Encoding of MP3 audio
The MPEG-1 standard does not include a precise specification for an MP3 encoder. The decoding algorithm and file format, as a contrast, are well defined. Implementers of the standard were supposed to devise their own algorithms suitable for removing parts of the information in the raw audio (or rather its MDCT representation in the frequency domain). This is the domain of psychoacoustics, which aims at understanding how human acoustical perception works (both in our ears and in our brain).
As a result, there are many different MP3 encoders available, each producing files of differing quality. Comparisons are widely available, so it is easy for a prospective user of an encoder to research the best choice. It must be kept in mind that an encoder that is proficient at encoding at higher bitrates (such as LAME, which is in widespread use for encoding at higher bitrates) is not necessarily as good at other, lower bitrates.
Decoding of MP3 audio
Decoding, on the other hand, is carefully defined in the standard. Most decoders are "bitstream compliant", meaning that the uncompressed output they produce from a given MP3 file will be the same (within a specified degree of rounding tolerance) as the output specified mathematically in the standard document. The MP3 file has a standard format which is a frame consisting of 576 samples and all the frames have associated header information(32 bits) and side information(17 bytes).The header and side information help the decoder to decode the associated huffman encoded data correctly.
To have balanced opinions here I add, "Frame size is the number of samples contained in a frame. It is constant and always 384 samples for Layer I and 1152 samples for Layer II and Layer III." [1]
Therefore, for the most part, comparison of decoders is almost exclusively based on how computationally efficient they are (i.e., how much memory or CPU time they use in the decoding process).
Volume normalization
As compact discs and other various sources are recorded and mastered at different volumes, it is useful to store volume information about a file in the tag so that at playback time, the volume can be dynamically adjusted.
A few standards for encoding the gain of an MP3 file have been proposed. The idea is to normalize the volume (not the volume peaks) of audio files, so that the volume does not change between consecutive tracks.
The most popular and widely used solution for storing replay gain is known simply as "Replay Gain". Typically, the average volume and clipping information about an audio track is stored in the metadata tag.
Alternative technologies
Many other lossy audio codecs exist, including:
* MPEG-1/2 Audio Layer 2 (MP2), MP3's predecessor;
* Ogg Vorbis from the Xiph.org Foundation, a free software and patent free codec.
* MPC, also known as Musepack (formerly MP+), a derivative of MP2;
* mp3PRO from Thomson Multimedia combining MP3 with SBR;
* AC-3, used in Dolby Digital and DVD;
* ATRAC, used in Sony's Minidisc;
* MPEG-4 AAC, used by Apple's iTunes Music Store and iPod
* Windows Media Audio (WMA) from Microsoft.
* QDesign, used in QuickTime at low bitrates;
* AMR-WB+ Enhanced Adaptive Multi Rate WideBand codec, optimized for cellular and other limited bandwidth use;
* RealAudio from RealNetworks, frequently in use for streaming on websites;
* Speex, free software and patent free codec based on CELP specifically designed for speech and VoIP.
mp3PRO, MP3, AAC, and MP2 are all members of the same technological family and depend on roughly similar psychoacoustic models. The Fraunhofer Gesellschaft owns many of the basic patents underlying these codecs, with Dolby Labs, Sony, Thomson Consumer Electronics, and AT&T holding other key patents.
There are also some lossless audio compression methods used on the Internet. While they are not similar to MP3, they are good examples of other compression schemes available. These include:
* FLAC stands for 'Free Lossless Audio Codec'
* Monkey's Audio
* SHN, also known as Shorten
* TTA
* Wavpack
* Apple Lossless
Listening tests have attempted to find the best-quality lossy audio codecs at certain bitrates. The tests have suggested that for some audio samples, newer audio codecs including Ogg Vorbis, mp3PRO, AC-3, Windows Media Audio, MPC and RealAudio perform better than MP3. Generally, these codecs achieve the equivalent of MP3 128kbit/s at around 80kbit/s. At 128kbit/s, Ogg Vorbis and MPC performed marginally better than other codecs. At 64kbit/s, AAC and mp3pro performed marginally better than other codecs. At high bitrates (128kbit/s+), most people do not hear significant differences. What is considered 'CD quality' is quite subjective; for some 128kbit/s MP3 is sufficient, while for others 192kbit/s MP3 is necessary.
Though proponents of newer codecs such as WMA and RealAudio have asserted that their respective algorithms can achieve CD quality at 64 kbit/s, listening tests have shown otherwise; however, the quality of these codecs at 64 kbit/s is definitely superior to MP3 at the same bandwidth. The developers of the patent-free Ogg Vorbis codec claim that their algorithm surpasses MP3, RealAudio and WMA sound quality, and the listening tests mentioned above support that claim. Thomson claims that its mp3PRO codec achieves CD quality at 64 kbit/s, but listeners have reported that a 64 kbit/s mp3PRO file compares in quality to a 112 kbit/s MP3 file and does not come reasonably close to CD quality until about 80 kbit/s.
MP3, which was designed and tuned for use alongside MPEG-1/2 Video, generally performs poorly on monaural data at less than 48 kbit/s or in stereo at less than 80 kbit/s.
Licensing and patent issues
Thomson Consumer Electronics controls licensing of the MPEG-1/2 Layer 3 patents in countries that recognize software patents, including the United States and Japan, but not EU countries. Thomson has been actively enforcing these patents.
In September 1998, the Fraunhofer Institute sent a letter to several developers of MP3 software stating that a license was required to "distribute and/or sell decoders and/or encoders". The letter claimed that unlicensed products "infringe the patent rights of Fraunhofer and THOMSON. To make, sell and/or distribute products using the [MPEG Layer-3] standard and thus our patents, you need to obtain a license under these patents from us."
These patent issues significantly slowed the development of unlicensed MP3 software and led to increased focus on creating and popularizing alternatives such as WMA and Ogg Vorbis. Microsoft, the makers of the Windows operating system, chose to move away from MP3 to their own proprietary Windows Media formats to avoid the licensing issues associated with the patents. Until the key patents expire, open source / free software encoders and players appear to be illegal for commercial use in countries that recognize software patents.
In spite of the patent restrictions, the perpetuation of the MP3 format continues; the reasons for this appear to be the network effects caused by:
* familiarity with the format, not knowing alternatives exist,
* the fact that these alternatives do not universally provide a definite advantage over MP3,
* the large quantity of music now available in the MP3 format,
* the wide variety of existing software and hardware that takes advantage of the file format,
* the lack of DRM-protection technology, which makes MP3 files easy to edit, copy and distribute over networks,
* the majority of home users not knowing or not caring about the software patent controversy, which is in general irrelevant to their choice of the MP3 format for personal use.
Sisvel S.p.A. [2] and Audio MPEG, Inc. [3], are suing Thomson for patent infringement on MP3 technology[4]. Audio MPEG also starts licensing MP3 to vendors of MP3, so legal status of MP3 is unclear.
Online music resources
Tools such as iRate try to make it easier to find music that matches the listener's tastes. There are several online music stores. Apple's iTunes store is presently the most popular commercial online music offering. Independent artists are able to use smaller sites to provide distribution.
There are also several online columnists who edit news sites focused on digital music and the grassroots community it spawned. They include Richard Menta's MP3 Newswire, an early MP3 news site started in 1998, Jon Newton's P2Pnet, and Thomas Mennecke's Slyck.com. Other sites like Download.com and Vitaminic.com which allow artists to choose to post their own music for free download.
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