About FFmpeg alimiter filter

[rowjekto]

Can anyone please explain the options, especially the first 3, and the settings ( e.g. what is "default is 1" in the first 3 options?) of FFmpeg alimiter filter?

[pandy]

Can anyone please explain the options, especially the first 3, and the settings ( e.g. what is "default is 1" in the first 3 options?) of FFmpeg alimiter filter?

gain is simple multiplier - 1 mean no modification of audio (2 multiplied by 1 stay 2) - 0.5 will reduce signal level by half (2 multiplied by 0.5 will be 1), gain 2 will "amplify" signal by 2 (so 2 multiplied by 2 will be 4).

input gain will be applied before signal processing, output gain will be applied to signal after processing.

limit (third option) set desired maximum level 1 can be considered as maximum in digital scale i.e. 0 dBFS.

[rowjekto]

Which options and settings should I use in order to get a constant volume?

[sneaker]

That's usually done using the dynaudnorm or loudnorm filters.

https://superuser.com/questions/323119/how-can-i-normalize-audio-using-ffmpeg

[pandy]

Which options and settings should I use in order to get a constant volume?

I would reduce level on input and on output by half i.e. alimiter=level_in=SQRT(2)/2:level_out=SQRT(2)/2 - reducing level shall prevent clipping during processing and prevent clipping after processing (assumption signal internally will be limited to 1 i.e. 0dBFS) - signal reduction by SQRT(2)/2 is comparable to reduction of signal by half bit and it is equal approximately to -3dBFS so after all you should be on safe side (no digital clipping).

[rowjekto]

Apparently, the alimiter filter is not for me since the minimum limit value is 0.0625 (-24.08 dBFS), and the dialogue volume in some movies is way lower.

That's usually done using the dynaudnorm or loudnorm filters.

https://superuser.com/questions/323119/how-can-i-normalize-audio-using-ffmpeg

If I understood correctly, loudnorm is a more like a normalizer, and I'm looking for a compressor/limiter/whatever else that can make the volume completely constant.
About dynaudnorm, it's pretty good but I still didn't manage to find the right settings for me. Recently, I did a test with dynaudnorm: I set the target peak value to 0.02 (-34 dBFS) and checked the maximum volume level with volumedetect filter. The result was -28 dBFS. Pretty odd. (Full settings: dynaudnorm=p=0.02:f=50:g=11:m=1)

[sneaker]

Have you read all of the dynaudnorm documentation?

s

Set the compress factor. In range from 0.0 to 30.0. Default is 0.0. By default, the Dynamic Audio Normalizer does not apply "traditional" compression. This means that signal peaks will not be pruned and thus the full dynamic range will be retained within each local neighbourhood. However, in some cases it may be desirable to combine the Dynamic Audio Normalizer’s normalization algorithm with a more "traditional" compression. For this purpose, the Dynamic Audio Normalizer provides an optional compression (thresholding) function. If (and only if) the compression feature is enabled, all input frames will be processed by a soft knee thresholding function prior to the actual normalization process. Put simply, the thresholding function is going to prune all samples whose magnitude exceeds a certain threshold value. However, the Dynamic Audio Normalizer does not simply apply a fixed threshold value. Instead, the threshold value will be adjusted for each individual frame. In general, smaller parameters result in stronger compression, and vice versa. Values below 3.0 are not recommended, because audible distortion may appear.

Not saying this is the ideal filter for what you want but it sounds like this could fit your use case.

[rowjekto]

Tried it some time ago and iirc it didn't change much and low values (even higher than 3.0) damaged the quality.