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I have a confusion about the two concepts of velocity sensitivity and attack.

Considering these two apps:

When it detects how fast the touch screen is pressed, is this velocity sensitivity?

How would it detect attack?

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up vote 3 down vote accepted

It's important to understand that what many people call a "keyboard" or a "synthesiser" is two elements connected together: an input device (the keyboard) and a sound generator (a synthesiser).

Velocity is a property of a moving object, such as a piano key as it strikes a hammer mechanism, or a finger striking a touch screen.

Velocity sensitivity is when something measures that velocity, such as when a piano note plays louder because the key was struck harder; or when an electronic keyboard sends a larger number to the sound generation hardware because it measured a faster keystroke.

So velocity sensitivity is something that a keyboard can have.

Attack is a property of a produced sound, using one simple method of generalising the overall volume envelope of a sound. There are lots of ways the volume of a sound could change, and not all fit this model, but synthesisers often simplify it to four knobs or sliders:

  • Attack: how long it takes the sound to go from silence to its loudest volume
  • Sustain: How long it remains at full volume while the key is held
  • Decay: How long it takes to quieten to silence while the key is held
  • Release: How long it takes to quieten to silence after the key is let go

So, attack is something you configure in a synth. And a synth gets to decide what it does with the velocity information it gets from a velocity-sensitive keyboard.

Velocity is not normally linked to attack. It's more likely that you'd make velocity map to the loudness of the note, rather than the time it takes to reach that loudness. However, keyboards and synthesisers are very flexible, and I'm sure it's possible to make velocity control attack.

The news article you have found talks about velocity sensitivity in touch-screens.

It's true that touch screens aren't made to measure how fast your finger is moving when you touch them. However some applications (including iPad GarageBand) do try to guess how hard you tapped. My guess is that they do this by measuring the size of the finger contact it measures. If you press gently, only a small amount of your finger is in contact, making a small circle. If you press hard, it makes a bigger circle. If it can measure frequently enough, it can also detect how quickly the circle changes size.

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+1 How could I forget sustain??! – luser droog Mar 17 '14 at 21:01

Attack is a property of a played note. It is part of the sound of the note, the initial part. For a plucked string, the attack is the very short period between the initial contact of the plectrum and the scraping of the string. After this, the string resounds (reverberates) and begins to decay. For a hammered string, the attack is period between the initial contact and the rebounding of the hammer (or mallet). And similarly for other kinds of musical instrument. The attack is just the initial moment of the sound. There are frequently more non-harmonic elements in the attack period than in the resounding period, thus some synthesis techniques use the addition of noise to bolster the attack. These three parts of the sounding of a musical tone, the attack, the resounding (or resonance), and decay, are collectively called the amplitude envelope (or just envelope). And in a synthesized instrument the parameters of the envelope will mostly depend upon which instrument is selected. An electric guitar will have a very different envelope from a kettle drum, even if playing the same note at the same dynamic-level.

Velocity-sensitivity is a measurement of the capabilities of an analog input device. Can it detect different velocities? How precisely can it detect these differences? Velocity is speed + direction, but here we're mostly interested in the speed of the phenomenon being measured. Speed is distance (length) divided by time. So it's detecting (or inferring) the speed of the motion of the fingertip as it initiates contact with the surface. This should directly affect the dynamics (volume, loudness) of the note played, but may or may not have much effect upon the attack per se, which depends on the type of instrument.

Explaining the way a touch screen detects velocity is, unfortunately, beyond my current understanding of differential calculus. But I know it uses a lot of calculus.

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So, it's impossible to detect attack in an interface like above? But, we set or synthesise the attack depending on which instrument we are simulating? – jessica Mar 17 '14 at 5:08
Right, attack is part of the output, it cannot be detected by such a device. To detect attack you need a microphone and a harmonic analyzer, like a human ear. – luser droog Mar 17 '14 at 5:15
If I could be able to do signal processing to the sound of the 'tap' and find the harmonics, then I would be able to recognize the attack from the phone? Could I relate with the attack from other instruments in order to do synthesis? Or its better to just detect the velocity-sensitivity of the 'tap' and then mapping it with a known attack from an instrument? – jessica Mar 17 '14 at 5:20
I don't understand. A virtual instrument like the above is a synthesizer. It creates the sound using the attack, resonance, and decay properties from its bank of pre-defined instruments. I suppose the sound of the finger tapping the string has an attack of its own, but that's not what is usually meant by attack in this context, but rather the sound, the output. It is an intriguing idea for the phone to listen to the sound of the tap and alter the sound in some way, these apps don't do anything like this. They just associate velocity with dynamics. Attack is a parameter from the bank. – luser droog Mar 17 '14 at 5:29
@jessica By the way, there is no need to accept this answer so soon. It's usually better to wait at least a day to get more answers. – luser droog Mar 17 '14 at 5:31

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