THE C O N T R I B U T I O N OF P A R S I N G TO P R O S O D I C
P H R A S I N G IN AN E X P E R I M E N T A L
T E X T - T O - S P E E C H SYSTEM
ABSTRACT
While various aspects of syntactic structure have been shown to bear on the determination of phrase- level prosody, the text-to-speech field has lacked a robust working system to test the possible relations between syntax and prosody. We describe an implemented system which uses the deterministic parser Fidditch to create the input for a set of prosody rules. The prosody rules generate a prosody tree that specifies the location and relative strength of prosodic phrase boundaries. These specifications are converted to annotations for the Bell Labs text-to-speech system that dictate modulations in pitch and duration for the input sentence.
We discuss the results of an experiment to determine the performance of our system. We are encouraged by an initial 5 percent error rate and we see the design of the parser and the modularity of the system allowing changes that will upgrade this rate.
INTRODUCTION
We describe an experimental text-to-speech system that uses a deterministic parser and prosody rules to generate phrase-level pitch and duration information for English input. This information is used to annotate the input sentence, which is then processed by the text-to-speech programs currently under development at Bell Labs. In constructing the ,system, our goal has been to test the hypotheses (i) that information available in the syntax tree. in particular. grammatical functions such as subject-predicate and head-complement, is bv itself useful in determining prosodic phrasing for svnthetic speech, and (ii) that it ts possible to use a syntactic parser that specifies grammatical functions to determine prosodic phrasing for synthetic speech.
Although certain connections between syntax and prosody are well-known (e.g. the influence of part of speech on stress in words like
progress,
or the settingoff of parenthetical expressions) very little practical knowledge is available on which aspects of syntax might be connected to prosodic phrasing. In many studies, investigators have sought connections between constituent structure and prosody (e.g. Cooper and Paccia-Cooper 1980. Umeda 1982. Gee and Grosjean 1983) but, with the exception of Selkirk (1984). they tend to neglect the representation of grammatical functions in the svntax tree. Moreover, previous work has not been specific enough to provide the basis for a full system implementation. Based on our study of prosodic phrasing in recorded human speech, we
Joan Bachenko
Eileen Fitzpatrick
C. E. Wright
A T & T Bell L a b o r a t o r i e s M u r r a y H i l l , N e w J e r s e y 07974
decided to emphasize three aspects of structure that relate to phrasing: syntactic constituency, grammatical function, and constituent length. These findings. which we will discuss in detail, have been implemented as a collection of prosody rules in an experimental text-to-speech system.
Two important features characterize our system. First. the input to our prosody system is a parse tree generated by a version of the deterministtc parser Fidditch (Hindle 1983). The left-corner search strategy of this parser and, in particular, its determinism, give Fidditch the speed that makes online text-to-speech production feasible. 1 In building a parse tree, Fldditch identifies the core subject-verb- object relations but makes no attempt to represent adjunct or modifier relations. Thus relative clauses. adverbials, and other non-argument constituents have no specified position in the tree and no specified semantic role. Second. the rules in the prosody system build a prosody tree by referring both to the syntactic structure and to earlier stages of prosodic structure. The result is a hierarchical representation that supports the view, also proposed in Selkirk (1984). that grammatical function information is related to prosodic phrasin.g, but indirectly, through different levels of processing.
Informal tests of the system show that it is capable of producing a significant improvement in the prosodic quality of the resulting synthesized speech, Our investigations of the system's problems, which we describe, have not revealed any serious counterexample to our basic approach. In many cases. it appears that problems with the current version can be resolved by taking our approach a step further, and including lexical information required by the parser as another factor in the determination of prosodic phrasing.
TEXT-TO-SPEECH
Most text-to-speech systems comprise two components: pronunciation rules and a speech synthesizer. Pronunciation rules convert the input text into a phonetic transcription; this information mav also be supplemented by a dictionary that provides information about the part of speech, stress pattern. and phonetic makeup of particular words. The speech
I. With a ~rammar of about 600 rules and a lexicon of about 2400 words, "Fidditch parses the 25 sample sentences of Robinson (1982), averagin~ 7 words per sentence and chosen for their structural divers*t'¢, at an avera~hrate of .405 seconds per sentence on a Sv'mbolics 3670. ~ rate is approximately proportional to th~ number of words in a sentence.
synthesizer then converts this phonetic transcription into a series of speech p a r a m e t e r s which are subsequently processecl to produce digitized speech.
While these systems tend to p e r f o r m quite well on word pronunciation, they fall short when it comes to providing good prosody for complete sentences. Current text-to-speech systems have no access to the syntactic and semantic properties of a sentence that influence phrase-level prosody. Hence rules for sentence prosody, when they are provided at all typically depend on superficial aspects of text (e.g. punctuation) and on heuristics that vary widely in sophistication. Although such techniques often add a more natural quality to the resulting synthetic speech, !hey .can fail in important ways, for example, by xgnormg the prosodic event between a lengthy subject and a predicate, so that there is no clear prosodic boundary between right and m a r k in The characters on the right m a r k the salient f e a t u r e s . 2
Several authors (e.g. Allen 1976; Elovitz et al. 1976; Luce et al. 1983) have suggested that prosodic differences between synthetic and natural speech are the primary, unaddressed factor leading to difficulties in the comprehension of fluent synthetic speech. The relation between phrase-level prosody and its sources, however, is so poorly understood that we have no good sense of the degree to which different levels of explanation--syntactic, semantic, or pragmatic--are applicable. We currently have reasonable tools for automatic syntactic anal~,sis of a text. but there is nothing .equivalently well-developed for semantic or pragmatic textual analysis. Thus an obvious goal is to explore the extent to which phrase-level prosody can be explained by the syntax tree and develop a detailed description of that relation. A further goal is to convert the resulting insights about this relation into a system that can work with a speech synthesizer. This allows us to test our description more adequately and perhaps also produce something that will further text- to-speech technology.
SYNTACTIC S T R U C T U R E AND
PROSODIC PHRASING
Certain relations between syntax and prosody. especially at the word level, are well-known. For example, the syntactic category of a word may affect its phonetic realization, as in the verb/adjective distinction of separate, approximate, and the verb/noun distinction of house, wind, lives. Likewise, syntactic category affects word stress, so that verbs such as
progress, insert, object, and rebel receive final stress,
whereas the corresponding nouns receive penultimate stress.
