Music File Sizes
Hi Fred, I've been a Plus subscriber for some time and learn something new from each newsletter.
I am thinking about trying buying music downloads and don't know much. Walmart seems to be a decent choice, at least to start with. Walmart does not charge a subscription fee, only per song charge. But looking at the site I see that the music downloads are in the 4-5Meg for a song. On the original CD the same song is much larger. How is this done without loosing quality?
Bye, Ken
Music on CDs is usually stored as uncompressed WAV files: In the recoding studio, the live music is "sampled" at a very high rate, taking a series of rapid-fire audio "snapshots" of the music. These are converted into the familiar ones and zeros of a WAV file. Just as a movie creates the illusion of smooth motion from a series of still pictures, playback of digitized music creates the illusion of smooth sound from the series of samples. (But movies typically run at 28 frames per second; music is sampled at many tens of thousands of times per second. Digitized music is much "smoother" to the ear than movies are to the eye.)
In the WAV format, 10 seconds of silence will occupy the same amount of disc space as 10 seconds of the loudest, most raucous head-banging heavy-metal music; or 10 seconds of a full orchestral crescendo. The music doesn't matter, and has no real effect on the file size. It's the sampling rate that matters most in WAV files.
But clearly, 10 seconds of silence doesn't *have* to occupy much space at all, and that's where compression comes in. MP3, WMA, and other formats use various tricks to squeeze the duplicate or unnecessary bits out of the stored music, leaving behind instructions for the playback software to reconstruct the original data.
"Lossless" compression means that all the original data can be reconstructed, bit for bit, giving you a playback that should be indistinguishable from the original, uncompressed recording. The files can be smaller than WAV files, but sound just as good.
"Lossy" compression trades off some fidelity for even smaller file sizes; the more audio fidelity you're willing to give up, the greater the compression can be and the smaller the files can be. At very high compression rates, the music files will be tiny, but the resulting music will be somewhat muddy and muffled.
Microsoft's audio format--- WMA, used by Media Player--- is actually pretty good, offering a wide range of compression from lossless on down. And it also has "variable bit rate" compression that adjusts the compression level on the fly: Musically complex passages may get very little compression, for example, but silence or near-silence may get heavy compression. This provides a nice tradeoff between space-efficiency and audio fidelity. Some competing software now also offers variable bitrate compression.
So, in a nutshell, that's why there's a difference in file sizes: You're probably seeing MP3 and WMA files on the download sites; the files on the actual commercial CDs are in the larger WAV format. If the vendors have done their job right, your ears won't hear the difference.
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