DIY audio spectrum analyzer

MichaelaJoy · Jan 24, 2019

Hi All,
Recently, I saw this chip and thought it might be fun to tinker with.

https://www.ti.com/product/UAF42

Well it got me thinking. Since it is voltage controlled, why not set up a microcontroller to address the housekeeping issues
and make an audio spectrum analyzer for an oscilloscope?

Here's an existing project that draws a clock on an o'scope screen using a PIC.

https://www.micro-examples.com/articles/index.php/PicOClock

Please let me know what you all think. If it's plausable, I'll start picking up parts and chronicle the build here.

JonSea · Jan 24, 2019

You might want to look at the price of that chip. Ouch.

Are you thinking of using a single chip, tuning it to different frequencies to make an analyzer? That could work for steady signals, and in fact was done for machinery vibration analysis... 50 years ago. Not so effective for signals like music that are continuously changing.

Or are you thinking of a bank of these chips each set to a different center frequency? That would work well for music, but for the cost of even a few bands, you could find a better solution.

MichaelaJoy · Jan 24, 2019

JonSea said:
You might want to look at the price of that chip. Ouch.

Are you thinking of using a single chip, tuning it to different frequencies to make an analyzer? That could work for steady signals, and in fact was done for machinery vibration analysis... 50 years ago. Not so effective for signals like music that are continuously changing.

Or are you thinking of a bank of these chips each set to a different center frequency? That would work well for music, but for the cost of even a few bands, you could find a better solution.

JonSea: It might be good for doing gain plots on things like guitar pedals.
If if works, $20 USD is not a lot of money for a one-on project using one IC.

I've been looking at spectrum analyzers, and they seem to fall into two classes:

A ) Stand alone unit. Very nice, but very expensive

B ) Very cheap, but having no use as far as measurements go.

Just some thoughts. I'd be using a low distortion sine wave generator to sweep it.

wkrug · Jan 24, 2019

...audio spectrum analyzer for an oscilloscope...

I think that's a job for an DSP.
I know You are very familiar with the PIC Controllers.
Spectrum analyzing should be possible with an dsPIC a functioning source code could be found in the internet.

A few Years ago a user made an 128 Band audio spectrum analyzer with an 16MHz 8Bit AVR ( Elm Chan is an absolute crack ) .
Youtube Video
I guess that should work with an PIC also.

As source You can use a noise generator.

Another possibility is to take a look at JYE tech.
https://jyetech.com/index.php

Some guitar effects are delayed overlaps, with an spectrum analyzer You would see nothing.

ClydeCrashKop · Jan 24, 2019

I used a MSGEQ7, seven band Graphic Equalizer chip and LM3915 Dot Bar Display Drivers to make a night club style musical light show. That might make things easy for you.

MichaelaJoy · Jan 24, 2019

wkrug: Some nice stuff over there.

Bookmarked.

Edit: I just looked at that video. The firmware is still available, so I downloaded it.

ClydeCrashKop: I found it here.

https://www.sparkfun.com/products/10468

It looks pretty neat.

ronsimpson · Jan 24, 2019

ClydeCrashKop said:
I used a MSGEQ7, seven band Graphic Equalizer chip

Some where these is a note on using two MSGEQ7s to make 14 bands. You must set the two oscillators one octave apart.

Nigel Goodwin · Jan 24, 2019

There used to be a very old application note at MicroChip AN542, don't know if it's still there, but apparently it didn't work?.

Here's a corrected version, it drives a VGA monitor directly for the display:

Or another more modern PIC example:

https://www.waitingforfriday.com/?p=325

Or even a PIC32 one:

https://www.radiolocman.com/shem/schematics.html?di=67509

dr pepper · Jan 24, 2019

I used the msgeq7 for a project, it worked well, after I found some of the ic's I ordered were Doa.

ronsimpson · Jan 24, 2019

Read this for more information.
**broken link removed**

ClydeCrashKop · Jan 25, 2019

I found it practical to get this little module.

https://rheingoldheavy.com/product/msgeq7-graphic-equalizer-kit/

This board has 2 on it. Meant for stereo but maybe you could modify it for 14 bands.
SparkFun Spectrum Shield

https://www.sparkfun.com/products/13116

unclejed613 · Jan 25, 2019

MichaelaJoy said:
JonSea: It might be good for doing gain plots on things like guitar pedals.
If if works, $20 USD is not a lot of money for a one-on project using one IC.

I've been looking at spectrum analyzers, and they seem to fall into two classes:

A ) Stand alone unit. Very nice, but very expensive

B ) Very cheap, but having no use as far as measurements go.

Just some thoughts. I'd be using a low distortion sine wave generator to sweep it.

another, more flexible approach would be to use a laptop computer and Spectrum Lab software. SL has tone generators, etc... as well as a configurable set of virtual instrumentation.. i have used a laptop with SL on it to EQ some very challenging halls. https://www.qsl.net/dl4yhf/spectra1.html

MichaelaJoy · Jan 25, 2019

unclejed613: That would be the sensible approach.

I'd have to purchase a laptop and the software.
That's beyond my budget ATM.

I thought it'd be nice to cobble something up from parts I have lying around here.

unclejed613 · Jan 25, 2019

i once began work on a handheld unit that used an LM3915, a 4051 analog mux, and bandpass filters as a spectrum analyzer.. you could probably do something similar with a 4051 and bandpass filters, and feed the output of the mux into a scope. it would be easy enough to take a counter's reset to 0 to trigger the scope. some oscopes have a H sweep output, and the ramp from that could be used to operate a voltage controlled filter such as a state variable filter where the resistors that set the Fc of the filter are jfets. so there are many ways to make a spectrum analyzer... the most difficult part is choosing which one...

DIY audio spectrum analyzer

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