System uses lasers to detect the pitch of a guitar string before a note is played
By Paul Ridden
July 20, 2011
If you've tried to digitally capture a guitar performance or turn your six-string into a powerful MIDI synth using an interface or special pickup, you'll no doubt be familiar with signal lag and pitch accuracy issues. As you pluck a string, there is a very short delay before it reaches its final frequency - it might only be a few milliseconds but it can be enough to cause latency angst. Germany's M3i Technologies has developed an optical system for accurately detecting the pitch of a string - even before a player has plucked it. The Laser Pitch Detection (LPD) Pickup system runs a beam under each string, from the bridge to the end of the fingerboard, and monitors its return to photosensitive receptors in the unit. When a player's finger touches a string, the beam is shortened and the system quickly measures and calculates the exact pitch.
On those rare occasions when I've wanted to interface a guitar with the digital world of MIDI, I've headed down the somewhat problematic route of using a hexaphonic pickup and a pitch-to-MIDI converter. The issue with such methods lies with the guitar string not immediately vibrating at its final frequency upon contact with a pick or a finger, there's a short delay of a few milliseconds while it reaches actual pitch. Even the fastest system available is said to have a latency of around 12ms, and also introduces additional unwanted spikes and other uncertainties.
The LPD Pickup system from M3i Technologies (the company which recently partnered with Fraunhofer's Institute for Surface Engineering and Thin Films to develop a new sensing film for stringed instruments) is said to be capable of registering the correct pitch of a string before a player's pick gets anywhere near it. The deepest note of a guitar string is determined by its maximum length, between the bridge and the nut. Altering this length by pressing down somewhere along the fingerboard of the instrument produces a higher note - the LPD system simply measures this distance and calculates the correct frequency.
At the bridge end of the guitar, each string of the guitar has a laser diode sat underneath it that sends out a beam parallel to each string towards the end of the fingerboard. As a player places a finger on the string the beam is shortened and light is reflected back to photo elements at the bridge. The new distance is accurately measured and the appropriate frequency calculated, with a recognition latency of less than a millisecond. The string length doesn't matter, the system will work on bass guitars, too. In fact, the developers say that the LPD Pickup system can be used on any stringed instrument with a fingerboard.
Like the ATG-6 development, pitch detection doesn't depend on the instrument being in tune and imperfect playing style isn't really an issue as string noise like fret buzz does not interfere with pitch capture. The LPD Pickup system also includes pressure-sensitive, piezo-elements built into the tailpiece to register such things as string bends.
Thanks to multiplexing, only one laser is active at a time which ensures that the photo sensor only registers the reflection of one dedicated laser at any one time. The laser is on for a fraction of a millisecond and has an average output power of less than 500 microwatts, electronic circuitry ensures that diode power does not exceed the power level and shuts down the lasers if the switching process stops for any reason. The low power draw means that the system could run on batteries.
The continuous output of light power is less than 1 milliwatt, which is considered safe enough for use without protective eyewear and claimed to be safer to use than laser pointers or CD/DVD player and less likely to cause discomfort than a laser light show at the local discotheque.
The LPD Pickup system can operate wirelessly, and incorporating a Bluetooth interface could open up a world of smartphone app interaction - such as learning to play with something similar to, but no doubt more powerful than, The Way of H or the Gibson Guitar App.
The developers say that information from the system can be converted into MIDI signals to record directly to DAW software or drive synthesizers and that the technology could also enable real instruments to be used as gaming interfaces for music games like Rock Band and Guitar Hero.
M3i Industries says that the technology is still in development but has plans to make it commercially available in 2012. In the meantime, the company has prepared the following video demonstration of the LPD Pickup system in action:
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