Archive | spectrum

Project – LED Cube Spectrum Analyzer

Introduction

A few weeks ago I was asked about creating a musical-effect display with an RGB LED cube kit from Freetronics, and with a little work this was certainly possible using the MSGEQ7 spectrum analyser IC. In this project we’ll create a small add-on PCB containing the spectrum analyser circuit and show how it can drive the RGB LED cube kit.

Freetronics CUBE4 RGB LED cube kit

Assumed knowledge

To save repeating myself, please familiarise yourself with the MSGEQ7 spectrum aanalyserIC in Chapter 48 of our Arduino tutorials. And learn more about the LED cube from our review and the product page.

You can get MSGEQ7 ICs from various sources, however they had varying results. We now recommend using the neat module from Tronixlabs.

The circuit

The LED cube already has an Arduino Leonardo-compatible built in to the main PCB, so all you need to do is build a small circuit that contains the spectrum analyzer which connects to the I/O pins on the cube PCB and also has audio input and output connections. First, consider the schematic:

MSGEQ7 CUBE4 spectrum analyser schematic

For the purposes of this project our spectrum analyser will only display the results from one channel of audio – if you want stereo, you’ll need two! And note that the strobe, reset and DCOUT pins on the MSGEQ7 are labelled with the connections to the cube PCB. Furthermore the pinouts for the MSGEQ7 don’t match the physical reality – here are the pinouts from the MSGEQ7 data sheet (.pdf):

MSGEQ7 pinouts

The circuit itself will be quite small and fit on a small amount of stripboard or veroboard. There is plenty of room underneath the cube to fit the circuit if so desired:

MSGEQ7 LED cube

With a few moments you should be able to trace out your circuit to match the board type you have, remember to double-check before soldering. You will also need to connect the audio in point after the 1000 pF capacitor to a source of audio, and also pass it through so you can connect powered speakers, headphones, etc.

One method of doing so would be to cut up a male-female audio extension lead, and connect the shield to the GND of the circuit, and the signal line to the audio input on the circuit. Or if you have the parts handy and some shielded cable, just make your own input and output leads:

MSGEQ7 input output leads

Be sure to test for shorts between the signal and shield before soldering to the circuit board. When finished, you should have something neat that you can hide under the cube or elsewhere:

MSGEQ7 RGB cube LED spectrum analyzer board

Double-check your soldering for shorts and your board plan, then fit to the cube along with the audio source and speakers (etc).

Arduino Sketch

The sketch has two main functions – the first is to capture the levels from the MSGEQ7 and put the values for each frequency band into an array, and the second function is to turn on LEDs that represent the level for each band. If you’ve been paying attention you may be wondering how we can represent seven frequency bands with a 4x4x4 LED cube. Simple – by rotating the cube 45 degrees you can see seven vertical columns of LEDs:

MSGEQ7 LED cube spectrum analyzer columns

So when looking from the angle as shown above, you have seven vertical columns, each with four levels of LEDs. Thus the strength of each frequency can be broken down into four levels, and then the appropriate LEDs turned on.

After this is done for each band, all the LEDs are turned off and the process repeats. For the sake of simplicity I’ve used the cube’s Arduino library to activate the LEDs, which also makes the sketch easier to fathom. The first example sketch only uses one colour:

… and a quick video demonstration:

For a second example, we’ve used various colours:

… and the second video demonstration:

A little bit of noise comes through into the spectrum analyser, most likely due to the fact that the entire thing is unshielded. The previous prototype used the Arduino shield from the tutorial which didn’t have this problem, so if you’re keen perhaps make your own custom PCB for this project.

visit tronixlabs.com

Have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column, or join our forum – dedicated to the projects and related items on this website.

Posted in analyzer, arduino, com-10468, cube, freetronics, LED, MSGEQ7, projects, rgb, RGB LED, spectrum, tronixlabs, tronixstuff, tutorial

Tutorial: Arduino and the MSGEQ7 Spectrum Analyzer

This is a tutorial on using the MSGEQ7 Spectrum Analyser with Arduino, and chapter forty-eight of a series originally titled “Getting Started/Moving Forward with Arduino!” by John Boxall – A tutorial on the Arduino universe. The first chapter is here, the complete series is detailed here.

Updated 10/11/2014

In this article we’re going to explain how to make simple spectrum analysers with an Arduino-style board. (Analyser? Analyzer? Take your pick).

First of all, what is a spectrum analyser? Good question. Do you remember what  this is?

It’s a mixed graphic equaliser/spectrum analyser deck for a hi-fi system. The display in the middle is the spectrum analyser, and roughly-speaking it shows the strength of  different frequencies in the music being listened to – and looked pretty awesome doing it. We can recreate displays similar to this for entertainment and also as a base for creative lighting effects. By working through this tutorial you’ll have the base knowledge to recreate these yourself.

