## 3 Related Work
Furthermore, DAB+ proved to be an efficient replacement for traditional FM radio. Later on in 2017, a study concerning the difference in DAB and DAB+ broadcasting was carried out [28]. What is worth mentioning is that the author utilized his own multiplex that was particularly designed and launched in three different configurations for the purpose of this test. In each of the three described scenarios, the ensemble consisted of both MP2 and AAC-processed signal samples, offering the same content and available to the listener in the form of six radio programs. In this scenario, these were three programs simulcasted in DAB and DAB+, respectively. The utilized signal samples were categorized into three groups, speech and singing, musical instruments, and music genres, and processed in two bitrates, which is 64 and 128 kbps. This study clearly showed the superiority of DAB+, as this standard obtained higher grades when transmitting the same type of content compared to DAB. A previous study conducted in Sweden, concerning the audio quality of FM and DAB+ terrestrial systems, is described in [29]. This examination consisted of two parts. The first one involved audio content coded and broadcasted at bitrates between 96 and 192 kbps, whereas the second one involved signal samples processed at bitrates from 48 to 192 kbps, with a comparison to FM analog broadcasting. In this case, the quality was evaluated with the 100-step MUSHRA (Multiple Stimuli with Hidden Reference and Anchor) metric. According to this study, DAB+ offered comparable or similar quality relative to FM for bitrates of approximately 192 kbps. In particular cases, this bitrate was ranked even higher compared to analog radio. In 2015, an international group of authors investigated the impact of different audio codecs utilized in a number of terrestrial broadcasting as well as internet streaming services [30]. The perceived audio quality was investigated for a number of codecs, including: MP2, Opus, MP3, Ogg Vorbis, and AAC. The bitrate ranged from 24 to 320 kbps depending on the particular codec. Studies involved both subjective and objective metrics, including PEAQ and POLQA (Perceptual Objective Listening Quality Assessment) algorithms. The study provides a good introduction to currently utilized commercial services, including the pros and cons of different popular audio codecs. It is worth mentioning that when it comes to designing and evaluating digital broadcasting systems, the process of frequency allotment, resource allocation, and ensemble configuration is still a widely discussed topic [31; 32]. Advances in audio quality assessment are discussed in [33; 34; 35; 36; 37; 38; 39; 40; 41]. Until now, only Norway performed the so-called switchover, leaving analog FM radio behind. Currently, this is the only country that went fully digital with DAB+. The results of previous studies concerning subjective and objective tests, including both laboratory and real-time operating conditions along with the evaluation of AAC-processed speech and music samples, provide a good introduction to the state of the art regarding popular codecs and formats. The main contributions of this paper are as follows:  
* It provides an overview of the current situation of the digital radio market. * It provides a throughout analysis of previously published works on DAB/DAB+ of research groups from all around the world. * It describes and discusses modifications of a selected DAB+ multiplex ensemble (number of available services, their profile, type of audio content, assigned bitrate, etc.). It presents results of a subjective quality evaluation study of broadcasts simulcasted in analog FM and digital DAB+ radio, including a comparison of the same ensemble before and after modifications. * It shows the results of a questionnaire considering factors that seem most important for a typical user and that are necessary for a successful switchover from the analog to digital domain.