Other Synthesis Methods

There are many ways to create sounds, using different technologies and approaches to

synthesis. This section covers all the main methods, with reference to Logic Express

instruments where applicable.

Many of the outlined methods incorporate into their design at least some elements of the

subtractive synthesis approach covered earlier. The most common modern approach is based

on samples of real instruments and sounds.

Sample-Based Synthesis

Sample-based synthesis, which is sometimes known as Pulse Code Modulation (PCM), or

sampling and synthesis (S&S) synthesis, is differentiated from subtractive synthesis mainly

by the use of samples in place of oscillator waveforms.

The samplesdigital recordings of existing soundsare mapped across the keyboard.

Typically, each sample is mapped to a note in the center of a keyboard range that spans 5 or

so notes that are unique to that sample. The reason for this range of 5 or so notes is that

samples tend to sound much less like the source sound if played more than a few notes

higher or lower than the original pitchdue to the relationship between the pitch and

playback speed of samples.

The pitch of each sample isnt changed with a frequency control, unlike the oscillator

waveform of a synthesizer that is not sample based. Rather, a sample is played back at a

faster or slower speed to alter its pitch, which has a corresponding impact on the sample

playback time. For example, a sample played back at twice the speed requires half the time

to play through.

The EXS24 mkII is a sample player that can be used much like a sample-based synthesizer,

due to the subtractive synthesis facilities that it offers.

Popular instruments that use this synthesis approach include Korgs M1, O1/W, and Triton;

the Roland JV/XP instruments; Yamahas Motif series; and many others.

Frequency Modulation (FM) Synthesis

Put simply, FM synthesis involves the use of a modulator oscillator and a sine wave carrier

oscillator. The modulator oscillator modulates the frequency of the carrier oscillator within

the audio range, thus producing new harmonics. These harmonics are known as sidebands.

Typically, FM synthesizers dont incorporate a filter. You can generate some subtractive

synthesizer style sounds with FM synthesis, but it is difficult to recreate the sound of a

resonant subtractive synthesizer filter with this method. FM synthesis is extremely good,

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however, at creating sounds that are difficult to achieve with subtractive synthesizers-sounds

such as bell timbres, metallic tones, and the tine tones of electric pianos. Another strength of

FM synthesis is punchy bass and synthetic brass sounds.

Logic Express includes a simple FM synthesizer, the EFM1. Although it is minimalist, it is

capable of producing many of the classic FM sounds made famous by Yamahas DX series of

synthesizers (the DX7, sold from 1983 to 1986, remains the most commercially successful

professional-level hardware synthesizer ever made).

The ES2 also features some FM techniques that allow you to modulate one oscillator with

another. You can use these FM techniques to partially bridge the gap between the very digital

sound of FM synthesis and the fat analog sound that the ES2 is noted for.

Component Modeling Synthesis

Also known as physical modeling, this synthesis method uses mathematical models to

simulate instruments. Parameters are used to describe an instruments physical

characteristics, such as the materials the instrument is made of, the dimensions of the

instrument, and the environment it is played inunder water, in air. Equally important are

descriptions of how the player would interact with the instrumentfor example, whether it is

played by plucking, bowing, or strumming strings; by hitting it with sticks; by placing fingers

on sound holes; and so on.

To model a drum sound, for example, the following aspects would need to be taken into

account. Of primary importance would be the actual drum strikehow hard it is and whether

the drumhead is struck with a wooden stick, a mallet, a beater, and so on. The properties of

the drumhead (the skin or membrane) would include the kind of material, its degree of

stiffness, its density, its diameter, and the way it is attached to the shell of the drum. The

volume of the drum cylinder itself, its material, and the resonance characteristics of all of the

above would need to be mathematically described.

To model a violin, youd need to take into account the bow against the string, the bow-width

and material, the bow tension, the string material, the string density, the string tension, the

resonance and damping behavior of the strings, the transfer of string vibrations through the

bridge (materials, size, and shape of the bridge), and the materials, size and resonance

characteristics of the violin body. Further considerations would include the environment that

your modeled violin is played in, and the playing style-hammering or tapping with the bow

as opposed to drawing it across the strings.

Logic Express includes Ultrabeat, a drum synthesizer that incorporates physical modelling

components.

Wavetable, Vector, and Linear Arithmetic (LA) Synthesis

Wavetable synthesis uses a number of different single-cycle waveforms, laid out in what is

known as a wavetable.

