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Inside the MP3 Codec - Page 10
featured with permission from MP3: The Definitive Guide by Scot Hacker

Notes on Decoding

As noted earlier, the great bulk of the work in the MP3 system as a whole is placed on the encoding process. Since one typically plays files more frequently than one encodes them, this makes sense. Decoders do not need to store or work with a model of human psychoacoustic principles, nor do they require a bit allocation procedure. All the MP3 player has to worry about is examining the bitstream of header and data frames for spectral components and the side information stored alongside them, and then reconstructing this information to create an audio signal. The player is nothing but an (often) fancy interface onto your collection of MP3 files and playlists and your sound card, encapsulating the relatively straightforward rules of decoding the MP3 bitstream format.

While there are measurable differences in the efficiency-and audible differences in the quality-of various MP3 decoders, the differences are largely negligible on computer hardware manufactured in the last few years. That's not to say that decoders just sit in the background consuming no resources. In fact, on some machines and some operating systems you'll notice a slight (or even pronounced) sluggishness in other operations while your player is running. This is particularly true on operating systems that don't feature a finely grained threading model, such as MacOS and most versions of Windows. Linux and, to an even greater extent, BeOS are largely exempt from MP3 skipping problems, given decent hardware. And of course, if you're listening to MP3 audio streamed over the Internet, you'll get skipping problems if you don't have enough bandwidth to handle the bitrate/sampling frequency of the stream.

Some MP3 decoders chew up more CPU time than others, but the differences between them in terms of efficiency are not as great as the differences between their feature sets, or between the efficiency of various encoders. Choosing an MP3 player becomes a question of cost, extensibility, audio quality, and appearance. That's still a lot to consider, but at least you don't have to worry much about benchmarking the hundreds of players available on the market (unless you've got a really slow machine).

NOTE

If you're a stickler for audio quality, you've probably got a decent to excellent sound card already. However, if you've got an older sound card (such as a SoundBlaster 16) and a slower CPU (slower than a Pentium 133), be aware that the " look ahead" buffer in the MP3 player can easily become exhausted, which will result in an audible degradation of sound quality. However, sticking a better sound card (such as a SoundBlaster 64) in the same machine may eliminate these artifacts, since better sound cards perform more of the critical math in their own hardware, rather than burdening the computer's CPU with it.

 

While this situation won't affect many modern geeks, there's an easy way to test your equipment to determine if its lack of speed is affecting audio quality: Just pick a favorite MP3 file and decode it to a noncompressed format such as WAV, then listen to the MP3 and the WAV side-by-side. If the WAV version sounds better, you'll know that your machine isn't up to the MP3 playback task, since the uncompressed version requires very little processing power to play .

 

Next:  The Anatomy of an MP3 File

 

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