Countermelody or descant
Sometimes a piece of music that is basically melody-with-accompaniment (homophonic) will include a single part that is truly independent of the melody. For example, a choral piece might be chordal for a few verses and then, to keep the music interesting and fresh, add an independent part for a flute or for the highest sopranos on the third verse. This is a countermelody, sometimes called a descant part. Gospel and pop singers often add countermelodies, sometimes imrovised, and classical music also contains many, many examples of countermelodies.
Harmony may be added to a melody with with drones. A drone is a note that changes rarely or not at all (as opposed to pedal tones). Drones can be most easily found in bagpipes music, Indian Classical music and other musics that use instruments that traditionally play drone notes. (See Harmony with Drones.)
An equalizer is a device originally intended to “equalize” or flatten the different tonal characteristics of sound. Today equalizers are more often used to actively accentuate or attenuate certain frequency ranges for creative effect. Many equalizers allow for adjusting the frequency ranges that each knob or fader controls as well as the “Q” factor.
An equalizer that uses sliders to set frequency levels instead of knobs. It is “graphic” in that the frequency curve can be seen by looking at the position of the faders. Most often, these will have fixed “Q” and fixed frequency for each fader.
An equalizer with knobs, often having more adjustable parameters than a graphic equalizer.
An equalizer with a combination of the features of a graphic equalizer—faders/sliders to set frequency levels—and a parametric equalizer—knobs to adjust frequency centers of the faders and the “Q” factor of the faders.
The resonance factor of an electronic circuit. The “Q” stands for “quality factor”. In equalizers, it is equal to the half-power point of the frequency range being controlled divided by the bandwidth. In practice, Q controls on equalizers have much the same effect as adjusting the bandwidth. Essentially, they adjust how large of a frequency band a boost/cut control will affect. A high Q factor results in a narrow band with low Q factor resulting in a wide band. However, if the equalizer uses a true Q control, moving the frequency range will change the bandwidth as well. For example, a 4.5 Q at 1KHz gives a bandwidth around 222Hz. Moving the frequency range to 2KHz while keeping the same Q will widen the effect across a 444Hz range.
Polyphonic Note Recognition
The ability to detect multiple fundamental frequencies inside an audio signal. This is often used for tuners, which detect and tune multiple notes; or some effects, such as octave generation. The algorithms to recognize multiple frequencies are fairly difficult to do in real time, and the speed of such implementations is largely based on which factors, such as if the number of notes, or their expected frequencies, are known ahead of time. Alternatively, some effects that normally require note recognition may apply a flat, frequency altering filter to the whole signal, bypassing the need for specific frequency recognition. Some of the main software algorithms used for polyphonic recognition are MUSIC and ESPRIT.