Questions tagged [filters]

In signal processing, a filter is a device or process that transforms a signal by selectively choosing specific frequencies and leaving others with appropriate gain or as it is.

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86
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3answers
25k views

Why is it a bad idea to filter by zeroing out FFT bins?

It's very easy to filter a signal by performing an FFT on it, zeroing out some of the bins, and then performing an IFFT. For instance: ...
45
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5answers
60k views

What is the difference between phase delay and group delay?

I'm studying some DSP and I'm having trouble understanding the difference between phase delay and group delay. It seems to me that they both measure the delay time of sinusoids passed through a ...
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6answers
47k views

Simplest way of detecting where audio envelopes start and stop

Below is a signal which represents a recording of someone talking. I would like to create a series of smaller audio signals based on this. The idea being to detect when 'important' sound starts and ...
33
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6answers
57k views

Understanding the Matched Filter

I have a question about matched filtering. Does the matched filter maximise the SNR at the moment of decision only? As far as I understand, if you put, say, NRZ through a matched filter, the SNR ...
32
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2answers
32k views

What is the advantage of MATLAB's filtfilt

MATLAB's filtfilt does a forward-backward filtering, i.e., filter, reverse the signal, filter again and then reverse again. Apparently this done to reduce phase ...
32
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4answers
30k views

What is the true meaning of a minimum phase system?

What is the true meaning of a minimum phase system? Reading the Wikipedia article and Oppenheim is some help, in that, we understand that for an LTI system, minimum phase means the inverse is causal ...
29
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2answers
43k views

Filter order vs number of taps vs number of coefficients

I'm learning DSP slowly and trying to wrap my head around some terminology: Question 1: Suppose I have the following filter difference equation: $$y[n] = 2 x[n] + 4 x[n-2] + 6 x[n-3] + 8 x[n-4]$$ ...
27
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3answers
15k views

What should be considered when selecting a windowing function when smoothing a time series?

If one wants to smooth a time series using a window function such as Hanning, Hamming, Blackman etc., what are the considerations for favouring any one window over another?
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5answers
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What Is the Best First Order IIR (AR Filter) Approximation to a Moving Average Filter (FIR Filter)?

Assume the following first order IIR Filter: $$ y[n] = \alpha x[n] + (1 - \alpha) y[n - 1] $$ How can I choose the parameter $ \alpha $ s.t. the IIR approximates as good as possible the FIR which is ...
24
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2answers
34k views

Applying filter in scipy.signal: Use lfilter or filtfilt?

I saw in a SO thread a suggestion to use filtfilt which performs backwards/forwards filtering instead of lfilter. What is the ...
24
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1answer
15k views

Overlap-Add versus Overlap-Save

What differences or other criteria can be used to help decide between using overlap-add and overlap-save for filtering? Both overlap-add and overlap-save are described as algorithms for doing FFT ...
24
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4answers
18k views

Peak detection approach

What are the peak detection algorithms in existence? I have noisy data, and I would like to implement peak detection for this data. Data is in reverse, actually I am trying to determine the bottom. ...
22
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4answers
30k views

How many taps does an FIR filter need?

I am looking to design a set of FIR filters to implement a low pass filter. I am also trying to reduce the latency of the signal through the filter so I am wondering what the minimum number of taps I ...
22
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3answers
27k views

What is the Gabor filter? And what are its main uses?

I'm doing research on the Gabor filter. When I Googled it, I found very long and complicated articles. Can anyone help me to find a simple explanation about it, or recommend a website or article to ...
21
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6answers
27k views

Why is a linear phase important?

If symmetry conditions are met, FIR filters have a linear phase. This is not true for IIR filters. However, for what applications is it bad to apply filters that do not have this property and what ...
21
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4answers
4k views

Fast / Efficient Way to Decompose a Separable integer 2D Filter Coefficients without the SVD

I would like to be able to quickly determine whether a given 2D kernel of integer coefficients is separable into two 1D kernels with integer coefficients. E.g. ...
21
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5answers
3k views

Why would I leave a signal oversampled?

