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by Daniel Wee
Thu 05 Nov 05 2009 2:30 am
Forum: Projects
Topic: FFT Audio Spectrum Analysis
Replies: 11
Views: 38756

Re: FFT Audio Spectrum Analysis

I finally got down to building the FFT board. It's about 99% done here I think, but not tested yet:- IMG_1100a.JPG IMG_1101a.JPG IMG_1104a.JPG That last one is the VFD that I will be using. Actually I do have a wider display but we will try to go with this one first and see how it goes from there. D...
by Daniel Wee
Wed 04 Nov 04 2009 2:12 am
Forum: Projects
Topic: FFT Audio Spectrum Analysis
Replies: 11
Views: 38756

Re: FFT Audio Spectrum Analysis

The op-amps have arrived so I can probably built the level-shifter and filter for testing, once I get some time to do it.

Daniel
by Daniel Wee
Mon 02 Nov 02 2009 1:06 pm
Forum: Support
Topic: Getting PPM-stream data out of the receiver
Replies: 2
Views: 22398

Getting PPM-stream data out of the receiver

When a PPM-stream is needed from a radio, usually some kind of modification needs to be made. Most PPM receivers will have no problems with this since there will be the PPM-stream available somewhere in the receiver so what you basically need to do is to open the radio up and find the data stream an...
by Daniel Wee
Wed 28 Oct 28 2009 10:54 pm
Forum: Projects
Topic: FFT Audio Spectrum Analysis
Replies: 11
Views: 38756

Re: FFT Audio Spectrum Analysis

This is the shifter cascaded with a two-pole Butterworth filter:- Shifter-Filter.jpg The frequency response is as follows:- Shifter-Filter-Response.png As you can see, the roll-off isn't exactly steep, which is what you would expect for a low-order filter, but it does work. BTW, I reverted the gain ...
by Daniel Wee
Wed 28 Oct 28 2009 8:50 pm
Forum: Projects
Topic: FFT Audio Spectrum Analysis
Replies: 11
Views: 38756

Re: FFT Audio Spectrum Analysis

I thought I had better post the shifter schematics used in my simulation before I forget the details:-
Shifter-sch.jpg
Shifter-sch.jpg (35.91 KiB) Viewed 30757 times
Note that the gain here is 1.5x just because I wanted to test the neutrality of the offset point. It can easily be changed to unity gain or something more or less.

Daniel
by Daniel Wee
Tue 27 Oct 27 2009 12:53 am
Forum: Projects
Topic: FFT Audio Spectrum Analysis
Replies: 11
Views: 38756

Re: FFT Audio Spectrum Analysis

I've been tinkering with the input amplifier and level shifter. I'm thinking that in our application, phase inversion should not be a problem so I've used an inverting amplifier with a level shifter to Vcc/2. The SPICE simulation suggests that our circuit will work as expected. Shifter.jpg Note that...
by Daniel Wee
Mon 26 Oct 26 2009 11:32 am
Forum: Projects
Topic: FFT Audio Spectrum Analysis
Replies: 11
Views: 38756

Re: FFT Audio Spectrum Analysis

Given that most of the audio signals we normally find are AC coupled, we will need a level-shifter circuit (simple summing op-amp with unity gain) at the input. This gives us the opportunity to control the gain with some of the spare op-amps as well as to implement some low-pass filtering if we so c...
by Daniel Wee
Mon 26 Oct 26 2009 11:10 am
Forum: Projects
Topic: FFT Audio Spectrum Analysis
Replies: 11
Views: 38756

Re: FFT Audio Spectrum Analysis

Anti-Aliasing Filters I've mentioned the need for anti-aliasing filters, basically a low-pass filter to cut out frequency components above the Nyquist frequency so as not to corrupt the sampled data. The thing here, though, is that if we over-sample at a high enough frequency, you gain the benefit ...
by Daniel Wee
Mon 26 Oct 26 2009 12:23 am
Forum: Projects
Topic: FFT Audio Spectrum Analysis
Replies: 11
Views: 38756

Re: FFT Audio Spectrum Analysis

Okay, so the basic idea behind this project is to transform the input signal from the time domain into the frequency domain using some form of DFT. This can be done thanks to Fourier's discovery of signal decomposition into sinusoids of fixed periods but varying amplitudes. To put it in another way,...
by Daniel Wee
Sun 25 Oct 25 2009 9:25 pm
Forum: Projects
Topic: FFT Audio Spectrum Analysis
Replies: 11
Views: 38756

FFT Audio Spectrum Analysis

One more of my crazy projects involving some basic FFT (DFT) convolution filters. To begin with, what we want to do is to build a device that will graphically display the spectral output of an audio signal in near real-time. It is "near" real-time because the display always lags real-time ...