jejt / jmons

Engineering Tommorrow

Arduino Audio Filter


Whats the difference between Noise Makers and Audio Filters – well, I would expect a level of control. A Noise Maker implies a lack of control – the difference between “din” and “music”. I’ll start this post off with this image, which is a Waterfall image. Waterfalls are spectrum over time – In this case, the frequency is up the side, and the time is across the bottom. The intensity of the colour indicates a voltage at that particular frequency.

Waterfall of Arduino Noise Maker

Waterfall of Arduino Noise Maker

What we are seeing is how the frequencies are changing over time when the dials on the board are swizzled, and by comparing this with other audio projects, we are able to see how various features and code changes change the output.

The next stage of the project is Filters – Filters are basically devices which limit frequencies that go through them – there are essentially two types of filters, High pass (which allow high frequencies to go through) and Low Pass (which allow low frequencies through). By combining these together, you can generate Band Pass filters and Notch Filters (although you might make notch filters in a different way).

By adding filters together, you can make filters which let multiple bands (a comb shape) or ban multiple bands. If you have control of your filters, you can change them and manipulate them over time, and apparently, if you generate white noise on one side, and use two bandpass filters, you can make sounds which sound like words – by moving the two filters in and out and widen and narrow.

Anyways, when I was at Uni, I learnt how to build Digital Filters, that is I learnt the electronics behind the DSP, and its quite simple – well, its a little tricky to get them efficient, but a problem that’s been solved time and time again. The system works by multiplying a set of samples in a fashion. Unfortuantly, my choice of modules (50% electronics or low level computing, 25% maths, and about 20% computing theory) means that… I didn’t do the Signals and Systems module because my Maths skills are troubled. I loose the decimal place ok?

So I’ve had a crash course in Fourier Transforms, which are the very basics behind working out the numbers that you need in the sample multipliers… Which is what I’m about to build on this new circuit. So far, I have an analog input (which is my sampling system) and an 6 Bit R2R DAC shield. The first test will be to act as a buffer. The second will to act as an Echo Box, and the third, to Filter.

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