Nuts and Volts - January 2008

A Digital Color Organ

by Craig A. Lindley

In the September 1969 issue of Popular Electronics Magazine, Don Lancaster presented a color organ which he called Psychedelia I that I thought was the coolest thing I had ever seen. A color organ — as you may know — is a device that splits music up into numerous frequency bands and modulates colored lights (typically one color of light per band) according to musical content. In honor of Don Lancaster (my electronics hero) I named this color organ, Psychedelia II.

In Don’s design, all of the functions of the color organ were done with analog circuitry, typical of the time. Don coupled transistorized active filters for frequency selectivity to triac, solid-state switching devices to control 110 VAC incandescent lights. All in all, a very nice design.

The problem I have with all analog color organ circuits I have ever seen or built is that as the musical material applied to the color organ changes, manual adjustments need to be made

to sensitivity controls on each channel to achieve a pleasant, balanced display. This constant adjustment gets old after a while, so fixing this problem was one of the motivations for designing a digital color organ.

The other motivation was to implement the design with a very small but powerful microcontroller (see Photo 1) using a development kit from Texas Instruments that cost just $20. When I started down this path, I wasn’t sure it was possible. I now know it was.

There is analog circuitry in my design, as well. In fact, the complete audio front end is analog. What is different about this design is that all of the frequency selectivity is provided by digital filters in the digital domain and the output devices are now low voltage super bright LEDs instead of incandescent bulbs. In addition, a digital automatic gain control (or

AGC) was included to allow the color organ to adjust itself to varying musical material.

I described some of the technology used to implement this color organ in two previous articles for Nuts & Volts Magazine. See the articles entitled, “Floating Point Multiplication and Division without Hardware Support”

and “Lattice Wave Digital Filters” for additional background information.

How It Works

This color organ has both hardware and software (firmware) components. Each will be described briefly here. For this discussion, the term microcontroller has been abbreviated as μC.

PHOTO 1. TI products used for the color organ. The MSP430F2012 target board in the upper left is the brains of the color organ. Its four pin connector plugs into the TI eZ430 USB development system, lower middle, for programming and debugging.

The Hardware

As shown on the schematics, the hardware consists of three sections: the analog front end, the power supply section, and the output section. The analog front end is where the audio signals from either a built-in condenser microphone or a stereo line input are processed in preparation for analog-to-digital conversion inside the μC.

The built-in microphone has its own high gain pre-amp whose gain is controlled by trimmer pot R5. Stereo line level input signals are summed into a mono signal as this color organ is a mono device. Whichever audio source is selected for use, its bandwidth is limited to less than one half the sampling frequency of 16,000 samples/second as required by the Nyquist criteria.

Numerous first order low pass filters distributed throughout the circuitry, along with a second order Sallen-Key active filter, help limit the bandwidth to reduce aliasing effects when the audio signal is digitized. All of the analog circuitry runs single ended from a single five-volt power supply.