Intro

In computers, sound is pressure variations that are represented by a continuous waveform with peaks and valleys that vary from the average pressure level.

The degree of deviation is the amplitude (measured in units of pressure or decibels) and is perceived as the volume of the sound.

The number of peaks in a period of time is the audio frequency (measured in cycles per second aka hertz (Hz)) and is perceived as the pitch of the sound.   The human ear can detect frequencies from 50 to 20,000 Hz. The human voice usually falls between 300 to 3,000 Hz. A sound is typically a complicated mix of super imposed amplitudes and frequencies

The number of waveform samples taken in a period of time is the sampling rate (measured in Hz) and is perceived as the quality of the recording. EGs: The sampling rate for the telephone company is 8 KHz, but CDs (CDDAs) and DVDs sample at 44.1 or 44.8 KHz, while professional recordings are 88.2 KHz, 96 KHz, and 192 KHz. DSD (Direct-Stream Digital) can supposedly sample at 2.8224 MHz.

The number of bits used to represent the waveform is the sample size (measured in bits (b)) and is also perceived as the quality of the recording, since a greater sample size can capture a greater dynamic range (perceptible differences and extremes in sound). Voice cards are usually 4 or 8 bits, whereas sound cards are usually 8 bits or 16 bits or more.

The sampling rate times the sample size is the transmission rate (measured in b/s or Kb/s). The transmission rate times the duration of the recording is the size of the sound file (measured in b). EG: (60 seconds of recording) x (8 bit sample size) x (8,000 Hz) = 3,840,000 b = 480,000 B = roughly 0.5 MB.

There are several audio codecs or amplitude conversions schemes to compress sound files including linear PCM (Pulse Code Modulation), non-linear PCM, and ADPCM (Adaptive Differential PCM). EG: WAV files typically use linear PCM. Non-linear PCM uses more bits to represent the easier to hear lower amplitudes. Micron-law PCM is a non-linear PCM common in the US and Japan, whereas A-law PCM is common elsewhere. A common ADPCM scheme is OKI ADPCM.

Sound Files

Sound is encoded into digital format via a codec program. Encoding also frequently also compressed as either lossy or lossless. A codec produces the essence (digital sound itself) plus some metadata (synching, titles, length of file, etc.). The codec file is encapsulated/stored/wrapped in an audio file format. Some file formats store only one kind of codec while others take more. Some "super" file formats (ASF, Ogg, and Quicktime) store a wide variety of video or audio codecs.

Some audio codecs:

Some audio file formats:

Web Sounds

Here are tips on how to put some sound in web pages. [I've hidden this info here instead of in my section on webs because so far most people make hideous sites when they add sound.]

Microsoft Internet Explorer accepts two non-W3C tags for automatically playing sounds in the background. Both must be enclosed by the HTML <head> tag:

<bgsound src="sound/SomeSound.mid"
         loop="2" />
<!-- src can only be wav, au, or mid. If LOOP is -1 or INFINITE, then it loops forever. / --> 
<embed src="sound/SomeSound.mid"
       hidden="true"
       loop="false"
       playcount="1"
/>
<!-- Here are some notes about embed:
Only the attributes HEIGHT, SRC and WIDTH are always available.
HIDDEN="true"|"false" The default is "false".
WIDTH="n"
HEIGHT="n"
LOOP="true"|"false"|"n" Supported only by NS.
PLAYCOUNT="n" Supported only by IE. 
AUTOSTART="true"|"false" The IE default is "false" while the NS default is "true".
The default on the Mac is "false" for both browsers.

Netscape accepts just the <embed> method. If you want to be both IE and NS compatible, use both <bgsound> and <embed> but enclose the <embed> tag in an <object> tag so IE ignores it and doesn't play the sound file twice.

<bgsound src="sound/SomeSound.mid" />
<object>
<embed src="sound/SomeSound.mid"
       hidden="true"
       loop="false" />
</object>

It is of course possible to have sounds play when certain events occur using script. EG:

<embed src="sound/SomeSound.mid"
       hidden="true"
       loop="false"
       name="sndSomeSound"
       id="sndSomeSound"
       autostart="false"
       mastersound
/>
...
<input type="button"
       value="Play sound file"
       onClick="document.sndSomeSound.play(false);"
/>

Sound Cards

A sound card is a physical card that is connected to your computer's motherboard. Make sure your sound card is compatible with the right standards.

Voice cards are usually 4 or 8 bits, whereas sound cards are usually 8 or 16 bits or more.

At the least a sound card should have these ports.

See also GameSpot.com/gshw a guide for gaming hardware.

Links

Links that lead to off-site pages about audio. See also Music#Links.

2007-11-09 21:29:16Z