Understanding MP3s (and other compressed music) – Part 2

Welcome to Part 2 of my series of posts about the pros and cons of compressed audio. If you haven’t read Part 1, it’d be a good idea. Here’s a link: Understanding MP3s (and other compressed music) – Part 1

Wielding the Eraser

I explained in Part 1 that compression means pulling out sounds that we won’t actually hear, but think about this… The music is like a painting that we “see” with our ears. Compressing music is the equivalent to taking an eraser to the Mona Lisa. It’s like saying, “No-one will notice this brush stroke of stray colour or this tiny bit of shading.” Perhaps that’s true and, to a degree, no-one would notice, but at some point the whole painting’s just going to lose something. It’ll lose a little bit of soul. Sure, you might not pick exactly which parts are missing, but you’ll know something’s not right. Here’s an example:

Notice how the sky in the second image looks unnatural and full of lines? That’s because the process of compressing has removed some of the subtle shades of blue and replaced them with wider bands of other shades. For example, let’s number the different shades 1.1, 1.2, 1.3 and 1.4. During the compression process we would replace shade 1.2 with a second band of 1.1 and replace 1.4 with a second band of 1.3. Now that blue sky would be made of bands of shades 1.1, 1.1, 1.3, 1.3. You can see the evidence of this above in the second image.

So looking at the example photos, it’s clear that they’re both the same photo, but if you had to choose one to print and frame, I’m guessing you’d choose the first one because it’s closer to real life and therefore more pleasing to the eye. The same goes for music.

Think of music as a complex bunch of vibrations making a particular range of patterns. Any little detail you remove from those vibrations will permanently alter the overall “picture”. You’ll still recognise the sound or the song, but it won’t actually sound identical to the original.

Let’s talk about the ear again. Remember my description of how we hear? The ear perceives music like the eyes perceive a painting. You take it all in at once, you don’t pick out a particular colour here and a particular texture there, you just see it as a picture. When we compress sound we permanently alter the “picture” as if we had taken to it with an eraser. To our ears, the result is no different to the photo above on the right. It might not be as dramatic (depending on the level of compression), but it’s essentially the same. You don’t notice a loss of individual sounds, you notice a loss of overall quality and realism.

Here’s one final visual version to show you what I mean. The following charts are spectrograms that show sound as colour. The darker the colour, the louder the sound and the higher up the colour appears, the higher pitch the sound is. A bass guitar shows up down the bottom while a violin shows up further towards the top. There are 2 lines in each chart – these are the left and right stereo channels.

Spectogram - lossless

"This is How a Heart Breaks" - no compression

"This is How a Heart Breaks" - moderate compression

"This is How a Heart Breaks" - mid-high compression (128 kbps)

Notice the density of the yellow and orange colours reduces as you get more compression? The more blue you see, the less of the musical “picture” is still intact. You might also notice that there is more variety and clarity in the colours on the top chart and the colours all get more “blurry” as you move down the charts. That’s the effect of averaging things out. If you look at the first spectrogram and then the second, you might notice that the second one looks like a slightly out-of-focus copy of the first one.

By the time we get to 128 kbps, nearly every high frequency sound is removed. That’s because we lose those hearing at these frequencies first and are less likely to notice the missing sound… or at least that’s the theory. The key thing to notice here is that the musical pictures are different. This is the most visual representation of sound that I can provide and it illustrates exactly how the musical “picture” is gradually erased by compression.

In the Final Installment

Now that you know how we perceive sound and how compression works, you’re all ready to read about why compressed music loses its “magic”. In Part 3, I’ll explain a bit harmonics and their role in creating the soul of the music. I’ll also sum up what this all means when it comes to choosing the level of compression that’s right for you.

As always, I hope you’re enjoying this information and I welcome any feedback or questions you might have.

Ready for Part 3?

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4 comments on “Understanding MP3s (and other compressed music) – Part 2

  1. Pingback: Understanding MP3s (and other compressed music) – Part 1 « Passion For Sound

  2. Pingback: Understanding MP3s (and other compressed music) – Part 3… Finale « Passion For Sound

  3. Your compressed image has been compressed with an algorithm specifically not designed for photos – you should use jpeg for photo type images. You have used gif or png.

    You should read some more about audio compression – these explanations which talk about “removing the quiet sounds”, always make vast over simplification of the processes. Learn the Fourier transform.

    • Hi Chris,

      I appreciate the feedback, but was merely trying to provide a rough example (emphasis on rough) to try and paint a picture of why compression does degrade the original sound. I would encourage those interested in understanding more to seek out better explanations than mine as this post is intended as a rough starting point for someone new to the area.

      Regards,

      Lachlan

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