When I mix and render a single track, the contrast analysis RMS can change. Method:
record 10 secs voice to track A
mute track A
record 10 secs voice to track B
copy some of track A, paste into track B
measure RMS of track B
Mix and Render track B to new track C
measure RMS of track C - it’s different from #5 reading
If I invert track C, then mix and render B & C to track D, I get a flat line, so the reported value in #5 is off. If I analyse amplitude statistics, I get identical values for each track, matching the reading for track C. I infer from all of this that contrast analysis readings are approximate.
The reason I’m checking this is to set correct RMS levels for ACX audio. I can live with it as long as I understand what’s going on.
Download it and install it as a plugin. Analyze > ACX Check
There’s no contest between that and struggling step-by-step through Contrast and Amplify. This is what that looks like if you haven’t beat your way through the whole thing yet. I think this will only do a mono show, not stereo.
I’m not able to reproduce the issue. Is it repeatable for you?
If so, could you make a ZIP archive of the project (including the AUP file and its _data folder) and upload it somewhere, then post a link to it. There’s some suggestions of free file sharing sites near the bottom of this post: https://forum.audacityteam.org/t/how-to-post-an-audio-sample/29851/1
I cannot reproduce it either, but why is pasting part of A into B a necessary part of the steps?
I would guess an explanation could be that you have Transport > Overdub on, and some of track B is thus pushed behind zero, and that you then time shifted track B or C. It seems Contrast does not measure audio behind zero, but Amplify for example does.
Thanks Phil. Yes it seems even duplicating track A (or B), copy and paste from one duplicate to the other then Mix and Render to New Track can produce different RMS between original and rendered track.
I’m checking this is to set correct RMS levels for ACX audio.
There is a suite of tools for producing ACX audiobook readings. You don’t need to go through that Contrast thing any more and you don’t have to cycle through compressors or other gymnastics to set RMS levels.
If the goal is submission to ACX, the tool collection is indicated. The steps were tested as a suite and there is a warning to take them in order and not leave anything out. Each tool depends on the results of the tools that came before.
For example, the first effect, Low Rolloff is a rumble filter and gets rid of very common sub-sonic trash that will throw off RMS-Normalize and the Limiter.
Home recordings commonly suffer from noise and we decided to push that off to its own document. If you have difficult noise, no tools are going to be easy and the condition may be a showstopper. Noise Reduction may plain fail, or too much Noise Reduction will fail ACX Human Quality Control. Some common microphone noises have specialized repairs. If you find yourself going round and round, post a sample and we can give a diagnosis.