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RMS levels different - Stereo Bounce vs Duplicated Mono .

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RMS levels different - Stereo Bounce vs Duplicated Mono .

Postby theno1guitarman » Fri Apr 16, 2021 9:06 am

Hello,

I have a session with a single mono audio track. When bounced as mono summed or using commit to track, the RMS measurement of the file is the same as that on the RMS meter on the mono track. -17db RMS.

If I then duplicate the committed mono track and put both these mono files on a stereo track the RMS level still measures the same -17db, however, the audio sounds loud. I'm assuming due to the stereo effect.

However, if I simple bounce the mono track to a stereo interleaved file the RMS level registers at -21db.

I'm curious what causes the difference because shouldn't bouncing a mono track to stereo and combining two mono track to stereo track in a Pro Tools session produce the same results.

I'm aiming for an RMS of -17 so happy to use the first method but wanted to understand the difference. I'm sure pan comes into play somewhere.
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Re: RMS levels different - Stereo Bounce vs Duplicated Mono .

Postby CS70 » Fri Apr 16, 2021 9:14 am

What is your current pan law in the DAW?
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Re: RMS levels different - Stereo Bounce vs Duplicated Mono .

Postby theno1guitarman » Fri Apr 16, 2021 9:37 am

CS70 wrote:What is your current pan law in the DAW?

That's a good point. I believe it is set to Pro Tools default so I'll make that my first point of call.
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Re: RMS levels different - Stereo Bounce vs Duplicated Mono .

Postby Wonks » Fri Apr 16, 2021 10:00 am

If you duplicate one mono track and then copy each onto half of a stereo track, then you are effectively creating a track that, taking both sides into account, will be 3dB louder than the mono track, even though both the L&R tracks are of the same amplitude as the single mono track. The RMS meter will be showing the level per channel, but if you summed the master output to mono, you'd get a -14dB reading for the RMS, not -17dB.

A mono track that gets bounced to a stereo file will (probably) be affected by your pan law setting, so that each side is reduced by the pan law amount in order to try and keep the resulting stereo track at the same overall output level. So if your pan law setting is -4db, then both sides will be reduced in amplitude by 4dB, giving the -21dB RMS value per channel you experience.

This is probably why your first stereo method gave a louder result, and the second a quieter one.

You just need to understand it, and that it is doing what it is supposed to do, and then move on and just adjust the levels according to your ears once more.

You won't want to adjust the panning law to 0dB just to cure this problem, as it will give you even more problems once mixing and panning!
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Re: RMS levels different - Stereo Bounce vs Duplicated Mono .

Postby Hugh Robjohns » Fri Apr 16, 2021 10:54 am

theno1guitarman wrote:If I then duplicate the committed mono track and put both these mono files on a stereo track the RMS level still measures the same -17db, however, the audio sounds loud. I'm assuming due to the stereo effect.

The measurement will probably be based on the per-track analysis level, hence being the same, but it will sound louder because you now have two channels of audio feeding two speakers. It's dual-mono, rather than true stereo.

However, if I simple bounce the mono track to a stereo interleaved file the RMS level registers at -21db.

If the mono channel feeds a stereo bus it goes via a pan-pot, and pan pot's introduce a varying attenuation related to the control position. At the centre the signal is typically attenuated by either 0, 3, 4.5, or 6dB, with the 3dB option being a common default. It's called the 'pan law' and there will be an option to change it buried somewhere in the DAW preferences.

It sounds like that's what's happening in your case, with either a 3 or 4.5dB centre attenuation.

I'm curious what causes the difference because shouldn't bouncing a mono track to stereo and combining two mono track to stereo track in a Pro Tools session produce the same results.

Nope... Different approaches use different technologies and achieve different things.
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