ALLOPHONE GROUP SELECTION FACTORS FOR POLISH
SPEECH SYNTHESIS
Bożena Kozłowska, Janusz Rafałko and Mariusz Rybnik
Institute of Computer Sciences, University of Bialystok, Sosnowa str. 64, 15-887 Bialystok, Poland
Keywords: Speech synthesis, Allophone, Allophone group.
Abstract: The article concerns selection of allophone groups for Polish speech synthesis. It describes factors to be
taken into consideration while dividing allophones into certain groups. Thus, the presentation includes
classification suggested by the authors. Although the described factors regard Polish language, they may
facilitate any study on similar division concerning any other language. Each language has determined
specificity pronounces, therefore should choose suitable allophonic groups for the language. However
precise description on what elements we should special attention give, where later problems can appear in
pronunciation e.g. certainty will make easier work to persons making similar division in different languages.
1 INTRODUCTION
Issue of transforming text information into its voice
equivalent constitutes a relevant part of the today’s
world. The need for speech synthesis is present in
many fields of life. Therefore, it is important to
develop this particular domain and create better and
better speech synthesers that will allow us to gain as
natural speech quality as possible.
At present, the most common method, and
providing the best speech quality, is a method
consisting in linking acoustic units that have been
recorded and processed by an author beforehand. It
is a so-called concatenation method which may link
phones, diphones, triphones or syllables. A
phoneme, from a phonological point of view,
represents a speech sound i.e. the smallest
distinguishable element of human speech. A
phoneme is an abstract notion that defines a
collection of articulatory features of a speech sound
that allows one to differentiate it from other sounds.
Thus a speech sound as a real creation is practical,
audible realization of a phoneme. A phoneme may
have several sound representations occurring in
various contexts. They are so-called allophones. A
diphone is transition between two phonemes. By a
triphone one understands a sequence of three
consecutive phones.
Choosing the kind of acoustic units and their
number has a great impact upon the volume of
acoustic database and the quality of speech
synthesis. For instance, concatenation of phones
requires the smallest number of acoustic units, but it
produces the worst speech quality. However, in case
of syllables, good quality speech imposes an
enormous acoustic database.
Good quality speech may be also obtained using
allophones, but it is crucial that the allophone groups
are chosen in a way with possibly smallest number
of acoustic units while maintaining the best quality
speech.
From the most suitable allophone group point of
view, several main factors are of utmost importance.
First are articulatory properties of individual speech
sounds.
Each language has a limited number of sounds
which are uttered in a characteristic manner for a
given language. What is more, the tone of an each
speech sound is dependant on its environment; to be
more precise, on the preceding and the following
sound. Sound dependence on a language is essential,
that is why each language may have, more or less, a
different classification. Second important factor are
acoustic parameters such as energy and duration of a
speech sound. An important factor while selecting
allophone group is also the level of difficulty with
determining the boundary of duration or length of a
sound.
451
Kozłowska B., Rafałko J. and Rybnik M..
ALLOPHONE GROUP SELECTION FACTORS FOR POLISH SPEECH SYNTHESIS.
DOI: 10.5220/0003889804510456
In Proceedings of the 4th International Conference on Agents and Artificial Intelligence (ICAART-2012), pages 451-456
ISBN: 978-989-8425-95-9
Copyright
c
 2012 SCITEPRESS (Science and Technology Publications, Lda.)

