Sound CardsKarl Heinz Kurze

Sound Cards have a minimum of four functions. These functions are:

Synthesizer MIDI interface Analog-to-digital conversion during the recording (A/D). Digital-to-analog conversion during the playback (D/A).

The Synthesizer

The synthesizer delivers the sound. That is, the sound card generates the sounds. Here we have three systems:

FM synthesis, Frequency Modulation Wave table Physical-modeling

FM synthesis The oldest of these methods is called FM synthesis. FM

synthesis does not produce high quality reproductions of the instruments they are supposed to be. How it works is this: the sound card is equipped with a synthesizer chip. This chip produces sine waves which are the beginning of sound. By combining several different sine waves, the chip is able to create complex waveforms, and these waveforms are supposed to be as close as possible to the actual sound of the instrument. In reality, though, the sound is "canned". FM synthesis cards are older cards, and are not much used today.

Wave tables - sampling Most boards sold today use what is called wavetable

synthesis. What this means is that in the on-board ROM is stored actual recordings of the instruments. These recordings are then used, by changing the pitch of and blending, to create much more life-like sound reproductions. Such sound cards must have ample memory on board to store the sounds. Many sound cards are expandable so that you can add memory and create your own wavetable recordings. The quality of the wavetable synthesis is a function of the quality of the recordings. Short, low-resolution sound tables produce cheap sounds.

Physical-modeling synthesis The third method of sound reproduction is called physical-

modeling synthesis. In short, a card using this method would emulate the vibrating sound system of an instrument (such as a guitar string) by creating a similar model through software. The sound chip would create wave vibrations that would act like a physical wave. This method of sound creation is incredibly life-like and is thus increasing in popularity. It is usually found on higher end sound cards. Creative Labs was one of the first to implement the method. The Creative Labs AWE64 Gold has 14 instruments re-created through this method.

Analog-to-digital conversion (ADC) When a sound card records analogue

audio, it is converting the sound waveform into digital information and then copying this in real time onto the hard disk. Essentially, it is using the disk as a digital tapeless recorder. The process of converting analogue to digital is known as digitising or sampling. With audio, the analogue waveform is chopped into a number of slices per second. At each slice, the amplitude is measured and rounded to the nearest available value. Clearly the more chops per second (sampling rate) and the finer the values assignable to the amplitude (dynamic range), the better the representation of the original.

Digital-to-Analog conversion (DAC) During DAC, digital

information stored in the hard disk is sent back to the sound card where it is then converted back into its analog form by the synthesizer.

From ISA to PCI ISA bus bandwidth is rather limited (8 Mbps). A stereo-CD data stream can be 1.4

Mbps, a large demand for the ISA bus. The PCI bus, on the other hand, offers a 100-MBps bandwidth or higher. Several simultaneous sound channels are no problem for the PCI bus.

There are other benefits to PCI. The PCI bus allows cooperative signal processing, which means that tasks can be shared between the main system processor and a separate audio processor on the sound card. This means that the audio signals are free of interference from other tasks thereby making it 10 – 20 times more efficient than ISA based sound cards.

One pitfall of the PCI bus arises out of older DOS applications. Legacy DOS applications are designed so that they require a DMA and IRQ to move audio data from the main system memory. This means they use Sound Blaster compatible protocols. These are only features of the ISA bus, not PCI. For this reason, there is a risk that older DOS games will not run with PCI sound cards. There are workarounds for this ISA-PCI problem in DOS. Using special hardware, it is possible to redirect the PCI interrupts into the legacy space, in effect making them ISA IRQ's. The DMA requirement for DOS is being handled by enhancements to the PCI bus protocol. The total effect is to provide DOS support for PCI sound cards.

DirectX DirectX was written by Microsoft to allow low-level control of the

hardware and allow high performance multimedia within Windows instead of DOS. DirectX 5.0, for example, allows low-level control of audio files, giving the ability to mix multiple .WAV files, as well as control the balance, volume, and playback rate of each. DirectX API's are divided into sections, each section controls a different function. We have DirectDraw, DirectSound, DirectSound3D, DirectPlay, etc. DirectSound is what allows the low-level control of the sound equipment in the PC. DirectSound3D allows 3-D sound capabilities from only two speakers, giving rise to positional audio. Positional audio manipulates the characteristics of sounds to make them seem to come from a specific direction, such as from behind or from far to the left. In version 7.0 you find improved 3D acceleration of sound as well as picture with reduced CPU usage. The performance should be increased with 20% compared to version 6.1.

