MEMS – Bringing Digital Music to Life

The electronics inside digital pianos.

Lots of households with aspiring musicians have digital pianos. But do modern electronics really allow digital pianos to accurately mimic traditional instruments?

This question is reminiscent of the debate of the physical versus touchscreen as BlackBerry rolled out its BB10 hardware, since the central feature of any piano is its keyboard.

Image courtesy Kawai America Corporation

For any move from the analog realm into digital, you need to take a naturally occurring phenomenon – sound in this case – and convert it to digital bits for convenient storage and communication.

You could build a piano from a system that encompasses a key–hammer–string–transducer. In a real piano, the transducer is in the listener’s ear. For digital, the instrument requires a little something extra. How do you convert the motion of a vibrating string into the digital domain?

One option is strain measurement, perhaps using a piezoelectric component. Crystal strain would be converted to a voltage signal. Another approach would be to use the now ubiquitous microelectromechanical system ( MEMS ).

Among myriad other uses, MEMS devices are ubiquitous in cell phones and tablet computers. But they can do a lot more than detect screen orientation. A MEMS component would provide a sensitive and accurate interface from the vibrating piano string to the electronic system.

In very simple terms, the MEMS device used in the top digital pianos is an accelerometer. Special semiconductor processing is used to create a chip with a series of free polysilicon fingers interdigitated with a series of corresponding fixed fingers.

Movement of the device causes the polysilicon fingers to move with respect to each other, causing capacitance changes between electrical terminals attached to one sides’ stationary fingers and the moving fingers. These small capacitance changes are multiplied by the use of a large number or interdigitated fingers that create voltage signals sufficient to detect very subtle movements.

Image courtesy Soboyejo Reseach Group, Princeton Univ.

But converting the piano string adds unnecessary complexity and cost to the digital piano. Instead, the typical digital piano plays audio samples recorded from real world instruments. In addition to reducing the system complexity, the use of pre-recorded digital samples allows the manufacturer to record the best sounding physical instruments available.

That gets us one more step along the path to creating the digital instrument. Eliminating the piano strings and using a Steinway sound sample still leaves a huge gap for creating an instrument that will appeal to discerning pianists.

Far more than the BlackBerry Q10 user, the serious pianist needs a keyboard with the right feel. They also need the instrument to respond to subtle changes in input.

To create a digital instrument on par with a top-quality analog piano, the designers focus on the structure of the key and hammer mechanism. Although there are no strings to strike, the best feel is offered by a key connected to a hammer that is similar to what the finest grand pianos provide.

That’s why leading digital piano keyboard designs still incorporate hammers. This not only creates a similar weighting and feel, but also creates the quiet percussive sounds that a professional pianist feels and hears (even if the audience doesn’t) when playing the most expensive concert grand pianos.

Here is where MEMS devices make another appearance. The MEMS device attached to the key and hammer unit readily detects a wide range of key strikes. Through the use of high-quality electronics, the digital piano interprets anything from pianissimo to triple forte.

Like many of our modern digital items, the digital piano offers unparalleled convenience that makes it easy to overlook a reduction in sound quality. Although the best-engineered digital pianos come very close to the musical experience of expensive fully strung instruments, they can be far more portable, never need retuning, and the sound quality is unaffected by temperature or humidity. That’s not to mention the other standard of nearly every digital device. Digitals are a fraction of the cost.

So MEMS sensors really can create a digital instrument capable of taking an expert pianist’s interpretation of anything from the softest passages of Chopinto the wildest hammering of Beethoven.