Beyond the word level, however, there has been little investigation of systematic connections between syntactic structure and prosodic phrasing. The psycholinguistic and acoustic investigations of Cooper and Paccia-Cooper (1980), U m e d a (1982) and G e e and Grosjean ( 1 9 8 3 ) a n d the prosodic theory of Selkirk (1984) are among the more notable studies and represent the two main approaches to syntax/prosody
2. Note that without a syntactic anal,,sis that correctly identifies ~rammatical functions, it is impos'sible to determine whether tlae word mark is a noun ending the subject phrase or the verb of the predicate phrase. Simple 'surface" parsers, such as that described in Umeda and Teranishl (1974l. will still fail to identify, the prosodic boundar.~ correctly..
relations. In C o o p e r and Paccia-Cooper (1980) and U m e d a (1982), the connection from syntax to prosodic phrasing is u n m e d i a t e d by any filtering process, i.e.. they propose that the details of prosodic phrasing can be d e t e r m i n e d directly f r o m syntactic structure by associating particular syntactic nodes (or constituent boundaries) with a phonetic value, either pausing, segmental lengthening, or the blocking of the cross- word conditioning of phonological rules. By contrast, G e e and G r o s j e a n (1983) and Selkirk (1984) believe that the syntax-prosody relation is indirect: prosodic phrasing is derived by rules that refer to left-to-right ordering, length (or branching patterns), and, in the ca~e of Selkirk. grammatical function, as well as constituent membership in order to infer a hierarchical prosodic structure. But while their respective positions are quite clear, none of these studies is conclusive. All lack a syntactic f r a m e w o r k sufficiently detailed and formalized to allow extensive testing, and most consider 9nly a small number of sentences and sentence t y p e s ? .
To develop our analysis, we first examined prosodic phrasing in the speech of one of us reading prose from various texts, including four instruction manuals. These texts were later a u g m e n t e d by a
~
rofessional reading of a prose story. The boundariesetween prosodic phrases were identified and then classed according to their syntactic context and semantic function.
O u r results, which are outlined below, indicate an organization of the prosodic phrases that supports the 'indirect relationship' approach of G e e and Grosjean (1983) and Selkirk (1984). We found that, in our corpus, prosodic phrasing depends on three aspects of structure: the b r e a k d o w n into syntactic constituents, the .grammatical function of a constituent, and constxtuent length, Let us review each of these factors.
Syntactic Constituency.
The possible constituents recognized by our parser are Noun Phrase (NP). Verb Phrase (VP). Adjective Phrase (AdjP), A d v e r b Phrase (AdvP), and Prepositional Phrase (PP). In general, we found that syntactic constituency is partxcularly important for predicting points at which a prosodic phrase boundary is not produced, i.e., the words within a syntactic constituent cohere. For example, the italicized phrases in (1)-(5) had no perceptible boundaries at the locations indicated by # :
(1) Left-hand # p o w e r unit is connected ...
(2) This procedure shows # you ...
(3) A n extremely # n a r r o w opening ...
(4) To spread powerload more # evenly
(5) ... next # to any p o w e r e d di-group
The single exception to word cohesion within syntactic
constituents involved boundaries between the verb and its first or second object when the object in question was lengthy. We discuss this exception below.
Grammatical Functions.
Our sample indicated that phrase boundaries are also d e t e r m i n e d by the grammatical relations among the syntactic constituents, i.e. the argument structure of the sentence. Four grammatical relations concern us:
(a) subject-predicate, as in T h e 4 8 - c h a n n e l m o d u l e -- h a s t w o d i - g r o u p s .
(b) head-complement, where the head can be a noun, verb, or adjective and may have one complement, e.g. h a s -- t w o d i - g r o u p s , or two complements, e.g. s h o w s -- y o u -- h o w t o f l y y o u r k i t e .
(c) sentence-adjunct, as in I n s e r t u n i t i n t o c o r r e c t s h e l f l o c a t i o n -- p e r d e t a i l i n s t r u c t i o n s .
(d) head-modifier, where the head can be a noun, verb, adverb, or adjective and the modifier can be one of several things, depending on the head (e.g., for nouns, the modifier can be a relative clause; for verbs, it can be a prepositional phrase; for adjectives and adverbs, the modifier can be a comparative).
We observed a hierarchy among these relations with respect to the strength, or perceptibility, of a prosodic boundary, with the boundary between sentence and adjunct receiving the highest potential boundary strength, followed by the subject-predicate boundary, then the head-complement and head- modifier boundaries. Thus in (6), there is a strong boundary between subject and predicate, whereas in (7), due to the strong boundary between adjunct and core sentence, the subject-predicate boundary diminishes. (Dashes indicate the location of the boundary being discussed.)
(6) The name of the character -- is not pronounced.
(7) When this switch is off -- the name of the character is not pronounced.
Constituent Length.
While we may view each boundary as having an intrinsic strength based on constituency and grammatical function, the determination of actual strengths appears to depend on the interaction of the intrinsic strength of a boundary with the strengths of other boundaries in the sentence, as well as the distance between these boundaries. The most salient of the interactions we observed was between the placement of a boundary at the subject-predicate junction and the placement of a boundary following the verb-complement junction. The mediating factor in this interaction was the relative length of the subject with respect to the length of the verb's complements. Thus a sentence such as (8). with both a short subject and a single short object generally is produced without a boundary in either position.
(8) You have completed the task.
But if, as in (9), the subject is long relative to the object, then a break occurs between the subject and predicate. Conversely, if the subject is short relative
to the object, then a break will occur between the verb and the object, as in (10). Or, if there are two objects and the first is simple, the break will occur between them, as in (11).
(9) The materials required -- are one kite kit.
(10) How shall we judge -- the goodness of an algorithm?
(11) This procedure shows you -- how to fly your kite.
AN EXPERIMENTAL PROSODY SYSTEM
O u r findings confirmed that syntactic structure plays a major role in determining prosodic structure, but the relationship is indirect--the exact influence of syntactic constituency varies according to the length and grammatical function of each constituent. To refine and test this idea, we implemented an experimental text-to-speech system in which rules apply to a parse tree to infer prosodic structure and then annotate the input string with phrasing information derived from the prosodic structure; this annotated input string is submitted to the Bell Labs text-to-speech programs, which convert it into a speech file. O u r system comprises three components: a parser that builds syntactic structure, rules that derive prosody information from the syntactic structure, and the Bell Labs text-to-speech programs. The parser and speech programs are independent components. The prosody rules act as a filter between them, converting the syntactic information generated by the parser into prosodic information that can be supplied to the text-to-speech programs.
Parsing.
