25
votes
Accepted
Why use the fast Fourier transform for noise reduction instead of a classical electronic filter?
I'd like to know how to remove environmental noise from a speech recording.
Well it's stored digitally now, right? so are you planning on putting your microphone next to the speaker after an analog ...
- 7,959
16
votes
Audio spectrum analysis: PC Software vs Hardware Spectrum Analyzer
The deciding factor between stand-alone analyzer or soundcard would be the frequencies you're interested in.
I will record the sound it makes with a microphone (it sounds differently if it is ...
- 59.1k
14
votes
Why should I use digital filters rather than simply manipulate signals in the frequency domain and then recover them into the time domain?
The main reason that frequency-domain processing isn't done directly is the latency involved. In order to do, say, an FFT on a signal, you have to first record the entire time-domain signal, beginning ...
- 165k
14
votes
Accepted
Help me understand FFT and harmonic distortions
They say a picture is worth a thousand words. And a handful of animated pictures...
All of these from Wikipedia. You can see these in context in the articles for Fourier transform, square wave, ...
- 55.4k
14
votes
What's the junk at the end of my FFT in LTSPICE?
There are several parts to this answer. I base this answer on the characteristics of the FFT algorithm. I am not familiar with the specific LTSpice implementation, but the behavior you report is ...
- 427
13
votes
Performing FFT at low frequencies but high resolution?
I assume for "high speed" you mean a small delay from data collection to the resultant FFT. With a low sample rate, your computational ability isn't the limiting factor, given modern computers. The ...
- 2,798
12
votes
Strange extra frequency in crystal oscillator
This really looks like a sampling artifact on your end, not something the crystal is doing. Expand the scope time scale (lower time/division) until you only have a cycle or two per division at most. ...
- 306k
11
votes
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Performing FFT at low frequencies but high resolution?
One usually needs to acquire multiple samples per waveform period to get good results from an FFT. The Nyquist limit of 2 samples per period is a lower bound but usually 10 samples per period or more ...
- 9,735
9
votes
Accepted
Strange extra frequency in crystal oscillator
Two observations:
12.28 and 12.72 are exactly symmetrical about 12.50 MHz.
The displayed wave form seems to have "beats" in it
Beats are either real (you would see beats if you had a mixture of two ...
- 1,050
8
votes
Relationship of FFT Size, Sampling Rate and Buffer Size
There is a direct, and actually quite simple, relationship between all the figures.
Let's start with the sample size. The numbers of bins (or "buckets") is equal with half of the samples in ...
- 55.5k
8
votes
Accepted
Uniform Linear Array (ULA) beamwidth and angular resolution using FFT
Theoretical Derivation
The antenna electric field pattern of array antenna consists of isotropic radiator can be given by
where N is the number of antenna elements
d is the spacing between antenna ...
- 273
7
votes
Accepted
Confusions on FFT of a square-wave in theory and in scope and simulation
Welcome to the real world!
The "mathematically perfect" transform you show at the top, with the "discrete" harmonics is generated assuming that the rise and fall times of the waveform are zero, and ...
- 165k
7
votes
Why use the fast Fourier transform for noise reduction instead of a classical electronic filter?
But why can't you use a classical electronic filter to remove the noise frequencies?
Who says you can't? It is how this was done in the days before digital signal processing. The problem is that ...
- 78.8k
7
votes
Audio spectrum analysis: PC Software vs Hardware Spectrum Analyzer
I would personally recommend using a PC for several reasons.
From a monetary stand point, if there is sufficient information currently to identify issues using a trained ear, there is no need for an ...
- 1,786
7
votes
Accepted
Getting different FFT results in LTspice comparing to MATLAB and Python
Despite the fixed timings provided by the PWL file, LTspice is a SPICE engine, analog simulator, which means the simulation will not have even steps. In Matlab and Python, you can correctly account ...
- 17.6k
6
votes
Calculating FFT for only part of full frequency range?
I was under the impression that the FFT was inherently calculated at every frequency 0->(Sampling Frequency)/2 distributed in bins of width Fs/(2*N).
This is roughly correct. A discrete fourier ...
- 121k
6
votes
Accepted
How to calculate total power from spectrum?
For a band limited power spectral density \$S(f)\$ to obtain the total power all you need to do is integrate over the spectrum. In measured data the power is measured in discrete frequency steps, so ...
- 691
6
votes
Why use the fast Fourier transform for noise reduction instead of a classical electronic filter?
Well, the first step to understanding why we need FFT is to understand how digital filtering works.
So basically, you have a structure, like a shift register, with a number of memory elements, an ...
- 22.7k
6
votes
Accepted
Help interpreting strange spectrum
The multitude of spurious frequencies in your FF transform are reflections of undersampled higher harmonics of your signal. These are aliases, as you rightfully noted. See this EDN article for better ...
- 37.4k
6
votes
Guitar to MIDI conversion
There isn’t enough information in 10 mS of sampled audio to reliably tell the difference in frequency between a low E and a low F or D# notes.
10 mS is barely more than 1 full pitch cycle at those ...
- 4,471
5
votes
What is the meaning of my fourier transform results?
The first entry [1] in the FFT is your fundamental, but the zeroth entry [0] is DC!
A standard FFT will have the same number of output samples as input samples, and they will all be complex.
The ...
- 141k
5
votes
Accepted
What's the junk at the end of my FFT in LTSPICE?
@D.Brown's answer is already a very good one, so I'll only add a few minor things. LTspice's algorithm is custom and accepts a non-power-of-two number of points. This doesn't mean that the resolution ...
- 17.6k
5
votes
Guitar to MIDI conversion
Since your guitar string can only produce a handful of pitches, a full FFT is wasteful.
Take the lower E string. The tones are approximately 5Hz apart.
Assume a sampling rate of 11025 Hz. To get ...
- 58.2k
5
votes
FFT of a square wave
I'd make this a comment, but I don't have enough points to do that yet.
You should plot your FFT data starting at 0 Hz and go up to, say, 500 Hz. That will give you 10 or so harmonics. You are ...
- 91
5
votes
Accepted
FFT of a square wave
Your signal is a square wave with its base at 0V and its peak at 2.7V or so. So it has an average voltage of 1.35V. In the frequency domain, the overall average of a signal is its content at DC or ...
- 36.3k
5
votes
Accepted
FFT of time-compressed signal not outputting correct amplitude spectrum
You have 4 times as many cycles in the time -5:ts:5. If your y and g signals had the same ...
- 3,646
4
votes
Accepted
FFT Beat Detection Circuit
I think the youtube guy might have been on the right track, but the confusion is that "ground" is typically centered between two supplies for most analog circuits, and coexistent with the lower supply ...
- 5,356
4
votes
Accepted
MATLAB - FFT aliases occur sooner than expected
If you do a FFT in MATLAB, you get one complex valued output (bin) for every input sample. If you then take the magnitude of this complex vector, assuming your original input was real valued, you will ...
- 4,138
4
votes
How to model Phase noise?
Does this example prove to you that the spectrum spreads ?
The spectral plot uses dB (I find that more convenient) on the Y-axis, so consider it a logarithmic scale.
You can see that I need to add A ...
- 78.8k
4
votes
How to detect a number of frequencies with minimal circuitry?
If you know what frequencies you're looking for, run a Goertzel for each of those frequencies. https://en.wikipedia.org/wiki/Goertzel_algorithm This is much more efficient than an FFT. This algorithm ...
- 672
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