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Category Archives: Filters
A onepole filter
Here’s a very simple workhorse of DSP applications—the onepole filter. By comparison, biquads implement two zeros and two poles. You can see that our onepole simply discards the zeros (the feedforward delay paths) and the second pole (feedback path): We … Continue reading
Posted in DC Blocker, Digital Audio, IIR Filters, Source Code
38 Comments
Biquad C++ source code
I don’t want to get into the business of teaching people how to code—there are a huge number of free resources available on the internet to that do that. But I’ll give a small taste for those trying to get … Continue reading
Posted in Biquads, Digital Audio, Filters, IIR Filters, Source Code
77 Comments
Convolution—in words
Convolution is a convoluted topic—and that’s what it means (convoluted, from MerriamWebster : “Extremely complex and difficult to follow. Intricately folded, twisted, or coiled.”). Really, it’s more difficult to explain why you would want to use convolution than it is … Continue reading
Posted in Convolution, Digital Audio, FIR Filters, Impulse Response, Reverb
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Biquad formulas
For fixed filters, we can plug biquad coefficients into our programs. But often, we need to calculate them on the fly, to user settings or changes in sample rate. As a companion to the biquad calculator, here are the formulas … Continue reading
Posted in Biquads, Digital Audio, Filters, IIR Filters
48 Comments
Sample rate conversion: down
In doubling the sample rate, we inserted zeros between existing samples, then used a lowpass filter to remove the resulting alias in the audio band. To resample at half the current rate, we use a lowpass filter to remove audio … Continue reading
Posted in Aliasing, Digital Audio, Filters, Sample Rate Conversion
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Sample rate conversion: up
Once we have a suitable set of FIR filter coefficients from our windowed sinc calculator, it’s time to apply them. Again, our recipe for doubling the sample rate: 1) Insert a zero between existing samples. (This is the upsampling step, … Continue reading
Posted in Aliasing, Convolution, Digital Audio, Filters, FIR Filters, Sample Rate Conversion
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Towards practical resampling
In a previous article, we looked at sample rate conversion in the frequency domain. Let’s take a quick second look in the time domain as reinforcement of principles behind sample rate conversion, before developing a practical rate convertor. In an … Continue reading