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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1answer
210 views

How to perform median filtering in multi-label prediction

I'm trying to perform automatic pitch tracking for piano. It's easy to detect the note when there's only a single note pressed. There are 88 keys on the piano and I set the result to one of them. ...
11
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3answers
2k views

How to remove certain background from image?

This was the image obtained after Gabor filtering... Is there some way of removing the lines inside the image except the bright white rounded defect. Did try another orientation of gabor filter go ...
10
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2answers
66k views

Filtering 50Hz using a notch filter in MATLAB

I've read up a lot about this, but haven't been able to piece everything together successfully, so I'm looking for some help. I need to filter 50 Hz from a signal. It looks like the best options are ...
9
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5answers
1k views

How can I vectorize the computations for a first-order recursive filter?

I have a simple single pole low pass filter (for parameter smoothing) that can be explained by the following formula: $$ y[n] = (1-a) y[n-1] + a x[n] $$ The architecture that I'm using has access ...
8
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3answers
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Is Ideal LPF BIBO unstable?

In one of other discussions : How to find frequency response, stability, and causality of a linear system? I found a comment which was quite strong and definitely caught my attention. An ideal ...
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2answers
2k views

Multiband processing in one pass

I want to split signal in multiple bands and process each band. Number of bands is not yet determined, but it would be more then 6 and less then 11 (depending on the collected results) As ...
6
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1answer
739 views

How to estimate taps required for Parks-McClellan filters

I have some code based on Jake Janovetz's Parks-McClellan (Remez) filter generating code. How can I estimate the number of taps required to build a lowpass filter given requirements for pass band ...
14
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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?
12
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3answers
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“Low-pass filter” in non-EE, software API contexts

I am an experienced software engineer and am working on smartphone sensors. I've taken fundamental EE classes in DSP and am trying to apply my knowledge. I believe that I understand convolution, ...
10
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2answers
12k views

How to find smoothed estimates of the derivative and second derivative of a signal?

I have a signal sampled at $\Delta t$: $f_i(t_i=i\Delta t)$ where $i = 0,\ldots,n-1$. I want to find the first and second derivative of the signal: $f'(t)$ and $f''(t)$. My first thought was to ...
9
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1answer
240 views

Unit Problem in Designing a Filter for a Given Auto Correlation Function

Given a WSS process with the following Auto Correlation Function: $$ r\left ( \tau \right ) = {\sigma}^{2} {e}^{-\alpha \left | \tau \right |} $$ The Laplace Transform would be: $$ R \left ( s \...
11
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1answer
2k views

Smoothing signal / detecting bumps in a data stream

(EDIT: This question follows from Extracting Binary Magnetic-Strip Card Data from raw WAV) Here is my signal (top line) and a basic IIR filter applied (bottom line) (EDIT: my task is to break the ...
7
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1answer
625 views

Can I simultaneously remove salt and pepper noise and improve the image resolution?

What should my approach be towards designing a filter that can simultaneously remove salt & pepper noise and improve the resolution?
2
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1answer
102 views

What are the Elements of Filters for Highly Volatility Price Series with Jumps?

What are the best filtering techniques for a highly volatility price series with jumps. What needs to be considered in designing new filters for practical finance which are adaptive and highly ...
2
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1answer
174 views

Identifying Bad Sectors on magnetic stripe, or noise in signal

I am starting with an audio signal picked up from the microphone jack of my iPhone, generated by running a standard magnetic strip ATM card through a sensor. On an old card, there will be a few ...
10
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2answers
142 views

What is this sinc doing in my LP-turned-HP filter output?

I'm trying to HP-filter a signal by LP-filtering it and subtracting the output from the original filter. To obtain the LP-filtered signal, I backward-forward filtered using a third-order Butterworth ...
9
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1answer
256 views

What are $H(i\omega)$, $H(s)$ and $H(z)$ called?

What are the Fourier-, Laplace- and Z-transforms of the impulse response of a filter called? I've seen $H(s)$ referred to as the system function or transfer function, what about $H(i\omega)$ and the ...
10
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2answers
4k views

Why does this manual bilinear transformation yield different results from Matlab's?

I've got a first-order Butterworth filter with the cutoff frequency $\omega_c$. Its transfer function is then $$H(s) = \frac{\omega_c}{s+\omega_c}$$ Using the bilinear transform to find an $H(z)$ (...
12
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3answers
2k views

Is it numerically more stable to implement filtering as multiplication or convolution?

I'm writing a program to filter a 20,000-sample signal with a fifth-order Butterworth filter offline. I've got access to an FFT implementation. There seems to be two alternatives for implementing the ...
8
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2answers
6k views

How does MATLAB handle IIR filters?

MATLAB has a butter, which constructs a Butterworth filter given an order and relative cutoff frequency. The filter created can be used to ...
7
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2answers
1k views

Should I use ideal or non-ideal filters for offline filtering?

I've got an offline signal that I want to high-pass filter. Should I use a Butterworth filter, or could I use the fact that the whole signal is known and use an ideal (step) filter? Using a high-...
8
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2answers
5k views

Using the Inverse Filter to Correct a Spatially Convolved Image (Deconvolution)

As part of a homework assignment, we are implementing the Inverse Filter. Degrade an image then recover with an Inverse Filter. I convolve the image in the spatial domain with a 5x5 box filter. I FFT ...
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 ...
4
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3answers
3k views

Bitmap border/stroke alogirthm?

I am looking for an algorithm that adds borders to a bitmap image(very much like the photoshop stroke effect) but one that will also have antialiasing to the borders(or one that copies alpha from the ...
10
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6answers
4k views

High pass filter of 0.01 Hz for EEG slow drifts

I am trying to make a high pass filter for electroencephalographic (EEG) datasets, in order to get rid of very slow drifts. However frequencies around 0.3 Hz are very important for the research these ...
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5answers
16k views

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 ...
17
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2answers
9k 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 ...
19
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3answers
951 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 ...
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 ...
4
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1answer
168 views

Find the relation between two noisy periodic signals of equal length

[Thank you, @leftaroundabout for your helpful comments to my related question. I'm trying to ask a less confusing question here.] $0\leq\phi\leq 2\pi$ is a parameter that theoretically describes a ...
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3answers
2k views

Documentation about filters in general

I already posted this question on StackOverflow. I got a suggestion to go for a better answer here. To make the question more concise I'm interested in an introduction to digital filters, any ...
26
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3answers
14k 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
58k 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 ...
11
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3answers
526 views

Image analysis - fiber recognition

I am completely new to image analysis. Do you know how to binarize this image in such a way to get the fibers only? A step by step procedure would be awesome, but any hint is appreciated as well. I ...
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 <...
19
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1answer
524 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 ...
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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 ...
20
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7answers
7k 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 ...
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, ...
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 ...
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?
21
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2answers
10k 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 ...
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 ...
8
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2answers
969 views

How to apply a filter kernel

A filter that can be used for digital signals like audio, video or image processing can be defined using a matrix ("kernel") that weights the surrounding area (this is a description I read in lecture ...
9
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2answers
955 views

How to design a continuously time-variant digital filter?

I have discrete time series containing signal with smoothly varied frequency over time (called a "sweep"). How can I design a discrete filter (low-pass or band-pass in my case) of a finite length with ...

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