Author Topic: Anti-Aliasing filters  (Read 1357 times)

Offline PaulVK

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Anti-Aliasing filters
« on: January 26, 2016, 05:21:10 PM »
Hello,

If I am downsampling a hi-res file into redbook 44.1/16, does XiSRC apply the appropriate anti-aliasing filters?

Offline geozem

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Re: Anti-Aliasing filters
« Reply #1 on: January 27, 2016, 09:26:13 AM »
Dear PaulVK,

the anti-alias filter is applied in any case by default in this case. Even if you up-sample does the same. ISP correction is applied optional since the last version because there is an issue.

I think the steep one is apllied and you can not deactivate or change it to the other one like in Audiorepair if you want.

I have requested this as an optional feature, but Stefan said not possible due to the algorithm design today.
Still unhappy with this default setting. Probalby he makes me sometime happy with a good solution.
I don't give up the hope.

Probably something is in pipeline. So I'm waiting for his feedback.

Best regards
geozem


Offline Xi_Stephan

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Re: Anti-Aliasing filters
« Reply #2 on: January 28, 2016, 06:12:53 PM »
Hello PaulVK and Geozem,

Please allow me to explain how up-sampling and down-sampling works and why we need to apply effectively the same low pass (antialiasing) filters for both steps.

1.) Down-Sampling:
If we reduce the sample rate then the Nyquist theorem still applies, which means that the highest frequency sampled in the signal must be lower or equal to 1/2 x sample rate.
To assure that the signal must be low pass filtered and then down-sampled. By using dedicated decimation filters that happens in one step.

2.) Up-Sampling
To do an up-sampling we need to insert the right amount of samples.

That is a two step process:
a.) Insert zeros to achieve the target sample rate
b.) Apply a low pass (Antialiasing) filter to remove the mirror spectra created by inserting zeros. That filter has the same cut-off frequency as the filter used for down-sampling.

If we have to change the sample base (e.g. 44.1kHz => 48kHz) then we do an up- and down-sampling to achieve a rational fraction. That is the reason why the up-/down-sampling between the sample bases takes considerable more time.

So, it is not a matter whether we won't like to present an option to switch the filter off, in fact those filters are absolutely necessary to avoid any kind of aliasing.

Thanks

Stephan
« Last Edit: January 28, 2016, 06:15:37 PM by Xi_Stephan »

Offline PaulVK

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Re: Anti-Aliasing filters
« Reply #3 on: January 28, 2016, 10:10:58 PM »
Thanks Stephen, that all sounds to me like things are being done correctly.