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Positional sensing made easy Whether you’re building a trigger from scratch or doing a home repair, digitalDrummer can help. Philippe Decuyper will find the answers to your DIY dilemmas. Just email your questions to editor@digitaldrummermag.com. This month’s question is from Kaushik Nilakant: “I am a little lost over how positional detection/sensing works on Roland drum modules, and how the configuration of piezos contributes to that effect.” THE THEORY BEHIND positional sensing has been overviewed in a previous issue of digitalDrummer. To recap, let’s say that a piezo placed in a crossbartype trigger with mesh head receives more bass frequencies (long waves) if a strike occurs next to it. If this piezo is placed right in the centre of such a trigger, it will then get more bass on a centred hit and less bass from the edge. Let’s get into details now… In order to detect the position of a hit, a module must analyse the produced waveform. It needs to know how much bass there is in the signal from a piezo which is used as a microphone. Analysing frequencies can be done using various methods. Some methods are very precise but need a lot of costly mathematical operations (if you are curious enough, you can search the web for “Fast Fourier Transform”). The basic requirement is fast response to avoid latency. Roland owns a patent (US patent #7385135) which describes the extremely fast method it uses: 58
“Namely, when detection (of the) signal of the head sensor in the case when the head, composed of a net-like raw material, is ... observed, there is such a characteristic that a first half-wave time changes dependent on a position of percussion point in a certain frequency band.” Ouch! What does it mean? It means that what’s coming from a piezo must be cleaned up to be useful (“a certain frequency band”) and that the module just needs to know how long it takes for this clean signal to go from 0 volts to 0 volts as soon as a hit occurs (“first half-wave time”). We can then imagine how our module works: Our piezo starts to produce enough electricity… -> a hit is occurring …wait until the signal goes back to 0 volts. -> then we can know how strong this hit was (from the voltage “peak”) and how far from the centre it was (from the elapsed time). www.digitaldrummermag.com