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Digital File Formats
Digital Music File Formats
By Paul Katsande
An uncompressed digital music file (such as that on a music CD) uses 10MB or more per minute (at least 30MB for a 3 minute song).
These files have to be compressed to the more usual levels of 3-5MB per song. Compression rearranges the sequence of numbers, sometimes throwing away less significant information, in order to reduce the file size. The way the compression is achieved leads to the many different audio file formats available. Below is a list of the popular formats used by MP3 players.
MP3 (MPEG Layer 3)
MP3 is a perceptual audio coding algorithm, developed by the MPEG group. Although its a lossy compression, the digital audio sounds exactly as, or very close to, the original sound. The algorithm attempts to adapt the compression to the characteristics of the human perception of sound. This compression algorithm can handle both constant and variable bit rate compressions.
For good quality music, a bit rate of at least 192Kbps is recommended - this is near CD quality. 256Kbps per second is better. There is very little quality difference between 256Kbps and 320Kbps, but the later will have a larger file size. MP3 files have no Digital Rights Management (DRM) technology built in - meaning they are freely portable. That’s not to say its legal to move music files from one device to another.
AAC (Advanced Audio Coding)
A newer, lossy audio compression format developed by the same MPEG group. AAC is more efficient than the popular MP3 format, and requires less processing power to decode. Its also not backward compatible with MP3. AAC-compressed audio at 96 Kbps generally exceeds the quality of MP3-compressed audio at 128 Kbps.
AAC supports DRM, but you can have both protected or unprotected formats. The unprotected format is freely portable, while the protected format is locked by the DRM used. Apple uses the AAC format for its iTunes and in QuickTime. Audio compressed at 128Kbps using AAC sounds very close to the original sound, which means a smaller file size for the same audio quality compared to the MP3 format. The file will be about 75% of the size an MP3 compression would produce.
WMA (Windows Media Audio)
WMA is another lossy compression format, developed and owned by Microsoft, and used in Windows Media Player for DRM management. This compression is more efficient than MP3, and comparable to AAC in terms of efficiency and file size reductions.
WMA supports DRM, so can have both protected and unprotected audio files. Most devices and players support the unprotected WMA files - Apple iTunes software can convert them to AAC files.
Unlike the other formats, Ogg Vorbis is a completely open-source, patent-free, professional audio encoding and streaming format. There are no licensing fees for using this codec. If you are an artist that’s goods news. You won’t have to pay any fees if you distribute your songs, which you would if you use, say, MP3.
It is yet another lossy compression, but much better than MP3. An Ogg Vorbis audio file encoded at 110Kbps gives a smaller size and sounds better than an MP3 file encoded at 128Kbps. 160Kbps gives very-near-CD-quality audio encoding. Although it’s open-source, this codec compares very favourably to WMA, AAC, WMA Pro etc (see Wikipedia for more on this).
This format is rising in popularity due to its open-source nature and freedom from licensing issues.
MIDI(Musical Instrument Digital Interface)
MIDI is one of the old formats, but still very much in use today. Its not really a compression algorithm, but a synthesized music format. MIDI files consist of lists of commands which tell a synthesizer when to start/stop playing a specific note of a specific instrument, and may include the volume and modulation of the note. With MIDI you can play a number of instruments at once, or just play one instrument at a time.
These files are generally tiny compared to normal audio files such as MP3, WMA, AAC. Because this is synthesized music, its quality greatly depends on the quality of the synthesizer on your PC’s sound card or the quality of the synthesized instruments in your software. One application for MIDI files is as mobile phone ring tones.
This format from RealNetworks was at the fore-front of internet audio formats for many years. This format is usually streamed, rather than downloaded. It can be played using the RealPlayer or RealOne software. A good number of internet radio stations still use this format for their broadcasts.
The audio can be streamed at different bit rates to satisfy bandwidth constraints. If you are on a dial-up internet connection, the music quality you get is lower than if you are on broadband.
ATRAC(Adaptive Transform Acoustic Coder)
The ATRAC compression algorithm is owned by Sony, and was first used in the MiniDisc in 1992. The ATRAC family of lossy algorithms has three members: ATRAC, ATRAC3, and ATRAC3plus. ATRAC, the oldest of the three, can compress CD music to approximately 20% of the original size. ATRAC3 improves on this by a factor of two, compressing a music stream to about 10% of the original.
The youngest member, ATRAC3plus, can compress a music CD to about 5% of the original. In simple terms, ATRAC3plus can allow you to save over 350 music tracks on a 700MB CD. The ATRAC3plus is the default on Sony MP3 players, although they can also play other file formats.
Sony also has a lossless algorithm, imaginatively called the ATRAC Advanced Lossless algorithm. Perfomance-wise, ATRAC3 and ATRAC3plus are comparable to AAC and WMA. Being a proprietary algorithm, its not as widely supported as say MP3.
The author is the webmaster of Akama Music, where you can get free lyrics and tips on music technology.
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