Audio compression formats compared
- — 31 July, 2002 08:00
Although MP3 is the most popular format for compressing digital audio, there are literally dozens of other formats from which to choose, including AAC, Windows Media Audio (WMA), Ogg Vorbis and MPC, to name a few. The reasons for using a format other than MP3 would depend upon your requirements. For example, you may want a format that is extremely high quality, in which case you might choose MPC. If you were after a good quality format for streaming audio over a modem, WMA would probably be the best choice.
The audio codecs (codec stands for "encoder-decoder") discussed below all belong to a class of compression called 'lossy'. Effectively, this means that in order to achieve such high levels of compression, and consequently such small files, audio information is discarded. Lossy compression is to audio what JPEG compression is to images. By sacrificing a little bit of quality, much space can be saved in the resulting file size. The success of a lossy codec is based on how well it discards audio information considered to be imperceptible and therefore unnecessary. Some newer audio compression formats -- such as AAC and WMA -- do a much better job of this than the now ageing MP3 algorithm. Below is a summary of the major digital audio compression formats available and a comparison of how they rate out of 10*.
Played by almost every portable digital audio device and many DVD players, MP3 is still hard to go past if you're looking for maximum compatibility for your files. Whilst you can get much better compression from other formats, hard disks and blank CDs are cheap enough to justify the extra file size. Stereo imaging is not terrific and encoding quality differs from one software package to another.
This format is interesting as it combines a low bitrate^ MP3 file with what is called spectral band replication (SBR) data. The SBR component of the file supplies quality high frequencies, while the MP3 part of the file produces quality low frequencies. The combination of the two ensures very small file sizes. Interestingly, the MP3 component of MP3PRO files is backwardly compatible with all MP3 players, making it a tempting choice for general use.
This format is a joint project between Fraunhofer (the people responsible for MP3), AT&T, Lucent, Sony and Dolby. MP3 was part of the MPEG-1 video compression specification, but AAC belongs to the MPEG-2 specification. Generally speaking, AAC files are better quality and around 30 per cent smaller than the MP3 equivalent. Some portable devices will play this format but, generally speaking, it is not in common use.
Window's Media Audio is Microsoft's contribution to high quality, lossy audio compression. Like most other new formats, it outperforms MP3 in terms of quality and compression, particularly at lower bitrates. Consequently, WMA is probably the format of choice for streaming at low bandwidths. Like MP3, however, the stereo imaging is not very accurate. Additionally, WMA tends to overcompensate for its high compression with what is often called 'overbrightness'.
This format, developed by Dolby, is often used for video soundtracks due to its ability to handle surround sound formats such as 5.1 channel information. It was designed for use in consumer electronics such as high definition TV, cable TV and satellite broadcasts. Some DVD/MP3 players support AC3 playback, although it is not widely used as a stand-alone audio format. One of the top features of AC3 is that it provides excellent stereo imaging -- an area where most other lossy codecs fail.
Also known as MPEGplus, this is a much better MPEG-1 audio format than MP3, although it can only be used at high bitrates because it is designed for very high quality applications. The encoder is currently free but will become shareware. While not widely supported in general, there is a free decoder plug-in for Winamp. If quality is your main concern and file sharing isn't on the agenda, this may be the format to choose.
Ogg Vorbis is a project attempting to replace all proprietary audio formats with an open standard freeware codec. Version one was released in this past fortnight and has been demonstrated to be very high quality and outperforms MP3 by a long shot. At low bitrates it doesn't compete with WMA, and at high bitrates it falls short of MPC. Given that it is a work in progress, however, it has strong potential to become a widely used audio codec. Some portable device manufacturers are promising to support Ogg Vorbis in future software releases.
Real Audio is something of a dinosaur in the digital audio world. The player is free, but wastes a lot of system resources as well as being advertising intensive. That said, the recent version 8.5 release supports CD quality bitrates as well as good quality results at lower bitrates. On the downside, the codec is not integrated as a standard at an operating system level and is quite CPU intensive. To its credit, though, Real Audio encoding is exceptionally quick.
|Quality (estimate)||Format (compression type)||Bitrate (Kbps)||Filesize (KB/min)|
|CD Quality||Uncompressed WAV||1411||105,000|
Disclaimer: the above guidelines are suggested estimates only. Similar quality audio output will, in most cases, occur at the specified settings although actual values will differ according to the nature of the audio recording, the encoding software, and the playback equipment used.
* The Compression/Quality/Compatibility/Overall ratings are purely the author's opinion. There is no definitive way of proving any of it, as it comes down to the subjective experience of the listener. In this case, Daniel Potts has based his opinion on a mix of his personal preference as well as many reviews and codec comparisons by others - Ed.^ The higher the bitrate, the lower the compression. The lower the compression, the better the quality and the larger the file size.
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