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Article ED00007: Understanding MP3s

Summary:

MP3s have changed the way we store and listen to music, but what is an MP3 file and how does the sound compare with a CD recording. Something every serious musician should know about.

Application:

Music teachers and students and anyone interested in good quality music and audio recording.

Details:

The ubiquitous MP3 has become synonymous with music... and most of us would know that MP3 files are much smaller than the equivalent CD track or WAV or AIFF audio file on a computer. The smaller size has changed our world. We can now carry around hundreds or even thousands of songs in a device that fits in the palm of our hand. Even in mobile phones. And we can download MP3 songs from the internet in seconds and store 10 hours or more MP3 music on a single CD. The MP3 has greatly increased consumer access to music (legal and otherwise!) and the way music is distributed by musicians and recording companies has changed dramatically.

What about audio quality?

Besides the obvious advantages of MP3s many serious musicians and music educators question whether the audio quality is “up to scratch”... largely based on the fact that MP3 files are known to be “compressed”, or maybe they have listened to some low quality examples.

Things you SHOULD know about MP3s

It is true. MP3 files are compressed versions of the original or live recording. But as musicians, the term “compression” usually refers to a squeezing of the dynamics... ie reducing the difference between the softest and loudest passages of a piece of music. In “computer speak”  however, the term “compression” means reducing the amount of data and hence the size of a file. The smaller the file the more you can store in a given space and, most importantly, the faster you can transfer the file to somewhere else (eg via the internet or digital radio  transmission).

Masters of perception... your ears!

So how do you reduce the amount of data without removing some of the actual audio? Simple answer... you can’t (or at least we haven’t yet worked out a method that technically retains ALL the details of the original, but in a smaller file). But the good news is that we DO NOT  ACTUALLY HEAR ALL the details anyway. Or, more correctly, the translation of sound from our ears to our brain is filtered in some very complex ways... and these perceptual characteristics have been studied at great length (psychoacoustics).

Some very clever processing

MP3 is a perceptual audio coding algorithm. Perceptual encoding is a lossy compression technique, ie. the decoded data is not an exact replica of the original audio data. Instead, digital audio data is compressed in a way that - despite the high compression rate - the decoded audio sounds exactly - or as closely as possible - like the original. This is achieved by adapting the encoding process to the characteristics of the human perception of sound: The parts of the audio signal that humans perceive distinctly are coded with high accuracy, the less distinctive parts are coded less accurately, and parts of the sound which we do not hear at all are mostly discarded or replaced by quantization noise.

MP3 quality vs CD quality

Most people would agree that CD quality is a very good benchmark when comparing any form of digital audio. For reference, CD quality means digital audio stored at 16bit/44.1kHz. (Professional recordings these days are typically made at 24bit and up to 192kHz... and then reduced or “dithered” down to “ordinary” CD quality). MP3 quality is determined by the “bit rate” and allows the creator to determine whether they want low quality and very small files or very high quality and larger files. Bit rates are given in kbps or kb/s and refer to the number of kilobits per second.

MP3 QualityWithout getting too technical, the higher the bit rate the better the quality and the larger the file size. Normal good quality MP3 files today are 128kbps and will be approx 1MB in size for each minute of audio. In comparison, CD audio (stereo) or 16bit/44.1kHz stereo WAV files will take up approx 10MB per minute... or are around 10 TIMES THE SIZE.

High Quality MP3 (160kbps) will be perceived by most listeners as similar to CD quality. Critical listeners would choose 192kbps. Although you can go higher (standard encoders/decoders go up to 320kbps) it is very doubtful that improvements are perceptible. High end encoders can also create Variable Bit Rate (VBR) MP3s that increase the rate when required and reduce it in less complex passages (eg during silence or when fewer instruments are playing).

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