Audio Hardware - Playing and Creating Digitized Sound Files
(Page 8 of 17 )
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 by the Windows Sound Recorder are shown in Table 16.3.
Table 16.3 Windows Default Sound File Resolutions
Format | Resolution | Frequency | Bandwidth | File Size |
PCM | Telephone quality | 11025Hz | 8-bit mono | 11KBps |
| Radio quality | 22050Hz | 8-bit mono | 22KBps |
| CD quality | 44100Hz | 16-bit stereo | 172KBps |
If you have a sound card that supports DVD-quality (48000Hz, 16-bit stereo, 187KBps), you can also save sounds at that frequency with the Windows Sound Recorder, but you must select that setting manually. Note that the Windows Sound Recorder applet uses the Pulse Code Modulation (PCM) method for storing sounds as its default. PCM produces the highest quality of sound, but because it doesn't use any type of data compression, file sizes can be enormous.
Note - When you use the Windows Sound Recorder, you can reduce the size of the audio files you create by choosing a different format and sampling frequency and selecting stereo or mono recording. If you want to experiment with the results of various formats, save the audio at the highest quality setting using PCM, retrieve the original audio file, change one or more settings, and save the changes under a different name.
As you can see from Table 16.3, 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.
Note - 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 disc 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.
Tip - 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.
Audio Adapter Concepts and Terms To fully understand audio adapters and their functions, you need to understand various concepts and terms. Terms such as 16-bit, CD quality, and MIDI port are just a few. Concepts such as sampling and digital-to-audio conversion (DAC) are often sprinkled throughout stories about new sound products. You've already learned about some of these terms and concepts; the following sections describe many others.
The Nature of Sound To understand an audio adapter, you must understand the nature of sound. Every sound is produced by vibrations that compress air or other substances. These sound waves travel in all directions, expanding in balloon-like fashion from the source of the sound. When these waves reach your ear, they cause vibrations that you perceive as sound.
Two of the basic properties of any sound are its pitch and intensity.
Pitch is the rate at which vibrations are produced. It is measured in the number of hertz (Hz), or cycles per second. One cycle is a complete vibration back and forth. The number of Hz is the frequency of the tone; the higher the frequency, the higher the pitch.
Humans can't hear all possible frequencies. Very few people can hear sounds with frequencies less than 16Hz or greater than about 20KHz (kilohertz; 1KHz equals 1000Hz). In fact, the lowest note on a piano has a frequency of 27Hz, and the highest note has a frequency a little higher than 4KHz. Frequency-modulation (FM) radio stations can broadcast notes with frequencies as high as 15KHz.
The amazing compression ratios possible with MP3 files, compared to regular CD-quality WAV files, is due in part to the discarding of sound frequencies that are higher or lower than normal hearing range during the ripping process.
The intensity of a sound is called its amplitude. This intensity determines the sound's volume and depends on the strength of the vibrations producing the sound. A piano string, for example, vibrates gently when the key is struck softly. The string swings back and forth in a narrow arc, and the tone it sends out is soft. If the key is struck more forcefully, however, the string swings back and forth in a wider arc, producing a greater amplitude and a greater volume. The loudness of sounds is measured in decibels (db). The rustle of leaves is rated at 20db, average street noise at 70db, and nearby thunder at 120db.
Evaluating the Quality of Your Audio Adapter The quality of an audio adapter is often measured by three criteria: frequency response (or range), total harmonic distortion, and signal-to-noise ratio.
The frequency response of an audio adapter is the range in which an audio system can record or play at a constant and audible amplitude level. Many cards support 30Hz–20KHz. The wider the spread, the better the adapter.
The total harmonic distortion measures an audio adapter's linearity and the straightness of a frequency response curve. In layman's terms, the harmonic distortion is a measure of accurate sound reproduction. Any nonlinear elements cause distortion in the form of harmonics. The smaller the percentage of distortion, the better. This harmonic distortion factor might make the difference between cards that use the same audio chipset. Cards with cheaper components might have greater distortion, making them produce poorer-quality sound.
The signal-to-noise ratio (S/N or SNR) measures the strength of the sound signal relative to background noise (hiss). The higher the number (measured in decibels), the better the sound quality. For example, the top-of-the-line Sound Blaster Audigy 2 sound card features an SNR of 106db, whereas the older Sound Blaster Audigy is rated at 100db and the AWE64 series has an SNR of 90db.
These factors affect all types of audio adapter use, from WAV file playback to speech recognition. Keep in mind that low-quality microphones and speakers can degrade the performance of a high-quality sound card.
 | This chapter is from Upgrading and Repairing PCs, 16th edition,by Scott Mueller. (Que Books, 2004, ISBN: 0789731738). Check it out at your favorite bookstore today. Buy this book now! |
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