Environment Simulation DS3D may have supported positional audio, but it

didn't offer much support for adding reverb, let alone considering individual reflections, to simulate different environments. Fortunately DS3D does support extensions to the API, and this need was soon met by a couple of new sound standards which have gained widespread support from games developers: Aureal's A3D technology and Creative Technology's Environmental Audio Extensions (EAX).

Reverb Sound that is heard is a mixture of direct path

sound and reflected sound. Reflected sound might reach our ears after bouncing off a wall or object, and the material of these obstacles absorbs certain frequencies, along with reducing the overall volume. This "first-order reflection" arrives not only sounding different from the direct source, but also slightly after it. Second-order reflections and so on take this effect further still. The quality and delay of the reflected sound reveals a great deal about the surrounding environment and its size.

Most humans can perceive precisely where first-order reflections are coming from, and some can distinguish second-order reflections too. However, as more and more reflections arrive at the ear, the brain tends to combine them into one late-order reflection echoing effect known as reverb. Using reverb properly is the first key to simulating different environments.

Aureal A3D Originally developed in 1997 in collaboration NASA (National

Aeronautics and Space Administration) for use in flight simulators, Aureal's A3D technology has subsequently progressed through a number of versions.

ASD1 improved upon DS3D by providing hardware acceleration, a more advanced distance model allowing simulation of different atmospheric environments, such as thick fog or underwater and a resource manager that allows developers to take advantage of the number of 3D streams the sound card can handle and control use of Aureal's 3D sound algorithms.

The A3D2 version actually takes the geometry information of the room that is fed to the graphics card, and uses it to render realistic sonic reflections and occlusions. Using a technology called WaveTracing, A3D2 genuinely calculates up to 60 first-order reflections, which interact in real time with the environment, and then groups later-order reflections into overall reverb.

Aureal A3DASD3 takes the technology to the next level by adding a number of new features: Volumetric Sound Sources: When developers define an audio file to a sound source, the

sound source must have a location so that it can be rendered in relation to the listener. This is usually done via a point source: the point where the source is. However, some sources will not "reside" in a single point; flowing water, wind, crowd cheers, etc. will actually stretch out or extend in various areas. To more accurately model these sources, ASD3 allows them to be defined as volumetric sound sources, thereby positioning them better.

MP3 playback: Previously, audio streams for 3D audio have had to be WAV files. Now, MP3 files can be used, thereby both reducing their associated storage space and increasing their quality.

Reverb: The sum of all late order reflections. Aureal's geometric reverb will work on Vortex2 (and later) cards, as well as automatically translating to EAX or I3DL2 if a sound card does not have the appropriate A3D support.

Streaming Audio: Automatic support for streaming audio has been added, eliminating the complex layer of development normally required for client/server interactive entertainment applications that use existing audio solutions.

A3D2 was such a computationally complex system that Aureal developed a processor dedicated to the necessary number crunching. A3D3 requires even greater processing power, which is provided in the shape of an additional DSP to accelerate the new commands.

Creative Technology's Environmental Audio Extensions EAX EAX 1.0 was designed to provide developers with the ability

to create a convincing sense of environment in entertainment titles and a realistic sense of distance between the player and audio events. The approach Creative took to achieve this was, computationally, significantly easier than the one Aureal had taken with A3D.

EAX 2.0 enabled the creation of more compelling and realistic environments with tools that allow the simulation of the muffling effects of partitions between environments (such as walls) and obstacles within environments (such as furniture).

EAX 3.0 introduced the ability to "morph" between environments.

Creative Technology's Environmental Audio Extensions EAX In late-2000 a number of EAX effects were incorporated into

the DirectX Audio component - the functions of which were previously shared between the DirectSound and DirectMusic components - of the latest release of Microsoft's suite of multimedia APIs, DirectX 8.0. A few months later, Creative unveiled an API platform for games developers wanting to incorporate Dolby Digital content into their games. Earlier soundcards had allowed Dolby Digital to be passed directly through the card and decoded by an external decoder. However, with the "5.1" version of its successful SoundBlaster Live! sound card Creative supported decoding directly through one of their audio products for the first time, the card being able to output straight to six discrete analogue channels (5 speakers + 1 sub-woofer).