The notes produced by an acoustic piano have a physical point of origin in the instrument's strings, and can be heard moving from left to right as you play a scale from the left (bass) end of the keyboard to the right (treble) end. To preserve this spatial relationship, the samples in a digital piano are recorded in two-channel stereo. This feature, often called "panning," adds to the realism by physically positioning the sounds in ways similar to what is heard from an acoustic piano.
Number of Notes Sampled
Now we must decide how many notes to sample. The obvious answer would seem to be "all of them," and some manufacturers take this route. But in the interest of keeping the cost of the digital piano under control, many manufacturers seek alternatives to sampling all 88 notes.
In an acoustic piano, the tonal behavior of the longer, bass strings is different from that of the shorter, treble strings. In fact, this tonal variation goes through several changes as you play up the keyboard from the bottom. Some of these changes are due to the differences in string length, others to differences in the types and numbers of strings associated with different ranges of notes. In the lowest bass, the hammers strike a single string per note. This string is wrapped with heavy copper wire to slow its rate of vibration to produce the proper pitch. Depending on the piano's scale design, a couple of octaves up from the bottom of the keyboard it switches to two strings per note, each wound with a lighter copper wire. Finally, by mid-keyboard, three plain-wire strings are used for each note. (Each set of one, two, or three strings per note is known as a unison because all the strings in a given set are tuned at the same pitch to sound a single note.) The subtle changes brought about by these different string arrangements also figure in the tonal variations we hear as we move up and down the keyboard.
But the tonal changes from one note to the next are not always noticeable; sometimes, all that changes is the pitch. It turns out that it's a fairly simple matter for the digital piano to play back a sample at a different pitch. This makes it possible to save memory space by using one sample as the basis for two or three consecutive notes. Taken too far, this would result in obvious tonal problems. But if at least a third of the notes are sampled, with careful attention to areas of the keyboard where there are more noticeable changes, these shared samples can produce a convincing, if basic, tonal progression.
One more source of tonal variation — the effect of dynamics (variation in volume or loudness) — must be dealt with before we move on from our basic sample set. Striking a string harder results in a larger number and greater prominence of higher overtones, which, in addition to making the sound louder, give the tone more "edge." Currently, in all but the least expensive instruments, digital pianos use from three to five dynamic samples. As you play with varying degrees of force, the digital piano selects the closest appropriate dynamic sample for playback. Entry-level pianos that use a single sample level for dynamics also use variable filtering of a note's overtones to simulate these tonal differences, sometimes with remarkable success.
Sampling Other Effects
Many digital pianos incorporate additional types of samples aimed at capturing more of the nuance of an acoustic piano. At this time, the two most common such samples are string resonance and damper effect. As with so many features, different manufacturers seldom use the same terms for the same effects. String resonance is related to the strings' overtones. Each of the overtones generated by a vibrating string are at, or close to, the fundamental frequencies of higher notes whose frequencies bear a mathematical relationship to the one played. This results in a weak sympathetic or resonant vibration of the strings of the related notes, and adds another dimension to the sound. (To hear this effect, slowly press the keys of a chord — for this discussion, let's make it a C chord — without actually sounding them. While holding these keys down, quickly strike and release the C an octave below the held chord and you'll hear, faintly, the sympathetic resonance of the C chord above.)
In an acoustic piano, a note's felt damper moves away from the string(s) when its key is depressed, and returns to stop their vibration when the key is released. The effect on the sound is not instantaneous; it takes a fraction of a second for the strings' vibration to stop. During this time the tone is altered as its overtones rapidly decay. Damper-effect samples are triggered by releasing a key, and add another subtle dimension to the digital piano's sound.