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• ffmpeg : Downmixing 5.1 to stereo with phase invert

  1er novembre 2022, par anti-hero

  I'd like to use ffmpeg and Dolby Logic Pro II formula to downmix a 5.1 song to stereo :

  


  


  Lt = L + (–3 dB × C) – (–1.2 dB × Ls) – (–6.2 dB × Rs)

  


  Rt = R + (–3 dB × C) + (–6.2 dB × Ls) + (–1.2 dB × Rs)

  


  


  I came up with this long and probably redundant bash script :

  


  # L channel
ffmpeg -i "$filename" -map_channel 0.1.0 -c:a pcm_f32le L.wav
ffmpeg -i "$filename" -map_channel 0.1.2 -filter:a "volume=-3dB" -c:a pcm_f32le C.wav
ffmpeg -i "$filename" -map_channel 0.1.4 -af "volume=-1.2dB" -c:a pcm_f32le Ls.wav
ffmpeg -i "$filename" -map_channel 0.1.5 -af "volume=-6.2dB" -c:a pcm_f32le Rs.wav
ffmpeg -i Ls.wav -af "aeval=-val(0)" -c:a pcm_f32le Lsi.wav
ffmpeg -i Rs.wav -af "aeval=-val(0)" -c:a pcm_f32le Rsi.wav
ffmpeg -i L.wav -i C.wav -i Lsi.wav -i Rsi.wav -filter_complex "amix=inputs=4" -c:a pcm_f32le L_final.wav
rm L.wav Ls.wav Rs.wav Lsi.wav Rsi.wav

# R channel
ffmpeg -i "$filename" -map_channel 0.1.1 -c:a pcm_f32le R.wav
ffmpeg -i "$filename" -map_channel 0.1.4 -filter:a "volume=-6.2dB" -c:a pcm_f32le Ls.wav
ffmpeg -i "$filename" -map_channel 0.1.5 -filter:a "volume=-1.2dB" -c:a pcm_f32le Rs.wav
ffmpeg -i R.wav -i C.wav -i Ls.wav -i Rs.wav -filter_complex "amix=inputs=4" -c:a pcm_f32le R_final.wav
rm R.wav C.wav Ls.wav Rs.wav

# Mix
ffmpeg -i L_final.wav -i R_final.wav -filter_complex "[0:0][1:0] amerge=inputs=2" -c:a pcm_f32le "${filename%.*}_dm.wav"
rm L_final.wav R_final.wav


  


  Would this work ? And is there a simpler way to do this ?

  


  update : I also wrote Python code for the 90 degree phase shift version.

  


  import scipy, os, numpy as np

os.system(f'ffmpeg -i "{filename}" -c:a pcm_f32le temp.wav')
sample_rate, waves = scipy.io.wavfile.read("temp.wav")
os.remove("temp.wav")

L, R, C, LFE, LS, RS = waves.T

LS_shifted = -np.imag(scipy.signal.hilbert(LS))
RS_shifted = np.imag(scipy.signal.hilbert(RS))
L_out = L + 0.708 * C - 0.871 * LS_shifted - 0.49 * RS_shifted
R_out = R + 0.708 * C + 0.49 * LS_shifted + 0.871 * RS_shifted

scipy.io.wavfile.write("out.wav", sample_rate, np.vstack((L_out, R_out)).T)


  


  Hope someone finds this useful ! (and that someone verifies it's correct)

  


• Is there a way to (automatically) detect if the channels of a stereo video/audio are out of phase and canceling each other ?

  8 mars 2024, par Rhenan Bartels

  Background

  


  I've encountered an issue while attempting to convert an audio downloaded from a youtube video into a mono .wav format using ffmpeg. Despite using typical conversion commands (code below), the resulting audio is either silent or corrupted, indicating potential stereo channel cancellation.
I tried to load the audio into Python and calculate phase and cross-correlation, but this step uses a lot of memory, especially for a 6-hour-long audio.

  


  Goal

  


  I'm seeking a method, preferably using ffmpeg, to detect if stereo channels are canceling each other out due to phase misalignment. Additionally, I aim to automate the process of phase inversion before converting to mono to ensure the integrity of the resulting audio.

  


  Question :

  


  How can I automatically detect if stereo channels are canceling each other and subsequently invert the phase to ensure successful conversion to mono using ffmpeg ? Any insights, solutions, or alternative approaches would be greatly appreciated.

  


  Steps Taken :

  


  

  1. Downloaded audio from YouTube using yt-dlp.

  


  2. Attempted audio conversion to mono using ffmpeg, resulting in silent or corrupted output.

  


  3. Successfully converted audio to mono by manually inverting the phase of one channel prior to conversion.

  


  


  Download audio from youtube

  


  yt-dlp -f bestaudio https://www.youtube.com/watch?v=s3QB_rJzH08 -o input.webm


  


  Audio conversion

  


  The resulting file is silent or corrupted

  


  ffmpeg -i input.webm  -ac 1 -ar 16000 -c:a pcm_s16le output.wav


  


  Audio conversion to mono .wav with phase inversion

  


  The resulting file has audio

  


  ffmpeg -i input.webm -af "aeval=val(0)|-val(1)" -ac 1 -ar 16000 -c:a pcm_s16le output.wav


  


• Revision 17501 : only generate a random serial number for one stream, and increment it ...

  10 octobre 2010, par oggk — Log

  only generate a random serial number for one stream, and increment it for
  the subsequent streams, to avoid a possible (though unlikely) collision
  OK’d by j.