What Is the Ringing Effect in Resampling Filters?

When working with digital signals like audio or images, it's common to change the sampling rate. This process, called resampling, can sometimes introduce an unexpected issue known as the ringing effect.

In this post, we’ll explore what causes ringing, how it affects audio and images, and how to reduce it.

What Is the Ringing Effect?

Ringing refers to unwanted oscillations around sharp transitions in a signal. In audio, it may sound like a faint echo or high-frequency noise. In images, it appears as thin halos or fringes around edges.

Simply put, it’s a kind of artificial “echo” or “outline” created by the filter during resampling.

Why Does Ringing Occur?

Resampling usually requires a low-pass filter to remove high-frequency components and prevent aliasing. The most ideal low-pass filter in theory is a sinc function (sin(x)/x), which is:

• Infinite in length

• Oscillatory in nature

• Impossible to implement exactly in the real world

To use it practically, the sinc function is windowed (cut off). But since its oscillations continue beyond the window, ringing inevitably occurs.

Ringing in Audio

In audio, ringing is noticeable in:

• Sudden transients like drum hits or note onsets

• High-resolution audio being upsampled

• Quiet sections where artificial overtones or echoes may become audible

How to Reduce Ringing

Ringing can't be completely eliminated, but you can minimize it with the following techniques:

Method	Description
Shorter filter kernel	Reduces ringing but may sacrifice high-frequency detail
Different window functions	Use Blackman, Hann, Hamming, Kaiser, etc. to suppress oscillation
Minimum-phase filters	Confine ringing to one side (usually after the transient)
Oversampling	Perform filtering at a higher sample rate to reduce audible ringing
User-selectable filters	Let users choose a filter that suits their preference or content

Ringing is a mathematical side effect of sinc-based filters used in resampling. While unavoidable in theory, careful filter design and user control can significantly reduce its impact.

If you're working with high-quality audio or sensitive content, choosing the right resampling filter is essential for a natural and clear result.

Apps like Pine Player Pro offer a variety of resampling filters to help reduce ringing and improve audio quality. Try different options and find the one that suits your ears best.

OSF (Over Sampling Filter) automatically sets the most optimal filter algorithm, delivering the best sound quality without requiring user intervention. However, if you want to apply different settings for specific tracks, select CSF (Custom Sampling Filter) and try configuring it using the following options.

Resampling quality and ringing control can mainly be adjusted using these options

Option	Description	Ringing Impact
Filter Size	Sets the filter size (number of taps)	Larger sizes offer higher audio quality but may increase ringing
Phase Shift	Number of filter phase stages (4 ~ 30)	Lower values provide minimum-phase effects and reduce ringing; higher values give more precise frequency response but may increase ringing
Cutoff	Cutoff frequency ratio (0.0 ~ 1.0)	Lower values reduce aliasing and also reduce ringing
Precision	Internal calculation precision (default: 16)	Higher precision leads to cleaner audio results
filter_size	(libsoxr only) Sets the soxr filter size	Affects ringing suppression
Filter Type	Specifies the filter type	Kaiser is recommended by default, but Lanczos or Blackman Nuttall can reduce ringing