## 3 Related Work  
In a recent paper [12], a group of Italian authors presented a study on the quality of demodulated FM broadcasting. The subjective judgements involved a set of audio signals comprising different types of content and/or genre of music that included classical and modern/pop music as well as the spoken word. The obtained results, in a five-step MOS (Mean Opinion Score) scale, were linked with different SIR (Signal-to-Interference Ratio) and QoS (Quality of Service) characteristics. As pointed out, their findings on technical and subjective FM signal quality issues may be interesting sources of information during the switchover from analog to digital radio. A similar study concerning the current broadcasting market in the Czech Republic, along with DAB+ network coverage measurements and planning, is available in [13]. The author provides information about the implementation process from testing to regular broadcasting. He discusses different parameters and settings related with frequency allocation and the transmitter itself, as well as results of studies considering the efficiency of different variants of the AAC (Advanced Audio Coding) algorithm. This paper provides valuable data to both national and private broadcasters that would like to participate in the digitalization process. Another recently published study [14] describes both objective PESQ (Perceptual Evaluation of Speech Quality) and subjective MOS results, including the CCR (Comparison Category Rating) variant, of speech and music recordings transmitted in real-time via DAB+ radio. The authors discuss the process of quality assessments, including the overall audio quality as well as timbre with relation to utilized bitrate ranging from 24 to 128 kbps. The obtained results, involving the regional multiplex in Wroclaw (Southern Poland), are compared with other studies related with the Gdansk (Northern Poland) multiplex. Former studies on DAB+ performed by this group of authors may be found in [15; 16; 17; 18; 19; 20]. In 2018, a group of Czech researchers investigated the audio quality of speech and music signals, including mono and stereo, in both DAB and DAB+ using the PEAQ (Perceptual Evaluation of Audio Quality) algorithm [21; 22]. In this case, the audio samples were coded in MP2 (bitrates ranging from 56 to 320 kbps), which is utilized in DAB, as well as AAC (bitrates ranging from 24 to 256 kbps), which is utilized in DAB+. Their analysis showed a break point for each codec where objective scores did not increase significantly with increasing bitrate. Furthermore, the minimum required bitrate necessary to fulfill the broadcast quality criterion was used. Additional findings from the Czech Republic including the influence of bitrate on subjective quality perception in the broadcasting chain as well as the matter of coexistence between broadcasting of satellite television and digital radio may be found in [23; 24]. A subjective and objective study, concerning the comparison of real-time simulcasted FM and DAB+ radio programs in Poland, along with AAC-processed speech and music \begin{table}
\begin{tabular}{c c} \hline \hline
**Country** & **Percentage (\%)** \\ \hline Azerbaijan & 30 \\ Turkey & 30 \\ Thailand & 14 \\ Algeria & 7 \\ Ukraine & 7 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Emerging worldwide digital radio markets and their coverage. signal samples (bitrates ranging from 64 to 160 kbps, with a step of 32 kbps), is described in [25]. In this work, the authors analyzed whether the quality of DAB+ surpasses the quality of FM radio. The subjective quality assessment was performed using the MOS scale, whereas the objective part was carried out using the ViSQOLAudio (Virtual Speech Quality Objective Listener Audio) metric [26; 27]. Both subjective and objective quality metrics provided scores in the five-step variant. As a result, the authors noted a need for higher bitrates in the case of particular audio content, e.g., with a clear stereo separation between the channels.