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How does a Digital Piano work? – A Beginners Guide

Sam Baldry

It’s all very well talking about Linear Graded Weighted Keys, Velocity sensitive touch, Key-off Sampling, etc, but what’s actually going-on on the inside of a digital piano? This blog aims to answer the mysteries of how digital pianos work.

Part 1 – Feel and Weighted Keys

So let’s start with the most commonly asked question: what makes a digital piano actually feel like an acoustic grand piano?

First, let’s take a look at the how the Key/Hammer mechanism works for a grand piano.

A Grand Piano Key Mechanism

I’ve simplified the mechanism substantially so it’s easier to see how it works fundamentally (Also note that the front of the key has been cut-off for the purposes of the diagram). So in short, you have a key which moves around a pivot point, or ‘fulcrum‘, which, when depressed, triggers an intricate lever mechanism which pushes against the hammer assembly.

The hammer head then strikes the string, and quickly recedes to allow the string to vibrate freely.

Yamaha’s NWX Key Action Mechanism (Source: Yamaha)

Now let’s take a look at Yamaha’s NWX key mechanism. Like with an acoustic piano, they have a key which moves around a pivot/fulcrum, and a ‘hammer’ triggered by a simple lever mechanism which instead of striking a string, hits a felt pad positioned above it. The hammers for this mechanism are steel hooks, and it is their weight which gives the key-bed a ‘weighted feel’ as they return to their resting position via gravity.

(In addition to this, the NWX keyboards also feature a spring under the key, to provide a more direct and instantaneous counter-force against the key).

In this case, the weight of each key is related to the length of the steel hook, so on models with graded keys, you will find longer hooks for the lower notes, and shorter hooks for the higher notes.

For the escapement mechanism, Yamaha have designed an ingenious solution to mimic the feeling of the hammer retracting from a string, without having to move the hammer at all. Instead they use a rubber membrane underneath the key, which offers a brief resistance when the key is almost fully depressed, giving the impression that the hammer has receded.

In order for digital pianos to remain compact and lightweight, in almost all cases, the keys are far shorter than those found in grand pianos, and the hammer mechanism is often found underneath them, triggered by the front of the key pressing down, rather than the rear of the key rising up.

The key-bed from Yamaha’s AvantGrand N3X (Source: Yamaha)

A noticeable exemption to this however is found in Yamaha’s Premium AvantGrand models, which effectively use a full grand piano hammer mechanism (excluding the NU models however, as these aim to replicate the lighter touch of an upright acoustic piano), providing the most realistic and authentic feel of any of their digital pianos.

Part 2 – Touch (Velocity) Sensitivity

As opposed to strings, the majority of digital pianos use electrical sensors or contact pads to detect when a key has been pressed. Below is a rubber contact strip which a key would press against to activate an electrical contact pad.

On the underside of the strip, you can see that they are coated in a conductive ink, which will trigger the electrical contact by effectively closing a circuit which correlates to the specific key (this is also a very similar way in which membrane keyboards for computers work).

But you might be wondering, how does the sensor know how loud I’m playing? Well you may have noticed there are two points on the contact strip which are coated in the conductive ink, which correspond to two pads for each key.

A Key contact circuit board (Source: Audiotronic)

The contact strip is positioned in a specific way so that the rubber strip presses down on one pad before the other. This allows the piano to measure the time between the two pads being activated, and after some fancy maths, it calculates the speed at which the key was pressed.

A small number of digital piano models however utilise different key trigger methods, and once again Yamaha’s AvantGrand models are a great example. Instead of a membrane and electrical contacts, they instead opt for a fibre-optic sensor, through which a small metal tab on the top of the hammer passes.

Without the need of any physical contact, the fibre-optic sensor is able to detect when the hammer has reached a certain position, and a what speed it was travelling, by detecting when the sensor was obscured by the metal tab on the hammer. This use of a contactless trigger also means there is no interference with the feel of the key action.

