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MP3 files are encoded using what's known as Lossy Data Compression. First of all, Data Compression is not at all like the Audio Compression that we've been talking about so far in this course. Data Compression means decreasing the number of bits in a digital word to make the file smaller. MP3 encoding does this in a lossy manner which means that it literally throws away certain audio information that the encoder thinks isn't important and won't be missed. It's almost like letting the air out of a bicycle tire. Still the same tire, but it's a lot smaller.
Of course, if we compare an MP3 file to its original non-data compressed source file, we can usually hear a difference. That's why the following information and parameter settings are so important, so you can get the best-sounding MP3 file that's sounds as close to the uncompressed source files can be. Regardless of the encoder, there's one parameter that matters the most in determining the quality of the encode, and that's Bit Rate, which is the number of bits of the encoded data that are used to represent each second of audio.
Lossy encoders like MP3 provide a number of different options for its Bit Rate. Typically, the rate shows the number between 128 and 320kbps. By contrast, uncompressed audio is stored on a compact disc has a Bit Rate of about 1400. MP3 files encoded with lower Bit Rate will result in a smaller file and therefore download faster, but they generally playback at lower quality. With the Bit Rate too low, compression artifacts or sounds that were not present in the original recording may appear in reproduction.
A good demonstration of compression artifacts provided by the sound of applause which is hard to data compress, because it's so random. (audio playing) As a result, the failings of an encoder are more obvious and become audible as a slight ringing. (audio playing) Conversely, a high bit rate encode will almost always produce a better sounding file.
(audio playing) It also results in larger file which may take an unacceptable amount of storage space or time to download. In these days of seemingly unlimited storage and widespread high-speed Internet, that's becoming less and less of a factor. 128kbps has lowest acceptable Bit Rate, but may have marginal quality depending upon the encoder. This results in some artifacts, but a small file size. 160kbps is the lowest Bit Rate considered usable for a high-quality file.
320kbps revise the highest quality and may even be indistinguishable from a CD. There are three modes that are coupled to Bit Rate that have a bearing on the final sound quality of the encode. Constant Bit Rate mode, or CBR, maintains a steady Bit Rate regardless of the complexity of the program. CBR mode usually provides the lowest quality encode, but the file size is very predictable. Average Bit Rate mode, or ABR, varies the Bit Rate around the specified target Bit Rate.
Variable Bit Rate mode, or VBR, maintains a constant quality while raising and lowering the Bit Rate depending upon how complex the program. Size is less predictable than with ABR, but the quality is usually better. At a given Bit Rate range, VBR will provide higher quality than ABR which will provide higher quality than CBR. The exception to this is when you choose the highest possible Bit Rate of 320kb where depending upon the encoder the mode may have little bearing on the final sound quality.
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