Our parser is a version of Fidditch (Hindle 1983), a moderate coverage parser based on the deterministic model described in Marcus (1980). To build syntactic
structure, Fidditch uses a g r a m m a r that requires the representations produced by lexical and syntactic rules to be consistent with the (semantic) predicate- argument structure. The surface syntactic structures generated by the parser represent the argument structure of a phrase or sentence, i.e. the "core" constituents of a sentence (its subject (NP), modality ( A U X ) , and predicate (VP)) and the complements of phrasal heads. The structure is determined, for the most part, by rules that refer to argument information that is specified in the lexicon for the content words !nouns, verbs, adjectives, adverbs), and by rules that insert null terminals such as the "trace" of wh- movement. In general, the g r a m m a r is consistent with the government and binding f r a m e w o r k of C h o m s k y (1981), as adapted to the needs of a parser.
The input to the parser is a phrase or sentence (punctuation is optional). Its output is a surface structure tree in which the status of a constituent with respect to the predicate-argument structure of the sentence is indicated by the constituent's attachment to higher nodes in the tree. Thus only constituents that belong to the core are attached to the S node, and only complements of a phrasal head can become righthand sisters of the head. Adjuncts and modifiers.
whose role depends on semantic and pragmatic i n f o r m a t i o n about the discourse domain, have no
assigned position within a structure and so are
r e p r e s e n t e d as "orphan" nodes in the tree.
For example, Figure 1 shows the parse tree for
L e f t - h ' a n d p o w e r u n i t on e a c h s h e l f in 4 8 - c h a n n e l m o d u l e c a n p o w e r o n l y t h e e c h o c a n c e l e r s t h a t a r e in t h a t s h e l f .
4 The structure in Figure 1 contains a single core
sentence -- u n i t c a n p o w e r t h e c a n c e l e r s -- with left-
branching modifiers -- l e f t - h a n d , p o w e r , and e c h o . The
sentence also contains three modifiers -- the PPs o n
e a c h s h e l f and in 4 8 - c h a n n e l m o d u l e , and the adverb
o n l y -- which are u n a t t a c h e d constituents. This is the significance of the u n l a b e l e d node d o m i n a t i n g each of these constituents. The PPs are not attached because
u n i t is not lexically m a r k e d to take a PP headed by on
or in as a c o m p l e m e n t , and s h e l f is not lexically
m a r k e d to take a PP c o m p l e m e n t headed by in. Nor is
any constituent lexically m a r k e d to accept o n h ' as an
argument.
Figure 1 also contains a relative clause, t h a t a r e in
t h a t s h e l f . In the relative clause, T is a null t e r m i n a l that stands for the trace of the relativized subject NP; the * in tense stands for a null Aux e l e m e n t . Because n o u n s do not select relative clauses as a r g u m e n t s (any n o u n can be relativized), the parser does not identify the relations of the modifier c o n s t i t u e n t to the e l e m e n t s of the core sentence. H e n c e the relative clause is not attached to any other syntactic node in the tree.
Text-to-speech Synthesis.
The programs that make up the speech c o m p o n e n t are described in L i b e r m a n and Buchsbaum (personal c o m m u n i c a t i o n ) . These programs take English text as
input and produce digitized speech output. By
a n n o t a t i n g the input text to this system, m a n y aspects of its o p e r a t i o n can be o v e r r i d d e n or modified: e.g. the location of major and m i n o r phrase boundaries, the stress given to words, the t r a n s c r i p t i o n of words and the b o u n d a r i e s b e t w e e n them, the timing of segments, and details of the pitch contour. As we will show, with our prosody system we are able to produce strings in which four b o u n d a r y levels are identified a n d p e r c e p t u a l l y distinguished, using the c u r r e n t text- to-speech system annotations.
Prosodic Phrasing.
The prosody rules use i n f o r m a t i o n about
c o n s t i t u e n t structure, g r a m m a t i c a l role, and length to m a p a surface structure such as that in Figure 1 onto a prosody tree such as that in Figure 2. The prosody tree identifies the location of phrase b o u n d a r i e s (signified by the • nodes) and the relative strength of each b o u n d a r y (signified by a n u m b e r in the • node). It is this i n f o r m a t i o n that is used to a n n o t a t e the input text with escape sequences that provide the text-to-
speech system with instructions about prosodic
phrasing.
In f o r m u l a t i n g our rules for building the prosodic
structure, we began with the idea of simply
i m p l e m e n t i n g the model of G e e and G r o s j e a n (1983). This model, initially proposed to predict a form of
psychological data describing subjective sentence
structure k n o w n as p e r f o r m a n c e s t r u c t u r e , d e t e r m i n e s
prosodic b o u n d a r i e s from a syntactic tree, but assumes r a t h e r than explicitly presents a syntactic c o m p o n e n t .
We were initially attracted to the G e e and G r o s j e a n model because of its emphasis on relative b o u n d a r y weighting, i.e., on the d e t e r m i n a t i o n of the strength of a given b o u n d a r y with respect to the other b o u n d a r i e s in the sentence. We found that in the data we had collected, this weighting played an i m p o r t a n t role. In fact, we i n c o r p o r a t e d directly into our system one m e t h o d of doing this weighting, n a m e l y G e e and G r o s j e a n ' s rule to d e t e r m i n e the strengths of the
prosodic phrase b o u n d a r i e s a r o u n d a verb using
relative length (as m e a s u r e d by t e r m i n a l node count).
As we e x t e n d e d G e e and G r o s j e a n ' s model to create an algorithm a d e q u a t e for use in a g e n e r a l purpose system, our algorithm diverged from its starting point, reflecting our a t t e m p t s to correct weaknesses and l a c u n a e that we e n c o u n t e r e d in the G e e and G r o s j e a n model. That we e n c o u n t e r e d these
p r o b l e m s is not surprising given the d i f f e r e n c e
b e t w e e n our goals and those of G e e and Grosjean. The most i m p o r t a n t d i f f e r e n c e b e t w e e n the G e e
and G r o s j e a n model a n d our c u r r e n t algorithm
involves the factors d e t e r m i n i n g b o u n d a r y weight. G e e a n d G r o s j e a n assume that this weighting is d e p e n d e n t only on the n u m b e r of syntactic nodes, their left-to-right ordering and, in the case of the verb phrase, on c o n s t i t u e n t length. In contrast, our data, in a g r e e m e n t with Selkirk's (1984) theoretical analysis, i n d i c a t e d that b o u n d a r y strength is d e p e n d e n t on the g r a m m a t i c a l functions that the c o n s t i t u e n t s in a given s e n t e n c e play. In p a r t i c u l a r , we observed a hierarchy a m o n g these functions with respect to b o u n d a r y strength, as discussed below. 5
In addition to i n c o r p o r a t i n g g r a m m a t i c a l f u n c t i o n i n f o r m a t i o n into our system, we fleshed out the model of G e e and G r o s j e a n to deal with syntactic structures that they do not explicitly consider. In p a r t i c u l a r , G e e and G r o s j e a n ' s strictly left-to-right building of the
5. As an example of the effect that grammatical functions have
on prosodic phrasing, consider the sentence Finalh" the strange
young man left. We view this sentence as consisting of two lgrammatical relations: subject-predicate and adjunct-sentence. m our hierarchy of grammatical relations, the boundary between the adjuhct and the sentence is more salient than the boundary between the subject and the predicate. The system
reflects this by assigning a stronger boundary following Finally
than following man.
