Bringing Yourself Up to Speed with AAC, MP3, and Digital Audio - Choose Between CBR and VBR for MP3

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After choosing the bitrate at which to encode your MP3 files, you need to choose between constant bitrate (CBR) and variable bitrate (VBR).

NOTE:  This choice doesn’t apply to AAC encoding.

CBR simply records each part of the file at the specified bitrate. CBR files can sound great, particularly at higher bitrates, but generally, VBR delivers better quality than CBR. This is because VBR can allocate space more intelligently as the audio needs it. For example, a complex passage of a song will require more data to represent it accurately than will a simple passage, which in turn will require more data than the two seconds of silence before the massed guitars come crashing back in.

The disadvantage to VBR, and the reason why most MP3 encoders are set to use CBR by default, is that many older decoders and hardware devices can’t play them. If you’re using iTunes and an iPod, you won’t need to worry about this. But if you’re using an older decoder or hardware device, you may need to check that it can manage VBR.

Choose Between Normal Stereo and Joint Stereo for MP3

As you’ll know if you’ve browsed through ancient vinyl records, early recordings used mono— a single channel that didn’t deliver any separation among the sounds. On a song recorded in mono, all the instruments sound as though they’re located in the same place. Many mono recordings made by competent recording engineers sound pretty good, but the effect is very different from listening to a live band. In most cases, you’ll want to stick with stereo, which produces the effect of different instruments sounding as though they’re in different locations. But if your sound source is mono (for example, a live recording with a single microphone), stick with mono when encoding the audio.

Once you’ve decided to use stereo, your next choice is between normal stereo and joint stereo. If you haven’t met these terms before, don’t worry: the last few conventional forms of consumer audio—LPs, cassette tapes (including digital audio tapes, or DATs), and CDs—essentially removed this choice from you, so you didn’t need to worry about it. But computer audio, and specifically MP3, has reequipped you with this choice. So it’s a good idea to understand your options and use them as necessary—even if that means simply choosing the best setting for your needs and sticking with it through thick and thin.

Stereo delivers two channels: a left channel and a right channel. These two channels provide positional audio, enabling recording, and mixing engineers to separate the audio so that different sounds appear to be coming from different places. For example, the engineer can make one guitar sound as though it’s positioned on the left and another guitar sound as though it’s positioned on the right. Or the engineer might fade a sound from left to right so it seems to go across the listener.

NOTE:  Surround sound goes much further than stereo, enabling the sound engineer to make sounds seem like they’re behind you, moving through your mouth from molar to incisor, and so on. MP3 doesn’t support surround sound, but AAC does.

Normal stereo (sometimes called plain stereo) uses two tracks: one for the left stereo channel and another for the right stereo channel. As its name suggests, normal stereo is the normal form of stereo. For example, if you buy a CD that’s recorded in stereo and play it back through your boom box, you’re using normal stereo.

Joint stereo (sometimes called mid/side stereo) divides the channel data differently to make better use of a small amount of space. The encoder averages out the two original channels (assuming the sound source is normal stereo) to a mid channel. It then encodes this channel, devoting to it the bulk of the available space assigned by the bitrate. One channel contains the data that’s the same on both channels. The second channel contains the data that’s different on one of the channels. By reducing the channel data to the common data (which takes the bulk of the available space) and the data that’s different on one of the channels (which takes much less space), joint stereo can deliver higher audio quality at the same bitrate as normal stereo.

TIP:  Use joint stereo to produce better-sounding audio when encoding at lower bitrates, and use normal stereo for all your recordings at your preferred bitrate. Where the threshold for lower-bitrate recording falls depends on you. Many people recommend using normal stereo for encoding at bitrates of 160 Kbps and above, and using joint stereo for lower bitrates (128 Kbps and below). Others recommend not using normal stereo below 192 Kbps. Experiment to establish what works for you.

The results you get with joint stereo depend on the quality of the MP3 encoder you use. Some of the less capable MP3 encoders produce joint-stereo tracks that sound more like mono tracks than like normal-stereo tracks. Better encoders produce joint-stereo tracks that sound very close to normal-stereo tracks. iTunes produces pretty good joint-stereo tracks.

Using the same MP3 encoder, normal stereo delivers better sound quality than joint stereo—at high bitrates. At lower bitrates, joint stereo delivers better sound quality than normal stereo, because joint stereo can retain more data about the basic sound (in the mid channel) than normal stereo can retain about the sound in its two separate channels. However, joint stereo provides less separation between the left and right channels than normal stereo provides. (The lack of separation is what produces the mono-like effect.)

Load Uncompressed Files on Your iPod for Unbeatable Sound Quality

To get the ultimate music quality, you can load uncompressed audio files—either WAV files or AIFF files—on your iPod. If you rip these audio files directly from the CD at full quality, you’ll have truly CD-quality files. (You can also create lower-quality WAV files or AIFF files if you choose.) Such files provide the equivalent of carrying your CDs with you on your iPod.

The disadvantages to loading uncompressed files on your iPod are as follows:

• You can’t fit as much music on your iPod. You’ll be able to fit only about 60 full-length CDs on a 40GB iPod—or 6 full-length CDs on a 4GB iPod mini.

• You’ll wear out your iPod’s battery much more quickly. This is because the uncompressed files will be too large for your iPod’s memory to cache effectively, so the hard disk will be used much more often than if you’re playing smaller files.

This is chapter three of How to Do Everything with Your iPod & iPod Mini, by Guy Hart-Davis (McGraw-Hill/Osborne, ISBN 0072254521, 2004). Check it out at your favorite bookstore today. Buy this book now.

Next: Copyright Law for Digital Audiophiles >>
 

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