Yet another aspect of touch weight is that it varies from one end of the keyboard to the other. In an acoustic piano, the hammers are significantly heavier at the bass end of the keyboard than at the treble end, which results in heavier touch weight in the bass and lighter touch weight in the treble. Enter the graded hammer action: To replicate the touch weight of the acoustic piano keyboard, most digital piano actions employ in their designs the equivalent of graduated hammer weights. Rather than using 88 different weights across the span of the keyboard, which would be cost-prohibitive and of questionable value, it's common to use four different touch-weight values, each one used uniformly throughout one touch-weight zone.
Some high-end digital pianos employ wooden keys to subtly move you closer to the feel of an acoustic action. The physical properties you may detect would be a slight flexing of the key, a difference in the mass of the key, and possibly a very slight difference in the shock absorption of wood vs. plastic when the key is depressed and bottoms out (although this is mostly masked by the felt pad under the key).
Another aspect of key design is the tactile property of the keytop material. Ivory is so prized (and missed) by acoustic piano players not for its appearance, but for the fact that it's porous, and thus offers a degree of "grip" that slick-surfaced plastic keytops don't. This grip is particularly valued when the playing gets serious and the pianist's fingers become sweaty, which typically occurs during demanding passages, when the pianist's accuracy and control are pushed to their limits. Ivory substitutes, such as Kawai's Neotex, Roland's Ivory Feel, and Yamaha's Ivorite, provide the positive properties of ivory without the discoloring, cracking, and chipping for which ivory is equally famous. Other manufacturers have since added this feature, and it's one worth considering when comparing instruments.
Dynamic (Velocity) Sensors
The final aspect of the digital piano action we'll explore is how it measures the force the player's fingers apply to the keys. This is typically done using two electrical contact switches that are closed in rapid succession as the key is depressed. Alternatively, some high-end digital hybrids use optical sensors to sense the key's motion — a small flag attached to the key breaks a beam of light as it descends. However, what these sensors actually measure is not force — that is, how hard the key is depressed — but the speed or velocity with which it is depressed. This is why you'll sometimes see the term velocity sensing in the keyboard specifications. As the key moves to the bottom of its travel, the instrument measures how much time has elapsed between the signals received from the first and second sensors. A longer time indicates that the key was traveling slowly and tells the instrument to produce a softer tone; a shorter time means a faster, harder keystroke, and thus a louder tone — it's that straightforward. Some actions employ additional switches to trigger other sample types, such as the damper effect mentioned earlier.
Modern acoustic pianos have three pedals. Let's take a look at how they work, and how their functions translate to the digital piano.
In the common three-pedal arrangement of an acoustic piano, the pedal on the right is the sustain pedal. In the case of digital instruments having only one pedal, it is the sustain pedal. Some refer to this as the damper pedal, because its mechanical function on an acoustic piano is to lift the dampers away from the strings. On a digital piano, the sustain pedal is an electronic switch. When depressed, it tells the instrument to allow played notes to gradually decay as they would on an acoustic piano.
The most frequent question about a digital piano's sustain pedal is whether it can perform a function called half pedaling. The acoustic piano's sustain-pedal mechanism can move the dampers from a position of rest on the strings to a position completely clear of the strings — or anywhere in between. Between these two positions is the highly useful half-pedal position, which allows the player more control of tone and sustain. While half-pedal capability is now commonly found on upper-end digitals, it is not always present on lower-priced instruments, where the sustain pedal is more likely to be a simple on/off switch that allows full sustain or no sustain, but nothing in between. Some lower-priced digitals come with a separate square plastic or metal foot switch rather than something that looks like a piano pedal. However, even if the piano itself is capable of half-pedal control, the foot switch may provide only on/off sustain. The same may be true even with some pedals that have the appearance and movement of a piano pedal. It's always worth checking the specifications to be sure that both instrument and pedal are capable of half-pedal control.