Category Archives: Digital Audio

Biquad formulas

Posted on January 2, 2011 by Nigel Redmon

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 | 5 Comments

A biquad calculator

Posted on December 20, 2010 by Nigel Redmon

Something useful: a biquad filter coefficient calculator… Continue reading →

Posted in Digital Audio, Filters, IIR Filters | 7 Comments

Sample rate conversion: down

Posted on December 20, 2010 by Nigel Redmon

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 | Leave a comment

A closer look at upsampling filters

Posted on December 11, 2010 by Nigel Redmon

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 →

Posted in Aliasing, Digital Audio, FIR Filters, Filters, Impulse Response, Sample Rate Conversion | Leave a comment

Sample rate conversion: up

Posted on December 5, 2010 by Nigel Redmon

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

Posted on December 5, 2010 by Nigel Redmon

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 →

Posted in Digital Audio, FIR Filters, Filters, Impulse Response, Sample Rate Conversion | 2 Comments

Towards practical resampling

Posted on November 23, 2010 by Nigel Redmon

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 →

Posted in Digital Audio, FIR Filters, Filters, Impulse Response, Sample Rate Conversion | 4 Comments

The sound of dither

Posted on November 7, 2010 by Nigel Redmon

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

Sample rate conversion

Posted on July 3, 2007 by Nigel Redmon

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 | Leave a comment

Sampling in-depth

Posted on July 3, 2007 by Nigel Redmon

Here we lay the foundation. We’ll look at analog to digital conversion, and we’ll look at the spectrum of the resulting digital signal. We’ll use that knowledge to help understand the conversion process back to analog. Though we can build … Continue reading →

Posted in Digital Audio, Sampling Theory | Leave a comment