All About Bitrates

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, or eight times the size we started at. You’d get thirteen and a half hours of this on a CD.

[mejsaudio src="http://blog.naxos.com/wp-content/uploads/2012/02/La-Mer-Short-Excerpt-128kbps.mp3"]

Double that again, and you’re at sixteen times the size we started at. At 256kbps, you’d get six hours and 47 minutes of music on a CD. The Amazon MP3 store delivers music in this format.4

[mejsaudio src="http://blog.naxos.com/wp-content/uploads/2012/02/La-Mer-Short-Excerpt-256kbps.mp3"]

Finally, the highest bit rate supported by the MP3 format is 320kbps. That’s 20 times the size we started at, and 22% of the size of the original. You’d get about five hours and twenty minutes of this on a CD. If you buy music from ClassicsOnline this is what you’ll get.5

[mejsaudio src="http://blog.naxos.com/wp-content/uploads/2012/02/La-Mer-Short-Excerpt-320kbps.mp3"]

You can try all this with your own music, indeed I’d encourage you to. Hook up your computer to your stereo, make some MP3s (and other files), shuffle them up, and try to tell them apart. Remember: bigger is not always better. If you want a fast car, you don’t buy the one that uses the most petrol. You buy the one that goes fastest. If you’re looking for an audio format that sounds good,  don’t go for the one that uses the most data. Go for the one that sounds best, and have fun.

 

Corrections:

I originally (and wrongly) wrote that the Amazon MP3 store delivered 320kbps files, not 256kbps files. This has now been corrected.

A typo in one of the footnotes said of Variable Bit Rate encoding “there’s no good reason to now use it” which is the exact opposite of what I meant, which is “there’s no good reason to not use it.”

 

Footnotes:

1All the files on this page are MP3s. This has become the format of choice for many mainstream download stores (including ClassicsOnline and Amazon) because it works on almost everything. Many of the same basic principles apply to both AAC (used by iTunes) and Ogg Vorbis (used by Spotify). These are both more sophisticated formats that avoid some of the more complex inherent weaknesses of MP3 at the expense of ubiquitous compatibility with all players. In general, either AAC or Ogg Vorbis should sound better than MP3 at a given bit rate, so if getting the best possible sound out of the smallest possible file is a priority for you, I’d suggest you check them out.

2Simple sounds are easier to encode than complicated music, so it tends to be that you only notice that you’re listening to encoded music when something complicated or sudden happens. It’s in these places where the bit rate isn’t high enough. To overcome this, modern MP3 encoders use “variable bit rate” encoding, where a small amount of data is used for the easy bits, and a lot of data is used for the difficult bits. It averages out at the overall target bit rate. I haven’t addressed variable bit rate encoding in the main body of this post because it’s pretty ubiquitous now, and there’s no good reason to not use it.

3This album was produced and engineered by Tim Handley, who has won numerous Grammy awards as a producer.

4iTunes also delivers 256kbps files, but in the AAC format1.

5If that doesn’t sound good to you, then you might like to try theclassicalshop.neteclassical.com or hdtracks.com - all of which will sell you full CD-quality downloads of just about any Naxos record, and many of those from the labels we distribute.

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Categories: Tips

11 Comments

  • Andy Gardiner says:

    Amazon bitrates are only 256kbps…

  • “Hook up your computer to your stereo”!? Wouldn’t any stereo owner who listened to MP3s have done this ages ago? What percentage of MP3 consumers own a stereo system, in the old sense of a component stereo system? And if you need a component stereo system to tell the difference between a 128 and a 320, and you don’t have a component stereo system, why would you care?

    • Andy Doe says:

      People ask us “How do I connect my computer to my stereo?” all the time, and some of the options are pretty complicated, so I’ll take any opportunity to mention the article about it.

      I don’t know what percentage of MP3 consumers owns a traditional stereo system. I don’t have one (although there are lots of speakers in my house). There are, though, lots of people who do own a stereo and don’t download music. If that’s because they love CDs and they’re happy to get their music that way, then of course that’s fine with me. If it’s because they don’t know how to get the music out of their computer, then that’s something I’m well-placed to help them with.

      As for the last question, you’re quite right that there’s no point in owning something you can’t hear. It takes quite an investment of time to encode your whole CD collection, though, and you don’t want to do that twice. It would be a real shame for somebody to hook up their computer to their stereo only to discover they could hear all sorts of encoding artefacts. Better to discover them now, and encode everything at a rate that sounds good to them from the outset.

  • Randall says:

    Great post.. reminds me of your Pepsi / Coke musical challenge from a few years back. It really is all about the juxtaposition of quality VS storage space..

    • Andy Doe says:

      Thanks. It has always been about quality vs. playing time. If LPs went at 78rpm, they’d sound amazing, but not for long*. If we used up a whole C90 cassette in five minutes, it would sound gorgeous. If we were happy with 25-minute CDs, we could double the sample rate and record at 24-bit.

      There have always been compromises. The big difference with digital audio is that it gives you, the user, the opportunity to decide for yourself where you want to draw the line. This is an important point, and I should’ve put it higher up.

