Compressed Versus Uncompressed
Digital audio can be split into two types of formats, compressed and uncompressed.
Uncompressed formats like CD Audio, WAV, or AIFF are pretty much perfect representations of the audio that was recorded. If you convert something from one uncompressed format to another, it's a completely perfect copy. So, for instance, if you rip audio off a CD into WAV format on your computer, then burn another CD using the WAV you created, those two CDs will be virtually identical, despite the fact that you copied it multiple times.
The only problem with uncompressed formats is that they're huge. If all the audio in your 4 Gig iPod Nano were in an uncompressed format, you'd only be able to store about 4 or 5 albums.
Compressed formats are those that cut corners in order to keep the file size relatively small. The most famous type of compressed format is MP3, but there are a ton of others, including WMA (what Windows Media Player uses) and AAC (popular in the iTunes music store). A program encoding audio into an MP3 for instance, will get rid of some high frequencies it thinks are mostly out of the range of human hearing. It also perform a lot more complicated calculations, but that example is one of the simplest. The advantage to these types of files is that they don't take up nearly as much space as an uncompressed file.
Why we ask you for uncompressed files before you convert them into MP2
To the average listener, there isn't much difference in the sound quality between high quality compressed and uncompressed formats. Unfortunately, every time an audio file is converted into a compressed format, it's not a perfect copy and it loses information. That means, just like making a copy of a copy on a Xerox machine, the quality of the audio will rapidly deteriorate as it get converted again and again. It doesn't matter if the audio is compressed in an extremely high bitrate, you're still losing information on the audio rapidly.
Say you have an MP3 (compressed format) of your radio show that you created using your audio editing software. In order to get it onto PRX, you convert that file into an MP2 (another, albeit lightly, compressed format). A station buys and downloads your piece, then broadcasts it on their Internet stream. Since the software for their stream is based on MP3 compression, the file is compressed yet AGAIN. By the time listeners actually hear the piece, the audio has been compressed at least three times, and at this stage the drop in quality is immediately noticeable to even an untrained ear.
How much a station cares about this drop in quality varies, but it's always a good idea to avoid compressing your audio if you have the choice. So, for these reasons and because its the industry standard for public radio, we have you upload MP2 audio.