How can you use ffmpeg's afftdn -sn flag to sample noise from a reference noise file?

Question

The FFmpeg documentation for the afftdn audio filter says for the -sn flag:

sample_noise, sn
Start or stop measuring noise profile. Syntax for the command is : "start" or "stop" string. After measuring noise profile is stopped it will be automatically applied in filtering.

It's a little unclear as to the basic implementation of the start/stop values in the sample_noise -sn flag. I tried:

ffmpeg -i inputfile.wav -af afftdn=nt=c:bn=sn=start=0=stop=1 noiseout.wav

as an attempt to sample the first second of sound as a noise profile, which errors with

[afftdn @ 0x7fefbb800000] Custom band noise must be integer.

So the questions are:

1. What's the correct input format for sample_noise?

2. Is there a way to sample the noise from a noise reference .wav file that I have, e.g. noiseprofile.wav?

Answer 1

Commands are instructions sent from an interface external to the filter, usually an external program using a ZMQ protocol. There is a filter that can execute commands which is what I'll use here.

Base syntax is

ffmpeg -i in -af "asendcmd=c='0.0 afftdn@n sn start; 1.0 afftdn@n sn stop',afftdn@n" noiseout.wav

To use another source for noise profile, you'll have to append it, measure noise, and then remove it.

ffmpeg -i in -i profile -filter_complex "[1][0]concat=n=2:v=0:a=1,asendcmd=c='0.0 afftdn@n sn start; 1.0 afftdn@n sn stop',afftdn@n,atrim=1,asetpts=PTS-STARTPTS" noiseout.wav

Above, I assume the profile is 1 second long, which is removed with atrim after denoising.

Answer 2

This worked beautifully, many thanks to @Gyan.

Just to add to the accepted answer, as I needed to do this for a large number of mp3 audio files with similar noise profiles and with a naming scheme of 001_somefile.mp3. I wrote myself a batch script to do so. The -y confirms overwrite and the || pause causes the script to halt if the command was bad. I don't know if there's any available constants to automagically obtain the length of the noise_profile.mp3 input, so I hard-coded the 0.89 seconds length:

@echo off

for /F "usebackq delims=" %%f in (`dir /B ???_*.mp3`) do (
   ffmpeg -y -i "%%f" -i noise_sample.mp3 -filter_complex "[1][0]concat=n=2:v=0:a=1,asendcmd=c='0.0 afftdn@n sn start; 0.89 afftdn@n sn stop',afftdn@n,atrim=0.89,asetpts=PTS-STARTPTS" "0%%f" || pause
)