If we exclude any effects of grammatical functions and assume a simple l.eft-to-right attachment of the three
constituents Finally, the stranee voune man and left, to the
prosody tree,.we ~,ould assigr/ a -strofiger boundary following
manGr .man Imiowing . . . . Finally. . It is not .clear that Gee and oslean make this lett-to-rlght assumption in such examples.
They view adverbial phrases-like Fina[Iv as dominated by the
comi~lementizer node in the s)ntax tree. and it is difficult to determine .whether the)' integrate the material in the comptemennzer Wltla the material in the core sentence as they are analy.zing the material in the core bentence or after that analysis IS completed. If they integrate the complementizer
with the core sentence, then they assume that Finally bundles
with the sentence in a left-td-right manner and- predict, incorrectly, that the stronger boundary occurs after man. If they complete the prosodic analysis of the core sentence before bundling the sentence with the complementizer, then they incorrectly predict that there is a strong boundary after
wh- phrases in'the complementizer. In particular, they would
incorrectly predict that in sentences like At the outset what
problems d i a y o u expect the most perceptible boundary would
be after problems.
prosodic tree left c e r t a i n questions open, F o r e x a m p l e , their m o d e l does not d e a l with s e n t e n c e s e m b e d d e d in the m i d d l e of a m a i n sentence (as-in The notion [that he would refrain f r o m such an act] was incorrect.) W e i n c o r p o r a t e e m b e d d e d s e n t e n c e s into the prosodic tree in a cyclic m a n n e r to insure that the m a t e r i a l in the e m b e d d e d sentence is p r o c e s s e d b e f o r e that in the main sentence. 6 In a d d i t i o n . G e e and G r o s j e a n leave o p e n the t r e a t m e n t of the multiple r i g h t w a r d e m b e d d i n g of n o n - s e n t e n t i a l constituents, e.g., the NP e m b e d d i n g in The destruction o f the good name of his f a t h e r . O u r a p p r o a c h is to handle these cases recursively, from the most d e e p l y e m b e d d e d p h r a s e up, in o r d e r to p r e s e r v e the prosodic cohesion of the entire NP.
O u r adjunction rules are d e r i v e d for the most p a r t from S e l k i r k ' s account. We have also m a d e use of the idea, which G e e and G r o s j e a n ([983) t a k e largely f r o m the work of S e l k i r k , that c e r t a i n syntactic heads m a r k off phonological phrase b o u n d a r i e s , and provide the basic prosodic constituents for higher level analysis.
O u r p r o s o d y rules run in four i n d e p e n d e n t stages. E a c h stage builds on the previous stage, so that the rules can r e f e r to both syntactic and prosodic s t r u c t u r e as they build successively higher levels of prosodic structure.
(i) Adjunction Rules combine o r t h o g r a p h i c a l l y distinct words into phonological c o n s t i t u e n t s with no internal w o r d b o u n d a r y , T h e y join a w o r d to its left or right neighbor d e p e n d i n g on (a) the c a t e g o r y of the word, and (b) its s t r u c t u r a l r e l a t i o n to o t h e r words. In g e n e r a l , adjoinable words are the function words-- articles, c o m p l e m e n t i z e r s , auxiliary verbs, conjunctions, p r e p o s i t i o n s and pronouns (except for the "strong" possessives, mine, hers, theirs, yours, ours, which are t r e a t e d as r e g u l a r NP's).
A d j u n c t i o n occurs six times for the s e n t e n c e in F i g u r e 2 to c r e a t e six multiple word groups, all right- adjoining: on each, in 48-channel, can power, the echo,
that are and in that. These groups of adjoined words
a p p e a r as t e r m i n a l s in the p r o s o d y tree in F i g u r e 2. In subsequent processing the b o u n d a r i e s b e t w e e n the words in these groups are m a r k e d so that the text-to- speech system does not p r o d u c e the prosodic indications of a word b o u n d a r y . In addition, these groups are t r e a t e d as single words in f u r t h e r analyses.
(ii) ~-phrasing Rules construct phonological (or 6p) phrases, which are the building blocks of the p r o s o d y tree. These rules identify groups of words that c o h e r e strongly in speech and thus should not be s e p a r a t e d by phrase boundaries. In the p r e s e n t i m p l e m e n t a t i o n , each • phrase is c o n s t r u c t e d by a left-to-right process that collects the words f o r m e d by adjunction until it reaches a noun or verb. A t this point, a • p h r a s e is c r e a t e d that consists of the c o l l e c t e d words plus the noun or verb, which acts as head of the phrase. F o r e x a m p l e , in that shelf, in F i g u r e 2. is a single • p h r a s e consisting of two words.
In Figure 2, the • nodes m a r k e d with a syntactic c a t e g o r y are the minimal phonological constituents with respect to l a t e r rules that build the prosodic
s. Having taken this strona approach, we now understand the limited exceptions to this~mechanism, which we discuss below'.
phrases; these @ phrases have an internal s t r u c t u r e , but the s t r u c t u r e plays no role in f u r t h e r processing. Note that n e i t h e r adjectives nor adverbs are allowed to be the h e a d of a • p h r a s e , so that three additional open slots is a single • phrase consisting of four words. E x a m p l e s such as Someone tall walked into the room, however, suggest that our t r e a t m e n t of these categories is not d e t a i l e d enough and that, in future versions of the system, some adjectives and adverbs should act as • heads.
