How It Works: Audio Compression

04/15/2014

The term “compression” is often a source of confusion when discussing digital music. There are two kinds of compression. The first is the kind used to compress the size of files; this is data compression. There is lossy compression, using with MP3 and AAC files, and lossless compression, used with FLAC and Apple Lossless formats.

But the other kind of compression, dynamic range compression, is the much derided method of limiting the amount of dynamic range in music. The point of dynamic range compression is to make less of a difference between the quietest parts of a piece of music and the loudest parts. Most music is compressed as part of the recording and mastering process, because it does sound a lot better, and keeps you from blowing out your speakers. But over-compressing music makes it sound like crap.

The best way to understand dynamic compression is to look at a couple of audio waveforms. The screenshots below were made using Rogue Amoeba’s Fission audio editor.

Here’s a song which is free on iTunes today. I chose this one because, well, any free pop single is likely to be heavily compressed, and this example shows that I’m not wrong.

001.png

You can see two things in this waveform. The first is that the song is almost universally loud; the waves show the loudness. The second thing to notice is that there is a lot of clipping; audio volume that hits the top of the available limit. This is bad. As Wikipedia says:

Music which is clipped experiences amplitude compression, whereby all notes begin to sound equally loud because loud notes are being clipped to the same output level as softer notes.

Excessive compression has led to what is known as the loudness wars. This is when record producers make their songs louder and louder so they stand out against other songs. Generally, the human brain perceives louder music to be better, so additional loudness can make a song more compelling. But, in the end, all this has done is made lots of loud, clipped songs.

Here’s an example of a song which is not compressed. This is Pink Floyd’s Wish You Were Here:

002.png

You can see the difference in two places in this screenshot. In the overall timeline at the top of the window, you can see that the music has a shape; in the first screenshot of the free pop single, it’s just one long mass of sound. And in the actual waveform, you can see that there is modulation, and no clipping, in the Pink Floyd song.

The difference is that you may play your Pink Floyd song at a louder volume, in order to hear the quiet parts of the song, but the louder parts will be, well, loud. In the first song, the entire song is loud, and you’re likely to become fatigued more quickly after listening to music like that.

For good examples of audio that is not compressed – or only very slightly – watch a movie. In general, movie audio is not compressed; this is why the dialog is often too soft, but the special effects are too loud. This is why you often need to adjust the volume for movies with lots of explosions, otherwise your ears hurt. (You may have an AV receiver which has a dynamic range compression feature; if you’ve turned this on, you may not hear such large differences in volume.)

Dynamic range compression isn’t a bad thing; it’s just bad when it’s overdone, as is the case in much popular music today.