No anti-aliasing filter for linear resampling

[jpaasen]

There is no anti-aliasing filter applied for the Linear resampling mode.

So if you create the following chirp:

and you resample (down) with Linear you get:

This might be intentional, but it is not straightforward from the documentation that this is the case.

The documentation says:

SRC_SINC_FASTEST - This is the fastest bandlimited interpolator and has an SNR of 97dB and a bandwidth of 80%.
SRC_ZERO_ORDER_HOLD - A Zero Order Hold converter (interpolated value is equal to the last value). The quality is poor but the conversion speed is blindlingly fast.
SRC_LINEAR - A linear converter. Again the quality is poor, but the conversion speed is blindingly fast.

The line This is the fastest bandlimited interpolator for SRC_SINC_FASTEST implies that the latter two options are not band-limited.

The documentation could be improved with a line or two, stating the latter two options should NOT be used without the user taking care of the anti-aliasing filtering him/herself.

If you downsample using SRC_SINC_FASTEST. libsamplerate takes care of the anti-aliasing filtering:

[evpobr]

Hi @jpaasen . Pull request?

[jpaasen]

I can do a PR with improvements to the doc. I just wanted to establish if this behavior was intentional or not.

Looking at librosa.resample, that exposes libsamplerate for some of its supported res_type values, there are no warnings about using ‘linear’ for down-sampling. So the misconception seems "widespread".

[evpobr]

We need to ask @erikd. But he doesn't answer.

[erikd]

The linear sample rate converter is provided for completeness and for something to compare actually good converters against.

If you want sample rate conversion without aliasing, then used the SRC_SINC_* converters.

[evpobr]

Hi @erikd :smile: