a class of words. The classification is hierarchical, so many of the classes of words are themselves divided into sub-classes. The sub-classes contain a number of word categories which correspond to the word tags  95  applied to words in the input text  40  by the parsing process  94 . Some of the sub-classes contain only one member, so they are not divided further. Part of the classification (the part relating to verbs, prepositions and punctuation) used in the present embodiment is given in Table 4 below. 
   
     
       
             
             
             
           
         
             
               TABLE 4 
             
             
                 
             
           
           
             
               verbs 
               &amp;FW 
                 
             
             
                 
               BTO22 
             
             
                 
               EX 
             
             
                 
               II22 
             
             
                 
               RA 
             
             
                 
               RGR 
             
             
                 
               beverbs 
               VBO VBDR VBG VBM VBN VBR VBZ 
             
             
                 
               doverbs 
               VDO VDG VDN VDZ 
             
             
                 
               haveverbs 
               VHO VHG VHN VHZ 
             
             
                 
               auxiliary 
               VM VM22 VMK 
             
             
                 
               baseform 
               VVO 
             
             
                 
               presentpart 
               VVG 
             
             
                 
               past 
               VBDZ VDD VHD VVD VVN 
             
             
                 
               thirdsingular 
               VVZ 
             
             
                 
               verbpart 
               RP 
             
             
               prepositions 
               iopp 
               IO 
             
             
                 
               iwpp 
               IW 
             
             
                 
               icspp 
               ICS 
             
             
                 
               iipp 
               II 
             
             
                 
               ifpp 
               IF 
             
             
               punctuation 
               minpunct 
               comma rhtbrk leftbrk quote ellipsis dash 
             
             
                 
               majpunct 
               period colon exclam semicol quest 
             
             
                 
             
           
        
       
     
   
   It will be seen that the left-hand column of Table 4 contains the classes, the central column contains the sub-classes and the right-hand column contains the word categories.  FIG. 4  shows part of the classification of verbs. The class of words ‘verbs’ includes four sub-classes, one of which contains only the word category ‘RP’. The other sub-classes (‘beverbs’, ‘doverbs’, and ‘Past’) each contain a plurality of word categories. For example, the sub-class ‘doverbs’ contains the word categories corresponding to the word tags VDO, VDG, VDN, and VDZ. 
   In carrying out the classification process  100  the computer first identifies a core word contained within each chunk in the input text  40 . The core word in a prepositional chunk (i.e. one labelled ‘pp’ or ‘vpp’) is the first preposition within the chunk. The core word in a chunk labelled ‘WH’ or ‘WHADV’ is the first word in the chunk. In all other types of chunk, the core word is the last word in the chunk. The computer then uses the classification of words  100 A to label each chunk with the class, sub-class and word category of the core word. 
   Each non-chunk word is similarly labelled on the basis of the classification of words  100 A, as is each piece of punctuation. 
   The classifications  101  for the elements generated by the classification process  100  are stored in RAM  12 . 
   Returning again to the example sentence, after classification of the elements of the input sentence would be as shown in Table 5 below 
   
     
       
             
           
             
             
           
             
           
         
             
               TABLE 5 
             
             
                 
             
           
           
             
               CLASS = [sentstart ] 
             
             
               phrasetag(&lt;ADV) CLASS = [adv ] Similarly — RR 
             
             
               CLASS = [punct minpunct ] , — , 
             
             
               phrasetag((NR) CLASS = [nonreferent proper ] Britain — NP1 
             
             
               phrasetag([VG) CLASS = [vg past ] became — VVD 
             
             
               phrasetag(&lt;ADJ) CLASS = [adj ] popular — JJ 
             
             
               phrasetag([pp) CLASS = [pp icspp after ] after — ICS 
             
             
               phrasetag ([pp) CLASS = [pp icspp after ] after — ICS 
             
           
        
         
             
                 
               &lt;&lt; SUBCAT phrasetag((NR) CLASS = [nonreferent ] a — AT1 
             
           
        
         
             
               rumour — NN1 &gt;&gt; 
             
             
               phrasetag([VG) CLASS = [vg verbpart] got — VVD about — RP 
             
             
               CLASS = I [lex coords cst ] that — CST 
             
             
               phrasetag(NR CLASS = [nonreferent proper place titular] Mrs — NNSB1 
             
             
               Thatcher — NP1 
             
             
               phrasetag([VG) CLASS = [vg past ] had — VHD declared — VVN 
             
             
               phrasetag(NR CLASS = [nonreferent locative ] open — JJ house — NNL1 
             
             
               NR) 
             
             
               CLASS = [punct majpunct ] . — . 
             
