Playing and Creating Digitized Sound Files
You can use two basic types of files to store audio on your PC. One type is generically called a sound file and uses formats such as WAV, VOC, AU, and AIFF. Sound files contain waveform data, which means they are analog audio recordings that have been digitized for storage on a computer. Just as you can store graphic images at different resolutions, you can have sound files that use various resolutions, trading off sound quality for file size. The default sound resolution levels used in Windows are shown in Table 16.2.
Table 16.2. Windows Default Sound File Resolutions|
PCM | Telephone quality | 11,025Hz | 8-bit mono | 11KBps | | | Radio quality | 22,050Hz | 8-bit mono | 22KBps | | | CD quality | 44,100Hz | 16-bit stereo | 172KBps |
If you have a sound card that supports DVD-quality (48,000Hz, 16-bit stereo, 187KBps), you can also save sounds at that frequency, but you must select it manually if you are using the Windows Sound Recorder to digitize sounds. Note that the Windows Sound Recorder applet uses the default Pulse Code Modulation (PCM) method for storing sounds. PCM produces the highest quality of sound, but because it doesn't use any type of data compression, file sizes can be enormous.
As you can see, the difference in file sizes between the highest and lowest audio resolution levels is substantial. CD-quality sound files can occupy enormous amounts of disk space. At this rate, just 60 seconds of audio would require more than 10MB of storage. For applications that don't require or benefit from such high resolution, such as voice annotation, telephone-quality audio is sufficient and generates much smaller files. To achieve a balance between high quality and smaller file sizes, you can convert conventional WAV files into compressed formats, such as MP3 or WMA audio files.
The other type of file is a MIDI file, which consists of a musical score that is played back by synthesized or sampled musical instruments incorporated into the sound card's MIDI support.
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To learn more about the differences between MP3, WMA, and MIDI files, see "Audio Compression and MIDI Files" in the Technical Reference located on the DVD-ROM packaged with this book. |
On a multimedia PC, it is often possible for two or more sound sources to require the services of the audio adapter at the same time. Any time you have multiple sound sources you want to play through a single set of speakers, a mixer is necessary.
Most audio adapters include a mixer that enables all the different audio sources, MIDI, WAV, line in, and CD to use the single line-out jack. Starting with Windows 95 through the latest Windows versions (XP Pro/XP Home), Windows uses a single mixer for both recording and playback features, instead of using separate mixers as with Windows 3.x. Normally, the adapter ships with software that displays visual sliders like you would see on an actual audio mixer in a recording studio. With these controls, you can set the relative volume of each of the sound sources.
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Whenever you change from analog to digital speakers or add speakers to a two-speaker configuration, you must adjust the mixer controls to match your current speaker configuration. If you don't, you will be unable to hear anything through your speakers. |
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