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Chapter 2: The Digital Representation of Sound,
OK, now we know about bits and how to copy them from one place to another. But where exactly do we put all these bits in the first place?
One familiar digital storage medium is the compact disc (CD). Bits are encoded on a CD as a series of pits in a metallic surface. The length of a pit corresponds to the state of the bit (on or off). As weve seen, it takes a lot of bits to store high-quality digital audioa standard 74-minute CD can have more than 6 billion pits on it!
Putting Everything Together
Now that we know about sampling rates, bit width, number systems, and a lot of other stuff, how about a nice practical example that ties it all together?
Assume were composers working in a digital medium. Weve got some cool sounds, and we want to store them. We need to figure out how much storage we need.
Lets assume were working with a stereo (two independent channels of sound) signal and 16-bit samples. Well use a sampling rate of 44,100 times/second.
One 16-bit sample takes 2 bytes of storage space (remember that 8 bits equal 1 byte). Since were in stereo, we need to double that number (theres one sample for each channel) to 4 bytes per sample. For each second of sound, we will record 44,100 four-byte stereo samples, giving us a data rate of 176.4 kilobytes (176,400 bytes) per second.
Lets review this, because we know it can get a bit complicated. There are 60 seconds in a minute, so 1 minute of high-quality stereo digital sound takes 176.4 * 60 KB or 10.584 megabytes (10,584 KB) of storage space. In order to store 1 hour of stereo sound at this sampling rate and resolution, we need 60 * 10.584 MB, or about 600 MB. This is more or less the amount of sound information on a standard commercial audio CD (actually, it can store closer to 80 minutes comfortably). One gigabyte is equal to 1,000 megabytes, so a standard CD is around two-thirds of a gigabyte.
One good rule of thumb is that CD-quality sound currently requires about 10 megabytes per minute.
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