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Author Topic: Convolution question  (Read 2420 times)

thomaspf

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Convolution question
« on: September 02, 2012, 05:56:37 pm »

I recently build a new stero system for my son and I wanted to try out the build-in convolution system for room correction. In my main system I have been using Acourate to create the impulse response and when VST support became stable in MC I started to use Pristine Space as the convolution engine.

Pristine Space allows you to take a impulse response in a single sample rate and internally resamples that impuls so it works with any media stream presented to it. How does the convolver in MC behave when the correction impulse was generated lets say in 96 Khz sample rate and I play a stream with 44.1 Khz?

If that is not supported what would a config file look like that has a correction impulse for the typical sample rates?

44.1, 48, 88.2, 96, 176.4, 192, 352.8, 384


Cheers

   Thomas
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Matt

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Re: Convolution question
« Reply #1 on: September 02, 2012, 06:35:58 pm »

The convolution engine in Media Center resamples filters automatically when necessary using our audiophile grade resampler.

We're talking about how to allow creating discrete filter files for each sample rate for v18, but haven't made any decisions yet.  This is an advanced option that you'd only use if you wanted.
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Matt Ashland, JRiver Media Center

thomaspf

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Re: Convolution question
« Reply #2 on: September 02, 2012, 08:08:02 pm »

This is excellent! I am trying it our right now and it works by simply loading a stereo impulse file. Very nice ...

If you only use one correction impulse which sample rate would you choose? My collection is mostly 16bit/44.1Khz with about 200 albums in varying formats 24/88.2, 24/96, 24/176.4 and 24/192.

Cheers

  Thomas
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hulkss

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Re: Convolution question
« Reply #3 on: September 02, 2012, 10:08:07 pm »

If you only use one correction impulse which sample rate would you choose? My collection is mostly 16bit/44.1Khz with about 200 albums in varying formats 24/88.2, 24/96, 24/176.4 and 24/192.

24/88.2    That way, upsampling from 16/44.1 is essentially perfect and downsampling will be inaudible.
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thomaspf

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Re: Convolution question
« Reply #4 on: September 03, 2012, 12:04:16 am »

I do not believe that JRiver will resample the actual music stream but only the correction impulse?

What I do not have a good understanding of is whether resampling the correction impulse introduces any negative effects and whether going to the highest rate is the best choice or not.

My microphone does not much over 20Khz (with any level of precision anyhow) so I am wondering whether a 44.1Khz impuls is sufficient.

Cheers

   Thomas
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thomaspf

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Re: Convolution question
« Reply #5 on: September 04, 2012, 09:13:43 pm »

Anyone here using the convolver? Which sample rate are you using for your correction impulse?

Correction results for my new system attached. The corrected curves are generated by convolving the logsweep and recording the resulting file. So this is actual performance not just calculated result.
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Skogkatt

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Re: Convolution question
« Reply #6 on: September 05, 2012, 12:27:27 am »

Quote
Anyone here using the convolver? Which sample rate are you using for your correction impulse?

Correction results for my new system attached. The corrected curves are generated by convolving the logsweep and recording the resulting file. So this is actual performance not just calculated result.

As the majority of my music is sampled at 44.1 kHz / 16 bit I decided to use a correction impulse at 44.1kHz as any other sampling frequency would be
anyhow resampled at the sampling rate of the music you are playing.

Quote
I do not believe that JRiver will resample the actual music stream but only the correction impulse?

What I do not have a good understanding of is whether resampling the correction impulse introduces any negative effects and whether going to the highest rate is the best choice or not.

My microphone does not much over 20Khz (with any level of precision anyhow) so I am wondering whether a 44.1Khz impuls is sufficient.

Cheers

   Thomas

Note that a convolver needs an impulse sampled at the same sampling rate of the music, so if the music is sampled at a different rate than the impulse, MC has to take care of that (impulse resampling).
Music sampled at 44.1kHz has a theoretical analog bandwidth of 22050Hz and that's your limit imposed by the sampling theorem. So, in this case an impulse sampled at 44.1kHz is not only sufficient but
mandatory. If you play mostly music with CD quality (16/44.1) as I do, then the impulse sampled at 44.1 is the logical choice.

Also, I've noticed that impulse resampled internally by MC, to play music at higher sample rate, is transparent enough in terms of sound quality even if an option to
store in MC impulses for the various sample rates would be highly desirable. IMHO MC's convolver is sonically the best I've heard.

BTW: your measured curve with correction shows an excellent linearity. Congratulations!
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