I can't think of a better way for asking this question so I will start with an example. Suppose that I have an input signal with a max frequency of 50Hz (sampled at 100Hz). Now the signals of interest ...
21
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3answers
5k views

FIR Filter Design: Window vs Parks McClellan and Least Squares

Are there any advantages to use a window approach over Parks-McClellan (further abbreviated here as PMcC) or Least Squares algorithms for FIR filter design of a low pass filter? Assume with today's ...
21
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2answers
11k views

How do I interpret the DCT step in the MFCC extraction process?

In most audio processing tasks, one of the most used transformations is MFCC (Mel-frequency cepstral coefficients). I mostly know the math that's behind the MFCC: I understand both the filterbank ...
20
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3answers
36k views

What is the difference between linear and non-linear filters?

How a mean filter is called as linear filter and a median filter is called as non linear filter? I understand how a mean and median filter operates, but I was not able to relate with the term linear ...
20
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3answers
10k views

How does cascading biquad sections for higher order filters work?

I'm trying to implement an 8th order IIR filter and every application note and textbook I've read says that it is best to implement any filter of order more than 2 as second order sections. I used <...
20
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7answers
8k views

Where can I get an open source implementation of the Parks-McClellan optimal FIR filter design algorithm?

Background: Often times I am doing some sort of signal processing task that requires a unique filter. Usually at this point I go to MATLAB and generate a new unique filter using $\tt firpm()$. The ...
19
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3answers
965 views

Should the input of a Kalman filter always be a signal and its derivative?

I always see the Kalman filter used with such input data. For example, the inputs are commonly a position and the correspondent velocity: $$ (x, \dfrac{dx}{dt}) $$ In my case, I only have 2D ...
19
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1answer
532 views

Are there any analytic approaches to the design of quadratic polynomial filters?

The following paper describes an application of the Teager-Kaiser energy operator to x-ray image enhancement: Reinhard Bernstein, Michael S. Moore and Sanjit K. Mitra, "Adjustable Quadratic ...
18
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3answers
4k views

A good textbook for designing signal filters

Since a couple of months ago I started being actively involved in the area of control of dynamical systems. In most cases, designing a controller for a given dynamic system will require the ...
17
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3answers
19k views

human speech noise filter

Does anyone know of a filter to attenuate non-speech? I am writing speech recognition software and would like to filter out everything but human speech. This would include background noise, noise ...
17
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2answers
1k views

How can I design Nyquist interpolation filters with the Parks-McClellan algorithm?

We can easily design interpolation filters that obey certain frequency-domain constraints using the Parks-McClellan algorithm. However, it's not immediately clear how to enforce time-domain ...
17
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2answers
3k views

Are there any standard implementation forms for tunable Butterworth filters?

One of the standard ways to implement a Butterworth filter is with a cascade of second-order sections, each corresponding to a pair of complex-conjugate poles. For a fourth-order filter, for example, ...
17
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2answers
10k views

Is a Kalman filter suitable to filter projected points positions, given Euler angles of the capturing device?

My system is the following. I use the camera of a mobile device to track an object. From this tracking, I get four 3D points that I project on the screen, to get four 2D points. These 8 values are ...
16
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9answers
23k views

Why do we use window in time domain rather than do FFT modify the spectrum and than inverse FFT

I thought that DSP would be done by using FFT of portions of a signal, modify the samples that result from FFT (since they represent the spectrum of our signal + noise) and remove any unwanted signals ...
16
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2answers
28k views

How do I manually plot the frequency response of a bandpass Butterworth filter in MATLAB without freqz function?

I have code like below that applies a bandpass filter onto a signal. I am quite a noob at DSP and I want to understand what is going on behind the scenes before I proceed. To do this, I want to know ...
16
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3answers
49k views

FIR filter with linear phase, 4 types

I know there are 4 types of FIR filters with linear phase, i.e. constant group delay: (M = length of impulse response) Impulse response symmetrical, M = odd Imp. resp. symmetrical, M = even Imp. resp....
16
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6answers
12k views

Savitzky-Golay smoothing filter for not equally spaced data

I have a signal that is measured at 100Hz and I need to apply the Savitzky-Golay smoothing filter on this signal. However, on closer inspection my signal is not measured at perfectly constant rate, ...
16
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1answer
22k views

How to Extract High Frequency and Low Frequency Component Using Bilateral Filter?

