EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 1 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC,...

EE2F1 Speech & Audio

Technology

Sept. 26, 2002

SLIDE 1

THE UNIVERSITY OF BIRMINGHAM

ELECTRONIC, ELECTRICAL &

COMPUTER ENGINEERING

Digital Systems&

Vision Processing

EE2F1Multimedia (1): Speech & Audio

Technology

Lecture 7: Speech Synthesis (1)

Martin RussellElectronic, Electrical & Computer Engineering

School of EngineeringThe University of Birmingham


Technology

Sept. 26, 2002

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Vision Processing

Stages in “text-to-speech” synthesis Text normalisation Text-to-phone conversion Linguistic analysis Semantic analysis Conversion of phone-sequence to sequence

of synthesiser control parameters Synthesis of acoustic speech signal


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Sept. 26, 2002

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Approaches to synthesis

Final stage is to convert ‘phone’ or word sequence into a sequence of synthesiser control parameters

Two main approaches:

– Waveform concatenation

– Model-based speech synthesis (inludes articulatory synthesis)


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Waveform Concatenation Join together, or concatenate, stored sections of

real speech Sections may correspond to whole word, or sub-

word units Early systems based on whole words

– E.G: Speaking clock - UK telephone system, 1936

Storage and access major issues Speech quality requires data-rates of 16,000 to

32,000 bits per second (bps)


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1936 “Speaking Clock”

From John Holmes, “Speech synthesis and recognition”, courtesy of British

Telecommunications plc


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Whole word concatenation (1)

Whole word concatenation can give good quality speech (as in speaking clock), but has many disadvantages:– pronunciation of a word influenced by

neighbouring words (co-articulation)– prosodic effects like intonation, rate-of-speaking

and amplitude also influenced by context.– interpretation of a sentence will be strongly

influenced by details of individual words used (“Mary didn’t buy Sam a pizza”)


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Disadvantages (continued):– words must be extracted from the right sort of

sentence– most suitable for applications where structure of

the sentence is constrained, e.g., announcements, lists…

– may need to record more than one example of each word, e.g., raised pitch at end of a list, pre-pause lengthening…


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Example – original recording

The next train to arrive at platform 2 will call at Bromsgrove, Droitwich Spa, Worcester Foregate Street and Malvern Link


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Example – trivial concatenative synthesis

The next train to arrive at platform 2 will call at Malvern Link, Worcester Foregate Street, Droitwich Spa and Bromsgrove


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Example repeated

Original recording ‘Concatenative synthesis’


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Disadvantages (continued):– to add new words the original speaker must be

found, or all words must be re-recorded– even with specialist facilities, selection and

extraction of suitable words is labour intensive and time consuming


Technology

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Digital Systems&

Vision Processing

Sub-word concatenation (1)

Limitations of word-based methods suggest concatenative speech synthesis based on sub-word units

Need well-annotated, phonetically-balanced corpus of speech recordings

Extract fragments from waveforms in the corpus which represent ‘basic units’ of speech, and can be concatenated and used for speech synthesis


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Difficulties include:– identification of a set of suitable units– careful annotation of large amounts of data– derivation of a good method for concatenation


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Sub-word concatenation overcomes difficulties with adding new words to the application vocabulary,

But, other problems exacerbated. In particular, coarticulation and pitch

continuity problems occur within, as well as between, words.

Necessary to use several examples of each phone (corresponding roughly to different allophones).


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Natural to select fragments that characterise the phone target values, but modelling transitions between these targets is a significant problem


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Example: sub-word concatenation

“stack” (original)

“task” sub-word concatenative synthesis


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Transitional units (1)

Central regions of many speech sounds are approximately stationary and less susceptible to coarticulation effects.

Hence select fragments which characterise transitions between phones, rather than phone targets.

e.g., diphone - transition between two phones.


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There are contextually-induced differences between instantiations of the central region of phone, which cause discontinuities if they are not attended to.

Possible solutions are:– use several different examples of each

diphone– store short transition regions, and– interpolate between end values


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Coping with coarticulation effects by modelling transitions and– (a) using multiple examples to cope with variation in the

instantiation of the phone centres, and– (b) by interpolation between short transition regions


Technology

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More on prosody

Discontinuity in the fundamental frequency exacerbated for sub-word methods.

Can use source-filter model to separate-excitation signal from vocal-tract shape.

Vocal-tract shape descriptions can then be concatenated and an appropriately smooth fundamental frequency pattern can be added separately.


Technology

Sept. 26, 2002

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PSOLA: Pitch Synchronous Overlap and Add PSOLA (Charpentier, 1986) Most successful current approach to

concatenative synthesis In PSOLA, the end regions of windowed

waveform samples are overlapped pitch-synchronously and added

BT’s Laureate is an example


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PSOLA

From: John Holmes and Wendy Holmes, “Speech synthesis and recognition”, Taylor & Francis 2001


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Speech modification using PSOLA In addition to speech synthesis from

segments, there are two other common applications of PSOLA:– Pitch modification– Duration modification


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Increasing pitch using PSOLA



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Decreasing pitch using PSOLA



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The ‘Laureate’ System

The BT “Laureate” system is a modern, PSOLA-based synthesiser

See Edington et al. (1996a), also look at the web site

Demonstration


Technology

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PSOLA strengths and weaknesses Strengths

– Produces good quality speech

Weaknesses– Large, annotated corpus needed for each ‘voice’– Requires accurate pitch peak detection– Inflexible – new voices can only be produced by

recording and labelling significant speech corpora from new speakers

Automatic annotation of corpora using techniques from speech recognition


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Summary

Concatenative speech synthesis Whole word concatenation Importance of prosody Sub-word concatenation Choice of sub-word units PSOLA

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 1 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC,...

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