The difference between lossy and lossless compression is a critical one which deserves to be better understood. We often have clients asking what the best format to send their images, audio, or video in. The answer, however, is not so simple. Virtually any format is fine, just as long as it does not use lossy compression!
Take image files as an example. Maybe you have an original Photoshop file. Photoshop files take up less space on disk than they do in your computer memory because they are compressed. But they are compressed losslessly, so every single bit of information is there again when you open the file. You can save and reopen the file as many times as you want, and it will still look as good as it did when you started.
But if you save that same file in the JPEG image format and look at it again, it looks different. Not quite as good. But it takes up far, far less space than the Photoshop file did, which is ideal for images on the Web. However, if you send us a JPEG file to work with, we are at a disadvantage, because we do not have the high-quality original material to work with. JPEG files use lossy compression, throwing away some of the image detail so that it is a much smaller image, but it’s a one way street. If you keep saving and reopening that JPEG image, it will eventually become unrecognizable!
The same holds true for all types of media. iPods are often maligned for sounding “worse than cassette tapes,” but that is because most people put MP3 files on their iPod. Like the JPEG format, MP3 is a lossy compression format, and depending upon your compression settings, they can sound pretty terrible. They sound terrible by default when iTunes imports them, because iTunes tries to compress audio into very small files. But you don’t have to let iTunes compress them that much! iTunes offers both uncompressed audio (AIFF) and losslessly compressed audio (Apple Lossless) formats.
But Apple’s reason for using the MP3 format, despite its poor audio quality, is because you can fit perhaps 10 or 20 times the number of files on your computer and your iPod with lossless compression, and for most people it’s worth it. Not to mention that if a file is 10 or 20 times bigger, your iPod’s battery will likely wear down a lot faster too!
There is a time and a place for both of these kinds of compression, but it is important to know the difference, and to make your choice accordingly.