Doing resampling, how can we choose lower cutoff frequency for LPF filter? I would like to resample an audio signal.

Sampling frequency of an original audio signal is 50k Hz.

What I want to down sampling frequency is 30k Hz.

In this case, I followed the procedures as below using LabVIEW.


  '1. IIR low pass filter
  
  Butterworth / 6th / Lowpass / up to 15k Hz
  
  '2. Resample to 30kHz


Why I used LPF is to prohibit the anti-aliasing.

 from now on, I have some questions.

1. Do we need to do LPF for prohibiting anti-aliasing?

2. If we have to do, how can we choose the lower cutoff frequency?
   In my case, I choose as 15k Hz because this is the new Nyquist frequency.


Thank you in advance.

OR is there the standard procedure for resampling?
One thing to consider is the existing spectral content of the signal being downsampled.   If there is no significant energy above 15kHz, then the anti-alias filtering requirements are certainly relaxed since it won't have to do much for a 30kHz output sampling rate.

So where to put the filter cutoff frequency also depends on assumptions about the spectral content of the input signal as well as the transition band rolloff characteristics of the filter.  For a 30kHz sampling rate the 3dB cutoff point of the filter may need to be much lower than 15kHz to prevent aliasing from 15.5kHz or 16kHz (for example).

Much of this depends on your tolerance for the level of aliasing and at what frequencies.