Essentially sound is the perception of the movement of air particles in a somewhat periodic or repetitive fashion. The faster the particles move the higher the perceived frequency. We can create a graphic representation of the movement of particles in time. This is called a Waveform Displacement Graph of simply a Waveform.
The frequency (pitch) is determined by the number of cycles (complete waves) in each second of sound, and the amplitude (volume) is determined by the height of the wave above and below the centre line. The timbre or quality of the sound can also be determined by the waveform -- generally speaking, simple periodic waveforms (such as sine waves) have a smooth, clear timbre, while complex aperiodic sounds are noisier and brighter in timbre. All sounds, no matter how complex, can be expressed as a waveform.
A Sine Wave and a Violin Tone
A Complex Waveform (human speech)
If we view the entire waveform of a sound, we can see the change in amplitude over time. This is the envelope of the sound.
The entire waveform of a piano note
Since waveforms are usually symmetrically above and below the 0 line, we normally graph envelopes by displaying only the maximum positive values of the sound over time.
The envelope of a piano note
A Wind Instrument Envelope and A Percussion Instrument Envelope
The line segments of the envelope are labelled:
Most synthesizers allow the envelopes of their voices to be modified. For example, you could prolong the attack of a piano sound, or alter the release time of the instrument so that the sound decays more slowly. The exact method of performing the alterations is dependent upon the synthesizer.
An audio system consists of
The sound producer generates an alternating current that is analogous to the fluctuating values in the waveform of the sound - high frequency sounds have fast alternations, low frequencies have slow alternations. This current is amplified (i.e. the amplitude is increased) and causes the loudspeaker cone to move back and forth as the current is varied. The loudspeaker pushes and pulls air particles which are, in turn, perceived by the listener as sound. Electronically produced sound is sometimes called analog sound (since the fluctuating current is analogous to the waveform).
In digital audio, sound is represented as a series of numerical values. These values correspond to the waveform of the sound. The amplitude values of the waveform are stored as a sequence of numbers. These numbers are called samples, and are stored at regular intervals (called the sampling rate). In order for the sound to be accurately produced or reproduced the sampling rate should be above 40,000 samples per second.
The highest perceived frequency is one half the sampling rate. Since we can hear sounds up to about 18,000 cycles per second, a sampling rate below 36,000 may not reproduce components of the sound that we can hear.
Digital Representation of a Waveform
Virtually all synthesizers use a digital (i.e. a computer) process to generate sound. The digital information must, however, be transformed to audio signal (voltages) so we can send the signal to a loudspeaker. This conversion is done by a device called a Digital to Analog Converter (DAC). All synthesizers, as well as CD players, computers and digital sound processors contain a DAC.
In order to create sound, therefore, a synthesizer must be able to generate digital samples which, in turn, are passed to a DAC and converted to an analog signal (i.e. voltages). The process used to create this essential digital data is called the synthesis algorithm. Different synthesizers use different algorithms in order to generate their samples, and each manufacturer claims to have the superior algorithm. The algorithm is a computer program which is imbedded in the synthesizer's memory and which is capable of generating digital samples when the appropriate MIDI message is received. Since samples must be calculated and converted to audio in order for sound to be heard, the algorithm must run quickly enough to produce approximately 30,000 - 60,000 samples per second.
Frequency ModulationEvery synthesizer has a method by which it can be modified or customized. Exactly what can be altered and how one alters it is dependent upon the synthesizer. In almost all cases, the envelope of the sound can be altered, and features like vibrato can be added. In most, the components that are being used to generate the sound can be altered. The editing can be done directly on the synthesizer, or it can be done on a computer (such as a Macintosh or IBM) running the necessary editing software. The changes are passed between the computer and synthesizer as system exclusive MIDI data. It is usually much easier to edit a synthesizer on a host computer since it has better graphics capabilities and a larger screen.
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