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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"><HTML><HEAD> <TITLE>Frequently Asked Questions about MPEG Audio Layer-3</TITLE> <META NAME="GENERATOR" CONTENT="Mozilla/3.0b6Gold (Win95; I) [Netscape]"></HEAD><BODY><H2>Frequently Asked Questions about MPEG Audio Layer-3, Fraunhofer-IIS,and all the rest...</H2><H3>Version 2.61</H3><P>This text will be continously upgraded: step by step, more answers andmore information will be included. Yes, we definitely know that there area lot more questions to answer! But we cannot do that all at once. So,some parts may remain "under construction" for a while, and otherparts may be modified due to new results of our research work or new applications.You find the latest release at</P><CENTER><P><A HREF="http://www.iis.fhg.de/departs/amm/layer3/sw/">http://www.iis.fhg.de/departs/amm/layer3/sw/</A>or<BR><A HREF="ftp://ftp.fhg.de/pub/layer3/l3faq.html">ftp://ftp.fhg.de/pub/layer3/l3faq.html</A></P></CENTER><H3>Table of Contents</H3><UL><LI><A HREF="#intro">Introduction - or: What is "MPEG Audio Layer-3"?</A></LI><LI><A HREF="#applications">Applications - or: Layer-3, what is it goodfor?</A></LI><LI><A HREF="#aboutMPEG">Overview about the ISO-MPEG Standard - or: Whatis MPEG all about?</A></LI><LI><A HREF="#aboutLayers">Some Basics about MPEG Audio - or: What aboutLayer-1, Layer-2, Layer-3?</A></LI><LI><A HREF="#aboutLayer3">Advanced Features of Layer-3 - or: Why doesLayer-3 perform so well?</A></LI><LI><A HREF="#aboutCoding">Basics of Perceptual Audio Coding - or: Whatis the trick?</A></LI><LI><A HREF="#references">References - or: Where to find more information?</A></LI><LI><A HREF="#aboutFraunhofer">About us - or: What is going on at our FraunhoferInstitute?</A></LI></UL><H3>Introduction - or: What is "MPEG Audio Layer-3"?<A NAME="intro"></A></H3><P>Today, efficient coding techniques are a must for cost-effective processingof digital audio and video data by computers. Data reduction of movingpictures and sound is a key technology for any application with limitedtransmission or storage capacity. In the recent years, a lot of progresshas been achieved. While there (still) exist several proprietary formatsfor audio and video coding, the ISO/IEC standardisation body has releasedan international standard ("MPEG") for powerful audio and videocoding tools (see: <A HREF="l3faq260.html#aboutMPEG">Overview about theISO-MPEG Standard - or: What is MPEG all about?</A>).</P><P><B>Without data reduction</B>, digital audio signals typically consistof 16 bit samples recorded at a sampling rate more than twice the actualaudio bandwidth (e.g. 44.1 kHz for Compact Disks). So you end up with morethan <B>1400 kbit</B> to represent just <B>one second of stereo music inCD quality</B>. By using MPEG audio coding, you may shrink down the originalsound data from a CD by a factor of 12, without losing sound quality. Factorsof 24 and even more still maintain a sound quality that is significantlybetter than what you get by just reducing the sampling rate and the resolutionof your samples. Basically, this is realized by "perceptual coding"techniques addressing the perception of sound waves by the human ear (see:<A HREF="l3faq260.html#aboutCoding">Basics of Perceptual Audio Coding -or: What is the trick?</A>).</P><P>Using MPEG audio, one may achieve a typical data reduction of</P><PRE> 1:4 by Layer 1 (corresponds with 384 kbps for a stereo signal), 1:6...1:8 by Layer 2 (corresponds with 256..192 kbps for a stereo signal), 1:10...1:12 by Layer 3 (corresponds with 128..112 kbps for a stereo signal),</PRE><P>still maintaining the original CD sound quality.</P><P>By exploiting stereo effects and by limiting the audio bandwidth, thecoding schemes may achieve an acceptable sound quality at even lower bitrates.Layer-3 is the most powerful member of the MPEG audio coding family. Fora given sound quality level, it requires the lowest bitrate - or for agiven bitrate, it achieves the highest sound quality (see: <A HREF="l3faq260.html#aboutLayer3">AdvancedFeatures of Layer-3 - or: Why does Layer-3 perform so well?