We’ll be using the MSGEQ7 “seven band graphic equaliser IC” from Mixed Signal Integration. Here’s the MSGEQ7 data sheet (.pdf).  This little IC can accept a single audio source, analyse seven frequency bands of the audio, and output a DC representation of each frequency band. This isn’t super-accurate or calibrated in any way, but it works. You can get the IC separately, for example:


and then build your own circuit around it… or like most things in the Arduino world – get a shield. In this case, a derivative of the original Bliptronics shield by Sparkfun. It’s designed to pass through stereo audio via 3.5mm audio sockets and contains two MSGEQ7s, so we can do a stereo analyser:

As usual Sparkfun have saved a few cents by not including the stackable header sockets, so you’ll need to buy and solder those in yourself. There is also space for three header pins for direct audio input (left, right and common), which are useful – so if you can add those as well.

So now you have a shield that’s ready for use. Before moving forward let’s examine how the MSGEQ7 works for us. As mentioned earlier, it analyses seven frequency bands. These are illustrated in the following graph from the data sheet:

freqresponse

It will return the strengths of the audio at seven points – 63 Hz, 160 Hz, 400 Hz, 1 kHz, 2.5 kHz, 6.25 kHz and 16 kHz – and as you can see there is some overlap between the bands. The strength is returned as a DC voltage – which we can then simply measure with the Arduino’s analogue input and create a display of some sort. At this point audio purists, Sheldonites and RF people might get a little cranky, so once again – this is more for visual indication than any sort of calibration device.

However as an 8-pin IC a different approach is required to get the different levels. The IC will sequentially give out the levels for each band on pin 3- e.g. 63 Hz then 160 Hz then 400 Hz then 1 kHz then 2.5 kHz then 6.25 kHz  then 16 kHz then back to 63 Hz and so on. To start this sequence we first reset the IC by pulsing the RESET pin HIGH then low. This tells the IC to start at the first band. Next, we set the STROBE pin to LOW, take the DC reading from pin 3 with analogue input, store the value in a variable (an array), then set the STROBE pin HIGH. We repeat the strobe-measure sequence six more times to get the rest of the data, then RESET the IC and start all over again. For the visual learners consider the diagram below from the data sheet:

strobing1

To demonstrate this process, consider the function

in the following example sketch:

If you follow through the sketch, you can see that it reads both left- and right-channel values from the two MSGEQ7s on the shield, then stores each value in the arrays left[] and right[]. These values are then sent to the serial monitor for display – for example:

If you have a function generator, connect the output to one of the channels and GND – then adjust the frequency and amplitude to see how the values change. The following video clip is a short demonstration of this – we set the generator to 1 kHz and adjust the amplitude of the signal. To make things easier to read we only measure and display the left channel:


Keep an eye on the fourth column of data – this is the analogRead() value returned by the Arduino when reading the 1khz frequency band. You can also see the affect on the other bands around 1 kHz as we increase and decrease the frequency. However that wasn’t really visually appealing – so now we’ll create a small and large graphical version.

First we’ll use an inexpensive LCD, the I2C model from akafugu reviewed previously. To save repeating myself, also review how to create custom LCD characters from here.

With the LCD with have two rows of sixteen characters. The plan is to use the top row for the levels, the left-channel’s on … the left, and the right on the right. Each character will be a little bar graph for the level. The bottom row can be for a label. We don’t have too many pixels to work with, but it’s a compact example:

lcdfullon

We have eight rows for each character, and the results from an analogueRead() fall between 0 and 1023. So that’s 1024 possible values spread over eight sections. Thus each row of pixels in each character will represent 128 “units of analogue read” or around 0.63 V if the Arduino is running from true 5 V (remember your AREF notes?). The sketch will again read the values from the MSGEQ7, feed them into two arrays – then display the required character in each band space  on the LCD.

Here’s the resulting sketch:

If you’ve been reading through my tutorials there isn’t anything new to worry about. And now for the demo, with sound –

That would look great on the side of a Walkman, however it’s a bit small. Let’s scale it up by using a Freetronics Dot Matrix Display – you may recall these from Clock One. For some background knowledge check the review here.  Don’t forget to use a suitable power supply for the DMD – 5 V at 4 A will do nicely. The DMD contains 16 rows of 32 LEDs. This gives us twice the “resolution” to display each band level if desired. The display style is subjective, so for this example we’ll use a single column of LEDs for each frequency band, with a blank column between each one.

We use a lot of line-drawing statements to display the levels, and clear the DMD after each display. With this and the previous sketches, there could be room for efficiency – however I write these with the beginner in mind. Here’s the sketch:

… and here it is in action:

Conclusion

At this point you have the knowledge to use the MSGEQ7 ICs to create some interesting spectrum analysers for entertainment and visual appeal – now you just choose the type of display enjoy the results. And if you enjoyed this article, or want to introduce someone else to the interesting world of Arduino – check out my book (now in a fourth printing!) “Arduino Workshop”.

visit tronixlabs.com

Have fun and keep checking into tronixstuff.com. Why not follow things on twitterGoogle+, subscribe  for email updates or RSS using the links on the right-hand column, or join our forum – dedicated to the projects and related items on this website.

Posted in analyser, analyzer, arduino, BLIPTRONICS, com-10468, dev-10306, education, graphic, lesson, MSGEQ7, sparkfun, spectrum, tutorial


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