Since the introduction of the Compact Disc in the 1980's, digitally encoded audio has become the most popular method of playing audio in history. Today, a new evolution in audio is quickly changing the way the world buys, distributes, and listens to music. This evolution refers to digital compression.
A CD holds music in what is known as the Pulse Code Modulation (PCM) format, which is a common method for digitizing analog signals. PCM signals are uncompressed, which means that after the conversion, the resulting digital stream will take up a great deal of space. In fact, CDs stream data off the disc at a rate of 1.41 million bits per second. This means that 30 minutes of audio will take up approximately 318 Megabytes of space, which is simply too large to efficiently store in a computer, load into a portable, or send over the internet.
Compression is the process of reducing the size of the digital signal, while still keeping the quality as close as possible to the original source. Throughout the 80's and much of the 90's, there was no great way to accomplish this task at home. Then it happened.
The arrival of the MP3 format shook the world. Since its release, the ability to compress music into tiny sizes has created many benefits, and many problems at the same time. Also, the market has since been flooded with many different audio compression methods that have made the process much more complicated than it has ever been before. This article will focus on the most popular digital compression formats.
MP3 is the one that started it all. MP3 is an abbreviation for the full descriptive name, International Standards Organization - Motion Picture Expert Group Audio Layer 3 (ISO-MPEG Audio Layer 3). MP3 is a very powerful compression codec (encode-decode system). Originally conceived in 1987, the format's popularity increased in the mid 90's when it became a popular method of storing music on a computer and sending it over the Internet.
MP3 compresses audio files relatively small. Depending on the bit rate chosen to encode the MP3 file, the size can vary. Though MP3 files can be found anywhere from 32 to 360 kbps, the most popular bit rates vary between 128 to 192 kbps. However, MP3 is a lossy codec, which means that audio information that is deemed to be unneeded is thrown away at the encoding stage based on the principles of perceptual coding. Most people will agree that MP3 files do not sound as good as the original CD for this reason.
MP3 is the most common audio compression format, which means that nearly all software players and portables are compatible to this format.
WMA is the abbreviated term for Windows Media Audio, which is a proprietary audio compression format developed by Microsoft. The latest release of the WMA format (version 9) has gained considerable momentum in the past year. You'll find that many of the newer software players and portable players are including support for WMA.
WMA is comprised of many different levels of compression, with each handling the compression of the original audio differently. In whole, those who have done direct comparisons feel that WMA sounds better than competing formats at a given bit rate. Though many feel the difference is negligible at bit rates over 128 kbps, most feel that WMA is substantially better at lower bit rates.
WMA is not just a lossy encoder like MP3 because it offers many different kinds of compression tools in one package. A brief explanations of its options is below:
- WMA VBR
- WMA Lossless
- WMA Voice
Similar to MP3, this lossy codec compresses audio information to a specific bit rate at the encoding stage. Also like MP3, there are many different bit rates to choose from.
VBR stands for variable bit rate, which means that the encoder will detect and encode harder passages at a higher bit rate, and the easier passages at a lower bit rate. This method helps keep the file size small, but more efficiently uses the space to make everything sound better. Many devices handle VBR, but there are still many that do not.
The lossless version of the codec ensures that no audio information is lost in the encoding stage. When these signals are decompressed, the result will be exactly the same quality as the original source. Lossless always uses variable bit rates, but the resulting file sizes are still large.
WMA Voice is designed specifically for encoding voice signals like e-books and radio. Since these signals do not require a huge amount of space to begin with, WMA Voice compresses these to a very small file, which will playback at fantastic quality.
AAC (.mp4, .aac, .mpe)
AAC stands for Advanced Audio Coding, which is part of the MPEG-2 AAC standard. AAC has been highly popularized by Apple, who uses this standard in their wildly successful iPod portable music player. The latest version, MPEG-4 AAC, includes additional tools for even better quality compression.
AAC is a more advanced and efficient codec than MP3. It yields better quality results, especially at lower bit rates. Like WMA, it also supports variable bit rate encoding.
AAC, though successful due to the iPod, is not one of the most compatible compression formats in the market. Other than Apple's iPod and their QuickTime Media Player, few portable and software players support this format.
OGG is the nickname for Ogg Vorbis, which is an open source, royalty free audio compression format. OGG has gained ground since its launch, mostly because it produces exceptional quality results, and is completely free for manufacturers to implement in their hardware and software players. OGG contains many of the same features that the major formats like MP3, WMA, and AAC contain, as well as a few unique features to itself.
Though it's the most popular of the free formats, it has a long way to go before it becomes a widely compatible format. Most of the software players have a plug-in to support OGG, but few portables devices play OGG files natively.
ATRAC stands for Adaptive Transform Acoustic Coding, and was initially developed by Sony in 1991 for the MiniDisc. The original ATRAC format does not reduce the size of the original file anywhere near as much as other formats. However, since ATRAC was developed for the MiniDisc, it was successful in packing the same playing time of a CD onto a MiniDisc, with the sound quality nearly indistinguishable from the source. The higher compression versions of ATRAC made their debut in 2000 and 2003, which were designed to pack more playing time on a MiniDisc. However, the discs would remain unplayable in older MiniDisc players.
The ATRAC format is only compatible with Sony portable audio products. In many cases, it is the only audio format it will accept. Owners of these products use bundled software to convert other audio formats to ATRAC before it is loaded into the player. With Sony's recent announcement to begin offering native support for MP3, it is likely that the ATRAC format will quickly be phased out.
The formats explained here are only the most popular. There are plenty of other audio formats to pick from if you are looking for the best sounding one to you.
Choosing a format ultimately depends on personal preferences. An important consideration is to find the best format that is compatible with the hardware you own. On a computer, you have the ability to playback all formats, but your portable will probably only be compatible with a few of them. MP3 is currently the most compatible of formats, but the sound quality of newer compression schemes might make them a better choice for you.
It is important to note that players may tout support for a variety of audio formats, but in reality they don't support them natively. Sony's NetMD players, for example, state they are compatible with ATRAC, MP3, and WMA, but are actually only compatible with ATRAC. The software converts WMA and MP3 into ATRAC when loaded into the player. An extra conversion could mean a hit in sound quality. Always make sure that the player natively plays the audio formats in question, and does not rely on additional software to make the conversion.