Sample based synthesis

Sample-based synthesis is a form of audio synthesis that can be contrasted to either subtractive synthesis or additive synthesis. The principal difference with sample-based synthesis is that the seed waveforms are sampled sounds or instruments instead of fundamental waveforms such as sine and saw waves used in other types of synthesis.

Unlike analogue or FM, sample synthesizers utilize samples in place of the oscillators. These samples, rather than consisting of whole instrument sounds, also contain samples of various stages of a real instrument along with the sounds produced by normal oscillators. For instance a typical sample-based synthesiser may contain five different samples of the attack stage of a piano, along with a sample of the decay, sustain and release portions of the sound. This means that it is possible to mix the attack of one sound with the release of another to produce a complex timbre.

Commonly, up to four of these individual 'tones' can be mixed together to produce a timbre and each of these individual tones can have access to numerous modifiers including LFOs, filters and envelopes. This obviously opens up a whole host of possibilities not only for emulating real instruments, but also for creating complex sounds. this method of synthesis has become the de facto standard for any synthesizer producing realistic instruments. By combining both samples of real-world sounds with all the editing features and functionality of analogue synthesisers, they can offer a huge scope for creating both realistic and synthesized sounds.

Granular Synthesis

One final form of synthesizer that has started to make an appearance with the evolution of technology is the granular synthesizer it is rare to see a granular synthesizer employed in hardware synthesizers due to it's complexity, but software synthesizers are being developed for the public market that utilize it. Essentially, it works by building up sounds from a series of short segments of sounds called 'grains'. This is best compared to the way that a film projector operates, where a series of still images, each slightly different from the last, are played sequentially at a rate of around 25 pictures per second, fooling the eyes and brain into believing there is a smooth continual movement.

A granular synthesizer operates in the same manner with tiny fragments of sound rather than still images. By joining a number of these grains together, an overall tone is produced that develops over a period of time. To do this, each grain must be less than 30 ms in length as, generally speaking, the human ear is unable to determine a single sound if they are less than 30-50 ms apart. This also means that a certain amount of control has to be offered over each grain. In any one sound there can be anything from 200 to 1000 grains, which is the main reason why this form of synthesizer appeals mostly in the form of software. Typically, a granular synthesizer will offer most, but not necessarily all, of the following five parameters.

• Grain length: This can be used to alter the length of each individual grain. As previously mentioned, the human ear can differentiate between two grains if they are more than 30-50 ms apart, but many granular synthesisers usually go above this range, covering 20-100 ms. By setting this length to a higher value, it's possible to create a pulsing effect.

• Density: This is the percentage of grains that are created by the synthesizer. Generally, it can be said that the more grains created, the more complex a sound will be, a factor that is also dependent on the grains shape.

• Grain shape: Commonly, this offers a number between 0 and 200 and represents the curve of the envelopes. Grains are normally enveloped so that they start and finish at zero amplitude, helping the invidigual grains mix together coherently to produce the overall sound. By setting a longer envelope (a higher number) two individual grains will mix together, which can create too many harmonics and often result in the sound exhibiting lots of clicks as it fades from one grain to the other.

• Grain pan: This is used to specify the location within the stereo image where each grain is created. This is particularly useful for creating timbres that inhabit both speakers

• Spacing: This is used to alter the period of time between each grain. If the time is set to a negative value, the preceding grain will continue through the next created grain; however, if this space is less than 30 ms, the gap will be inaudible.

The sound produced with granular synthesizers depends on the synthesizer in question. Usually, the grains consist of single frequencies with specific waveforms or occasionally they are formed from segments of samples or noise that have been filtered with a bandpass filter. Thus, the constant change of grains can produce sounds that are both bright and incredibly complex, resulting in a timbre that's best described as glistening. After creating this sound by combing the grains, the whole sound can be shaped by using envelopes, filters and LFOs.

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