SOUND SYNTHESIS TUTORIAL (II)
Tutorial about synthesis of sound: the ADSR envelope.
ADSR Envelope
The ADSR (Attack-Delay-Sustain-Release) is probably the most important aspect for shaping any sound. The four components of the ADSR form what is known as the envelope of the waveform. An envelope represents the variations in a certain magnitude that a given sound experiments over time. In this case I am referring to amplitude envelope, which dictates the volume or loudness of a sound over time and contributes so much to its personality that sounds are often categorized in base of their amplitude ADSR.
Having a clear idea about which kind of sound we want to create in a synthesizer, after selecting the correct waveforms and adjusting other settings in the oscillators, we would proceed to tweak the ADSR values in the amplificator (AMP) until finding the amplitude envelope which better suits the desired personality of the sound. The sound of a musical instrument is characteristic not only because of its timbre, but also because of its amplitude envelope. The following pictures show a representation of how Attack, Delay, Sustain and Release parameters shape the envelope of the sound (left picture) and the ADSR controls (for both loudness and filter contours) of the filter bank of the MiniMogue Luxus analog synthesizer (right picture).
Looking at that particular ADSR graphic we can see that it represents a sound whose whole length can be differentiated in two parts; the first part (Attack and Decay) shows a sort of brief and loud sonic impact, such as that generated by a percussion hit or a plucked string, whereas the second part (Sustain and Release) shows how the sound, after quickly losing loudness, has a stable amplitude during a certain amount of time, until being finally extinguished after a gradual decrease of amplitude. It is easy to understand that Attack and Decay correspond to the moment when a note is plucked or struck in a musical instrument, whereas Sustain and Release are related to the sustained vibrations that might follow just after. Hence, the ADSR envelope depends on the characteristics of the musical instrument that we want to recreate in our electronic sound generator.
The concept of ADSR envelope is crucial for achieving a proper synthesis of musical sounds. The combination of these four parameters provides a large amount of variations which can be used to imitate the sound of real instruments or otherwise create fantastic sounds. To recreate the effect of a percussion hit or a plucked/struck note we should adjust a short Attack time followed by a short Decay time, a minimal or relatively low Sustain level and a short Release time. In the opposite case, to create a pad sound which can be used as a sonic texture that fills the sonic space with a deep or dramatic atmosphere, we should use a long Attack time followed by a long Decay time, a pronounced Sustain level and a long Release time. Some percussion instruments, such as a snare, are characterized by insignificant amounts of Sustain and Release, whereas others which generate more prolonged vibrations when hit, such as a crash cymbal, are characterized by larger amounts.
Looking at the picture we can see that whilst Attack, Decay and Release are defined by an horizontal span, Sustain is defined by a vertical span. This means that the ADSR controls of an electronic instrument would adjust the time span of Attack, Decay and Release, and, on the other hand, the amplitude level of Sustain in relation to that of Attack, which represents the maximum amplitude level of any sound. When, for example, we strike a key in a piano keyboard, Attack starts and ends on its own and then Decay does the same, but the sound remains sustained as long as we press the key, and Release starts only after releasing the key; this means that a long Release time often causes the sound of successive notes played to be merged. If Sustain level is minimal (and hence Release, which is dependant upon the former) the sound is quickly attenuated after striking a key, regardless of it being released or not.
This is how the classical ADSR envelope works, but different instruments or the utilization of pre-processed waveforms (called samples) would allow to use different ADSR rules. If we are playing a sampler, the length of the sample waveform assigned to the key would define the length of the sound, because the regions that we know as Attack, Decay, Sustain and Release are built in the very waveform. Of course, the behavior of the classical ADSR could be recreated as long as the sampler includes such feature. The following pictures show several waveforms as they are displayed within an audio editor; these waveforms are samples of a single note played in different musical instruments.
This waveform corresponds to a snare drum; it has a very fast Attack, and after a pronounced Decay it shows rather prolonged vibrations ranging from low to very low amplitude. The sound is short and incisive, being capable of standing out over almost any other sound.
This waveform corresponds to a mid tom, which being a percussion instrument as well, produces a waveform which has some resemblance to that corresponding to a snare drum, being less extreme in its parameters. Attack is still fast, but Decay is much less pronounced and vibrations are somewhat stronger, so Sustain and Release should have slightly higher values.
The waveform produced by a hi-hat showcases the ability of said instrument of sustaining its vibration over time; Decay is progressive and hardly distinguishable from Sustain and Release, whilst Attack is short albeit slightly longer than in the previous examples.
This waveform corresponds to a saz, a string instrument of Turkic origin; Attack is short and Decay is relatively fast, whereas Sustain and Release are represented by a soft and long vibration.
This waveform corresponds to another string instrument of Asian origin; the differences with the previous example are obvious, having a notably softer Attack and a more intense Sustain.
These last two waveforms correspond to chimes (upper) and wind (lower) samples; they both have a prolonged Attack time and a slow, hardly distinguishable Decay.
The following picture shows the ADSR Envelope function as implemented in Cool Edit Pro 2. It allows to apply the desired ADSR envelope to any waveform, altering so the evolution of the intensity of sound over time. A related but different tool is the Amplitude Envelope, which uses multiple-point splines to shape the intensity of sound over time.
