Rigidity
The hardness of the keys on a keyboard is the amount of resistance they provide when pressed.
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Unweighted. Very low rigidity: the keys literally "fail" under the fingers. Found mostly in entry-level keyboards; this is due to the fact that unweighted mechanics are inexpensive, but the low pressure resistance makes it difficult to select the optimal pressure and control the dynamics of the sound.
— Semi-
weighted. Keyboards with medium resistance — not as high as on weighted keys, but noticeably higher than on unweighted ones. This option is considered optimal for active keyboards (see "Mechanics"): at a low cost, it gives quite good responsiveness and, with a minimum skill, allows you to accurately control the pressing force.
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Weighted. Keyboards with high resistance, which is comparable to the resistance of classical piano keys. Note that high effort in this case is an advantage: it allows you to bring the response of the keyboard as close as possible to the response of a classical piano and makes it easier to control the pressure and dynamics of the sound. Actually, by definition, all hammer-action models are made weighted (see "Mechanics"), but among active keyboards this option is rare — due to complexity and high cost.
Aftertouch
The presence of the aftertouch function in the keyboard (aftertouch). The essence of this function is to track the pressure force on the key after it is pressed. In fact, this means that in models with aftertouch, you can control the sound not only due to the force and speed of pressing the key, but also due to the change in pressure on it after pressing; this gives additional features and allows you to use various specific tricks of the game. The specific ways of changing the sound “tied” to aftertouch can be different, depending on the synthesis settings set: for example, when recording a guitar part, you can tie a small change in tone to this function and imitate string bending, when recording a saxophone, change the volume of notes, and etc.
Relatively inexpensive models use a common sensor to monitor aftertouch, which monitors the average force of pressure on all keys pressed; in more advanced ones, separate sensors are installed for each key.
Transport control
The presence in the keyboard of
the transport control function.
In this case, we are usually talking about support for the MIDI Machine Control standard, designed to control external recording and playback equipment. Such support actually turns the keyboard into an external remote control, allowing you to give commands from it to "Play", "Stop", "Pause", "Rewind" (forward and backward) and "Record". To do this, the design provides a separate panel with a set of appropriate buttons.
MIDI out
A specialized output for transmitting MIDI signals to an external device — a sequencer, a computer sound card (if there is a MIDI In input), etc.
MIDI signals are a kind of "digital notation": they tell which key was pressed, with what force and for how long, and also carry information about various additional parameters: timbre, tempo, key, etc. To convert such signals a "ready" sound requires an external sequencer (software or hardware).
Note that despite its specialized purpose, the
MIDI Out output is not mandatory for modern MIDI keyboards: MIDI signals can also be transmitted via other interfaces, such as USB or Bluetooth (see the relevant paragraphs). So it makes sense to specifically look for a keyboard with such a connector if you plan to connect it to equipment with a MIDI In input.