What does High frequency and low frequency component signify in an image. How to separate out the high frequency as well as low frequency component of an image using Bilateral filter.
16
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1answer
5k views

Digital filter design basic principles (IIR/FIR)

Although I have a solid experience in designing audio engines and such, I am fairly new to the realm of Digital Filter Design, particularly IIR and FIR filters. In other words, I'm trying to learn as ...
16
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5answers
2k views

What factors should I consider when choosing an IIR filter realization?

IIR filters may be implemented as direct form 1 or 2. But how do you decide which form to use? What are the pros and cons of each?
16
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2answers
16k views

Wiener Filter for Image Noise Reduction (Image Denoising)

I'm trying to get my head round the operation of the Wiener filter for the purpose of image noise reduction. In my case I'll have used another noise reduction filter first and will then use the result ...
16
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1answer
1k views

How to deal with a negative pole (unstable) in the pre-filter of a control system?

So while answering how to design a PI controller for a first order time delayed system (Question Here ) Here is the closed loop equation to a control system: $$ G_C(s) = \frac{\frac{K}{T}(1-sT)(s)...
15
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5answers
25k views

Why are FIR filters still stable even though they contain poles?

How come FIR filters are always stable? Since they contain poles, shouldn't they be more affected by stability issues than others?
15
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4answers
8k views

How to extract traffic signs from a photograph?

What image analysis techniques can I use to extract the traffic signs from an image such as the one below? Edit: After Anisotropic diffusion: The background that i don't want gets cleared a little ...
15
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5answers
13k views

What are the problems with designing an FIR filter using FFT?

I'm trying to get an understanding of the relationship between an FIR filter designed from "first principles" using a filter kernel with convolution, and a filter designed in one of two ways using FFT ...
15
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3answers
1k views

How can I design a very narrow filter?

Suppose I've got an audio signal sampled at $48000$ Hz, and I'd like to design a low-pass filter that isolates everything below ~$60$Hz. In the digital world, this is a low-pass filter with the ...
15
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3answers
7k views

How to Remove Gaussian Noise from an Image without Destroying the Edges?

What is the best filter for removing Gaussian noise without destroying the edges? I am using the standard Lena images with additive Gaussian noise and I want to denoise before applying anisotropic ...
15
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2answers
6k views

Which IIR filters approximate a Gaussian filter?

So it recently dawned on me that Bessel filters, despite being listed along with the other common types, are really an oddball that belongs in a different "class", and I'm trying to learn more about ...
14
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3answers
895 views

What does it mean in image processing when a filter is called non-linear?

In image processing, what does it mean when a filter is called non-linear? Does it mean the equation of the filter contains derivatives and if it didn't, it would have been called linear?
14
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4answers
16k views

could anybody explains me why IIR filters do not have linear phase?

Please tell me. Also I dont know why phase is linear with FIR filters. I would like quantitative analysis. And why linear phase is not achieved by IIR filters ?
14
votes
1answer
2k views

Why is $y(n) = x(n) + x(n-1)$ a low pass filter?

I have found that this is a simple, but bad lowpass filter: $$y(n) = x(n) + x(n-1)$$ However, I can't understand why it is a lowpass filter. What is its cutoff frequency?
14
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2answers
1k views

What bandpass filter design will yield the shortest impulse response?

Designing a simple 2nd order IIR bandpass Buterworth filter with a centre frequency of 500 Hz and a 1 octave bandwidth gives me the following frequency response ... Now, if I take the impulse ...
14
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2answers
6k views

Bitmap alpha bevel algorithm?

I am looking to create an algorithm which adds a bevel effect to a bitmap using it's alpha as a bump map. How would I go doing something like this? I have tried specular lighting but I only get the ...
13
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5answers
6k views

Is up-sampling prior to cross-correlation useless?

Consider a simple case where two signals from two different sensors are cross-correlated, and the time-delay-of-arrival computed from the absissa of the peak of their cross-correlation function. Now ...

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