EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 1 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing EE2F1 Multimedia (1): Speech & Audio Technology Lecture 7: Speech Synthesis (1) Martin Russell Electronic, Electrical & Computer Engineering School of Engineering The University of Birmingham

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 2 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & 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

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 3 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing 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)

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 4 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing 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)

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 5 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing 1936 “Speaking Clock” From John Holmes, “Speech synthesis and recognition”, courtesy of British Telecommunications plc

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 6 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing 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”)

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 7 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Whole word concatenation (2)  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…

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 8 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Example – original recording The next train to arrive at platform 2 will call at Bromsgrove, Droitwich Spa, Worcester Foregate Street and Malvern Link

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 9 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Example – trivial concatenative synthesis The next train to arrive at platform 2 will call at Malvern Link, Worcester Foregate Street, Droitwich Spa and Bromsgrove

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 10 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Example repeated  Original recording  ‘Concatenative synthesis’

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 11 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Whole word concatenation (3)  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

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 12 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING 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

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 13 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Sub-word concatenation (2)  Difficulties include: –identification of a set of suitable units –careful annotation of large amounts of data –derivation of a good method for concatenation

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 14 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Sub-word concatenation (3)  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).

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 15 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Sub-word concatenation (4)  Natural to select fragments that characterise the phone target values, but modelling transitions between these targets is a significant problem

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 16 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Example: sub-word concatenation “stack” (original) “task” sub-word concatenative synthesis

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 17 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing 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.

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 18 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Transitional units (2)  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

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 19 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Transitional units (3)  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

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 20 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing 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.

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 21 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing 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

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 22 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing PSOLA From: John Holmes and Wendy Holmes, “Speech synthesis and recognition”, Taylor & Francis 2001

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 23 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Speech modification using PSOLA  In addition to speech synthesis from segments, there are two other common applications of PSOLA: –Pitch modification –Duration modification

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 24 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Increasing pitch using PSOLA From: John Holmes and Wendy Holmes, “Speech synthesis and recognition”, Taylor & Francis 2001

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 25 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Decreasing pitch using PSOLA From: John Holmes and Wendy Holmes, “Speech synthesis and recognition”, Taylor & Francis 2001

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 26 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing 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

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 27 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing 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

EE2F1 Speech & Audio Technology Sept. 26, 2002 SLIDE 28 THE UNIVERSITY OF BIRMINGHAM ELECTRONIC, ELECTRICAL & COMPUTER ENGINEERING Digital Systems & Vision Processing Summary  Concatenative speech synthesis  Whole word concatenation  Importance of prosody  Sub-word concatenation  Choice of sub-word units  PSOLA