audio format (file format)
Audio formats include the modulation methods relevant in the audio field, such as Pulse CodeModulation( PCM) and Adaptive Delta Pulse Code Modulation( ADPCM), but also file formats such as the Audio Interchange File Format( AIFF), Wave Format, Musical Instrument Digital Interface( MIDI), manufacturer-specific and proprietary file formats, as well as compression methods and compression algorithms such as Dolby Digital, RealAudio and MPEG.
A distinction is made between uncompressed and compressed audio data, respectively between lossless and lossy, which is ultimately reflected in the sound quality and file size.
The uncompressed, lossless audio is similar to the way pulse code modulation (PCM) works. The digitized data is stored in its original form. The quality depends on the sampling rate and quantization: for compact discs( CD), the sampling rate is 44.1 kHz and the sampling depth is 16 bits; for DVDs, the sampling rate is 96 kHz and the sampling depth is 24 bits, resulting in much higher quality. The data volumes for uncompressed audio are correspondingly high; for example, one second ofDVD-quality audio has a data volume of 288 kB. Uncompressed audio is therefore completely unsuitable for transfer over networks. Typical representatives for uncompressed audio are Waveform Audio File(WAV) and Audio Interchange File Format (AIFF).
In compressed, lossy audio, certain inaudible frequency components are filtered out of the audio signal, thus significantly reducing the data rate. The most important representatives of compressed audio files are: RealAudio, WindowsMedia Audio(WMA), QuickTime, MPEG, MP3, Audio Code Number 3( AC-3), Extensible Music Format(XMF), Advanced Audio Coding( AAC) and Adaptive Transform Acoustic Coding( ATRAC). In addition to uncompressed and compressed audio files, there are also markup file formats. In this technique, one does not transmit the audio, but a functional description. With the help of this description, the computer generates the audio sequence. The quality depends on the hardware and software of the computer and is usually very poor. The amount of data is very small. The best known representative of this group is Musical Instrument Digital Interface (MIDI), another VoiceXML.