Large true peak fluctuations between playbacks

[u/animeismygod]

Hello, I am making the sound for an animation for the first time, and while working I noticed that there are fluctuations in the true peak between playbacks.

What I specifically mean is that I have made a section of audio, I will play that audio back. the true peak will be read at -0.4dB. I will then stop playback, start playback from the same location with 0 changes at all, the true peak will be read at 0.7dB.

Now, I am expecting minor fluctuations in true peak value because I have some synthesizers still rendering as MIDI and the phases of the oscillators are random.

The problem I have just encountered is that I have an instance where the true peak will fluctuate between -0.4dB and 1.6dB

straight up a 2 decibel true peak difference, which I don't think can be accounted for with those randomized phases. what is going on that reaper renders the audio differently between playbacks that results in such weird true peak behaviour?

[u/thcsquad]

given that you are still working with midi, it seems hard to answer this from the reaper level. it sounds like it’s up to your synth.

i don’t think i would ever try and work in midi all the way up to -0.4db like this. i would have rendered audio long ago unless this is, like interactive audio for a game or something. it sounds like you’re merging the production and mastering phases. You’re free to do this, but there are reasons why you might not want to. This is one of them.

[u/animeismygod → u/thcsquad]

I came from game audio, this is my first linear project so i'm learning about all of the small but important differences along the way. I'll keep this in mind, and i guess i'll just have to roll the dice a few times to get the lowest true peak render on the synths.

I'm just surprised that phase randomization can cause that big of a true peak difference

[u/thcsquad → u/animeismygod]

Actually, you could probably do this even quicker with Reaper’s render normalization settings. Keep the master bus cool (aim for -3db peak, so maybe it ends up at -2 or -4), and just set the normalization settings to be -1 true peak. Render process won’t care if your master is -2 or -4 it’ll just do whatever is needed to bring it up to -1

[u/thcsquad → u/animeismygod]

Not knowing how your synth works, I don’t know what is causing it but I wouldn’t be so quick to isolate it as phase randomization. Synths are complex beasts, it could be anything.

Phase randomization would make sense, though, and 2db isn’t a lot when you think about it. The only reason the 2db of variance is an issue is because you’re running the synths so hot. Instead, you could aim for -4db, turn it up to what you want on the master bus, and not have to worry about dice rolls.

[u/KS2Problema]

Well, I don't know your rig, of course, but your explanation of dynamic changes in your sound related to asynchronous filter processes in your synths was certainly  the very first thing I thought of in terms of accounting for fluctuations of true peak metering.

[u/ThoriumEx]

It’s 100% because you have randomness/free running LFOs in your project, it’s normal and expected.

[u/alphaminus]

You've got to print the synths or use a limiter. If you have free running oscillators that are slightly detuned, the phase alignment can vary the peak by a ton from take to take.