Sound Physics

WaveTable Synthesis doesn't use carriers and modulators to create sound, but actual samples of real instruments. A sample is a digital representation of a waveform produced by an instrument. ISA-based cards generally store samples in ROM , although newer PCI products use the PC's main system RAM, in banks which are loaded when Windows starts up and can theoretically be modified to include new sounds.

The first step in the process of converting an analogue signal into a digital representation. This is accomplished by measuring the value of the analogue signal at regular intervals called samples. These values are then encoded to provide a digital representation of the analogue signal.

A common method for generating sound electronically on a PC. Output is produced using a table of sound samples -actual recorded sounds - that are digitised and played back as needed. By continuously rereading samples and looping them together at different pitches, highly complex tones can be generated from a minimum of stored data without overtaxing the processor.

Industry Standard Architecture: the architectural standard for the IBM XT (8-bit) and the IBM AT (16-bit) bus designs. In ISA systems, an adapter added by plugging the card into one of the 16-bit expansion slots enables expansion devices like network cards, video adapters and modems to send data to and receive data from the PC's CPU and memory 16 bits at a time.

Peripheral Component Interface: the 32-bit bus architecture (64-bit with multiplexing) developed by DEC, IBM, Intel, and others, that is widely used in Pentium-based PCs. A PCI bus provides a high-bandwidth data channel between system board components such as the CPU and devices such as hard disks and video adapters. Superseded the VL-Bus.

Whereas one FM sound card will sound much the same as the next, WaveTable cards differ significantly in quality. The quality of the instruments is determined by several factors:

Most instrument samples are recorded in 16-bit 44.1kHz but many manufacturers compress the data so that more samples, or instruments, can be fit into small amounts of memory. There is a trade-off, however, since compression often results in loss of dynamic range or quality.

When an audio cassette is played back either too fast or too slow, its pitch is modified. The same is true of digital audio. Playing a sample back at a higher frequency than its original results in a higher pitched sound, allowing instruments to play over several octaves. But when certain timbres are played back too fast, they begin to sound weak and thin. This is also true when a sample is played too slow: it sounds dull and unrealistic. To overcome this, manufacturers split up the keyboard into several regions and apply the relatively pitched sample from the instrument to it. The more sample regions recorded results in a more realistic reproduction.

Every instrument produces subtly different timbres depending on how it is played. For example, when a piano is played softly, you don't hear the hammers hitting the strings. When it's played harder, not only does this become more apparent, but there are also changes in tone.

Many samples and variations have to be recorded for each instrument to recreate this range of sound accurately with a synthesiser. Inevitably, more samples require more memory. A typical sound card may contain up to 700 instrument samples within 4MB ROM. To accurately reproduce a piano sound alone, however, would require between 6MB and 10MB of data. This is why there is no comparison between the synthesised sound and the real thing.

Upgrading to WaveTable Synthesis sound doesn't always mean having to buy a new sound card. Most 16-bit sound cards have a feature connector that can connect to a WaveTable daughterboard . The quality of the instruments such cards provide differs significantly, and is usually a function of how much ROM the card has. Most cards contain between 1MB and 4MB of samples, and offer a range of digital effects.


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