## 1 Introduction  
Parkinson's disease (PD) is the second most prevalent neurodegenerative disease [1]. PD mostly affects people over 50 years of age, although the number continues to decline, and the number of affected subjects only increases as the population increases. With more than 10 million people diagnosed each year in the world, 1 million in the USA and comparable rates of occurrence have been observed in Europe [2]. The pathogenetic mechanisms of the disease are variable and not yet fully understood [3]. The disease presents itself with a wide spectrum of motor and non-motor symptoms, including those that directly affect a person's capability to operate. Clinically manifested PD is preceded by a prodromal period, lasting for decades, when there are no clinical signs or unspecific symptoms, such as constipation, apathy, daytime sleepiness, inattention, depression, anxiety, smell impairment, pain, motor slowing, etc., may be present [4, 5, 6, 7]. Motor symptoms appear relatively late in the course of neurodegeneration, only when compensatory mechanisms of smooth motor control are exhausted and do not manifest until more than half of the dopaminergic neurons in the substantia nigra region in the midbrain are dead [8]. Nevertheless, they are the signs that make the disease recognizable. The cardinal symptoms of PD are bradykinesia, which represents impairment of voluntary motor control and is characterized by slowness and gradually decreasing range of movement, muscle rigidity, and tremor [9]. To date, the diagnosis of PD is exceptionally clinical and faces many challenges. Firstly, it comes late in the context of the neurodegenerative process. Secondly, the diagnosis requires experience in recognizing clinical signs, especially when motor symptoms are mild, as mild motor symptoms as well as non-motor symptoms are prevalent in the healthy elderly population [10]. The symptoms of PD overlap with other neurodegenerative, toxic, or vascular diseases that might have a different prognosis and require different approaches to treatment, and even with aging, as mentioned above. Early identification of PD is important, as disabling motor symptoms, which otherwise may be attributed to aging or other causes, could be treated highly effectively when recognized. Earlier disease intervention may result in longer uncompromised working capacity, as well as the prolonged quality time of the patient's life. The prospect of neuroprotective treatment that is expected to emerge in the near future demands identification of those who need it even earlier, before the symptoms appear. Many studies in the area of neurodegeneration have recently been directed toward finding the disease biomarkers [11]. Speech is very vulnerable to degeneration of neural structures [12] and quality decline has been extensively reported in the PD research literature [13; 14]. It is therefore not surprising that speech acoustic analysis in PD has been receiving an exponentially growing scientific interest in recent years as it has the potential to reveal a lot of information about fine motor control. The ease and broad availability of the recording technique makes it an excellent candidate for becoming a diagnostic biomarker as well as a progression marker for PD. Fundamental frequency variability has been discovered as early as 5 years before diagnosable symptoms [15]. [PERSON] et al. have repeatedly shown that subjects with early PD [16; 17] and even with rapid eye movement behavior disorder (RBD) [18] (which is recognized as the strongest predictor of neurodegeneration, including [PERSON]'s disease) manifest various articulatory, phonatory, and prosodic speech deficits, a combination of which can discriminate between the people with PD and the control group. Early detection and diagnosis can help with therapy, but diagnosis usually necessitates an interview with a healthcare professional or the completion of a formal diagnostic questionnaire.