(iii) Prosody-phrasing rules use i n f o r m a t i o n about phrases and syntactic s t r u c t u r e to c r e a t e a new o r g a n i z a t i o n of the sentence and to assign strength values to the b o u n d a r i e s b e t w e e n successive • phrases. The process of building the prosody tree starts with the sentence node (S or Sbar) that is most deeply e m b e d d e d in the u t t e r a n c e , t r a n s f o r m i n g it into a prosody subtree. This process continues through successively higher levels of sentence nodes until all top-level sentences have b e e n t r a n s f o r m e d into prosody subtrees. All the processing of each successive sentence is done b e f o r e the relation of the sentences to e a c h other is c o n s i d e r e d 7
W i t h i n a s e n t e n c e , the • phrases are p r o c e s s e d from left to right. This stage of the analysis uses a window that allows access to t h r e e a d j a c e n t nodes. P a t t e r n - a c t i o n rules, which are d e s c r i b e d below, apply to the nodes in the window and build p r o s o d y subtrees that r e p l a c e the syntax nodes. T h e s e subtrees are h e a d e d by a • node containing a n u m b e r that r e p r e s e n t s node count; the n u m b e r is d e t e r m i n e d by counting the n u m b e r of nodes c o n t a i n e d in the p r o s o d y a s u b t r e e , plus 1 for the • node that heads the subtree. In g e n e r a l , the prosody p h r a s e rules do t h r e e things:
(a) Balance prosodic phrases by r e f e r r i n g to c o n s t i t u e n t length. This rule only applies for building the p r o s o d y subtree that contains the verb. If the node count for subject plus verb is less than the node count of the verb's c o m p l e m e n t , then subject and verb are g r o u p e d t o g e t h e r in a prosodic subtree; this gives the phrasing in The characters on the right -- mark the salient f e a t u r e s . O t h e r w i s e , the verb is g r o u p e d with its c o m p l e m e n t in a prosodic subtree; an e x a m p l e of this grouping is the s u b t r e e for can p o w e r only the echo cancelers in Figure 2,
(b) C o m b i n e the • p h r a s e d a u g h t e r s of the major constituents, excluding VP, into a prosodic subtree. A t p r e s e n t , this rule only applies to NP and PP since adjectives and adverbs are c u r r e n t l y not t r e a t e d as @ heads. F o r e x a m p l e , the name o f the character, which forms two d~ phrases under NP (the name and of the
character), become a single prosody phrase that
r e p l a c e s the NP.
7, We have found at least one class of phrases for which this order of processing appears inappropriate. In these, the head of the top-level phrase is epistemlc -- e.g., believe, know, belief, knowledge -- andits complement is a sentence. In most cases, the current processing order for embedded sentences will produce a break between a head and a following embedded sentence. For this class of sentences, however, thd break does not seem to be appropriate. "~Vhile it wot ld be straightforward to handle this as an exception, we are currently examning whether there is a more principled wa? to describe what must be done in these cases.
s Onl,~ the top-level • nodes, those which contain the head of the ~ ntactic phrase, are counted in computing the node count. LnU~,~'- ~y~:Lv~ . . . . ~am~lev • in Fi,,ure -, "~ the sub-phrasal branching' ot" Left-hand and power unit c~oes not contribute to the node count.
(c) Bundle t o g e t h e r prosodic constituents ( ~ p h r a s e s ) from left to right if no o t h e r rules apply. This rule i n t e g r a t e s the constituents left u n a t t a c h e d by the p a r s e r into the prosodic s t r u c t u r e . It accounts for the prosodic s t r u c t u r e of left-hand power unit on each shelf in 48-channel module in figure 2, which is f o r m e d by first bundling left-hand power unit with on each
shelf, into q~-3, and then bundling the result with in
48-channel module into ~ - 5 . The final a p p l i c a t i o n of bundling r e p l a c e s the Sigma node with the top level p r o s o d y node, which is q5-13 in F i g u r e 2.
(iv) Prosody conversion rules m a p the b o u n d a r y strength indices o n t o t h r e e p h o n o l o g i c a l m e c h a n i s m s . B o u n d a r y indices in the low r a n g e , e.g. the ~ - 3 nodes in F i g u r e 2, are r e a l i z e d as a p h r a s e accent ( P i e r r e h u m b e r t 1980). M i d - r a n g e indices such as ~-5 and ~ - 9 in F i g u r e 2 are r e a l i z e d as c h a n g e s in pitch range. H i g h indices are r e a l i z e d with m o d u l a t i o n s in b o t h p i t c h range and d u r a t i o n . Thus the h i e r a r c h i c a l o r g a n i z a t i o n of a s t r u c t u r e such as that in F i g u r e 2 can be r e f l e c t e d d i r e c t l y in the s y n t h e s i z e d speech.
P H E N O M E N A NOT T R E A T E D
Several p h e n o m e n a have b e e n o m i t t e d f r o m this p r e l i m i n a r y version of the system. Some of these omissions arise f r o m the fact that we c o n c e n t r a t e d on sentence analysis r a t h e r t h a n discourse analysis. O t h e r s involve p h e n o m e n a that c h a r a c t e r i z e s p o k e n English, and thus did not occur in our original corpus of t e c h n i c a l r e p a i r manuals.
C o n t r a s t i v e stress is an e x a m p l e of prosodic phrasing based on discourse analysis. In our system's analysis, the p h r a s e f r o m India does not r e c e i v e c o n t r a s t i v e stress in (12).
(12) P a s s e n g e r s f r o m s e v e r a l c o u n t r i e s e n t e r e d
the t e r m i n a l .
Finally a m a n f r o m I n d i a w a l k e d in.
In designing the c u r r e n t s y s t e m , we have c o n c e n t r a t e d on the level of s e n t e n c e analysis. H a n d l i n g the c o n t r a s t s involved in d a t a like (12) n e c e s s i t a t e s an a d d i t i o n a l level of discourse analysis.
In a d d i t i o n , the s y s t e m n e v e r explicitly m a n i p u l a t e s s e g m e n t d u r a t i o n s or o v e r a l l s p e e c h rate. F o r e x a m p l e , we have vet to e x p l o r e w h e t h e r l e n g t h e n i n g of the s e g m e n t b e f o r e a m i d - r a n g e b o u n d a r y value is a p p r o p r i a t e , or w h e t h e r increasing the d u r a t i o n of constituents of the core s e n t e n c e might e n h a n c e the n a t u r a l sound of the system.
RESULTS AND FUTURE RESEARCH
To date. our s y s t e m has b e e n t e s t e d s y s t e m a t i c a l l y on a set of 39 s e n t e n c e s , and its p e r f o r m a n c e has b e e n o b s e r v e d less f o r m a l l y on a set of a p p r o x i m a t e l y 300 sentences. 9 The test corpus covers a r e p a i r m a n u a l for t e l e p h o n e switching systems and an i n t r o d u c t o r y d e s c r i p t i o n of the Prose 2000 t e x t - t o - s p e e c h system. W e a d d e d sentences cited in U m e d a (1982) and s e n t e n c e s that we c o m p o s e d in o r d e r to e x t e n d the range of syntactic constructions r e p r e s e n t e d in the test. In g e n e r a l , we have o b s e r v e d a significant i m p r o v e m e n t of prosodic quality in those test
9 The 39 sentences are listed in the appendix to this paper.
s e n t e n c e s w h e r e the p a r s e r and the prosodic c o m p o n e n t have r e t u r n e d a c c e p t a b l e results.
