●●●●
AUDIO SPECTRUM
ANALYZER
BY LARRY CICCHINELLI
The device described in this article
is a configurable audio spectrum
analyzer. Many of you have probably
seen spectrum analyzers on some
higher-end audio equipment, as well
as within the GUI of many audio deck
programs on your PCs. The function
of a spectrum analyzer is to view the
frequency components of a signal.
One way to state the difference between an oscilloscope and a spectrum
analyzer is that with an oscilloscope, you see time domain presentations;
with a spectrum analyzer, you see into the frequency domain.
Photo courtesy of www.visualizationsoftware.com.
When I first thought about this project, I simply
wanted to see if I could do it. I had never worked
with Fast Fourier Transforms (FFTs) before or developed an
interface and software for a graphics LCD. It was a very
interesting and rewarding experience! One of my primary
uses for this device will be to monitor the output of a
public address (PA) system I am responsible for. It lets
me see if any frequencies are being over or under
emphasized. I can also see the frequency of any feedback
and then adjust the equalizer accordingly. Used in
conjunction with my DDS (N&V, November ‘06), it can
be used to determine the acoustic characteristics of a
room or PA/recording system.
This unit has been designed to view audio signals up
■ FFT analysis display of a complex audio signal.
44
August 2008
to about 20 kHz. The data is displayed on a graphics LCD
with a pixel matrix of 64 x 128. It uses a microprocessor-based module with a clock speed of either 29. 5 MHz or
59 MHz that enables a maximum display update rate of
about 5 or 10 Hz. The audio processing circuit allows you
to select one attenuation value or any combination of
three gain values.
The program uses several separate — but coordinating
— files to complete the software for the system: the main
program, a library for the LCD, and a library specifically
for the menu functions. Provision has been made to allow
the user to store up to 10 custom configurations. Since
the source code is available, you can customize the
program to meet your specific needs. To do this, you will
need access to a development system for the
microprocessor. If you do not want to incur the expense
of a development system, I will be happy to program the
microprocessor module for you if you send it to me with a
SASE. (Contact me at k3pto@arrl.net)
Although the program uses the FFT, I will not be going
into any details on how this works. There are good
descriptions of this algorithm available on the web, as well
NOTE:
Circuit board artwork is available on the
Nuts & Volts website at www.nutsvolts.com.
