Making VOX

So we are now two weeks through BREATHE: a celebration of air and we’ve had loads of questions about our audiovisual installation VOX. This is the third time that Output Arts have built a wind harp installation and this is, by far, the most technically challenging iteration.

In the past we have basically just strung some metal strings across a lump of wood with an electric pickup hooked to an amplifier. This time we decided to make things substantially more difficult for ourselves. The first difficulty was that we decided to make the lump of wood a 1.6m-side cube, with the strings at right-angles to each other across the centre. This means that we need a pickup per string rather than being able to stretch a set of strings across one pickup. Further, since the harp is across the road from our venue, The Old Church in Stoke Newington, high in the tower of (new) St Mary’s Church, we’re not going to be just dragging a few cables between them.

I tried using off-the-shelf, bass-guitar pickups, but the shape of them is awkward for a single string and they just don’t have enough copper in them to pick up the fine vibrations caused by the wind across the strings. So I went back to hand-winding pickups; luckily I’ve gotten better at doing this, producing fairly consistent results across the three pairs of coils and making a special effort to spin them the right way each time (one clockwise and one counter-clockwise for a hum-bucking pair). I use a pretty low-fi approach — a sawn-off nail with glued plastic caps spun in a hand-drill, and a neodymium magnet — but the result is pretty decent and the making process is very satisfying (see the video at the bottom of this article for some clips of me winding them).

Each of the pickups is connected to a balanced input of a 4-channel digital audio interface and a small computer. This computer digitises the signals from the strings, parcels these up into packets (25 per second, per string) and transmits these from the tower of the new church across to a receiver in the tower of the old church using point-to-point WiFi. The equipment we found is designed for hooking up ranches in the middle of nowhere to an Internet provider 20 miles away – so a little over-specced for the problem at hand, but better safe than sorry! Having got these packets of signal over to the venue we can then reassemble them into audio ready for passing into the PA system.

A key tenet of the installation is that the strings are tuned to resonant frequencies of the space (see my article on resonant frequencies). This is where the second, substantial difficulty comes in: a musician will typically retune a stringed instrument multiple times during an evening of performance to account for the changes in the condition of the instrument or the environment. Unfortunately our instrument is up a spiral staircase so high and so narrow that I feel not just tired, but actually dizzy by the time I get to the top of it; our performance is three weeks long and our environment is the endlessly changing weather. Maintaining the tuning of the harp is effectively impossible.

So we cheat.

It’s a small cheat, but one worth explaining if you are not to feel cheated. The process of digitising the signal at one end, breaking it up into pieces, reassembling and then re-synthesising audio at the other end is done by converting the signal from the time domain to the frequency domain (and vice versa); this means changing a stream numbers representing the movement of the string into a stream of numbers describing the frequencies and amplitudes of those movements. Done correctly, the translation process is invisible to the ear. It is, in fact, a similar process to that used to compress audio in MP3s. The cheat is that when we reconstruct the audio at the other end we are able to shift the frequencies slightly to match the resonant frequency that we’re trying to hit.

Effectively, we are auto-tuning the strings: the dynamism of the vibrations is retained, but the pitch is kept constant. In fact, deliberately not quite constant: much of the richness of the sound comes from the fact that the pitch of each string varies slightly – a sort of warble. The code monitors the long-term base frequency of each string, calculates an adjustment factor against the desired frequency, and then multiplies each of the actual frequencies detected moment-by-moment by this adjustment before resynthesising the output. The result is equivalent to twisting the machine heads to change the tension of the strings, but without the endless climbing up and down the tower.

A side benefit of this process is that we can discard the inevitable hiss that comes with electric pickups and high-gain amplification. Hiss is just a mass of low-amplitude signals spread across the entire frequency spectrum. Switching into the frequency domain allows us to separate out the individual higher-amplitude signals that represent the actual vibrations of the strings from this background noise. The result is both a smaller amount of information needing to be transmitted across the road and clearer sound at the other end.

The same numbers that describe the sound of each string are easily turned into a power, or volume, figure for each string. It is these that are used to drive the visual part of the installation. The amount each string vibrates over time is tracked and converted into a change in volume. These relative changes in volume are turned into brighter or dimmer values of the three colours associated with the three strings: red, green and blue. By mixing these colour values together we get a constantly changing hue and brightness that is projected onto a screen in the church.

The shape of the screen echoes the space and, while being deliberately monumental, by being forward of the altar and at ground level is not apart from the viewer. As the wind picks up, the screen glows brighter and as it dies down, it dims. The hue shifts and moves with the wind, but not so directly as to mirror it. The visual component is deliberately designed to be subtle and gentle, often appearing to be doing nothing, but rewarding watching for longer periods. The attention focused on it results in a more meditative experience and a deeper listening to the sound of the wind harp.

The result is an immersive artwork that is better experienced than described. If you can make it to Stoke Newington before the festival ends on 20 March 2016, then please take a look. More information about the festival can be found on the festival website; below is a video you can watch showing the construction and installation of the wind harp.