Sound Synthesis
MIDI instructions and digital audio rely on sophisticated Synthesizers to reproduce sounds of pre-recorded actual instruments and unique synthesized sounds. An important evaluation of a synthesizer, whether as part of a keyboard instrument or attached to the soundcard in the computer is Multi-Timbral and Polyphony. These capabilities are important as a Sequencer can create many tracks (of different instruments) with each track having a sequence of notes playing simultaneously.
A multi-timbral synthesizer can produce more than a single instrument sound at a time. Thus 16 channels of MIDI information can represent 16-tracks of instrument sounds when coming from the Sequencer to the Synthesizer.
A polyphonic synthesizer (or instrument) is capable of playing more than a single note at a time, and there is normally a maximum amount of notes that any device or application is capable of playing simultaneously. Many high-end synthesizers are capable of 64-note polyphony. Thus the Sequencer performance playback represents several instruments playing similar or different melodies simultaneously.
- Synthesizer originally used electronic circuits to generate tones. The performance of these circuits are duplicated by digital circuits in hardware and by digital modeling in software applications. VCO (Voltage Controlled Oscillator)
- VCF (Voltage Controlled Filter)
- VCA (Voltage Controlled Amplifier)
These circuits have their signals shaped by an Envelope Generator.
The synthesizer on a soundcard utilizes the digitized data passed on by the Sequencer to produce sounds. The polyphonic synthesizer can play several notes simultaneously.
FM (Frequency Modulation) synthesis is an older method used by sound cards and computers to mimic musical instrument sounds through algorithmic mathematical representation and electronic means. The electronic means are circuits known as oscillators. FM synthesis requires several oscillators, each producing individual sine waves. These sine waves are combined (modulator waves are combined with a carrier wave) to create representations of complex sounds which are then transferred to analog output. The amplitude of the modulator waveform is used to modulate the frequency of the carrier waveform. By adjusting both the amplitude and the frequency of modulator, and utilizing an envelope generator the timbre of the produced sounds change.
Wavetable synthesis produces sounds which are stored digital sample reproductions of actual musical instrument sounds which are played back by digital oscillators and the pitch is then manipulated by filters and envelope generators. The samples (preferably originally recorded at 44.1 kHz / 16-bit Resolution) are read from the soundcard ROM into the RAM, combined and processed by the DSP capabilities of the soundcard, and played at the appropriate speed (frequency Hz) in order to attain the correct pitch. In this manner not every note an instrument can produce is recorded and memory can be conserved (a sound card has finite memory). Wavetable synthesis reproduces a better sounding quality of sound than Frequency Modulation. A polyphonic synthesizer can play more than one tone at a time.
There are several types and approaches to sound synthesis:
- Subtractive Synthesis
- Additive Synthesis
- Physical Modeling
- Harmonic Synthesis
- Sampler Synthesis
- Granular Synthesis
Synthesizers produce sound by generating a wave pattern and then allowing for the manipulation of wave form by the various modules of the synthesizer.