Used Digital Piano Selection

Part 3 – Samples

So now that we’ve got a nice feel, and the piano is able to determine how loud or soft we’re playing, how does this actually translate to sound being produced?

You may have heard that digital pianos will never go out of tune, but many people are often curious as to why this is.

When you press a key on a digital piano, it will play a ‘Sample’, which is a specialised sound file of an instrument playing the same note at the same volume. Samples for Yamaha’s CFX grand piano for example, will have been recorded on a hand picked CFX piano, in a certain room kept under very specific conditions.

A grand piano being recorded for samples for Native Instruments (Souce: Native Instruments)

It is common to use a configuration of multiple microphones when recording samples of (particularly acoustic) instruments, and especially for pianos, in order to capture the sound from every part of the cabinet and strings, as well as from different positions within the room in which it was recorded.

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Multiple variations of each note will then be recorded, from loud to soft and sustained to dampened etc, in order to build a vast spectrum of different volume intonations and range of expression for each.

When you play a note on a digital piano, multiple samples will be played from the real piano which was recorded, in order to provide an accurate note intensity, but also include more nuanced aspects such as the adjacent strings resonating next to the note you are playing.

Having a collection of multiple samples for each key not only improves the authenticity of sound, but also provides scope in allowing the player to tweak the sound of the piano as they wish, and alter different parameters such as the lid position.

An illustration of high vs low quality samples (Source: SlideShare)

It is important to note that the sample quality is a big factor in judging the quality of a digital piano, as more expensive models will have higher quality samples and/or better ‘sampling technology’ which take up far more space in the instruments internal memory. The sample quality is primarily affected by the data compression being used on the sample files, and to some extent, the microphones and room the instrument was initially recorded in can have an effect.

Used Keyboard Selection

Part 4 – Speakers

So, we can replicate the feel of a real grand piano, and have captured its sound, but then how does do we actually translate this into the sound we hear from a digital piano?

For acoustic pianos, when a string is hit by a hammer, it vibrates, generating sound energy which is reflected inside the instrument, primarily off of a wooden ‘soundboard‘, which resonates in unison with the strings. It is the soundboard which gives acoustic pianos their distinct warm rich sound.

A soundboard from a Yamaha grand piano (Source: Yamaha)

But how does this work in a digital piano? Well, each digital piano model uses a specially designed speaker system to play the stored sample files.

Unlike with normal HiFi speakers, which are designed to play music and audio as close to the source material as possible, digital piano speakers are placed in specific locations, and are ‘tuned’, in order to give the most realistic sound compared to that of an acoustic piano. The N3X is a brilliant visual example of how its speakers are positioned in order to recreate the sound of a grand piano as closely as possible.

Where high end HiFi speaker systems are usually designed to have what is referred to as a ‘flat‘ response, digital pianos are designed to have a slightly ‘coloured‘ response, to match the harmonic qualities of an acoustic piano.

The upwards facing speakers on the Yamaha N3X (Source: Yamaha)

Because of this, if you were to plug your digital piano into a pair of professional studio monitor speakers, the samples would sound almost identical to the source recording, but it would not replicate the effect of actually sitting in front of a grand piano and playing it for yourself like the digital piano would.

In Yamaha’s high spec digital piano models, including the CLP775 and CLP785, they have also introduced the use of tactile transducers. These are devices which work similar to speakers, but instead of using a cone or diaphragm to produce sound, they attach directly to one of the panels of the digital piano, causing the entire panel to vibrate, replicating the effect of a soundboard on an acoustic piano.

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I hope you found this blog helpful, take care everyone.

Sam Baldry
Sam divides his time between sales and assisting with stock maintenance and other logistics in our warehouse. He also occasionally writes for the ePianos blog, using his keen interest in engineering to help explain product technology in everyday terms.
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How does a Digital Piano work? – A Beginners Guide

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