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.
Power for the color organ is
provided by a wall wart power supply
which provides between 8 and 12
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January 2008