             
               CLASS = [sentend ] 
             
             
                 
             
           
        
       
     
   
   It will be seen that each element is labelled with a class and also a sub-class where there are a number of word categories within the sub-class. 
   Returning to  FIG. 2A , as stated above, the syntactic information  48  and word grouping data  46  are stored in the RAM  12  by the text analysis process  42 . The syntactic information  48  comprises word tags  95 , syntactic groups  96 , chunk markers  99  and element classifications  101 . The word grouping data comprises the sentence markers  86  and paragraph markers  87 . 
   Similar processing is carried out in forming the prosodic structure corpus  52  stored on the CD-ROM  32 . Therefore, each of the reference sentences within the corpus is divided into elements and has similar syntactic information relating to each of the elements contained within it. Furthermore, the corpus contains data indicating where a human would insert prosodic boundaries when reading each of the example sentences. The type of the boundary is also indicated. 
   An example of the beginning of a sentence that might be found in the corpus  52  is given in Table 6 below. In Table 6, the absence of a boundary is shown by the label ‘sfNONE’ after an element, the presence of a boundary is shown by ‘sfMINOR’ or ‘sfMAJOR’ depending on the strength of the boundary. The start of the example sentence is “As ever, | the American public | and the world&#39;s press | are hungry for drama . . . ” 
   
     
       
             
             
           
         
             
                 
               TABLE 6 
             
             
                 
                 
             
           
           
             
                 
               CLASS =[sentstart ] sfNONE 
             
             
                 
               phrasetag(&lt;ADV) CLASS = [adv ] As — RG ever — RR sfNONE 
             
             
                 
               CLASS = [punct minpunct ] , — , sfMINOR 
             
             
                 
               phrasetag((NR) CLASS = [nonreferent ] the — AT American — JJ 
             
             
                 
               public — NN sfMINOR 
             
             
                 
               CLASS = [lex coords cc ] and — CC sfNONE 
             
             
                 
               phrasetag((NR) CLASS = [nonreferent ] the — AT world — NN1 ‘s — $ 
             
             
                 
               press — NN sfMINOR 
             
             
                 
               phrasetag([VG) CLASS = [vg beverbs ] are — VBR sfNONE 
             
             
                 
               phrasetag( &lt;ADJ) CLASS = [adj ] hungry — JJ sfNONE 
             
             
                 
               phrasetag([pp) CLASS = [pp ifpp for ] for — IF &lt;&lt; SUBCAT phrase 
             
             
                 
               tag((NR) CLASS = [nonreferent ] drama — NN1 sfNONE &gt;&gt; 
             
             
                 
                 
             
           
        
       
     
   
   The prosodic structure prediction process  50  involves the computer in finding the sequence of elements in the corpus which best matches a search sequence taken from the input sentence. The degree of matching is found in terms of syntactic characteristics of corresponding elements, length of the elements in words and a comparison of boundaries in the reference sentence and those already predicted for the input sentence. The process  50  will now be described in more detail with reference to  FIG. 5 . 
     FIG. 5  shows that the process  50  begins with the calculation of measures of similarity between each element of the input sentence and each element of the corpus  52 . This part of the program is presented in the form of pseudo-code below: 
                                                                   FOR each element(e i ) of the input sentence:                FOR each element(e r ) of the corpus:                calculate degree of syntactic match between elements ei and er           (=A)           calculate no. — of — words match between elements ei and er (=B)           calculate syntactic match between words in elements ei and er           (=C)           match(ei,er) = w1 * A + w2 * B + w3 * C                NEXT er            NEXT ei                    
where e l  increments from 1 to the number of elements in the input sentence, and e r  increments from 1 to the number of elements in the corpus.
 
