HELSINKI UNIVERSITY OF TECHNOLOGY 30.11.2004 Telecommunications Software and Multimedia Laboratory T-111.550 Multimedia Seminar Fall 2004: Mobile Multimedia Application Platforms Mobile Audio – from MP3 to AAC and further Henri Autti Johnny Biström 51194K 21548C
Mobile Audio – from MP3 to AAC and further Henri Autti and Johnny Biström HUT, Telecommunications Software and Multimedia Laboratory henri.autti@hut.fi johnny.bistrom@hut.fi Abstract The purpose of this paper is to evaluate the advanced audio codec’s and reflect over their suitability for mobile needs of today and tomorrow. The historical development of different codec’s for different purposes is analyzed. The features of the most common codec’s are discussed in parallel with performance and other criteria. The capabilities of mobile devices and the telecommunication possibilities now and in the future are also considered in the analysis. Finally some comparisons of the codec’s performances are done. Some existing applications on mobile audio are presented and ideas for audio applications in the future are discussed. Some of the questions to be answered here are. Can one codec, that is superior for the mobile world, be found or do we have to prepare for a wide diversity in the future? Will the codec’s continue to develop as rapidly as they have done so far? 1 INTRODUCTION A look at audio applications for the mobile world today reveals that the diversity in implementations is wide. The solutions chosen for the representation of audio streams in certain situations differ greatly and the number of codec’s used for the purpose of encoding and decoding audio data streams is large. It is not obvious which codec should be used for what purpose. The selection of codec depends on several factors as on content type of the audio material, the available communication speed and the quality requirements of the listening situation. Other factors that might influence the selection of codec are the standardization situation, the licensing policy and the competitors’ choices in the market. During the last years the mp3 format has been a great success but it does not fit well into mobile devices. Lately more efficient codec’s as AAC and AMR have been presented and they have been refined for mobile audio purposes. The purpose of this paper is to evaluate the most important audio codec’s by revealing the technical principles of the en- and decoding, the standardization situation and the suitability of the codec in relation to technology available and the market needs. The analysis also takes into consideration the development situation of mobile and telecommunication hardware and software. Using technical literature and documented 1
listener testing combined with mobile manufacturer specifications and published white papers we try to find out if there is one superior codec for mobile audio applications and which codec it would be. To do that, an analysis of existing and future audio applications in the market has to be done to clarify the needs and expectations on mobile audio. Finally the result is reflected against the development trends of the mobile technology and the persistency of the chosen solution is judged. 2 BACKGROUND In this chapter we take a look at the background of mobile audio formats. First we present some basic facts about the development of audio codec’s and the reasons for developing them. Then we discuss some facts about the development of mobile devices including phones, PDA’s and Laptops. Later in this part we take a quick look at the applications available today and the demands these applications pose The short history of audio codec’s dates back to the mid-1980s, in the Fraunhofer Institut in Erlangen (Fraunhofer, 2004) , Germany, which first began working on a high quality, low bit-rate audio coding with the help of Dieter Seitzer, a professor at the University of Erlangen. Their project was financed by the European Union as a part of the market-oriented Eureka research program where it was commonly known as EU- 147. In Germany in 1989, Fraunhofer was granted a patent for mp3, which we are going to discuss more thoroughly in the next chapter. A few years later it was submitted to the International Standards Organization (ISO), and mp3 was introduced as a part of the official MPEG-1 standard in 1992. It was in January 1995 that Fraunhofer applied patent on mp3 in America as well and it was granted in November 1996. The revolutionary thing created was, that using mp3-compression PC-users were for the first time in history able to compress an ordinary music-CD to one tenth of its original size, with only a small sacrifice in the sound quality - thus 12 hours of music could be stored on a recordable CD that on the other hand could be played by a mp3-CD-player or an ordinary PC. In the rapidly evolving world of mobile content development things have changed a lot since those days. Nowadays ranging from small laptops through palmtops to phones, these devices are more available, and high-speed wireless networks are getting better day by day. At the same time speech and audio compression have advanced rapidly in recent years spurred on by cost effective digital technology and diverse commercial application. Wideband speech and high fidelity audio compression have also made great progressions in recent years, accelerated by the commercial success of consumer and professional digital audio products. Telephone speech, wideband speech and wideband audio signals differ not only in bandwidth and dynamic range, but also in listener expectations of the offered quality. Using of wideband not only improves the intelligibility and naturalness of speech, but also adds a feeling of transparent communication and eases speaker recognition. The commercial applications in the mobile content area of today are also developing at a growing rate. Mobile device services contain, according to Ericsson (Bruhn, 2004) streaming, messaging, downloading and broadcasting. Streaming scenarios include news listening, monitoring of sports events, audio books, music listening, commercial 2
advertisements, access to information systems and interactive gaming. Broadcasting scenarios are very close to screaming scenarios including web casting or Internet radio broadcasting. They have become especially popular allowing listeners to "stream" audio on their computers. Unlike downloaded audio files, streamed audio files are not stored on the user’s hard drive, but are broadcasted like traditional radio through the user’s audio player. Messaging scenarios are also similar to streaming, but with size limitations, including business-to-person and person-to-person scenarios. Download scenarios include music, books and comics downloading over the network. Important for all of these scenarios named above, is to be able to handle mixed content - covering, music, speech, speech-between-music and speech-over-music. The demands these applications pose today on audio codec’s for mobile services include the ability to cope with generic content, sufficient and consistent quality at lowest rates, best quality at lowest rates, and high quality operation with relaxed bit rate requirement. The new audio codec’s also have to be optimized for low-resource devices (low memory and computational resources) and have to be supportive for a variety of operating systems, e.g. Symbian, WinCE, Palm OS5 and OS6. Developing and standardizing the codec’s is at the moment focusing on 3GPP, which is the body standardizing GSM, evolved GSM UMTS and 3G. In the next chapter we are going to introduce some of the most important audio standards and codec’s, which play an important role in the 3GPP. 3 AUDIO STANDARDS AND CODEC’S In this chapter we describe what we consider the most important audio standards and codec’s at the moment. In the first part of this chapter, we are going to discuss the 3GPP audio standard format families AAC and AMR, introducing the underlying technology. First we present mp3 (predecessor to AAC), AAC, HE-AAC and EAAC+ and then the challengers; AMR, AMR-WB and AMR-WB+. In the second part we are going to discuss an open source codec Vorbis Ogg ACM and some of the most important non- standard audio formats, using the streaming technology, Windows Media Audio and RealMedia. For terminology, architecture and technology see Wales (2004) and ARM Developer’s Guide (2004). 3.1 MPEG-1 (mp3) Mp3 stands for MPEG-1 Audio Layer III. It is not a separate format, but a part of the MPEG-1 video encoding format, as described earlier. Mp3 is a lossy data compression method (meaning that compressing a file and then decompressing it retrieves a file that may well be different to the original, but is "close enough") to store good quality audio into small files by using psychoacoustics in order to get rid of the data from the audio that most humans can't hear. Mp3's bit rates vary from 8 kbps to 320 kbps. When the mp3 phenomenon began in 1996, most of the audio files were encoded using 128 kbps bit rate, which is still the most popular bit rate in the world - although most of the people agree that by using slightly higher bit rates, like 192 kbps or 256 kbps, the audio quality can be compared with the CD quality. 3
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