17.4 Sound Card Components
The key function of a sound card is playback accepting a digital data stream or MIDI instructions from the PC and converting them to an analog audio signal that can be reproduced on speakers or headphones. Most sound cards can also do the converse accept an analog audio signal and convert it to a digital data stream that can be stored on a PC. Sound cards use the following components to support these functions:
Sound cards contain at least one Digital-to-Analog Converter (DAC) and one Analog-to-Digital Converter (ADC) for each of the two stereo channels, and some contain more. A DAC converts a digital audio stream into the analog audio delivered to the Line-out port. An ADC digitizes analog sound received from the Line-in or Microphone port. CD-Audio sound, generally the highest quality supported by sound cards, requires 16-bit resolution. The converters used in better-quality sound cards usually support higher resolution, typically 18- or 20-bit. Resolution sometimes differs between the DAC and ADC. For example, a card might use an 18-bit DAC and a 20-bit ADC. Internal resolution is often higher than that supported by the DAC/ADC, typically 24- or 32-bit.
- Sample rate generator
The sample rate generator provides the clock for the converters under the control of the PC. While nothing prevents using arbitrary or continuously variable sample rates, most sample rate generators instead support discrete sample rates, which are usually even fractions of 44,100 Hz and 48,000 Hz. A sample rate generator might support sample rates of 48,000, 44,100, 32,000, 24,000, 22,050, 12,000, 11,025, and 8,000 Hz. Many sound cards support differing rates for record versus playback. For example, a card may support playback rates of 48,000, 44,100, 22,050, 11,025, and 8,000 Hz, but record only at 44,100 Hz.
The processor (also called the sound generator or synthesis engine) creates analog output from MIDI input by reading, interpolating, and combining wavetable samples into the composite audio waveform represented by the MIDI instructions. Most sound cards use a custom digital signal processor (DSP) like the E-mu Systems EMU10K1 or EMU10K2, or the Crystal/Cirrus Logic CS4630 or CS8420. The processor used directly or indirectly determines several key capabilities of the sound card, including how many MIDI channels, voices, hardware-accelerated sound streams, and so on it supports. DSPs provide useful supplementary capabilities in hardware, such as reverb and chorus effects, text-to-speech processing, and compression. Because a DSP is programmable, some DSP-based sound cards support related functions, such as faxmodem or TAD functionality.
Sound cards typically provide at least the following connectors:
Line-out is a line-level (unamplified) stereo output intended to be connected to Line-in on amplified speakers, headphones, home audio equipment, or a tape or DAT recorder. Most sound cards provide one stereo Line-out port, but some provide two mono Line-out ports, designated Left and Right. Sound cards that support four speakers usually have two stereo Line-out ports, one each for front and rear speakers. The PC 99 standard color code for Line-out is lime, although for this and other color codes makers often pay scant attention to the exact hue. The standard icon usually stamped into the card bracket is three concentric circle segments (to represent audio vibrations) with an outward-pointing arrow anchored in the center.
Line-in is a line-level stereo input intended to be connected to Line-out of external analog audio sources such as a CD player or VCR. Some microphones can also be connected to Line-in. The standard color is light blue. The standard icon is the same as for Line-out, but with the arrow head pointing to the center.
Microphone-in, sometimes labeled Mic, is a monaural input that supports inexpensive microphones for recording voice. The standard color is pink, although red is commonly used, and the standard icon resembles a microphone.
MIDI/gameport is a DB-15 connector whose primary purpose is to connect a game controller. However, the standard gameport (see Chapter 21) also supports connecting external MIDI devices such as keyboards and synthesizers. Doing so requires a special octopus cable, available from the manufacturer, which extends the MIDI signals present on the gameport to MIDI-standard 5-pin DIN MIDI-in and MIDI-out connectors, which resemble standard AT keyboard connectors. The gameport should fully support analog and digital game controllers, including force feedback and DirectInput devices. Standard color is gold.
