RADIOENGINEERING, VOL. 23, NO. 2, JUNE 2014 665 The Effects of the Active Hypoxia to the Speech Signal Inharmonicity Zoran N. MILIVOJEVIĆ 1 , Darko BRODIĆ 2 , Marina Z. MILIVOJEVIĆ 3 1 College of Applied Technical Sciences Niš, Aleksandra Medvedeva 20, 18000 Niš, Serbia 2 University of Belgrade, Technical Faculty in Bor, V. J. 12, 19210 Bor, Serbia 3 University of Niš, Medical Faculty, Bulevar dr Zorana inđića 81, 18000 Niš, Serbia zoran.milivojevic@vtsnis.edu.rs, dbrodic@tf.bor.ac.rs Abstract. When the people are climbing on the mountain, they are exposed to decreased oxygen concentration in the tissue, which is commonly called the active hypoxia. This paper addressed the problem of an acute hypoxia that affects the speech signal at the altitude up to 2500 m. For the experiment, the speech signal database that contains the articulation of vowels was recorded at different alti- tudes. This speech signal was processed by the originally developed algorithm, which extracted the fundamental frequency and the inharmonicity coefficient. Then, they were subjected to the analysis in order to derive the effects of the acute hypoxia. The results showed that the hypoxia level can be determined by the change of the inharmonicity coefficient. Accordingly, the degree of hypoxia can be estimated. Keywords Acute hypoxia, inharmonicity coefficient, fundamen- tal frequency, speech analysis, speech processing. 1. Introduction Hypoxia is a condition of decreased concentration of oxygen in blood, cells and tissues of an organism [1]. The brain is the most sensitive organ, so the condition of hy- poxia causes disturbances of mental activities (disturbances of memory, sight and speech, etc.). Among other factors which can cause hypoxia there is inhaling the air with lowered oxygen concentration. This condition can occur in tunnels, during diving, during flights with planes, and during stay on high mountains [2]-[4]. There are: (i) acute, and (ii) chronic hypoxia conditions. Under acute hypoxia, we understand the situation when someone whose natural environment is at low altitude, is exposed the stay on high mountains. In the period of adaptation we talk about the acute hypoxia. Under the chronic hypoxia, we understand the condition of the lowered amount of oxygen in people whose natural environment is on high mountains. Refs. [3], [4] showed that the fundamental frequency of the speech signal changes due to hypoxia. According to [5], the processing of the fundamental frequency as well as its relation to the other frequencies in the spectrum can designate the emotional state of the speaker (happiness, sadness, anger, anxiety, boredom, disgust, and neutral) and the stress [6]-[9]. Ref. [10] shows the results of digital signal processing made by filtering of dissonant spectral ranges. This approach is incorporated in order to improve quality of real speech in different ambient noise. The process included the fundamental frequency estimation, which was the base for determination of disso- nant interval limit. According to that, the spectral compo- nents from defined intervals have been eliminated by fil- tering. The dissonant ranges known as Devils intervals are defined by the theory of music [11]. Refs. [12], [13] show that along with the fundamental frequency change, hypoxia has repercussions on the amplitude speech characteristic. This way, it changes the energy of the spectral components that belongs to the dissonant intervals. In this paper, the influence of the acute hypoxia to the real speech inharmonicity is described. Till now, this problem hasn’t been completely studied. The basic idea is connected to the research of the inharmonicity in the stringed musical instruments [14]. Although, the inhar- monicity linked with the oscillations of stretched strings was well known, the effects of the musical instruments inharmonicity weren’t investigated. First important results that analyzed the piano strings inharmonicity were pub- lished in [15], [16]. Up to now, the inharmonicity of differ- ent musical instruments like piano and guitar has been made [17]-[23]. These works show that due to the finite elasticity of the string the higher harmonics deviate from the position given by the integer multiple of the funda- mental frequency. This deviation is modeled by the coeffi- cient . Hence, the higher value of  initiates the higher level of instruments inharmonicity. Hypoxia effects on the fundamental frequency and the energy of the spectral components [12], [13]. Furthermore, it changes the frequency of the partials which leads to the inharmonicity. To determine the degree of the inharmonic- ity, the inharmonicity coefficient has been calculated for the persons under the effect of hypoxia. This is made by the originally proposed algorithm. Then, the real speech