MP3 is a file format, used to encode audio, that's been with us since 1994. It was specifically created to be able to send audio files across the Internet. While it's possible to send "full bandwidth" audio (like a WAV or AIFF file) across the Internet, it's usually not very convenient because of the file size. MP3 compression reduces the size of the file by a ratio of approximately 10 to 1. In other words, MP3 files are 10 times smaller. The way this is done is with algorithms that discard a lot of the data in the audio signal that's in the upper ranges of human hearing.
MP3s can be encoded in a variety of different ways. The 3 main parameters you can adjust are:
1. mono/stereo
2. sample rate
3. bit rate (expressed in kbps)
Most programs that encode MP3s have presets that enable you to choose common combinations of these settings. For example, Apple's iTunes software gives you a choice of 128, 160 or 192kbps. Usually, for a product with someone speaking (like most info products), the 128 setting works just fine, while the higher settings would be used for encoding music. Sometimes you have constraints on how big a file can be (for example, your shopping cart may have a file size limit on downloadable products). In this case, you will have to go into the custom encoding settings in your MP3 encoder and do "trial and error" until your audio file is compressed down to an acceptable size. In this situation, you'll want to make sure you're encoding in mono, since that will cut your file size in half.
Basically, the rule of thumb is that the higher the bit rate, the bigger the file and the better the quality. The converse is the lower the bit rate, the smaller the file and the lower the quality. You're looking for a "happy medium" between quality and file size.
When working with audio, it's always a good idea to work with the highest quality format possible and then convert to MP3 as the last step in the process. This way, you can preserve your full-bandwidth recording for formats that support it, like audio CDs.
MP3 is considered a "lossy" compression format, which means that data is discarded in order to make the MP3. Once the data is discarded, you can't get it back. In other words, it's a "one-way" conversion. You can have a "full bandwidth" format like WAV that you convert to an MP3, but if you were to take that MP3 and convert it back to a WAV, the quality would be no better than the MP3 that it was converted from.
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