2 SOUND ARTICULATORY
CLASSIFICATION IN POLISH
LANGUAGE
Articulatory classification relates to an organ
arrangement of articulatory apparatus, so-called
articulators, characteristic of a particular speech
sound. They comprise: palate, a tongue, lips, a
uvula, and vocal ligaments. Depending on a kind of
a sound classification criteria differ a bit.
In principle, sounds in Polish language may be
divided into 3 groups: vowels, consonants and
semivowels, on the understanding that “l” and “ł”
are semivowels.
Common features of the vowels are sonority,
openness and syllable creation. Apart from this, one
may group them according to articulatory
classification criterion.
Vowel classification
 with regard to horizontal movement of a
tongue:
o front – e, ę, i, y
o central – a
o back – ą, o, u
 with regard to vertical movement of a tongue:
o high – i, u, y
o mid – ą, e, ę, o
o low – a
 with regard to central tongue’s position:
o soft – i
o hard – a, ą, e, ę, u, y
 with regard to lips formation:
o flat – e, ę, i, y
o rounded – ą, o, u
o neutral – a
 with regard to uvula position:
o oral – a, e, i, o, u, y
o nasal – ą, ę.
Consonant classification
 with regard to vocal ligaments activity:
o voiced – b, b’, d, d’, g, g’, z, z’, ź,
ż, dz,
dź, dż, w, w’, j, m, m’, n, ń, l, l’, ł, r, r’
o voiceless – p, p’, t, t’, k, k’, s, s’, ś, sz, c,
c’, ć, cz, f, f’, h, h’
 with regard to a center tongue’s position:
o soft – b’, ć, s’, ś, z’, ź, ć, c’, dź, ń, p’, f',
w’, m’, l’, k’, g’, h’, j
o hard – b, c, s, z, dz, p, f, w, m, l, k, g, h, t,
d, sz, ż, cz, dż, r, ł
 with regard to manner of articulation i.e. speech
organs closure:
o plosive – b, b’, p, p’, d, t, k, k’, g, g’
o affricate – c, dz ,dź, cz, dź, ć
o fricative – f, f’, w, w’, z, s, z’, ż, sz, ź, ś,
s’, h, h’
o unobstructed – m, m’, n, ń, r, l, l’ (ł)
 with regard to place of articulation:
o bilabial – p, b, m
o labio-dental – f, w
o apico-dental – t, d, c, dz, s, z
o alveolar – l, r, sz, ż, cz, dż
o palatal – ś, ż, ć, dź, ń
o velar – k, g, h
 with regard to uvula position:
o nasal – m, m’, n, ń
o oral – the others.
The above-presented articulatory classification
will facilitate understanding of further proceedings
during the selection of allophone groups. While
uttering a word one does not voice each sound
separately but “passes on” fluently from one speech
sound to the other. In this connection the position of
articulators before voicing the particular sound as
well as the position they assume to voice another
sound is very important.
3 INFLUENCE OF ADJACENT
SOUNDS ON SOUND TONE
The basic parameter, on which a speech sound tone
depends, is adjacent sounds influence. That
influence is not equal in every case i.e. some sounds
affect their environments to a greater extend than the
others.
This very sound tone dependence on its
environment determines selection of allophone
group and what follows is the number and kind of
allophones. Each allophone is described by its
environment, left context i.e. the preceding sound,
and right context i.e. the following sound. It is
presented in figure 1.
Observation of adjacent sounds influence has
indicated several important aspects:
1. Strength of influence of adjacent sounds is
greater for vowels than consonants. Thus, there
is need to create different allophone group
classifications for vowels and consonants.
2. Intensity and the way the preceding and the
following sounds affect voicing is not the
same. The fact that a particular group of the
preceding sounds have a similar impact on a
speech sound does not mean that the same
group, as the following sounds, will influence
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
leftcontext(b)arightcontext (r)
Figure 1: Left and right contexts of allophones.

boundary
Figure 2: Precise setting the boundary between two allophones.
the sound similarly. Moreover, the strong
influence of a given preceding sound does not
also imply that it will have the same strength as
the following sound. For this reason, left
context groups and right context groups may
differ.
3. The preceding sound has a lot more greater
influence on vowels – left context, than the
following sound – right context. Therefore, for
vowels, the number of left context groups will
be greater than the number of right context
groups.
4. The manner one voices adjacent sounds greatly
influences the tones of vowels. During the
process, central tongue’s position (softness,
hardness), and the place of articulation
(labiality, frontness, centralness and backness)
are especially important.
5. In case of consonants,
voicedness/voicelessness of adjacent sounds is
important. The preceding voiced sounds affect
a speech sound similarly, as is the case with the
following voiced sounds. The preceding
voiceless sounds affect a speech sound
similarly, as is the case with the following
voiceless sounds.
4 DIFFICULTY LEVEL OF
SETTING ALLOPHONE
BOUNDARIES
Another crucial factor affecting selection of an
allophone group is precise determining allophone
boundaries which is indispensable for separating an
allophone from a natural speech signal. Incorrectly
determined allophone boundaries will cause
distortion in synthesized speech.
In most cases precise setting the boundaries is
not a difficult task as the points where an allophone
begins and ends can be seen clearly. The figure 2
presents an example of clear boundaries between
allophones of “a” and “b” sounds.
There are cases, however, in which it is difficult
to determine the boundary. It is connected with the
influence of some left context sounds on the
following vowel. Those sounds make the boundary
between them and the vowel indistinct and it is
problematic to define the point where one allophone
ends and the other begins. As shown in the figure 3.
First of all, this very problem concerns sonore
sounds i.e. m, n, r, l, ł, j and vowels. Their influence
upon the following speech sound causes the
boundary, between one sound and the other, to fade
away. Transition area appears. One can hear two
sounds simultaneously. You cannot link the two
sounds in a single group as they affect each other’s
tones in a unique manner. Taking it into
consideration, each of the above-mentioned sounds
comprises an individual left context group.
5 IMPORTANCE OF ACOUSTIC
PARAMETERS
Acoustic parameters which have greatly affected the
selection of allophone groups are energy and the
duration of a sound. They are especially important in
ALLOPHONE GROUP SELECTION FACTORS FOR POLISH SPEECH SYNTHESIS
453


