CompTIA A_ Certification All-In-One Exam Guide, Seventh Edition - Michael Meyers [345]
The number of characteristics of a particular sound captured during sampling is measured by the bit depth of the sample, the number of bits used to describe the characteristics of a sound. The greater the bit depth used to capture a sample, the more characteristics of that sound can be stored and thus re-created. An 8-bit sample of a Jimi Hendrix guitar solo, for example, captures 28 (256) characteristics of that sound per sample. It would sound like a cheap recording of a recording, perhaps a little flat and thin. A 16-bit sample, in contrast, captures 216 (65,536) different characteristics of his solo and reproduces all the fuzzy overtones and feedback that gave Hendrix his unique sound.
The last aspect of sound capture is the number of tracks of sound you capture. Most commonly, you can capture either a single track (monaural) or two tracks (stereo). More advanced captures record many more sound tracks, but that’s a topic for a more advanced sound capture discussion.
The combination of sampling frequency and bit depth determines how faithfully a digital version of a sound captures what your ear would hear. A sound capture is considered CD quality when recorded at 44.1 KHz, with 16-bit depth and in stereo. Most recording programs let you set these values before you begin recording. Figure 20-1 shows the configuration settings for the Windows Sound Recorder.
Figure 20-1 Sound Recorder settings
Hey, wait a minute! Did you notice the Format setting in Figure 20-1? What’s that? You can save those sampled sounds in lots of different ways—and that’s where the term format comes into play.
Recorded Sound Formats
The granddaddy of all sound formats is pulse code modulation (PCM). PCM was developed in the 1960s to carry telephone calls over the first digital lines. With just a few minor changes to allow for use in PCs, the PCM format is still alive and well, although it’s better known as the WAV format so common in the PC world. WAV files are great for storing faithfully recorded sounds and music, but they do so at a price. WAV files can be huge, especially when sampled at high frequency and depth. A 4-minute song at 44.1 KHz and 16-bit stereo, for example, weighs in at a whopping 40-plus MB!
What’s interesting about sound quality is that the human ear cannot perceive anywhere near the subtle variations of sound recorded at 44.1 KHz and 16-bit stereo. Clever programmers have written algorithms to store full-quality WAV files as compressed files, discarding unnecessary audio qualities of that file. These algorithms—really nothing more than a series of instructions in code—are called compressor/decompressor programs or, more simply, codecs. The most famous of the codecs is the Fraunhoffer MPEG-1 Layer 3 codec, more often called by its file extension, MP3.
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NOTE Using MP3 compression, you can shrink a WAV file by a factor of 12 without losing much sound quality. When you compress a WAV file into an MP3 file, the key decision is the bit rate. The bit rate is the amount of information (number of bits) transferred from the compressed file to the MP3 decoder in 1 second. The higher the bit rate of an MP3 file, the higher the sound quality. The bit rate of MP3 audio files is commonly measured in thousands of bits per second, abbreviated Kbps. Most MP3 encoders support a range of bit rates from 24 Kbps up to 320 Kbps (or 320,000 bits per second). A CD-quality MP3 bit rate is 128 Kbps.
WAV and MP3 are only two among a large number of file formats for sound. Not all sound players can play all of these formats; however, many sound formats are nothing more than some type of compressed WAV file, so with the right codec loaded, you can play most sound formats.
MIDI
Every sound card can produce sounds in addition to playing prerecorded sound files. Every sound card comes with a second processor designed to interpret standardized