card must be able to generate these sounds using a synthesizer chip. Through
MIDI signals, an external MIDI device can control the sound card’s synthesizer
chip. Synthesizer chips vary widely in capabilities and sound quality, but many
newer sound cards now incorporate Wavetable synthesis that produces a higher
quality sound by using digital samples of actual instruments in place of other
synthesized sounds.Amplifier
After digital audio data is converted into an audible (analog) signal, it must be amplified
before it can be played back on speakers or headphones. Nearly all sound cards have an
amplifier that can produce a sound level compatible with a set of headphones or a set of
small PC speakers. Because the amplifier on the sound card is usually weak, PC speaker
systems may include an amplifier in one or both of its speakers to enhance the sound. By
adding the correct cabling and jacks, the sound card’s output signal can be sent to your
home stereo or home theater, in case you desire very high quality sound reproduction.Speakers
Like all speakers, PC speakers are categorized into two general groups: passive and ac-
tive. Passive speakers do not include an amplifier; active speakers do. A passive speaker,
which is what most standard PC speakers are, receives a signal that has been amplified
enough to generate motion in the speaker’s diaphragm and produce sound. An active
speaker includes a built-in amplifier and typically does not require external amplifiers.
An active speaker can accept low-level (line-level) signals.
Nearly all PC speakers are passive speakers, with the exception of subwoofers.
Subwoofers are speakers that generate only very low frequency sounds, like bass tones.
A subwoofer usually includes a large built-in amplifier, which makes it an active
speaker. The benefit of having a subwoofer on your system is that it will handle all of the
low bass sounds, leaving the system’s passive speakers and the amplifiers driving them
to handle higher-level sounds, which are much easier to reproduce.
Exactly how audio speakers work is beyond the scope of this book, but if you’re curious
about it, check out Joel Antonini’s Audio Visual 101 Web site atwww.audiovideo101.
com/learn.
PC speakers come in a wide range of configurations, from small passive systems
poweredbythesoundcard’sheadphoneoutputtoactivethree-wayandsurround-sound
systems that would rival many home theaters. Some PC monitors have integrated speak-
ers either incorporated into their cases or designed to attach to its sides.
The speakers I have been discussing to this point all connect to the PC via the sound
card using 1/8-inch jacks. However, the USB (Universal Serial Bus) speaker system is a
recent development. USB speaker systems do not require that a sound card be installed in
the PC. Digital audio is sent directly to the speakers via the USB cable, and all signal pro-
cessing is done within the speaker enclosure itself, outside of the PC. There are many ad-
vantages to this type of system, not the least of which are the available expansion slot andChapter 21: Audio/Visual Devices^543