Summary: My last post was about the frequencies missing from MP3s.1 Today, I want to talk about bit rates. To make sensible decisions about bit rates, it helps to understand a bit about how MP3 encoding works. When your computer makes a CD into an MP3, it has three main ways of making the music take up less space: 1) It throws out sounds you probably can't hear - either because they're "masked" by louder sounds, or because they're only audible to a very small proportion of humans. Done right, this is an elegant exercise in efficiency. Done wrong (or too much) you music sounds tiny, thin and empty. 2) It describes the sound in terms of the shape of the wave, instead of as a big long list of values. If the wave isn't a very complicated shape, it can do this with virtually no loss of quality. Here are two MP3s of the same sound - a simple 440Hz sine wave. This is just about the easiest thing to make into an MP3. Although the first file is 10x the size of the second, they sound identical because you don't even need 16kb to record 1 second of sine wave. Like a stick of rock, the file just says "440hz at -3dBFS" all the way through. Here it is at 160kbps (mono) [mejsaudio src="http://blog.naxos.com/wp-content/uploads/2012/02/Sine-Wave-160kbps.mp3"] Here it is at 16kbps (mono) [mejsaudio src="http://blog.naxos.com/wp-content/uploads/2012/02/Sine-Wave-16kbps.mp3"] With our nice simple sine wave, there's no extra data to throw out, so they sound the same. If we give it something really complicated, though, we'll start to notice a difference.2 Here's a bit of Debussy's La Mer3, as a very high quality MP3: [mejsaudio src="http://blog.naxos.com/wp-content/uploads/2012/02/La-Mer-Excerpt-320kbps-MP3.mp3"] That sounds pretty good to me, but here it is again, a tenth of the size: [mejsaudio src="http://blog.naxos.com/wp-content/uploads/2012/02/La-Mer-Excerpt-32kbps-MP3.mp3"] Suddenly it sounds like it's being played down a telephone. A file this size can happily hold a simple sine wave, but in trying to describe the complex harmony and sonority of Debussy's orchestration, it has to make some cuts. All the notes are still there, but we've lost a lot of what is beautiful about it. When we're looking for a bit rate that works for us, this is the outcome we're looking to avoid. 3) Once the fat is trimmed off (1) and the important sounds reduced to their component waves (2), the computer looks for commonly-occuring patterns in what remains, so the information in them only needs to be recorded once. In our first example above, that means saying "440Hz, -3bDFS" to define the single note, and "ditto" for the rest of the file. With La Mer, the opportunities are less obvious, but if you see time in 44100ths of a second like the computer does, there's plenty of repetition here. By itself, this third type of compression is lossless - you get exactly the same data out as you put in, but it takes up less space while being stored. The combination of these three techniques allow us to make the files much, much smaller. Even the highest-quality MP3s are just a fifth of the size of the original files, but they can be much smaller. The goal is to find the smallest file size that sounds good to you. Let's start with a 16kbps file. At this size, you could fit more than five days of music on a single CD: [mejsaudio src="http://blog.naxos.com/wp-content/uploads/2012/02/La-Mer-Short-Excerpt-16kbps.mp3"] I want you to make up your own mind, but I think you'll agree that sounded pretty bad. This next one is twice the size, at 32kbps. This would let you put 54 hours of music on a CD. [mejsaudio src="http://blog.naxos.com/wp-content/uploads/2012/02/La-Mer-Short-Excerpt-32kbps.mp3"] This one is twice the size again: 64kbps. You'd get 27 hours of this on one CD. [mejsaudio src="http://blog.naxos.com/wp-content/uploads/2012/02/La-Mer-Short-Excerpt-64kbps.mp3"] Next is 128kbps,