- MPC-3 CD-in
Although some recent CD-ROM drives can deliver CD audio digitally across the bus to the sound card, listening to CD audio on most CD-ROM drives requires connecting the line-level Audio-out connector on the back of the CD-ROM drive to the Audio-in connector on the sound card. Older CD-ROM drives and sound cards used a chaotic mix of proprietary and nonstandard connectors, which sometimes made it very hard to find the right cable. Recent CD-ROM drives and sound cards all use a standard cable. The sound card uses a shrouded, keyed, four-contact header-pin MPC-3 connector, often called a Molex or ATAPI connector. The CD-ROM drive uses an unkeyed version of the same connector, sometimes called a Sony connector. MPC-3 CD-in is usually color-coded black to differentiate it from other connectors, described below, that use the same physical connector.
Some sound cards also provide some or all of the following connectors:
Speaker-out/Subwoofer is an amplified stereo output intended to be connected to unpowered speakers, or to some powered subwoofers that expect a high-level input. Do not use this output to connect to standard powered speakers. Although it will usually work without damaging the speakers or the sound card, sound quality will be noticeably inferior to that provided by the Line-out connector. Standard color is orange.
- MPC-3 Aux-in
MPC-3 Aux-in uses the same physical connector as MPC-3 CD-in, described above, and is usually color-coded white to differentiate it. This connector accepts any line-level input, although it is most commonly used on systems that already have one CD-ROM drive to connect audio from a second CD-ROM, CD-R(W), or DVD drive.
- MPC-3 Modem-in/out
MPC-3 Modem-in/out uses the same physical connector as MPC-3 CD-in and MPC-3 Aux-in, and is usually color-coded green to differentiate it. MPC-3 Modem-in is a line-level audio input/output intended to be, connected to a line-level connector on a modem. This allows modem tones to be played through the speakers, and also supports such functions as Internet telephony and audio conferencing via the modem.
- Daughtercard connector
Some sound cards include one or more daughtercard connectors, which usually take the form of a large header-pin connector similar to an IDE connector. Daughtercards, which are proprietary to the make and model of sound card, may be used to add wavetable support, to extend ROM-based wavetable samples, or for other similar purposes.
Sony-Philips Digital InterFace (S/PDIF) is an RCA coax jack that provides a direct digital connection between the sound card and an external device with an S/PDIF jack (typically, a DAT deck). All S/PDIF ports support 48,000 Hz; most support 32,000 Hz; some support lower rates. S/PDIF is a standard feature on most high-end sound cards, and may be an option on midrange sound cards. Some sound cards have both S/PDIF input and output ports, but others have only an S/PDIF output. Because of limited room on the expansion bracket of the sound card, S/PDIF ports are often present as a header connector on the sound card, which uses an extender cable to a cliffhanger bracket where the S/PDIF connectors reside. Some sound cards use a proprietary connector that joins the sound card to a remote head, which often contains S/PDIF connector(s), line-in connector(s), and MIDI connector(s).
- CD interface connector
Some older sound cards include interface connector(s) for a CD-ROM drive, which made it easier and cheaper to upgrade older systems to multimedia capability. These connectors may be any of five types: the three proprietary interfaces used by early-model Mitsumi, Panasonic, and Sony CD-ROM drives; a second ATA interface, intended for use on early ATA systems that had only one embedded ATA interface; and/or a SCSI interface. Using any of these interfaces is a bad idea. The proprietary interfaces are used only by CD-ROM drives too old to be usable. The IDE interface is normally slow and conflicts with the second ATA interface present on all modern motherboards. The SCSI interface is slow, probably supports only one SCSI device, and often causes conflicts. Indeed, the presence of any of these connectors indicates that the sound card is so old that it should be replaced.
Early sound cards were intended for use with unamplified speakers, and so included an on-board amplifier. Nearly all modern computer speakers have built-in amplifiers that require only line-level input. Sound card amplifiers usually produce inferior sound because they are inexpensive, underpowered, poorly shielded, and must operate in the electrically noisy environment inside a PC. Some modern sound cards include an amplifier, primarily for use with devices (like some powered subwoofers) that expect an amplified rather than line-level signal. A sound card that lacks an amplifier is not necessarily low quality. In fact, the converse is often true. Many inexpensive sound cards include amplified outputs, and many high-end sound cards do not.