   In order to calculate the degree of syntactic match between elements, the program controls the computer to find:
         a) whether the core words of the two elements are in the same class; and   b) where the two elements are both chunks whether both chunks have the same phrasetag (as seen in Table 2).       

   A match in both cases might, for example, be given a score of 2, a score of 1 being given for a match in one case, and a score of 0 being given otherwise. 
   In order to calculate the degree of syntactic match between words in the elements, the program controls the computer to find to what level of the hierarchical classification the corresponding words in the elements are syntactically similar. A match of word categories might be given a score of 5, a match of sub-classes a score of 2 and a match of classes a score of 1. For example, if the reference sentence has [VG is — VBZ argued — VVN VG] and the input sentence has [VG was — VBDZ beaten — VVN VG] then ‘is — VBZ’ only matches ‘was — VBDZ’ to the extent that both are classified as verbs. Therefore a score of 1 would be given on the basis of the first word. With regard to the second word, ‘beaten — VVN’ and ‘argued — VVN’ fall into identical word categories and hence would be given a score of 5. The two scores are then added to give a total score of 6. 
   The third component of each element similarity measure is the negative magnitude of the difference in the number of words in the reference element, e r , and the number of words in the element of the input sentence, e i . For example, if an element of the input sentence has one word and an element of the reference sequence has three words, then the third component is −2. 
   A weighted addition is then performed on the three components to yield an element similarity measure (match (e l , e r ) in the above pseudo-code). 
   Those skilled in the art will thus appreciate that the table calculation step  102  results in the generation of a table giving element similarity measures between every element in the corpus  52  and every element in the input sentence. 
   Then, in step  103 , a subject element counter (m) is initialised to 1. The value of the counter indicates which of the elements of the input sentence is currently subject to a determination of whether it is to be followed by a boundary. Thereafter, the program controls the computer to execute an outermost loop of instructions (steps  104  to  125 ) repeatedly. Each iteration of the outermost loop of instructions corresponds to a consideration of a different subject element of the input sentence. It will be seen that each execution of the final instruction (step  125 ) in the outermost loop results in the next iteration of the outermost loop looking at the element in the input sentence which immediately follows the input sentence element considered in the previous iteration. Step  124  ensures that the outermost loop of instructions ends once the last element in the input sentence has been considered. 
   The outermost loop of instructions (steps  104  to  125 ) begins with the setting of a best match value to zero (step  104 ). Also, a current reference element count (e r ) is initialised to 1 (step  106 ). 
   Within the outermost loop of instructions (steps  104  to  125 ), the program controls the computer to repeat some or all of an intermediate loop of instructions (steps  108  to  121 ) as many times as there are elements in the prosodic structure corpus  52 . Each iteration of the intermediate loop of instructions (steps  108  to  121 ) therefore corresponds to a particular subject element in the input sentence (determined by the current iteration of the outermost loop) and a particular reference element in the corpus  52  (determined by the current iteration of the intermediate loop). Steps  120  and  121  ensure that the intermediate loop of instructions (steps  108  to  121 ) is carried out for every element in the corpus  52  and ends once the final element in the corpus has been considered. 
   The intermediate loop of instructions (steps  108  to  121 ) starts by defining (step  108 ) a search sequence around the subject element of the input sentence. 
   The start and end of the search sequence are given by the expressions:
 
 srch   —   seq   —   start= min(1,  m−no   —   of   —   elements   —   before )
 
 srch   —   seq   —   end= max( no   —   of   —   input   —   sentence   —   elements, m+no   —   of   —   elements   —   after )
 
   In the preferred embodiment, no — of — elements — before is chosen to be 10, and no — of — elements — after is chosen to be 4. It will be realised that the search sequence therefore includes the current element m, up to 10 elements before it and up to 4 elements after it. 
   In step  110  a sequence similarity measure is reset to zero. In step  112  a measure of the similarity between the search sequence and a sequence of reference elements is calculated. The reference sequence has the current reference element (i.e. that set in the previous execution of step  121 ) as it core element. The reference sequence contains this core element as well as the four elements that precede it and the ten elements that follow it (i.e. the reference sequence is of the same length as the search sequence). The calculation of the sequence similarity measure involves carrying out first and second innermost loops of instructions. Pseudo-code for the first innermost loop of instructions is given below:
 
