Thursday, July 10, 2008

Impulse Response Convolver test

I read an interesting comment which written by Cyaoeu on my previous post about Impulse Response Modification 2 . He found something weird when using KeFIR as an impulse response convolver. He wrote that there are new frequency appeared at 10000 Hz and 12000 Hz when tested using a sine wave. I gave a simple answer that "Two very narrow spikes you found are ussualy happen when the impulse response that loaded in KeFIR are to short or we didn't set the LENGTH knob properly". Is it true? Let's prove it.

I made a simple test based on what Cyaoeu did. I'm using 512 sample points impulse response. 1000 Hz sine wave from mda TestTone VST plugin. Voxengo Span v1.9.1 as the spectrum analyzer and KeFIR v1.2. This test is done under Tracktion v1.6.0.1.

The plugin sequence:

mda TestTone (1000 Hz) -> KeFIR v1.2 -> Voxengo Span v1.9.1

Lets take a look to the picture below:

When the impulse response is loaded into KeFIR for the first time, it's yes... what Cyaeou has reported is true there are new frequencies appeared at 10000, 12000 and 21000 Hz. I assume that few last samples are uncounted by the plugin. We can see it's 10 % on KeFIR full length, so there still enough space to add more length on KeFIR parameter.

I add more length to 13 %, as the result, you can see the picture below:

Yup, those new weird frequencies are gone. KeFIR is pure fast convolution plugin without any coloration added. In my opinion it's one of the best free plugin for impulses response convolver plugin or impulse response modification.

From what I've tested, these are some advices when using KeFIR as convolver plugin:

  • Make sure the length parameter is sets properly
  • If your IR is longer than 8192 samples on KeFIR v1.2 or 32768 samples on KeFIR v1.3, you should cut your IR first. Read Shorten The Impulse Response.

Thank you for reading everybody...

Thanks for your great comment Cyaoeu...

Related Posts:
Shorten The Impulse Response
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Impulse Response Modification
Guitar Cabinet Simulator


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