      *They’d also wear out quickly, but then if LPs had been designed to turn at 78rpm, they’d probably be made of something else.

  • I’ve had my iMac hooked up to a decent component stereo system for years, and I can tell you, Neil Young and his fellow golden ears notwithstanding, iTunes’ old 128 AAC’s, to take an example of a low-end bit rate, sound JUST DANDY. I’ve done the lossless/lossy CD ripping comparison; the only difference I noticed was the incredible amount of space that was being lost to lossless.

  • Julio says:

    “The big difference with digital audio is that it gives you, the user, the opportunity to decide for yourself where you want to draw the line. This is an important point, and I should’ve put it higher up.”

    Absolutely true in theory, but in practice this hasn’t happened yet.

    I actually ended up viewing this blog, and this post, completely by chance. What set off the string of events is somewhat relevant.

    I had just finished listening to the Toscanini 1953 recording of Dvorak’s 9th symphony on LP. I own it as part of a collection released decades ago by The Franklin Mint called “The 100 Greatest Recordings of all Time”. Looking for a 24 bit / 96 or 192khz download of this piece, I eventually found something from Naxos, but it was only available as a standard mp3. The LP still sounds better, despite its status as a senior citizen.

    The computer audiophile community is growing, and it’s not necessarily limited to people with huge budgets. Good DACs are getting cheaper, as are decent speakers. We are starved for good downloads and it seems that all the content owners are dragging their feet on this.

    It will take a generation before all the individual constraints (storage, bandwidth, DACs, shielding, speakers, etc) are insignificant enough that everything can be listened to at the limits of the best auditory systems. For now, us fussy types will have to endure the subtle (and sometimes not so subtle) taunting from people like Mr. Kattleman.

    • Andy Doe says:

      Julio,

      You can get a full CD-quality (16-bit, 44.1kHz) download of that album here:

      http://www.theclassicalshop.net/Details.aspx?CatalogueNumber=NS%200474

      For independent labels, at least, there are several classical specialist download stores offering lossless and hi-res downloads, and we try to send them the highest-quality masters we have. We handle digital distribution for a number of important audiophile labels, and I can assure you, they’re not dragging their feet. There are, though, some commercial hurdles to overcome.

      Unless something is scheduled for release on SACD or some other high-quality format, we’ll normally record it in 24-bit at a high sample rate, and convert it to 16-bit, 44.1kHz when the album is mastered for CD. To release high resolution digital tracks, we’d have to go back and remaster them from the high resolution source, which would be a big one-off fixed cost.

      When there’s a big enough demand to justify these costs, that’s exactly what we’ll do, but for now the demand is fairly small, and we’d have to charge too much for them.

      In the meantime, we’re looking at ways to cost-effectively bring you the highest-possible quality from our new releases.

      -Andy

  • Cindy Parrill says:

    Andy, I read this blog with great interest because I both rip my Classical CD collection into FLAC but also am tempted to both listen to streaming audio as well as purchase digital classical music online but don’t want to limit my future options by making an inferior choice now.

    I have two questions, You said “I haven’t addressed variable bit rate encoding in the main body of this post because it’s pretty ubiquitous now, and there’s no good reason to now use it.” Can you elaborate? If there is no good reason to use it why is it ubiquitous?

    How does this article and the one on Frequencies relate to a statement like “full CD-quality (16-bit, 44.1kHz)” as you mentioned to your reply to Julio? If I decided that all the frequencies I can hear will be captured in a 192 kbps MP3 and I can’t hear a quality jump between 256kbps and 320kbps then how does that fact relate to (16-bit, 44.1kHz)?

    • Andy Doe says:

      Cindy,

      Your first question highlights a typo in that footnote, now corrected. What I meant to say is there’s no good reason to NOT use VBR encoding. This is why it is pretty much ubiquitous. Sorry for the confusion.

      As to your second question, it’s complicated. There’s more to compression than missing frequencies. A CD and a high-quality MP3 will both give you 16-bit, 44.1kHz playback, but they’ll store the music in different ways. The CD takes a brute-force approach to storage that guarantees the almost exact reproduction of the input. This is the way CD players like to get their data, but on a computer it is inefficient. Silence takes up as much space as complicated music, and that can’t be necessary. The MP3 will try to give you 100% of the audible data in a fraction of the space.

      Rather than looking at the difference between different MP3 bit rates, I’d suggest comparing each bit rate to CD. The thing to do here is to try to tell a CD (or FLAC file) from an MP3 when you don’t know which is which. No peeking. If you can’t tell the difference, then there’s not a lot of point in you storing those big files.

      It’s tempting to think we need to get the best-possible quality now, because we plan to become more discerning listeners over time. That’s a perfectly good approach so long as your hard drive won’t fill up before you next decide to buy a computer.

      In truth, though, our hearing generally gets worse, not better. If you can’t hear the difference on a moderately-priced stereo now, you will probably never be able to hear the difference, even if you buy a much more expensive stereo later.

      I hope this helps,

      -Andy