Digital Piano Basics, Part 1
Imitating the Acoustic Piano
IN PART 1 OF THIS ARTICLE, we describe how a digital piano performs its most basic function — imitating the acoustic piano. We begin with tone production, then move on to controls — the keyboard and pedals — and conclude with the instrument's audio system. In Part 2, we explore all the ways that digital pianos can go beyond simply duplicating the functions of the acoustic piano.
Sample Rate and Bit Rate
The technology now used in most digital pianos to emulate the complex tonal behavior of the acoustic piano is called sampling. Sampling, in its simplest form, is the process of making a digital recording of a sound for later playback. A collection of samples, such as those needed to reproduce the tone of a piano, is called a sample set. There are many decisions to be made in compiling a sample set for an instrument as sonically complex as a piano, perhaps the most important being the sample rate and bit rate.
The sample rate determines how many times per second the sound will be measured. The sound must be sampled often enough to avoid missing changes that occur between sample times. This rate, in turn, depends on the frequency of the sound being sampled. The fundamental frequency of the highest note on the keyboard is 4,186 cycles per second, or hertz (Hz). But the overtones that accompany these fundamentals vibrate at multiples of the fundamental's frequency, and must be properly recorded in order to accurately reproduce the tone. Fortunately, the inventors of the Compact Disc were well aware of this requirement, and long ago adopted the sampling rate of 44,100Hz for audio CD recordings.
The other decision is how finely to measure at each of those 44,100 times per second. Just as we don't want to miss changes in the sound that occur between the times we measured it, we also can't afford to miss the details of those changes. In digital recording, this is called the bit rate, or, as recording pros call it, the bit depth. The higher the bit rate, the finer the detail that can be recorded. In computers, an 8-bit number represents up to 256 levels of detail, a 16-bit number can represent 65,536 levels, and a 24-bit number tops out at 16,777,216 levels. Once again, we will bow to the decision of the developers of the Compact Disc and go with the choice of a 16-bit number as our standard.
What all of this means is that, under the audio-CD standard, every second of sound sampled is measured 44,100 times at a degree of detail that can represent up to 65,536 individual levels. This one second of sample information takes up just over 86 kilobytes (KB) of memory space. Because digital piano manufacturers do not release information about their sampling standards, there's no basis for comparison with the audio-CD standard. However, the rates stated by developers of software pianos tend to be higher than this standard, so it's reasonable to assume that some digital piano manufacturers may exceed these rates as well.
One interesting characteristic of a piano note is that it can sustain for several seconds, but after the first couple of seconds much of the initial complexity of the sound is gone; the remaining seconds of sustained sound go through very little change other than gradually decreasing in volume. This opens up the possibility to save some memory space, and thus some money, by introducing a process called looping. Looping involves selecting a short duration of the sound that remains essentially unchanged over a period of time, and repeating it over and over at gradually reduced volume levels. Done with care, the result is barely detectable when listening intently to the sustain of one note, and becomes completely lost in the commotion when playing normally.