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Goldilocks Analogue Synthesizer




For the past year, I’ve been prototyping an Arduino clone, the Goldilocks Analogue, which incorporates advanced analogue output capabilities into the design of the original Goldilocks with ATmega1284p AVR MCU and uSD card cage. Recently the design scope crept up to include two SPI memory devices (EEPROM, SRAM, FRAM), and microphone audio input. But, before I go through another prototype cycle, I thought it would be a good idea to build some demonstration applications, showcasing the capabilities of an arduino compatible platform with integrated analogue output and have some fun with audio.Goldilocks Analogue Synthesizer

Some of the initial tests I’ve built include some 8 bit algorithmic music and, using two Goldilocks Analogue prototype devices, a digital walkie talkie using Xbee radios. They were fun, but don’t really demonstrate the full range of the audio capabilities of the platform.

It seemed appropriate to build a synthesizer using the Goldilocks Analogue as the platform, and a Gameduino 2 shield incorporating a FDTI FT800 EVE GPU, and see how close I could get to a musical outcome.

Research

Before randomly building something that made a bunch of squeaky sounds, I thought the best thing to do is to learn something about the field of analogue synthesizers and synthesizing audio.

I also obtained some simple analogue synthesizers from Korg to see exactly what they produce, so I could copy them. Some people write that this monotron analogue synthesizer family are good examples of a low cost musical instrument. I found it very interesting to examine the wave forms produced by the various settings.

Using the features of the two Korg devices, I was able to define the goal for the synthesizer that I wanted to build using the Goldilocks Analogue.

The Korg monotron DUO has two voltage controlled oscillators (VCO1 and VCO2), which produce square waves. The VCO1 has a pitch setting, which defines the basic frequency at which the ribbon keyboard operates. The ribbon keyboard can be set to have a major scale, a minor scale, a full chromatic scale, or be a ribbon with no set notes. For clarity, the pitch on the DUO is analogue, so there is no guarantee that the notes generated by the ribbon keyboard will be in tune.

The VCO2 pitch can be modified either below or above the pitch of the VCO1. In its middle section, with some care, it can be matched exactly to the VCO1 setting. The switch allows either just the VCO1 or both VCO1 and VCO2 to produce sound. A separate XMOD intensity knob allows the VCO2 to modulate the frequency of the VCO1 oscillator, producing cross-modulation.

The monotron DUO contains the famous Korg MS-20 resonant low pass filter, which can be adjusted for both cut-off frequency and intensity of the resonant frequency. Setting the filter values allows the square wave noise generated by the two oscillators to be shaped into very interesting tones.

The Korg monotron DELAY is a very different device from the DUO. It has two oscillators, but only one at audio frequencies. The audio oscillator produces a saw-tooth wave at a frequency controlled by the ribbon keyboard. On the monotron DELAY there is no capability for playing specific notes as the keyboard is only available in ribbon mode. The second oscillator of the monotron DELAY is a low frequency oscillator (LFO), which can be adjusted from 1Hz up to about 30Hz. This LFO can produce either a triangle wave or a square wave to modulate the main audio oscillator. This is used mainly to apply vibrato to musical tones, or to produce very unusual tone ramps. The intensity and pitch of the LFO are controlled by knobs.

The Korg low pass filter present in the monotron DELAY is only adjustable for its cutoff frequency, so it is less flexible and interesting than the monotron DUO implementation.

The monotron DELAY is really built to showcase the analogue space delay functionality, which can be adjusted in both length of delay, and in intensity of feedback. With about 1 second of delay and 100% or more feedback possible, very short sequences of notes can be played and then built upon.

I’m not particularly musical, but I spent some very pleasant hours playing with the two Korg synthesizers experimenting with the sounds available from their very simple platforms, and used their capabilities to guide me in what to build into my Goldilocks Analogue synthesizer.

The next piece of research was to understand how to generate analogue wave forms using direct digital synthesis, and then how to modify sound of the wave forms using convolution or modulation in the time domain.Schematic Goldilocks Analogue Synthesizer

For more detail: Goldilocks Analogue Synthesizer

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