FOR  current   —   position   —   in   —   srch   —   seq  (= p )= srch   —   seq   —   start  to  srch   —   seq   —   end  
 
 s.s.m=s.s.m+ weight( p )*match( srch   —   element   —   p, corres   —   ref   —   element ) NEXT
 
   Where s.s.m is an abbreviation for sequence similarity measure. 
   In carrying out the steps represented by the above pseudo-code, in effect, the subject element of the input sentence (set in step  103  or  125 ) is aligned with the core reference element. Once those elements are aligned, the element similarity measure between each element of the search sequence and the corresponding element in the reference sequence is found. A weighted addition of those element similarity measures is then carried out to obtain a first component of a sequence similarity measure. The measures of the degree of matching are found in the values obtained in step  102 . The weight applied to each of the constituent element matching measures generally increases with proximity to the subject element of the input sentence. Those skilled in the art will be able to find suitable values for the weights by trial and error. 
   The second innermost loop of instructions then supplements the sequence similarity measure by taking into account the extent to which the boundaries (if any) already predicted for the input sentence match the boundaries present in the reference sequence. Only the part of the search sequence before the subject element is considered since no boundaries have yet been predicted for the subject element or the elements which follow it. Pseudo-code for the second innermost loop of instructions is given below:
 
FOR  current   —   position   —   in   —   srch   —   seq (= q )= srch   —   seq   —   start  to m−1
 
 s.s.m=s.s.m+ weight( q )* bdymatch ( srch   —   element   —   q, corres   —   ref   —   element ) NEXT
 
   The boundary matching measure between two elements (expressed in the form bdymatch (element x, element y) in the above pseudo-code) is set to two if both the input sentence and the reference sentence have a boundary of the same type after the qth element, one if they have boundaries of different types, zero if neither has a boundary, minus one if one has a minor boundary and the other has none, and minus two if one has a strong boundary and the other has none. A weighted addition of the boundary matching measures is applied, those inter-element boundaries close to the current element being given a higher weight. The weights are chosen so as to penalise heavily sentences whose boundaries do not match. 
   It will be realised that the carrying out of the first and second innermost loop of instructions results in the generation of a sequence similarity measure for the subject element of the input sentence and the reference element of the corpus  52 . If the sequence similarity measure is the highest yet found for the subject element of the input sentence, then the best match value is updated to equal that measure (step  116 ) and the number of the associated element is recorded (step  118 ). 
   Once the final element has been compared, the computer ascertains whether the core element in the best matching sequence has a boundary after it. If it does, a boundary of a similar type is placed into the input sentence at that position (step  122 ). 
   Thereafter a check is made to see whether the current element is now the final element (step  124 ). If it is, then the prosodic structure prediction process  50  ends (step  126 ). The boundaries which are placed in the input sentence by the above prosodic boundary prediction process ( FIG. 5 ) constitute the phrase boundary data ( FIG. 2A :  54 ). The remainder of the text-to-speech conversion process has already been described above with reference to  FIG. 2B . 
   In a preferred embodiment of the present invention, boundaries are predicted on the basis of the ten best matching sequences in the prosodic structure corpus. If the majority of those ten sequences feature a boundary after the current element then a boundary is placed after the corresponding element in the input sentence. 
   In the above-described embodiment pattern matching was carried out which compared an input sentence with sequences in the corpus that included sequences bridging consecutive sentences. Alternative embodiments can be envisaged, where only reference sequences which lie entirely within a sentence are considered. A further constraint can be placed on the pattern matching by only considering reference sequences that have an identical position in the reference sentence to the position of the search sequence in the input sentence. Other search algorithms will occur to those skilled in the art. 
   The description of the above embodiments describes a text-to-speech program being loaded into the computer from a CD-ROM. It is to be understood that the program could also be loaded into the computer via a computer network such as the Internet.