W e have o b s e r v e d p r o b l e m s , h o w e v e r , e s p e c i a l l y in the f o r m a l test corpus, much of which we chose for its p o t e n t i a l difficulty. Of the 39 test s e n t e n c e s , 38 p a r s e d correctly. Of these, the prosodic c o m p o n e n t r e t u r n e d 26 sentences with a c o m p l e t e set of a c c e p t a b l e p r o s o d y m a r k i n g s . In t e r m s of a c t u a l m a r k i n g s , the system m a r k e d 393 prosodic e v e n t s , of which 21 m a r k i n g s were u n a c c e p t a b l e . W e can a t t r i b u t e e r r o r s in those s e n t e n c e s with u n a c c e p t a b l e p r o s o d i c m a r k i n g s to t h r e e distinct p r o b l e m s discussed below.
Complement Sentences.
F i v e of the e r r o r s that arose from the p r o s o d y s y s t e m ' s t r e a t m e n t of the test c o r p u s result f r o m the fact that the system sets off all s u b o r d i n a t e s e n t e n c e s , including c o m p l e m e n t s e n t e n c e s , from the m a i n s e n t e n c e . I n f o r m a l testing of the p r o d u c t i o n s of four i n f o r m a n t s on the r e l e v a n t d a t a i n d i c a t e d that this a p p r o a c h w o r k s c o r r e c t l y for c o m p l e m e n t s e n t e n c e s such as (13)-(16). ( C o m p l e m e n t s e n t e n c e s are italicized):
(13) H e a l t h services c a u t i o n e d W e s t e r n r e s i d e n t s -- that they should ask where their
watermelons come f r o m before buying.
(14) W e have to satisfy p e o p l e -- that the crisis is past.
(15) The v e n d o r s e x p l a i n e d -- that this is the result of illness among 281 people who ate pesticide- tainted watermelons.
(16) W a t e r m e l o n g r o w e r s w o n d e r -- whether this will continue throughout the rest of the season.
H o w e v e r . the i n f o r m a n t test consistently i n d i c a t e d that the c o m p l e m e n t s e n t e n c e s in (17)-(19)" are not set off by a c o m p a r a b l e b o u n d a r y :
(17) T h e y b e l i e v e California sales are still o f f 75 percent.
(18) T h e y t h i n k the Southeast is shipping half its normal load.
(19) G r o w e r s and r e t a i l e r s c l a i m e d the incident hurt sales across the USA.
Cases like (17)-(19). in which no b r e a k is p e r c e i v e d b e t w e e n the v e r b and its c o m p l e m e n t s e n t e n c e , f o r m a s y n t a c t i c a l l y distinct class in F i d d i t c h . This class is c h a r a c t e r i z e d by the fact that the v e r b a l h e a d in e a c h case is one that does not r e q u i r e that its c o m p l e m e n t s e n t e n c e begin with a c o m p l e m e n t i z e r ( e i t h e r that, f o r , or a wh- word). T h e class includes e p i s t e m i c verbs, like those in (17)-(19), as well as a wide r a n g e of verbs that t a k e e i t h e r t e n s e d s e n t e n c e s , or various types of n o n - t e n s e d s e n t e n c e s as c o m p l e m e n t s ) ° The e x a m p l e s (20)-(26) d e m o n s t r a t e the range of this class ( c o m p l e m e n t s e n t e n c e s are italicized):
l0 Fidditch, in followin~ the outlines of Chomskv's (1981) Government and Binding theory, assumes that propositions, i.e., those elements that cBntain k]oth a prkdicate and a perhaps null subject, are syntactically represented as sentences, regardless of tensing.
(20) We had the ship's forces make temporary repairs.
(21) We saw the crew repairing the unit.
(22) He wants the units repaired by the ship's force.
(23) The c o n s t r u c t i o n of the unit makes detailed
investigation impractical.
(24) Try to give the names of the characters in
advance.
(25) They will help finish the job.
(26) The new e q u i p m e n t will facilitate making
repairs.
Sentence-Final Constituents.
F i f t e e n of the errors that arose from the system's t r e a t m e n t of the test corpus result from a high boundary value that sets final c o n s t i t u e n t s off from the main sentence. The high value is due to the system's purely left-to-right a t t a c h m e n t of syntactically u n a t t a c h e d constituents (see rule iii.d above). The high boundary value is acceptable in sentences like
(27)-(29). (The relevant final c o n s t i t u e n t s in these
examples are italicized).
(27) In these instances it may be desirable to use p h o n e m e characters instead of text characters
to represent a word -- each time it appears
in the input text.
(28) Phonemic characters can also be used to handle syntactic data such as boundaries --
which can improve speech quality.
(29) We were u n a b l e to finish the work -- due
to equipment failure.
However. the high b o u n d a r y value sets the final constituent off u n n a t u r a l l y from the m a i n sentence in data such as (30)-(32).
(30) The method by which you convert a word into p h o n e m e s is provided -- in
Chapter 7.
(31) The e x p e r i m e n t e r s instructed the i n f o r m a n t
to speak -- naturally.
(32) We discussed the techniques -- we had
implemented.
In many cases it appears that the g r a m m a t i c a l relation of the final constituent to the rest of the sentence d e t e r m i n e s the boundary value that sets off
this constituent. In particular, sentence adjuncts,
which bear no relation to any single item in a sentence, are set off by a minor phrase b o u n d a r y . whereas final constituents that modify a p a r t i c u l a r
item are less perceptibly set off. This is the
distinction b e t w e e n the final constituents in (27)-(29), which are adjuncts, and those in (30)-(32), which are modifiers. However, while the distinction b e t w e e n the
grammatical relations of the core sentence
( c o m p l e m e n t and subject) and those of the periphery (adjunct and modifier) is fairly straightforward, and handled directly bv the mechanisms of the Fidditch
parser, the distinctions b e t w e e n the peripheral
e l e m e n t s of adjunct and modifier are complex a n d require the addition of costly mechanisms.