rightboundary„Ł”transitionarealeftboundary„A”
Figure 3: Transition area between two allophones.
case of speech sounds that may be stressed. Polish
language vowels are the only sounds that may be
stressed. The research performed has portrayed the
position of a vowel in relation to a word stress to be
essential for the sound energy as well as its duration.
It is important whether the vowel is placed before, or
after the stress, or if the stress falls exactly on it.
The diagram below presents the average values
of vowel allophone durations with the division
according to the relation to a word stress. It can be
easily noticed that irrespectively of a lecturer the
duration time of an each vowel allophone is
enormously different. The shortest vowel allophones
are located before the stress. The time of duration, of
allophones on which the stress falls or the ones that
are located in word after the stress, is similar.
Figure 4: The average values of vowel allophone durations
with the division according to the relation to a word stress.
In case of energy, it has been noticed that
irrespectively of a lecturer, the highest value falls on
vowel allophones located before the stress, and the
lowest on allophones after it. The figure 5 also
presents the relationships.
Examining the energy and the duration of
individual allophones has shown that for the
selection of vowel allophone groups it is essential to
take into account the position of a vowel in relation
to a stress. Thus, it is necessary to add, to the
previously presented classification of allophone
groups, further division of vowels into vowels
located before and after the stress, as well as vowels
that are stressed.
Figure 5: The average energy of vowel allophone with the
division according to the relation to a word stress.
6 SUMMARY
In order to select proper allophone groups, it is
necessary to take into consideration all the described
factors and examine if it is sufficient for the speech
synthesis to be distinct, understandable and as
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
Vowel
Stress AfterthestressBeforethestress
Before After
Pause
Hard+a,e
Labial+u,o
Soft+i
Softlabial+y
Pause
Labial
Hard apico‐dental&palatal
Hardvelar
Soft
Softlabial
m
n
l
r
ł
m’
n’
l’
r’
j
a
u
y
i
o,ą
e,ę
Figure 6: Allophone group classification for vowels.

Consonant
Before Afte
r
Pause
Voicelessconsonant
Voicedconsonant
Notstressvowel
Stressvowel
Pause
Voicelessconsonant
Voicedconsonant
Vowel
Figure 7: Allophone group classification for consonants.
natural as possible. The figures below, present
allophone group classification for vowels (Fig. 6)
and consonants (Fig. 7) suggested by the authors.
“Before” index means that the allophone is before a
given sound group while “after” index means the
allophone is after a given sound group. “Pause”
means that before/after the allophone there is no
sound i.e. the allophone beginning or ending a word.
Such a division, theoretically, produces 3500
allophones. However, one will never achieve the
number like this as Polish language specification
does not allow for certain allophone clusters. For
instance, an “i” allophone will be present only after
soft consonants because it always palatalizes the
preceding consonant. Moreover, there are
allophones, theoretically existent, that in practice
have no representation in any word. In search for
such allophones approximately 4 million Polish
words were examined. Ultimately the acoustic
database includes about 2300 allophones for a
speaker.
Speech synthesis working on an acoustic
database with such classification brings good results.
The speech is quite clear and understandable.
Obviously the receiver of the certain text message
has no doubts it is not natural speech as it is
necessary to preserve proper rhythmics and
intonation of sentences. However, hearing single
words, in most events, it is difficult to assess
whether a particular word is synthesized or natural.
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ALLOPHONE GROUP SELECTION FACTORS FOR POLISH SPEECH SYNTHESIS
455

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