Section 17.5. Sound Card Characteristics

17.5 Sound Card Characteristics

Here are the important characteristics of sound cards:

Interface

Discrete sound cards are available in ISA and PCI models, although ISA cards are a vanishing breed and are increasingly hard to find. All recent embedded sound adapters use PCI. The much smaller bandwidth of ISA limits 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. The first PCI audio chipsets that emerged in the mid-90s were quite expensive, but current PCI cards are generally less expensive than ISA cards with similar functionality, primarily because they require little on-board memory.

Synthesis type

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.

Channels

Each MIDI interface supports 16 channels, each corresponding to one instrument. Low-end sound cards use a single MIDI interface, allowing up to 16 instruments to play simultaneously. Midrange and some high-end sound cards provide dual MIDI interfaces, allowing 32 simultaneous instruments. Some high-end sound cards, such as the Creative Labs SoundBlaster 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.

Polyphony

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. For example, the Creative Labs SoundBlaster AWE64 sound card supports 64-voice polyphony, 32 in hardware and 32 in software. High-end sound cards support 64 hardware voices, and add software voices for a total of 256 to 1024 voices.

Frequency response

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 PC 99 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.

Sampling rate

All current sound cards support waveform audio playback at 44,100, 22,050, 11,025, and 8000 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

Signal-to-Noise (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.

Duplex mode

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 tasks listening to CDs or playing games a 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 support full-duplex.

Standards compatibility

In the past, software wrote directly to the sound card. That meant that compatibility with proprietary standards initially AdLib and later SoundBlaster was 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:

SoundBlaster

SoundBlaster 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 SoundBlaster 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 SoundBlaster 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

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 SoundBlaster compatibility as an absolute requirement for any sound card.

Microsoft DirectSound3D

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 in an air combat game 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.

Aureal A3D

Although Aureal went bankrupt in spring 2000, many cards that use Aureal chipsets are still out there, and many were still being sold as of 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

Creative Labs EAX (Environmental Audio Extensions) is basically a proprietary Creative Labs extension to DirectSound3D. EAX is technically less ambitious than A3D2.0, but provides reasonable 3D imaging. Given the dominance of Creative Labs, EAX is widely supported by game software.

Windows 95, 98 and 2000/XP fully support DirectX, currently version 8.1. Windows NT supports a subset of DirectX 3 functions, including DirectSound (but not DirectSound3D). That means that, although DirectSound-compatible sound cards can be installed and used under NT, 3D functions are not available.

Hardware acceleration

Midrange and high-end sound cards have an on-board DSP, which is a general-purpose CPU optimized for processing digital signals, such as audio. In 2D mode, the DSP provides enhanced audio effects like chorus, reverb, and distortion. In 3D mode, it processes 3D-positional audio (e.g., A3D, DirectSound3D, EAX, Sensaura) 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. How many and which types of sound streams are hardware accelerated is an important specification, depending on how you will use the card. Aureal A3D2.0-based cards should provide hardware acceleration for 16 or more A3D2.0 and 64 or more A3D1.0 sound streams. A Creative Labs AEX card should accelerate 64 or more EAX sound streams. Any accelerated sound card should accelerate 32 or more DS and DS3D sound streams in hardware.