## 2 Related Work  
In this Section, we briefly describe some previous research that is related to our work. The research activities carried out in the last few years for the face-to-face communication between humans and humanoid robots, as reported in Reference [35], also involved the creation of mechanically speaking heads, as we shall describe below. At Waseda University, several prototypes for producing Japanese vowels and some consonant sounds, called WT-3, WT-2 and WT-1R, have been developed. WT-1R [36] has articulators (tongue, lips, teeth, nasal cavities and soft palates) and vocal organs (lungs and vocal cords). It can reproduce the human voice movement and has 15 degrees of freedom. The vocal movement of the WT-1R for vocals is constant. However, consonant sounds are produced by dynamic movements of the vocal tract, to generate transitions of resonant frequencies (formant frequencies). Therefore, since Japanese voice generally consists of two phonemes, where the first is a consonant sound and the last a vowel, researchers at Waseda University have proposed considering the concatenation of three types of sounds (constant consonant sound, transient and vocal consonant sound). WT-2 [37] is an improvement of WT-1R; has lungs, vocal cords, vocal cavity and nasal cavity and aims to reproduce the vocal trait of an adult male. WT-3 [26] is based on human acoustic theory for the reproduction of human language; it consists of lungs, vocal cords and articulators and it can reproduce human-like articulation movements. The oral cavity was designed based on the MRI images of a human sagittal plan. At the Kagawa University, a mechanical model of the human vocal apparatus was developed [38] using mechatronics technology. It implements a neural network mapping between the motor positions and the voice sound produced by auditory feedback in the learning phase. In Bib:Asada the mechanical vocal tract is excited by a mechanical vibrator that oscillates at certain frequencies and acts as an artificial larynx. The shape of the vocal tract is controlled by a neural network. However, the development of an anthropomorphic mechanical speaking robot is very difficult as shown in References [18; 38]. On the other hand, graphic representations of human talking heads have been developed since many years. Generally, these graphics systems are connected to a speech synthesizer that plays a signal synchronized with the facial movements. Many solutions to this synchronization problem have been proposed since now. However, very few of them go through the articulatory movements of the phonatory organs. In addition to controlling the mechanical phonemic organ of a mechanical speaking head, knowledge of articulating movements is essential to drive the movements of the face. In fact, many researchers have experimentally demonstrated that there is an important link between the movements of the internal organs of the vocal tract and the facial movements. Similar to the approach proposed in this document, [PERSON] et al. [23; 26] developed an algorithm that estimates articulator parameters using [PERSON]'s optimization. [PERSON] et al., describe a computational model that explains the development process of human infants in the first acquisition period of the language [39]. The model is based on recurrent neural networks. [PERSON] et al., propose a joint analysis of the prosody and head movements to generate natural movements [40]. [PERSON] et al., introduce a method to add nonverbal communication to the talking head. This communication is realized by automatic generation of different facial expressions from the prosodic parameters extracted from the voice [41]. The SyncFace [42] system uses articulation-oriented parameters to recreate the visible articulation of a talking head during phonation. The systems described so far require a pre-processing step. Real-time facial animation is described in Reference [43].