Perceived Audio Quality Analysis in Digital Audio Broadcasting Plus System Based on PEAQ

342 K. ULOVEC, M. SMUTNY, PERCEIVED AUDIO QUALITY ANALYSIS IN DIGITAL AUDIO BROADCASTING PLUS SYSTEM . . . Perceived Audio Quality Analysis in Digital Audio Broadcasting Plus System Based on PEAQ Karel ULOVEC, Miroslav SMUTNY Czech Technical University in Prague, Dept. of Radio Engineering, Technicka 2, 166 27, Prague, Czech Republic {xulovec, smutnmir}@fel.cvut.cz Submitted November 6, 2017 / Accepted February 1, 2018 Abstract. Broadcasters need to decide on bitrates of the services in the multiplex transmitted via Digital Audio Broadcasting Plus system. The bitrate should be set as low as possible for maximal number of services, but with high quality, not lower than in conventional analog systems. In this paper, the objective method Perceptual Evaluation of Audio Quality is used to analyze the perceived audio quality for appropriate codecs — MP2 and AAC offering three profiles. The main aim is to determine dependencies on the type of signal — music and speech, the number of channels — stereo and mono, and the bitrate. Results indicate that only MP2 codec and AAC Low Complexity profile reach imperceptible quality loss. The MP2 codec needs higher bitrate than AAC Low Complexity profile for the same quality. For the both versions of AAC High-Efficiency profiles, the limit bitrates are determined above which less complex profiles outperform the more complex ones and higher bitrates above these limits are not worth using. It is shown that stereo music has worse quality than stereo speech generally, whereas for mono, the dependencies vary upon the codec/profile. Furthermore, numbers of services satisfying various quality criteria are presented. Keywords Objective audio quality assessment, Digital Audio Broadcasting Plus (DAB+), Perceptual Evaluation of Audio Quality (PEAQ), MP2, AAC, number of services 1. Introduction Radio broadcasting is in use for almost a hundred years. Recently, there is a worldwide transition from analog to digital radio systems. One of the systems for digital radio is Digital Audio Broadcasting (DAB) [1]. In many countries, there is experimental broadcasting, which aims to persuade listeners to switch to digital. However, the frequency spectrum is limited and is cluttered with many types of systems, which forces the broadcasters to include more services in DAB multiplexes. This results in a lower bitrate reserved for each service, which is one of the drawbacks in transition from DOI: 10.13164/re.2018.0342 analog to digital, because insufficient bitrate leads to inferior audio quality compared to analog systems [2]. On the other hand, the digital radio systems are evolving. They adopt new more efficient standards for source coding. Hence, the quality is higher or the number of services in one multiplex is increased. New error protection coding techniques are also adopted in modified specifications to further enhance robustness of the systems. With audio quality in mind, the broadcasters or service providers have to decide on bitrate for each service according to its content. Logically, some services could be broadcasted with lower quality than other ones (anticipated number of listeners could be one of the criteria), but beside that, without proper quality assessment results, the broadcasters can only use their own ears. Detailed quality analysis of source coding methods used in digital radio systems is needed to help broadcasters with this decision, there must be an appropriate balance between the number of services and their audio quality in order to face up to expensive radio spectrum [2]. Although there were many papers examining quality in DAB and other systems of digital radio, such as [2–6], none of them, to authors’ best knowledge, compared codecs or profiles of codecs, respectively, in sufficient detail regarding types of signal and number of channels. In this paper, these are investigated in detail. The main aim is to determine dependencies on the type of signal — music and speech, the number of channels — stereo and mono, and the bitrate. Furthermore, according to the results of the quality assessment analysis, the numbers of services in the multiplex are determined for various quality criteria. The rest of this paper is organized as follows. Section 2 contains background information about DAB and the audio quality assessment and the summary of the related work. Section 3 presents method and results of the audio quality analysis in DAB. Section 4 concludes the paper. 2. Background In this section, background information about topics discussed in this article is presented. SYSTEMS RADIOENGINEERING, VOL. 27, NO. 1, APRIL 2018 2.1 Digital Audio Broadcasting The Digital Audio Broadcasting system is intended for broadcasting radio services. According to [1], the DAB system is one of four types of the current digital radio systems deployed around the world. The DAB system is convenient for fixed, portable, and mobile receivers and is capable of operation at frequency from 30 MHz to 3 GHz. A terrestrial form (T-DAB) is dedicated to portable and mobile reception especially. Audio content distribution can be certainly realized by cellular networks. However, streaming media applies a big load on a telecommunication infrastructure, and also appreciably decreases battery life of mobile devices [3]. Currently, digital audio broadcasting systems are replacing conventional analog radio systems around the world and popularity of these systems is increasing [4]. The audio quality of digital broadcasting systems is an important issue for the successful deployment of the new digital systems. Generally, new digital systems are designed to be robust to errors introduced by transmission thanks to high efficient channel coding (Forward Error Correction, FEC) and modern modulation format. The DAB system is standardized by ETSI in [7], the first edition of specification was released in 1995. Generally, DAB uses industry standard encoding techniques for audio (and video) to remove redundancy and irrelevancy from source signals and error protection is applied during transmission. Multiplex of several digital audio services is carried via Radio-Frequency (RF) signal simultaneously in the bandwidth of about 1.5 MHz with total raw transport bitrate capacity of over 2.4 Mb/s. The range of the channel coding for error protection can be chosen for each service independently according to requirements of the broadcaster. The standard [7] allows several modes of FEC. The most used mode by the broadcasters is the option allowing protection levels labeled as 1-A, 2-A, 3-A and 4-A. The maximal available useful bitrate for broadcast services altogether (the useful capacity) ranges from 576 kbit/s to 1.728 Mb/s per one multiplex according to error protection. Services can contain audio, video and data (related or not to audio service). In this paper, only audio services with embedded data in the bit stream are supposed. This data directly linked to the audio program (...truncated)