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Publisher17.5 Sound Card Characteristics
Here
are the important characteristics of sound cards:
Discrete sound cards have been made in
ISA and PCI versions, but ISA cards are no longer available. All
recent sound adapters, embedded or standalone, use PCI. The much
smaller bandwidth of ISA limited ISA cards in many respects,
including generally requiring that wavetable data be stored locally,
placing an upper limit of about 16 on simultaneous sound streams, and
making effective 3D audio support impossible. If you encounter an ISA
sound card when stripping an older system for spares, pitch the ISA
card. It's not worth keeping.
FM synthesis is no longer used in current
sound cards. All current midrange sound cards use wavetable
synthesis, and some expensive sound cards use partial waveguide
synthesis. The quality and features of wavetable synthesis vary
depending on both the processor and the quality and size of the
wavetable samples, with more-expensive cards producing better
synthesis, as you might expect.
Each
MIDI interface supports 16 channels, each corresponding to one
instrument. Low-end sound cards have one MIDI interface, allowing up
to 16 instruments to play simultaneously. Midrange and high-end sound
cards usually have dual MIDI interfaces, allowing up to 32
simultaneous instruments. Some high-end sound cards, such as the
Creative Labs Sound Blaster Live! Platinum, use a triple MIDI
interface, which allows up to 48 simultaneous instruments. In
general, 16-channel cards are suitable for most uses, 32-channel
cards are useful for playing MIDI instrumentals realistically, and
48-channel cards are necessary only for the most complex MIDI
environments. It's worth noting that Creative
replaced its flagship 48-channel Sound Blaster Live! Platinum with
the 32-channel Audigy 2.
Polyphony
refers to the ability of a sound card to generate multiple
simultaneous voices when playing MIDI. A voice
corresponds to one note generated by one instrument. Do not confuse
number of voices with number of channels. The 16 channels of a
standard MIDI interface allow 16 instruments to play simultaneously.
However, some instruments require multiple voices. For example, a
piano occupies one MIDI channel, but if the musician is playing a
single-note melody with one hand and three-note chords as
accompaniment with the other hand, that channel requires four voices.
A large number of voices is important for reproducing complex MIDI
scores accurately. Voices may be hardware-based or software-based,
and some sound cards use both types. A basic sound card might support
64-voice polyphony, 32 in hardware and 32 in software. High-end sound
cards support 64 or more hardware voices, and may add software voices
for a total of 256 to 1024 voices.
The range
of human hearing is usually stated as 20 Hz to 20 kHz. All current
sound cards nominally support this range or close to it, which is in
fact required for PC99 compliance. However, few cards state
± dB for that range, which specifies how flat the
frequency response curve is. A good card may have frequency response
of 20 Hz to 20 kHz at 3 dB down. A professional-level card may have
frequency response of 20 Hz to 20 kHz at 1 dB down. Inexpensive cards
may claim frequency response of 20 Hz to 20 kHz, but that range may
turn out to be stated at 10 dB down or some similarly absurd number,
which in effect means that actual usable frequency response may be
something like 100 Hz to 10 kHz.
All current sound cards support waveform
audio playback at 44,100, 22,050, 11,025, and 8,000 Hz. Many also
support various intermediate playback rates and the DAT-standard
48,000 Hz. Some cards record only at 44,100 Hz, although most also
offer other standard rates.
Signal-to-Noise ratio (S/N
ratio), stated in dB, measures the amount of signal (data)
relative to noise, with higher numbers indicating better performance.
A low S/N ratio translates to audible hiss. The best sound cards have
95 dB or greater S/N for analog audio; midrange cards about 90 dB;
and inexpensive cards may have 85 dB or less. It's
not unusual for a card to have somewhat lower S/N ratio for digital
recording and digital playback. For example, an excellent
consumer-grade sound card may specify an S/N ratio of 96 dB FS
A-weighted for analog audio, 93 dB FS A-weighted for digital
recording, and 90 dB FS A-weighted for digital playback. In a typical
PC environment, noise level (both ambient external audible noise and
the electrically noisy inside of the PC) and the typical use of
low-quality speakers or headphones make it unlikely that anyone could
differentiate between cards with S/N ratios of 80 dB or higher if
that were the only difference. However, cards with higher S/N ratios
are generally better shielded and use better components, which
translates to better sound and less hiss.
