Wavetable Synthesis

A more sophisticated method of simulating musical instruments is known as wavetable synthesis. Rather than rely upon the combination of several pure tones to simulate an instrument's sound, wavetable synthesis uses a recording of the instrument in question. The manufacturer digitally records an instrument playing a single note. A tiny snippet of that recording is then isolated, and circuitry in the sound card can play it back in a loop to reproduce the continuous note. Furthermore, the recorded note can be changed in volume, compressed to produce higher tones, or expanded to produce lower tones. Between these manipulations, it's possible to simulate the sound of the instrument playing any note at any volume.

Wavetable Quality

Even the worst wavetable synthesis sound cards usually sound better than the best FM synthesis cards when playing MIDI files. The variation among wavetable boards is quite wide, however. Variables that can affect wavetable quality include

• Sample length All other things being equal, a larger sample is better than a smaller one.

• Sample rate Just as with digital audio, wavetable synthesis samples are recorded at some specific sample rate. If the sample rate is too low, the sample can lose some of the higher-frequency harmonics that help to define the nature of a sound.

• Number of samples When you play two notes on an instrument, their waveforms resemble one another, but they aren't identical, even when you adjust for the different frequencies. Therefore, many wavetable products actually record two or more samples for each instrument. Each sample handles a different range of notes. All other things being equal, more such samples are better than fewer.

• Source Manufacturers use different instruments for their source samples. If the source instrument was poorly built or maintained, the resulting wavetable suffers.

Aside from the last factor, these variables all affect the ultimate size of the complete 10

wavetable, as stored in ROM or RAM on the sound card. For instance, one product might have a 2MB wavetable, and another might have an 8MB wavetable. Chances are the second product o will sound better than the first, although other factors might intervene. The first card might use g better source instruments, for instance, or it might devote more space to instruments important CA

to you, whereas the second might devote a great deal of space to accurately reproducing g bizarre instruments you never use.

Part iii

Wavetable Storage

Wavetable sound cards store their samples in one of three locations:

• Sound card ROM Information might go on a ROM chip on the sound card. This location is convenient for the user because it requires no intervention on your part—simply plug the board in and it's ready for use. The drawback is that the ROM isn't easily upgraded, so you can't change to a better wavetable sample set.

• Sound card RAM Some sound cards include RAM, either built into the card or on SIMM sockets, as shown in Figure 10.4. This design provides flexibility, because you can download updated wavetable samples, or even maintain several dozen megabytes of samples and download only those instruments you're using at any given time. The drawback is that you must use drivers or auxiliary software that supports the wavetable download function.

Figure 10.4

Some wavetable sound cards include SIMM sockets for storing wavetable samples.

• Computer RAM A recent trend, particularly with PCI sound cards, is toward what's known as software wavetable. In this design, the wavetable functions are handled in software. Effectively, the computer generates a digital audio file from the MIDI file and a set of wavetable samples, and then plays that file on the sound card like any other sound file. Linux includes software wavetable support for all sound cards, as described shortly, so don't favor a sound card that advertises this feature over one that doesn't. Definitely don't consider such a sound card equal to any form of hardware wavetable support.

Chapter 10

Some sound cards include both a ROM-based wavetable and RAM chips or SIMM sockets for expansion or replacement of the samples included on the ROM. This design provides the best mix of convenience and flexibility, all other things being equal.

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