According to all this, the measurement of loudness (especially watching the true peak TP) seems a wise way to avoid unwanted peaks and distortions when converting digital audio to analog.
In the loudness measurements that I did with the Youlean Loudness Meter 2 plugin (in my first response), red marks are observed that indicate a true peak value above 0 dB (+0.5 dB).
However, the TP value in the loudness meter in Shotcut marks a positive value (728.5 dBTP) out of scale range (-50 to +3).
I should ask about this.
Finally I overcomplicated the answer to your problem.
Searching the forum, I found this.
I hope it helps.
OK, problem solved. All this was due to the version of Shotcut I was using (17.06.02 in the Debian Stretch release). With this version, when moving the video clip to the timeline, the audio level drops significantly: -13,5dB with the video I was working on (voice audio track) and -6dB with a pure 1kHz tone. This is no more the case in the latest Shotcut release (19.02.28) so I consider that this problem has been addressed and fixed. There is still a difference between the audio level of the video track and the level reported by the peak meter: I recorded a video with a 1kHz tone at 0dB and when played in Shotcut the peak meter shows apprx. +2dB. This is apparently due to oversampling (see this old topic: Shotcut peak meter reads incorrectly - lowers volume while rendering). Thank you all for helping me solve this and sorry for having put up a problem which no longer exists.
Still to be clarified is what exactly the peak meter displays in a frame. Have a look at this screenshot where you can see the audio in a frame. The audio is a 1kHz tone slightly peaking at -1db in Audacity, AM modulated at 99% with a square wave at apprx 50Hz. So what value does the peak meter show in the following screenshot? (if you go through the successive frames you will see the peak meter showing a maximum of 0db and a minimum of -18db with intermediate values of -2,5dB, -5dB, -10 dB, -15dB)
Oh, I think I have been a little bit too fast with my answer. With my sin signal am modulated at approx 50Hz normally Shotcut sees the same signal in each frame, only the phase is changed due to the fact that the modulating frequency is not exactly 50Hz. Also, with a sinus tone with its max at 0dB, the peak meter should show -3dB for the RMS value. Am I wrong? The following screenshot shows the peak meter at +2dB…
I take back what I said about RMS. I was going off of memory since I knew we were using the MLT normalize module (based on the normalize command line tool, which is heavily based on RMS). However, upon looking it up, now I recall I added a separate filter based on code from a meter written by Steven Harris, who wrote the LADSPA plugins we include in Shotcut for many of the audio filters.
We already know what the reference standard for this digital peak meter is.
Before reading this (my fault), I spent the whole morning doing tests to try to find out what was the standard reference (or measurement) method with respect to the most common options that I have in my DAW and in my loudness meters.
I generated a sinusoidal 1kHz tone with Audacity to contrast the measurement with different standards. However, it did not match any of the VST analyzer plugins that I use.
With this new information, I observe that this IEC standard 268-18-1995 (1995), has already been overtaken by other standards such as ITU-BS.1770 (2015) or EBU R128 (2014)The loudness meters that I have used for the comparison are based on these last two standards, so they will hardly coincide.
This is neither good nor bad, they simply can not match.
This applies only to the peak meter, so we have the Shotcut loudness meter (LUFS) based on EBU R128.
The use of this tool can help to standardize our criteria at the time of editing the audio of our video.
I did not delve into the advantages of one or another measurement standard (the different algorithms, the different commercial needs, the different platforms and entities involved). For me, each regulation is focused on a situation and that is how it should be understood.
I made some measurements of the peak meter response with 0dB sinus inputs created with Audacity (0dB=level1 on the Audacity peak meter). I did not plot on the graph the frequencies which are not multiples of the frame frequency (PAL 25Hz) because for these frequencies, especially at low values, the meter response varies a lot according to the phase of the signal (see the last values in the table). As you can see there are significant amplitude variations, most of them probably due to the ringing of the high pass filter. Notice also the dip at 12kHz (submultiple of the sample frequency):
This already exceeds my limited knowledge about this (although these days I learned some things that I did not know).
Anyway for a more professional audio edition surely an external audio edition is a possible option.
For my modest and scant videos, the audio filters in SC work perfectly.