2. Sound Card Technology

This section gives a very cursory overview of computer audio technology, in order to help you understand the concepts used later in the document. You should consult a book on digital audio or digital signal processing in order to learn more.

Sound is an analog property; it can take on any value over a continuous range. Computers are digital; they like to work with discrete values. Sound cards use a device known as an Analog to Digital Converter (A/D or ADC) to convert voltages corresponding to analog sound waves into digital or numeric values which can be stored in memory. Similarly, a Digital to Analog Converter (D/A or DAC) converts numeric values back to an analog voltage which can in turn drive a loudspeaker, producing sound.

The process of analog to digital conversion, known as sampling, introduces some error. Two factors are key in determining how well the sampled signal represents the original. Sampling rate is the number of samples made per unit of time (usually expresses as samples per second or Hertz). A low sampling rate will provide a less accurate representation of the analog signal. Sample size is the range of values used to represent each sample, usually expressed in bits. The larger the sample size, the more accurate the digitized signal will be.

Sound cards commonly use 8 or 16 bit samples at sampling rates from about 4000 to 44,000 samples per second. The samples may also be contain one channel (mono) or two (stereo).

FM Synthesis is an older technique for producing sound. It is based on combining different waveforms (e.g. sine, triangle, square). FM synthesis is simpler to implement in hardware that D/A conversion, but is more difficult to program and less flexible. Many sound cards provide FM synthesis for backward compatibility with older cards and software. Several independent sound generators or voices are usually provided.

Wavetable Synthesis combines the flexibility of D/A conversion with the multiple channel capability of FM synthesis. With this scheme digitized voices can be downloaded into dedicated memory, and then played, combined, and modified with little CPU overhead. State of the art sound cards all support wavetable synthesis.

Most sound cards provide the capability of mixing, combining signals from different input sources and controlling gain levels.

MIDI stands for Musical Instrument Digital Interface, and is a standard hardware and software protocol for allowing musical instruments to communicate with each other. The events sent over a MIDI bus can also be stored as MIDI files for later editing and playback. Many sound cards provide a MIDI interface. Those that do not can still play MIDI files using the on-board capabilities of the sound card.

MOD files are a common format for computer generated songs. As well as information about the musical notes to be played, the files contain digitized samples for the instruments (or voices). MOD files originated on the Amiga computer, but can be played on other systems, including Linux, with suitable software.

MP3 files are a popular format for distributing computer music and speech. MP3 uses a sophisticated encoding scheme (MPEG layer 3) to compress audio by roughly a factor of 10 with little reduction in quality as compared to CD audio.