T h e cost of adding a d j u n c t / m o d i f i e r distinctions is illustrated by the ambiguity that arises when both
adjunct and modifier readings are possible. For
example, on one reading of (31), naturally modifies the
verb speak; i.e., the i n f o r m a n t s were to speak in a
n a t u r a l m a n n e r . O n the other reading, naturally is an
adjunct equivalent to of course. (To see this m e a n i n g
more clearly, consider the r e a r r a n g e m e n t of this
sentence with the adjunct at the beginning: Naturally,
the3: instructed the informants to speak.) The context of speech analysis prefers the f o r m e r reading. However, the net benefit of adding sophisticated contextual analysis to our system, if a t t a i n a b l e , is, at best, unclear. The same may be said of adding selectional restrictions, or detailed i n f o r m a t i o n on logical form.
In contrast, a finer t r e a t m e n t of local syntactic
constraints on b o u n d a r y values preceding final
constituents is within reach. From the data we have e x a m i n e d , it appears that the character of the prosodic e v e n t before the final c o n s t i t u e n t can be locally d e t e r m i n e d to a great extent. For the most part. this d e t e r m i n a t i o n depends on the category type of the final constituent and on the contents of the leading edge of the constituent. For example, interjections
(however. moreover, therefore, alas, thus, of course, etc.)
and sentence adverbs (apparently, generally, luckih'
etc.) are u n i f o r m l y set off by a high b o u n d a r y value a n d should r e m a i n so. In contrast, the b o u n d a r y value of final prepositional phrases, particularly those with
a monosyllabic preposition (in, on. at, to. with, f o r ) as
11
the left edge of the phrase, should be reduced. We
are c u r r e n t l y engaged in categorizing the c o n s t i t u e n t
types and left-edge items that characterize final
constituents with respect to the prosodic event that precedes them.
A l t e r n a t i v e l y , we are considering the play-it-safe approach of reducing the high b o u n d a r y values that set off final constituents to m i d - b o u n d a r y values.
C u r r e n t l y these values are c o n v e r t e d to a
downstepping feature. This approach may also be
useful in conjunction with our local d e t e r m i n a t i o n approach for those constituents whose status is either undecidable or ambiguous u n d e r the latter a p p r o a c h J ~
11. In this view, expressions such as in principle, iJ~ eenerul, in
particular, in consideration of, etc. must be treated like interjections.
12. Reducing the final boundary ~alue leaves ambiguities unresolved. For sentences such as (i! and (ii), below, we believe this lack of resolution is appropriate:
(i) John saw a ~irl in the park with a telescope.
park.liThe telesccTpe is witli John or the girl. or it's in the
(ii) I need a woman to fix the sink.
[I need a woman so that I can fix the sink. I need a woman who can fix the sink.]
Our view, following. _Marcus. and.. Hinde (p.e.) is that in normal,
spoken Enghsh, such ambl~ulnes are not processed unless the speaker or listener is directly questioned re~,arding the
ambiguity, . . . . Likewise. the. _pr~osodic events . ~hat. mi g ht
dlsamblguate are inappropriate unless such questioning occurs. Other cases are less clear. For example, it is difficult to imazine that, in (28) the difference between the readin~ of the
whic~'h clause as a sentence adjunct and as a noun~phrase
modifier on boundaries is not processed. We would hope that in
such cases some local distinction, such as the presence or absence of the comma in (28), obtains.
151
Sentence-Initial Constituents.
W h e n a sentence contains both sentence-initial and sentence-final adjuncts, the sentence-initial adjuncts will be less prominently set off than the sentence-final adjuncts due to the left-to-right a t t a c h m e n t of adjuncts to the prosodic tree (see rule iii.b above). In data like (33), however, a more appropriate rendering would have the boundary after the adjunct 011 a clear day be strong relative to the boundary before the adjunct as it rises over the mountains.
(33) On a clear day you can see the sun as it rises over the mountains.
While it would be trivial to increase the value of the pertinent boundary, we are as yet unsure what the critical features are which require a more perceptible boundary. For example, while a higher boundary value after the prepositional phrase in (34) might b'e acceptable, it is not clear that it is necessary:
(34) In the morning John left.
Given the stylistically distinct nature of this data, we have not yet considered this question in detail.
Summary.
While we have systematically tested our system so far on a small set of examples, the number of prosodic events involved in those examples, 393. is high, due to the length of the sentences tested. We find the 5 percent error rate, representing 21 prosodic events, encouraging at this stage in the development of the system. In addition, we have delimited the problem areas of an approach that relies solely on information available in the syntax tree. Our initial investigation of these problems indicates that at least part of the necessary information about phrase-level prosody is conveyed in the lexicon per se. Additionally, due to the left-corner orientation of the Fidditch parser, which exists independently to optimize search strategies, the necessary lexical information is made easily available.
CONCLUSIONS
We have described an on-line experimental system that uses prosody rules to infer prosodic phrasing from constituent structure, grammatical functions, and length considerations. The system contains three modules: a deterministic parser, a set of prosodic phrasing rules, and an algorithm to convert the output of the prosodic phrasing rules into signals for the Bell Labs text-to-speech system.
In developing the experiment, our intention was to build a working system that would allow us to test various hypotheses about the connections between syntax and prosodic phrasing in human speech and to upgrade the prosody of existing synthetic speech. The modularity of our system enables us to alter each module independently in order to test different hypotheses. For example, the parser can be altered to reflect the difference between verbs that require a complementizer before a sentential complement and those that do not. 13 This alteration is independent of
13. Fidditch represents this as a difference in the level of the com- plement sentence. Verbs that require a complementizer take an S-bar complement, while verbs that do not require a com- plementizer take an S complement with an optional that preceding.
the workings of the prosody system or the prosody conversion rules.
The existence of this prosody system makes the problem areas in the syntax-prosody relation more tractable by allowing online testing of a large body of data. For example, the prosodically different character of the two classes of complement sentences discussed above became apparent after several examples from each class were run through the system. We therefore feel we have built a tool that will aid in designing better approximations of sentence prosody as it relates to syntacnc structure.
REFERENCES
Allen, J. 1976. Synthesis of speech from unrestricted text. Proceedings o f the IEEE, 4, 433-442.
C h o m s k y , N. 1971. Lectures on government and binding. Dordrecht: Foris Publications.
Cooper, W. and J. Paccia-Cooper. 1980. Syntax and speech. Cambridge, M A : H a r v a r d University Press.
Elovitz, H., R. Johnson, A. M c H u g h , and J. E. Shore. 1976. Letter-to-sound rules for automatic translation of English text to phonetics. I E E E Transactions on Acoustics, Speech, and Signal Processing, 6, 446-459.
Gee, J. P. and F. Grosjean. 1983. P e r f o r m a n c e structures: a psycholinguistic and linguistic appraisal. Cognitive Psychology, 15, 411-458.
