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Please help me understand what's happening here

February 24, 2013 | 4:37 am

I have an audio file that was recorded at 11.025kHz sampling rate. When I play it in Pro Tools (set to 44.1kHz) and examine the frequency spectrum, I get the results I expect–nothing above 5512.5Hz (top image). However, when I use the same plugin in Max (also set to 44.1kHz) to examine the same audio file, I get frequencies going all the way up to 20kHz, but with a dropout at 11.025kHz (bottom image).

In Pro Tools 10, I'm using the AAX version of this plugin, and in Max 6, I'm using the AU version loaded into the [vst~] object. I have the file loaded into a [buffer~] and playing through a [groove~] object with nothing between [groove~] and [vst~] but a patch connection. Setting the sample rate of the [buffer~] to 11025 doesn't seem to make a difference.

Can someone help me understand why I'm seeing the extra "stuff" above the Nyquist frequency? Thanks so much!

–Brian

[attachment=215696,5064]

Attachments:
  1. Spectrum.gif

February 24, 2013 | 10:26 am

could it be that you are playing the file with a speed different from 1.0 ?


February 24, 2013 | 11:12 am

Does it sound different? What happens when you play with the wave~ object with interp set to 0 (no interpolation)? Or alternatively you could use an index~ and count~.


February 24, 2013 | 11:54 am

Another wild guess: is it possible that the AU version of the plug-in is dithering the signal? (I.e., adding a small amount of noise, this is typically done when working with a soundfile at a higher sample rate than the original recording).

WG2: do you know for sure that the dials at the bottom of the analysis windows work identically on both versions of the plug-in?


February 24, 2013 | 6:16 pm

Thanks, everyone.
Roman: yes, the speed into the groove~ is 1.0
jvkr: they do sound the same. I tried using wave~ and index~ as you suggested, but got similar results
Peter: Those are great wild guesses. I don’t know the answer to either one. I know dither is used with higher sample-rate files, but I had not heard of it being used with lower sample-rate files. I’m going on the assumption that since the version number for the plugin is the same, but one is AAX and the other AU that the controls are the same. Of course I seem to remember an old saw about assuming…


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