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A Profile of the MP3 Format

Details on the MP3 format including what influences audio quality

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Introduction

 

  • Format name: MPEG Audio Layer 3
  • Format type: Audio
  • File extension: .MP3
  • Compression: Lossy

 

The MP3 format’s history is legendary when it comes to digital music and the Internet. It has single handedly revolutionized the way we listen to and share music. Even its name has become part of our everyday language; MP3 music and MP3 players are terms that are used to describe all kinds of formats and players. It is this impact on the world that makes the MP3 format truly impressive. Since its first release in 1995, the MP3 format continues to dominate over all other audio formats in terms of global usage.

 

Sampling Frequencies & Bit Rates

There are three sampling frequencies that are available for MP3 (MPEG-1) encoding which are:

  • 32, 44.1 and 48 kHz.

It is the 44.1 kHz sample rate that is used the most; this is the same frequency that is used for the compact disc.

 

There are a selection of bit rates that the MPEG-1 Layer 3 standard sets out, and these are:

  • 32, 40, 48, 56, 64, 80, 96, 112, 128, 144, 160, 192, 224, 256 and 320 kbps.
The most popular bit rate used for MP3 files is 256 kbps as this strikes a good balance between audio quality and file size.

 

Compression Algorithm

The compression algorithm that the MP3 standard uses is a lossy one. The MP3 encoding process takes advantage of psychoacoustic algorithms that filter out frequencies that humans are unlikely to hear. Low frequencies are either filtered out or are converted to mono signals - this requires less storage space. The human ear can’t detect directions of low frequencies very well and so a stereo signal isn’t necessary. Another way that MP3 compression can reduce file size while keeping the quality is to drop quieter sounds in preference for louder sounds; it is unlikely that the listener will notice any difference.

Audio Quality Influences

Factors such as, sampling frequency, bit rate and compression algorithms all influence audio quality. The MP3 format is old compared to modern audio formats, like AAC and so its performance in certain scenarios isn’t as optimized. Using MP3 to encode audio at low bit rates produces a lower quality of sound compared to newer formats like AAC or WMA. Using CBR as opposed to VBR can also degrade sound quality. CBR produces MP3 files that are typically not as good as their VBR equivalents. VBR encoding can produce a higher quality of sound because the bit rate varies to optimize quality of sound and file size.

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