</A>).</P><P>Some typical performance data of Layer-3 are:</P><PRE>sound quality bandwidth mode bitrate reduction ratio"telephone sound" 2.5 kHz mono 8 kbps* 96:1"better than shortwave" 4.5 kHz mono 16 kbps* 48:1"better than AM radio" 7.5 kHz mono 32 kbps 24:1"similar to FM radio" 11 kHz stereo 56..64 kbps 26..24:1"near-CD" 15 kHz stereo 96 kbps 16:1"CD" > 15 kHz stereo 112..128 kbps 14..12:1*: Fraunhofer uses a non-ISO extension of Layer-3 for enhanced performance ("MPEG 2.5")</PRE><P>All in all, Layer-3 is the key for numerous low-bitrate, high-qualitysound applications (see: <A HREF="l3faq260.html#applications">Applications- or: Layer-3, what is it good for?</A>).</P><H3>Applications - or: Layer-3, what is it good for?<A NAME="applications"></A></H3><P>A key technology like Layer-3 is useful for a pretty large spectrumof applications - practically almost any system with a limited channelcapacity may benefit from it. The following chapters identify some mainareas and list some companies that are actively exploiting the Layer-3technology. For product-related information, please contact these <A HREF="L3FAQ260.HTML#addresses">companies</A>directly.</P><H4>Music Links via ISDN</H4><P>Digital telephone networks (ISDN = Integrated Services Digital Network)offer reliable dial-up links with two 64 kbps data channels per basic rateadapter; other regional networks (in North-America) use 56 kbps data links.Transmission fees are often rather similar or identical to the traditionalanalog phone lines - those allow to transmit up to 28.8 kbps (V.34 modem)or even 32 kbps ("V.34+").</P><P>Using Layer-3, a low-cost narrowband ISDN connection allows to transmitCD-quality sound. Audio professionals, like broadcasting stations and soundstudios, benefit from the "music-by-phone" application in variousways. They save money, as they only pay transmission fees for the actualtime of usage (not 24 h a day in case of a leased phone line) and for arather small data channel (one ISDN phone connector for a stereo musiclink). Radio stations increase the attractiveness of their programs, asreporters transmit high-quality takes (e.g. an interview) or live newswithout annoying "telephone sound". And new applications becomepossible, e.g. a "virtual studio", where remote artists may playalong some preproduced material, without actually travelling to the studio.</P><P>Examples:</P><UL><LI>In 1992, Radio FFN, a private broadcasting station in Niedersachsen,Germany, replaced its leased phone lines with ISDN and Layer-3 codecs,to transmit 8 local programs 20 min per day to the central broadcastingstudio. This move saved them transmission fees of more than 300.000 US$per year.</LI><LI>As one of the first real-world trials, all private radio stations ofGermany very successfully used Layer-3 codecs during the Winter OlympicGames in Albertville (France) as reporter links between the various sportingevents and their central studio in Meribel.</LI><LI>At the International Music Festival 92 in Bergen, Arne Nordheim composeda piece of music, where an organ in the church of Trondheim played alongwith the symphony orchestra in Bergen; the sound of the organ was transmittedvia ISDN and a Layer-3 codec.</LI></UL><P>Since 1992, various manufacturers are producing equipment ("codecs")for studio applications: <A HREF="L3FAQ260.HTML#Dialog 4">Dialog 4</A>,<A HREF="L3FAQ260.HTML#Lucent">Lucent</A>, <A HREF="L3FAQ260.HTML#Telos">Telos</A>.</P><H4>Digital Satellite Broadcasting</H4><P>Pioneered by <A HREF="L3FAQ260.HTML#WorldSpace">WorldSpace</A>, a worldwidesatellite digital audio broadcasting system is under construction. Itsname is WorldStar⌨️ 快捷键说明
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