Playing a note on the keyboard triggers a predetermined sequence of waves. In general, this

is not a stepped transition but rather a smooth blend from one waveform into another,

resulting in a constantly evolving waveform. Multiple wavetables can also be used

simultaneouslyeither played one after the other, or blended togetherresulting in more

harmonically complex waveforms.

A single wavetable can emulate filter cutoff with a series of bright, less bright, then

dull-sounding waveforms played in sequencewhich resembles a reduction of the filter cutoff

frequency in a subtractive synthesizer.

Wavetable synthesis isnt particularly successful at emulating acoustic instruments. It is,

however, extremely successful at producing constantly evolving sounds; harsh and metallic,

or bell-like sounds; punchy basses; and other digital tones.

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Wavetable synthesis was championed by the PPG and Waldorf instruments. The ES2 also

includes wavetable facilities.

Roland LA (Linear Arithmetic) synthesizers such as the D-50 work on a similar principle. In

these synthesizers, however, complex sampled attack phases are combined with simple

sustain or decay phases to create a sound. In essence, this is a simple wavetable that

consists of two samples.

Where LA and wavetable synthesizers differ is that the latter were designed to create new,

original, digital sounds. LA synthesizer designers, in contrast, wanted to emulate real

instruments using a minimum of memory. To facilitate this, they combined samples of the

attack phasethe crucial part of a soundwith appropriate decay and sustain phases.

Vector synthesisused in the Sequential Circuits Prophet-VS and Korgs Wavestationallows

you to move through wavetables and sequences arranged on a two-dimensional grid (two

different vectors, or less technically, on the X or Y axis). The main benefit of this approach is

that the balance between samples and waves is achieved in real time by moving a joystick.

You can also use the ES2 to perform vector synthesis by modulating the Oscillator Mix

(Triangle) parameter with the Vector Envelope.

Additive Synthesis

Additive synthesis could be considered the reverse approach to subtractive synthesis. See

the beginning of this appendix, including the discussion about all sounds being a sum of

various sine tones and harmonics, for background information to provide insight into additive

synthesis.

In essence, you start out with nothing and then build up a sound by combining multiple sine

waves of differing levels and frequencies. As more sine waves are combined, they begin to

generate additional harmonics. In most additive synthesizers, each set of sine waves is

viewed and used much like an oscillator.

Depending on the sophistication of the additive synthesizer you are using, you will be

provided with individual envelope control over each sine wave, or you will be limited to

envelope control over groups of sine wavesone envelope per sound and its harmonics, or

all odd or all even harmonics, for example.

Logic Express does not contain any instruments that use additive synthesis methods.

Resynthesis

You can analyze the frequency components of a recorded sound and then resynthesize

(reconstruct) a representation of the sound using additive techniques. By calculating the

frequency and amplitude of each harmonic in the overall frequency spectrum of the sound,

an additive resynthesis system can generate a series of sine waves (with appropriate levels

over time) for each harmonic.

After the sound has been resynthesized in this fashion, you can adjust the frequency and

amplitude of any harmonic. Theoretically, you could restructure a harmonic sound to make it

inharmonic, for example.

Phase Distortion Synthesis

Phase distortion synthesis creates different waveforms by modifying the phase angle of a

sine wave.

In essence, you can bend a sine wave until it becomes a sawtooth wave, a triangle wave, a

square wave, and so on. The synthesizer engine beyond the waveform generation in general

follows the standard subtractive method.

Phase distortion synthesis was commercially introduced in 1984s Casio CZ series

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synthesizers.

Granular Synthesis

The basic premise behind granular synthesis is that a sound can be broken down into tiny

particles, or grains. These sampled grainsusually no more than 10 to 50 ms longcan then

be reorganized, or combined with grains from other sounds, to create new timbres.

In many respects, this is much like wavetable synthesis, but it works on a much finer scale.

As you might expect, this method is ideal for creating constantly evolving sounds and truly

unique tones.

The downside is that granular synthesis is extremely processor-intensive, and it wasnt

possible to do it in real time until relatively recently. For this reason, it has remained largely

ignored by all but a few in academic institutions. Todays computers, however, have sufficient

processing power to make this synthesis method a practicality, and there are a number of

commercial products now available.

Copyright 2009 Apple Inc. All rights reserved.

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