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Guide to Digital Audio Formats

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Digital Audio

Sound is stored on computers as thousands of snapshots of an audio signal called samples. When you play sound files back, your computer converts these samples back into an analogue audio signal with the help of a device called a DAC (digital-to-analogue converter).

These samples are basically numbers indicating the voltage of a waveform at a given point in time (think of them as points on a graph). The higher the number of samples and the depth (resolution) of each sample, the more accurate the reconstructed sound will be. CDs store 44,100 of these samples (at 16-bit depth) for every second of audio (actually twice as many, because CDs are stereo).

As you might imagine, storing this amount of data uncompressed takes up quite a lot of space on a hard-disk or personal media player. In fact, this equates to roughly 10MB of space for every minute of CD-quality audio.

Common File Formats

Audio can be stored on computers in a number of different formats, depending on what type of system you use. Windows systems tend to store uncompressed audio in .WAV format whilst Apple systems will use the equivalent .AIFF format. These file formats are uncompressed, full quality files analogous to a bitmap image. In order to save disk space, various compressed formats have been invented. MP3, WMA, AAC, FLAC and MP4 are all examples of compressed file formats. Compression basically trades file-size for quality (or for lossless formats, just CPU cycles).

Audio Compression

The amount of space taken up by uncompressed digital audio is quite substantial; as storage space increases, this is becoming less of a problem, but in order to fit as much music into as little space as possible, you will need to store it in a compressed format. Numerous algorithms exist that will take your raw audio and use clever tricks to compress the data down into a much smaller file size. When you play one of these files in a compressed format (such as an MP3), your computer will 'uncompress' the audio on the fly and play you the result.

'Lossy' vs 'Lossless' Compression

There are two approaches to compressing audio, depending on how much quality you want to retain. Formats which retain ALL of the original signal are known as lossless formats, the most common being FLAC (Free Lossless Audio Codec). These formats will reduce file size significantly, but to reduce the file size even further algorithms were developed which 'throw away' parts of the signal not deemed as important. These are known as lossy codecs, of which MP3 is an obvious example.

The WAV file format

WAV format has been around for many years and is the de-facto standard for storing raw uncompressed audio on PC. It is very versatile and capable of storing audio at virtually any sample rate or bit-depth.

The AIFF file format

As previously discussed, this format is typically found on Apple systems and is more or less equivalent to the WAV format on Windows. It too can support virtually any combination of sample rate and bit-depth, from 96kHz 24-bit down to 11kHz 8-bit.

The MP3 File Format

Developed by the Fraunhofer Institute as a means of compressing high-quality audio down into a manageable file size, MP3 is capable of up to 10:1 compression ratios whilst maintaining good quality sound. It is a lossy format, meaning that it discards some sonic information deemed superfluous in order to save disk space. Differing levels of quality can be specified when encoding in MP3 format, depending on how large you want the file to be.

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