Hindle. D. 1983. User manual for Fidditch, a deterministic parser. N R L Technical M e m o r a n d u m #7590-142.
Luce, P.A., Feustel, T.C., and Pisoni, D.B. 1983. Capacity demands in short-term m e m o r y for synthetic and natural speech. Human Factors, 25, 17-32.
Marcus, M. 1980. A theory o f syntactic recognition f o r natural language. Cambridge, M A : M I T Press.
Pierrehumbert, J. B. 1080. The phonetics and phonology of English intonation. Ph.D. Dissertation, MIT.
Selkirk, E. O. 1984. Phonology and syntax: the relation between sound and structure. Cambridge, M A : M I T Press.
U m e d a , N. 1982. Boundary: perceptual and acoustic properties and syntactic and statistical determinants. Speech and Language, 7, 333-371.
U m e d a , N. and R. Teranishi. The parsing program for automatic text-to-speech synthesis developed at the Electrotechnical L a b o r a t o r y in 1968. I E E E Transactions on Acoustics, Speech, and Signal Processing, 23, 183-188.
A P P E N D I X : T E S T S E N T E N C E S
1. T H E N A M E OF T H E C H A R A C T E R IS N O T P R O N O U N C E D .
2. L E F T - H A N D P O W E R U N I T ON E A C H S H E L F IN F O R T Y - E I G H T
C H A N N E L M O D U L E P O W E R S O N L Y E C H O C A N C E L L E R S IN T H A T
S H E L F .
3. THE C O N N E C T I O N MUST BE D E T E R M I N E D F O R THE L E F T - H A N D P O W E R UNITS ON E A C H S H E L F .
4. T H E C O N N E C T I O N MUST BE D E T E R M I N E D F O R T H E L E F T - H A N D P O W E R UNITS W H I C H A R E ON E A C H SHELF.
5. T H E M E T H O D BY W H I C H ONE C O N V E R T S A W O R D INTO P H O N E M E S IS P R O V I D E D IN C H A P T E R 7.14
6. WE DISCUSSED THE T E C H N I Q U E S WE H A D I M P L E M E N T E D .
7. T H E T E C H N I Q U E S WE H A D I M P L E M E N T E D W E R E TESTED ON A L A R G E R M A C H I N E .
8. THE M A N W H O M WE SAW Y E S T E R D A Y LIVES F A R A W A Y F R O M H E R E .
9. T H E Y T O L D HIM TO W A L K SLOWLY.
10. T H E D E S T R U C T I O N OF T H E G O O D N A M E OF HIS F A T H E R B O T H E R E D HIM.
11. L A T E L Y HE H A D H A S C O N T R O L O V E R T H E S I T U A T I O N .
12. I N E E D A W O M A N TO F I X T H E SINK.
13. JOHN MET A W O M A N H E T H O U G H T H E LIKED.
14. THE W O M A N I S A W C A M E F R O M H E R E ,
15. IN T H E S E INSTANCES IT M A Y BE D E S I R A B L E TO USE P H O N E M E C H A R A C T E R S I N S T E A D O F T E X T C H A R A C T E R S TO R E P R E S E N T A W O R D E A C H T I M E IT A P P E A R S ON T H E INPUT TEXT.
16. P H O N E M E C H A R A C T E R S G I V E M O R E C O N T R O L O V E R THE P A R T I C U L A R SOUNDS T H A T A R E G E N E R A T E D .
17. T H E M A T E R I A L S R E Q U I R E D A R E ONE KITE KIT.
18. P H O N E M I C C H A R A C T E R S C A N A L S O BE USED TO H A N D L E SYNTACTIC D A T A SUCH AS THE B O U N D A R I E S W H I C H C A N I M P R O V E SPEECH Q U A L I T Y .
19. IT M A Y BE D E S I R A B L E TO G I V E J O H N A H A N D .
20. A F T E R T H E S E Q U E S T I O N S , A D E T A I L E D D E S C R I P T I O N O F T H E USE O F P H O N E M E S W I L L BE
P R O V I D E D IN C H A P T E R 7.
21. T H E E N G L I S H T H A T IS SPOKEN IN A M E R I C A A T THE P R E S E N T DAY H A S R E T A I N E D A G O O D M A N Y C H A R A C T E R I S T I C S O F E A R L I E R BRITISH E N G L I S H T H A T DO NOT S U R V I V E IN BRITISH E N G L I S H T O D A Y .
22. P H O N E M I C C H A R A C T E R S C A N A L S O BE USED TO H A N D L E S Y N T A C T I C D A T A SUCH AS T H E L O C A T I O N O F T H E ENDS O F P H R A S E S W H I C H C A N I M P R O V E S P E E C H Q U A L I T Y . 23. T H E STUDENTS C O N S I D E R E D THE A S S U M P T I O N T H A T A B R E A K M I G H T O C C U R .
24. F I N A L L Y YOU MUST A S S U M E T H A T Y O U R C I G A R E T T E S W I L L B O T H E R T H E P A S S E N G E R S ,
25. TRY TO G I V E T H E N A M E S O F THE C H A R A C T E R S TO JOHN,
26. I P R E F E R F O R HIM TO G I V E T H E N A M E S O F T H E C H A R A C T E R S TO JOHN.
27. I B E L I E V E T H O S E P E O P L E TO BE I N T E L L I G E N T .
28. I P R O M I S E D HIM T H A T HE C O U L D COME.
29. T H E Y G A V E T H E BOY A BOOK.
30. T H E Y G A V E H I M A BOOK.
31. T H E 4 8 - C H A N N E L M O D U L E C A N H A V E O N L Y T W O D I - G R O U P S BUT C A N H A V E UP TO F O U R P O W E R UNITS IF BOTH D I - G R O U P S A R E E Q U I P P E D W I T H E C H O C A N C E L E R S .
32. I T O L D HIM Y E S T E R D A Y TO C L E A N HIS R O O M .
33. M O V E T H E P O W E R OPTION J U M P E R P L U G SO T H A T IT IS A D J A C E N T TO D I - G R O U P ONE ON P R I N T E D W I R I N G BOARD.
34. I W A N T A LOT M O R E C O O K I E S .
35. THE MINUS-SIGN P R O N U N C I A T I O N SWITCH IS IN T H E M I D D L E .
36. HE A S K E D T H E C H I L D R E N TO FINISH THE JOB.
37. HE A R G U E D T H A T IT WAS IMPOSSIBLE.
38. IS A M A N A T THE DOOR.
39. A D E T A I L E D D E S C R I P T I O N O F T H E USE OF P H O N E M E S IS P R O V I D E D IN C H A P T E R 7.
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