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• 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


  


• how to begin streaming while ffmpeg is still transcoding the file in PHP

  15 janvier 2013, par Joyal

  I want to transcode an AVI video to mp4 with ffmpeg, but while is still transcoding, I would like to watch the video transcoded on a flash video player in realtime , Im using jwplayer , I made some test with mp4 and works great , but Im not able to make it work while is transcoding

  I made a php script to run the command in background

  ffmpeg.exe -threads 1 -y -i "a.avi" -s 1280x720 -f mp4 -vcodec libx264 -b 2000000 -ab 128000 -ar 44100 "a.mp4"

  on the jwplayer i have as source "a.mp4"

• ffmpeg transcode to youtube live bad video container

  13 juillet 2017, par popcorn9499

  I’ve been attempting to transcode a stream produced by obs studio to my nginx server and send it off to youtube. Now I’ve made it work with twitch and I know these settings are actually transcoding it mostly correctly and is viewable. The problem being that youtube live picks it up as Bad video settings and tells me to change the current video container format. The other side effect that is probly unrelated is the stream looks really poorly on youtube. Looks like it was streamed at a poor bitrate and stuff but the real problem is the bad video settings error.

  The ffmpeg command being used is as follows

  ffmpeg -i rtmp://localhost/Private/Private1 -vb 6000k -minrate 6000k -maxrate 6000k -bufsize 6000k -s 1280x720 -c:v libx264 -preset faster -r 50 -g 100 -keyint_min 50 -x264opts nal-hrd=cbr:force-cfr=1 -sws_flags lanczos -tune film -pix_fmt yuv420p -c:a copy -f flv -threads 6 -strict normal  rtmp://a.rtmp.youtube.com/live2/{key}

  I’ve tried with different framerates and been googling for awhile and found nothing or interpreted everything wrongly. Either way I would be very happy for some help here.

  System info.

  • OS : Ubuntu Server 16.04 LTS
  • Ram : 10gb
  • Processor : AMD Phenom(tm) II X6 1090T
  • GPU : Geforce GT 520

  Internet.

  • Upload 15mbit
  • Download 150mbit

  If you need any more info I will gladly send it. Thanks for reading.

  Edit 1

  After some googling about what I’m doing wrong I decided to try and change stuff slightly and came up with this command

  ffmpeg -re -i rtmp://localhost/(app)/(key) -c:v libx264 -r 50 -g 100 -keyint_min 100 -x264opts "keyint=100:min-keyint=100:no-scenecut" -sws_flags lanczos -profile:v baseline -preset veryfast -vb 6000K -minrate 6000k -maxrate 6000k -bufsize 6000k -s 1280x720 -tune film,zerolatency -pix_fmt yuv420p -f flv -c:a copy -ac 1 -strict normal rtmp://(output site)/(output app)/(output key)

  which as of my current testing seems to at least have a healthy stream for longer than 2 minutes if i only output to youtube live directly. Ive found output to my nginx server then youtube live breaks things.

  my nginx rtmp settings are on this link https://pastebin.com/siE99Tv8

  Edit 2

  If I push the stream to a site like restream to stream it to youtube then it seems to be working. tested for 25 minutes with no change of them saying bad video container or anything. So I’m going to say nginx is partly to blame in how its distributing the files ? Unsure what I’m doing wrong. I am pretty sure ffmpeg isn’t to blame here at least