What is the better filter steepness for resampling from 24 kHz to 48 kHz?

Lower steepness makes the range of frequencies at the upper boundary quieter. At the same time, iZotope RX 9 manual does not recommend higher values:

Higher filter steepness means better frequency performance of the filter: wider passband retains more useful signal, while stronger stopband attenuation provides better rejection of aliasing. At the same time, higher steepness of the frequency response requires a longer filter, which produces more ringing in time domain and energy smearing near the cutoff frequency.

  • what sample rate converter are you using?
    – Mark
    Commented May 30, 2022 at 2:15
  • I'm using iZotope RX 9
    – dereks
    Commented May 30, 2022 at 23:01

2 Answers 2


If you consider the fact that you are resampling from 24KHz to 48KHz a lot of the issues that the question raises are moot as there is going to be no audio to speak of in the vicinity of the resampling filters.

Recall that the nyquist frequency for 24KHz is 12KHz, so that the very highest frequency available to you in your 24KHz material is likely to be just below 12KHz.

The Nyquist frequency for 48KHz is going to be 24KHz, which is going to be more than 12 KHz away from your highest incoming audio frequency. The resampling filters will not be playing in this region, so you are unlikely to be able to hear any difference at all in the audio output at 48KHz no matter what resampling parameters you use.


The filter proposed by Izotope is needed when you reduce the sampling rate and don’t want aliasing.

Here, you want to go increase the sample rate. There are no risk of aliasing (frequency under 12kHz are already under 24kHz).

Then I would use the lower steepness (or no filter at all) to avoid too much degradation of the signal.

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