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Frequency shift when going from stereo to mono?
Old 17th April 2019
  #1
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Frequency shift when going from stereo to mono?

Hi,

I recorded a stereo piano track (using the entire range of the piano) via MIDI and a VST ('The Maverick' from Native Instruments). I have the piano VST on a preset, with the stereo width set to the width that matches the original recording of the piano samples.

Now, when I concert the stereo track to mono I notice a frequency shift in the sense that there seems to be a lot less high-end frequencies in the mono version compared to the stereo version.

My question: what's the explanation for this and how can I get the mono version to sounds as close to the stereo version as possible in terms of frequency distribution? Is it due to some kind of phasing issue? Given that I just used a preset from the VST I would think phasing issues would be sorted out.

Thanks for your time!
Bart
Old 17th April 2019
  #2
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There are a ton of possible psychoacoustic effects that can result from converting a recording from stereo to mono that have nothing to do with phase issues. There's no simple answer to, "Why does this stuff sound so much different now?" There could be a million reasons.

If you haven't done so already, listen to your mono recordings on a mono system and see what you think then. Also check the stereo and mono files on an editing program and compare their frequency profiles, and see whether or not they differ in a significant way. Your ears might be fooling you.
Old 17th April 2019
  #3
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Mono typically reveals phase issues, so most probably that's what you're experiencing (cancelling).

Many virtual instruments use some kind of haas delay to achieve stereoness in their stereo patches.
Even when the stereo samples were made using stereo miking techniques, there will be some phasing somewhere in the frequency spectrum, given the extended range of a piano and its physical size.

A small delay between the left and right sides gives stereo width, but while a small delay causes coloration in the lower frequencies when collapsed to mono, in the higher frequencies, it can cause total cancellation, due to the small wavelengths.

The problem is that anything you do to solve the issue in one frequency band will probably cause the issue to arrise in a different frequency band.

One option is to try delaying one of the sides just a few milliseconds to get rid of the issue in the higher frequencies at the expense of some coloration in the lower frequencies (and probably loss of the stereo width). Polarity switching may have similar effect.
If this coloration is not acceptable to you, you must find a way to separate the higher frequency band and treat/delay it without affecting the lower bands.

Another option is to use one stereo side and apply haas delay to create your own stereo width.


I'm sure that there are others with better solutions for problems like this, but I hope my contribution is of some help.

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