Half-duplex
sound cards can either play sound or
record sound, but not both at the same time.
Full-duplex sound cards do both simultaneously.
For simple taskslistening to CDs or playing gamesa
half-duplex card is adequate. More advanced audio functions, such as
Internet telephony and voice recognition, require a full-duplex card.
Most midrange and all high-end sound cards are full-duplex.
In the past, software wrote directly to
the sound card. That meant that compatibility with proprietary
standardsinitially AdLib and later Sound Blasterwas
important because if your game or application didn't
explicitly support your sound card, you simply
couldn't use sound with that software. Microsoft
took the initiative away from sound card manufacturers by
incorporating standard sound APIs into Windows. Here are the
standards you should be aware of:
Sound Blaster
compatibility, formerly a
sine qua non for any sound card, is now largely
immaterial except to those who still use DOS software, including DOS
games. True Sound Blaster compatibility requires fixed IRQ, I/O port,
and DMA assignments, whereas PCI cards are assigned resources
dynamically. Within those constraints, all Creative Labs sound cards
and most competing cards boast (nearly) full Sound Blaster
compatibility. If you still use DOS applications, though,
it's worth verifying whether real-mode drivers are
available for a sound card before you purchase it.
Microsoft DirectSound (DS)
is a component of DirectX. Developers can write to the DS API, rather
than to the underlying hardware, with the assurance that their
software will function with any DS-compatible sound card. DS
compatibility has replaced Sound Blaster compatibility as an absolute
requirement for any sound card.
Microsoft DirectSound3D
(DS3D) is an extension to DS that supports 3D
positional audio, which is a technology that manipulates sound
information to extend stereo imaging to full surround sound, allowing
sounds to appear to come from any position around you. For example,
when you're playing an air combat game and your
missile hits a bandit in front of you, the sound of that explosion
comes from the front. But if you didn't notice his
wingman on your six, the sound of his missile blowing off your tail
comes from behind. The realism of DS3D imaging in any given situation
depends on the means used to reproduce the sound (two speakers, four
speakers, or headphones) and the hardware capabilities of the sound
card. But whatever the physical environment, DS3D provides noticeably
better imaging than older 2D technologies. If you intend to use
DS3D-enabled software, it's important to have
hardware support for DS3D in your sound card because DS3D positional
effects that cannot be processed in hardware are processed by the
main CPU, which can bog down system performance.
Although Aureal went bankrupt in spring 2000, many cards with Aureal
chipsets remain in use, and such cards were still available new as
recently as late 2001. Aureal
A3D is a proprietary 3D positional audio
standard that is available only on sound cards based on the Aureal
Vortex and Vortex2 chipsets, which have been made by Voyetra/Turtle
Beach, Diamond Multimedia, Aureal itself, and others. A3D is
available in two versions. A3D2.0 is supported only by the Vortex2
chipset, whereas the earlier and less-capable A3D1.0 is supported by
both the Vortex and Vortex2 chipsets. A3D1.0 provides realistic 3D
imaging even on dual-speaker systems or headphones. A3D2.0 provides
extraordinary 3D effects, particularly on quad-speaker systems. A3D
achieved broad support from game software manufacturers. For software
without A3D support, A3D hardware drops back to using DS3D.
Creative Labs EAX (Environmental Audio
Extensions) is basically a proprietary Creative Labs
extension to DirectSound3D. EAX 1.0 is technically less ambitious
than A3D2.0, but provides reasonable 3D imaging. EAX 2.0 and EAX
Advanced HD Multi-Environment are significant enhancements that match
A3D2.0 in most respects and exceed it in many. Given the dominance of
Creative Labs, the various flavors of EAX are widely supported by
game software.
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Midrange and high-end sound cards have an onboard DSP, which is a
general-purpose CPU optimized for processing digital signals, such as
audio. In 2D mode, the DSP provides enhanced audio effects such as
chorus, reverb, and distortion. In 3D mode, it processes
3D-positional audio (e.g., DirectSound3D or EAX) algorithms locally,
removing that burden from the main CPU. Inexpensive sound cards use
the host CPU, which reduces performance significantly, particularly
during complex operations such as 3D rendering. Any accelerated sound
card should accelerate 32 or more DS and DS3D sound streams in
hardware.
