Introducing Digital Audio
| The advent of digital audio enables anyone with a personal computer to make reasonably acceptable (and, depending on the format, bit-for-bit perfect) digital copies of songs from compact discs, and to store these copies on their computer's hard disk. Digital audio also lets any PC user with an Internet connection trade songs with other users, and download copies of songs that other users have made. In addition, digital audio lets users download their favorite songs to portable audio players and listen to them while they're away from the computers. It's ultimate user control over the music you listen to. In the previous chapter I explained how music and movies are now recorded digitally. There are many different ways to make a digital recording, which results in many different file formats for digital audio. Of all the available file formats, the one that has enjoyed almost universal popularity is the MP3 format. That's because MP3 was the first widely accepted format that combined good quality sound with reasonably small files. But MP3 isn't the only digital audio file format in use today. Microsoft is waging a strong campaign for its Windows Media Audio (WMA) format, which offers similar quality to MP3 at half the file size. I'll look at both these formats MP3 and WMA in the following sections. MP3 AudioMP3 is a digital audio file format that compresses music to fit within reasonably sized computer files while maintaining near-CD quality sound. A typical three-minute song in MP3 format takes up about 3MB of disk space. It's not an exaggeration to say that MP3 took the music industry by storm. Music fans love MP3 even though most record companies (and many artists) despise it. In particular, they fear that unauthorized copying of their music will deprive them of revenues from traditional CD sales. Whether or not that's the case, MP3 has quickly become the standard for music on the Web and there's no stopping it. Developed (and patented) by Thomson Multimedia and the Fraunhofer Institute, the MP3 digital audio format is an extension of Motion Picture Experts Group (MPEG) Layer 3 technology. (The MP3 file extension is short for MPEG Layer 3.) MP3 and other MPEG formats store music digitally, and in the process compress the original data to take up less space than it did originally. MP3's data compression reduces digital sound files by about a 12:1 ratio. Besides compressing the data in an audio file, MP3 also lets you choose the rate at which the original music is sampled. The lower the sampling rate, the smaller the filer size and the lower the sound quality. As an example, normal compact discs sample music at a 44.1KHz rate. In other words, the music is sampled, digitally, 44,100 times per second. Each sample is 16 bits long. When you multiply the sampling rate by the sample size and the number of channels (two for stereo), you end up with a bit rate. For CDs, you multiply 44,100 x 16 x 2, and end up with 1,400,000 bits per second (1,400Kbps). The space taken up by these bits can add up quickly. If you take a typical three-minute song recorded at 44.1KHz, you end up using 32MB of disk space. Although that song can easily fit on a 650MB CD, it's way too big to download over a 56Kbps dial-up Internet connection. The appeal of the MP3 format is that it combines compression with lower bit-rate sampling. By recording at lower bit rates, you get smaller files and faster downloads. For example, if you choose to encode an MP3 file at a 128Kbps bit rate, that same 32MB three-minute song will compress to just 3MB of storage. That's small enough to download easily, even on a dial-up connection. The problem with shrinking files to this degree, of course, is that the sound quality suffers. The lower the bit rate, the worse the sound. The higher the bit rate, the bigger the file. It's a trade-off. When you're recording an MP3 file, which bit rate should you use? Table 15.1 shows some of the more popular bit rates available, and describes the sound quality of each.
If you're encoding MP3 files for use at your computer or with a portable MP3 player, the 128Kbps rate is probably good enough quality. If you're encoding MP3 files for playback on a high-fidelity audio system, you'll probably want to move up to at least a 256Kbps bit rate. (Sound quality is highly subjective, as you can imagine some "golden-eared" audiophiles like myself find it difficult to listen to MP3s recorded at any bit rate!) Windows Media AudioTIP Another popular digital audio format is the AAC format, which Apple uses for music sold in its iTunes Music Store. AAC is also the native format for playback on its wildly popular iPod portable audio players. The problem is, WMP won't play AAC file and iPods won't play WMA files. So if you have an iPod, you're locked out of the WMA/WMP universe, and if you want to use WMP (or have a lot of WMA-format files), you're locked out of the Apple iPod/iTunes universe. If this annoys you, you're not alone; blame it on stubborn corporate incompatibility. As I mentioned earlier, several competing audio formats have been introduced with the goal of providing listeners with either better sound quality or smaller file size (or both) than the current MP3 format. Microsoft's entrée in this competition is Windows Media Audio (WMA for short). WMA files offer similar sound quality as MP3 files, but at about half the size. (Or so Microsoft claims….) WMA definitely offers better copyright protection than the MP3 format, which is a big issue for the music industry. Unlike MP3 files, the copyright holder can use digital rights management (DRM) technology to protect WMA files from unlawful distribution, copying, and sharing. (No more downloading of your favorite songs for free, anyway.) The way WMA copy protection works is that each file you download from an official site has a license. When you use Windows Media Player to copy protected content from your computer to another computer, the license is copied along with the song. When you try to play the copy-protected song on another computer, it won't play; you can only play DRM-protected songs on the original computer or device. Fortunately, most DRM schemes let you copy protected files to a limited number of portable devices, and to burn the files to a limited number of CDs. And, for what it's worth, DRM works invisibly, in the background until you exceed the license restrictions, that is. Digital rights management aside, when you're making WMA recordings, you have a similar choice of bit rate as you do with MP3 files. The higher the bit rate, the higher the sound quality and the larger the file size. Lower the bit rate and you lower the sound quality, but you also reduce the necessary disk space. WMA files can be encoded at lower bit rates than MP3 files, with comparable sound quality. Table 15.2 (next page) compares bit-rate quality between the two formats.
TIP Use lossless WMA files to rip your CD collection to hard disk for playback on a home audio or home theater system. The sound quality will be identical to the original CDs. As with the MP3 format, you can really hear the difference between the different WMA bit rates. A 48Kbps recording sounds a lot like AM radio, while a 192Kbps recording is near CD-quality. Pick something in the middle, like 96 or 128Kbps, and you get a recording that sounds like decent FM radio. And, starting with WMP 9, the WMA format offers two interesting and sometimes useful variations. The first type of new WMA files feature variable bit-rate recording, which lets you cut down on file size during low-fidelity sections of music, or ramp up the bit rate during high-quality sections. The second type of new WMA files utilize so-called lossless compression, which maintain the original sound quality while still compressing the file size although not nearly as much as with traditional WMA encoding. Both options are available from within WMP. MP3 Versus WMA: Which Format Is Best?Microsoft would like you to think that WMA is a better format than MP3 because it delivers similar quality at half the file size. Or, looking at it another way, you can get better sound quality out of similar-sized files. After several years of living with the WMA format, I definitely prefer WMAs to MP3s; the difference is subtle, but it's there. Of course, you also have to take into account the popularity factor. Most song files that are being traded today are in MP3 format. Millions of users can't be wrong. MP3 is the king, and that's just the way it is. Fortunately, Windows Media Player can play back both MP3 and WMA-format files. So whichever way you go, WMP has you covered! |
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