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Author Archives: Nigel Redmon
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 Digital Audio, Filters, IIR Filters
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A biquad calculator
Something useful: a biquad filter coefficient calculator… Continue reading
Posted in Digital Audio, Filters, IIR Filters
7 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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A closer look at upsampling filters
Interpolation type: None Zero-order hold Linear Sinc 1 Sinc 2 Sinc 3 Show impulse response Sweep! In this demonstration, we generate a sine wave sweep from low in the audio band to near the Nyquist Frequency, which is half the … Continue reading
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, FIR Filters, Filters, Sample Rate Conversion
4 Comments
Building a windowed sinc filter
As promised, here’s our windowed sinc calculator for building a 2x oversampling filter: Factor Length Rejection Gain Notes: Use the Tab or Enter keys to effect changes (most browsers), or press Calculate. The frequency axis is in multiples of the … Continue reading
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
The sound of dither
Dithering is about spreading errors out, so that they aren’t related to the sampled signal. A constant background hiss is easier to ignore than tones that change depending on signal frequencies and amplitude. Here’s a fixed-frequency sine wave, truncated to … Continue reading
Posted in Digital Audio, Dither
3 Comments
Blog birth
Sometimes I get email asking a question—a question that I think others might have. Sometimes I’d like to write some helpful notes, but maybe it doesn’t warrant a full article. Or sometimes I’d like to address a subject in pieces. … Continue reading
Posted in Uncategorized
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Sample rate conversion
Here we explain how sample rate conversion works. As an essential prerequisite, you must understand the principals of sampling. Even if you understand sampling already, read our explanation of the process here. The viewpoint and terms used there are mirrored … Continue reading
Posted in Digital Audio, Sample Rate Conversion
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