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• ffmpeg to muffle audio in .mp4 files (lowpass freq + amplify)

  4 juillet 2018, par martins

  I have different 40second mp4 files recording people speaking (1 speaker=1 mp4 file). I am interested in muffling the words of the speech so that the content is unintelligible. I run some tests with Audacity and it seems I found the right combination of effects, that is :

  • Low Pass Filter at about 200Hz (frequency) and a Rolloff (dB per octave) of 36dB : the default in Audacity is 1000Hz and 6dB, respectively
  • Amplify the audio 40 times (allow clipping)

  My question is whether ffmpeg can reproduce the same steps. I will finally need to loop over 250 speeches, so ffmpeg would be highly valuable. At the moment, I could only code this middle solution, which is very far (I suspect) from what I want :

  ffmpeg -i input.mp4 -vcodec copy -af "lowpass=200,volume=40" output.mp4

  I don’t know how to deal with the 36dB per octave I need and whether the "volume" filter is really doing the amplifying job...

  Since I am a beginner to ffmpeg and audio edition, I would greatly appreciate any help. Thank you in advance.

• How can I simulate OpenFile in FFmpeg ?

  16 juillet 2019, par Jason117

  Most gif capture software capture screen and then save them one by one single frame picture file on disk,then read them into memory and combine them to gif,makes the whole procdure very slowly.

  I got a idea to capture screen with DirectX(so we could also capture directx window faster since it direct operate the screen d3d device)API to got the bitmap，then save them to memory(such as buffer),then passing the memory location to ffmpeg to produce a video so we don’t need disk storge as a middle buffer so it could be ten more faster since the disk is now most slowly part on pc now.

  the directx capture screen part is already.But I found that ffmpeg using OpenFile to read the picture file,so here may we can simulate the OpenFile ?
  If answer is yes,how could we do it ?

• when ffpmeg drops frames some things aren't played back in real time

  8 février 2024, par Alex028502

  I am trying to run a bunch of ffpmeg processes that act as simulators for cameras, and something funny is happening when I the processor can't keep up with the configured frame rate.

  


  I have replaced the rtsp stream with an output file, and managed to reproduce the issue, so will just show that to keep it simple.

  


  First here is a makefile that creates my source movie :

  


  clock.mp4: Makefile
    rm -f $@
    ffmpeg -f lavfi -i color=c=black:s=4096x2160:r=25 -vf \
"drawtext=fontfile=/usr/share/fonts/truetype/dejavu/DejaVuSans-Bold.ttf:fontsize=72:fontcolor=white:x=(w-text_w)/2:y=(h-text_h)/2: \
text='%{eif\:trunc(n/25)\:d}':start_number=0:rate=25" \
-t 60 -r 25 $@


  


  that gives me a one minute long movie that prints the second number to the screen. I have tested it out and the seconds are close enough. I put a lot of pixels to make it easier to jam up my CPU.

  


  Here is the script that creates a processes similar to the one I am trying to debug (called experiment.sh)

  


  I am actually using H.264, but H.265 is easier to overwhelm the processor with

  


  #! /usr/bin/env bash

set -e

echo starting > message$1.txt

rm -f superclock$1.mp4
ffmpeg -re -stream_loop -1 -i clock.mp4 \
       -an -vcodec libx265 -preset ultrafast -sc_threshold -1 -x265-params repeat-headers=1 \
       -vf "drawtext=fontfile=/usr/share/fonts/truetype/dejavu/DejaVuSans-Bold.ttf:fontsize=24:fontcolor=white:x=10:y=10:text='%{localtime\:%X}', \
            drawtext=fontfile=/usr/share/fonts/truetype/dejavu/DejaVuSans-Bold.ttf:fontsize=24:fontcolor=white:x=10:y=(h-text_h-10):textfile=message$1.txt:reload=1, \
            scale=1920x1080,fps=25" \
       -b:v 3M -minrate 3M -maxrate 3M \
       -bufsize 6M -g 25 superclock$1.mp4 &
pid=$!

for x in $(seq 0 10)
do
    echo $x > message$1.txt
    sleep 10
done

kill -INT $pid || true


  


  It should

  


  

  • put the second in the middle of the screen - 'cause it gets it from the source video

  


  • put the approximate sixth of minute in the lower left corner
(only approximate because of the sleep but close enough)

  


  • put the the wall clock time in the upper left corner

  


  


  and it works

  


  make clock.mpg
./experiment.sh 0
vlc superclock0.mp4


  


  shows something like this


  


  Now here is the interesting part

  


  If I run the script in four different terminals at the same time

  


  ./experiment.sh 1
./experiment.sh 2
./experiment.sh 3
./experiment.sh 4


  


  It can't keep up with the frame rate, and I see this in the output :

  


  frame= 1515 fps= 16 q=0.0 size=     768kB time=00:00:59.96 bitrate= 104.9k


  


  I was hoping the end result would all look ok when I watch it except with fewer frames, but the timestamps of the frames would make it all work as expected

  


  However...

  


  

  • The time in the middle, that is inherited from the source video, the seconds in the middle of the screen, stays in sync with VLC's clock.

  


  • the wall clock in the upper left seems play at 150% speed

  


  • the every ten seconds incrementor in the lower left seems to increment every 7 seconds

  


  • the video is only 1:25 long even though it was recording for at least 1:40 according to sleeps

  


  • the wall clock in the upper right hand corner makes it more than 1'40" and then counter in the lower left makes it to 10.

  


  


  

  


  Here are four states to compare

  


  |                  | Start    | 30" in   | end      |
|------------------+----------+----------|----------|
| Video Time       | 00:00    | 00:30    | 01:24    |
| Wall Clock Time  | 15:05:50 | 15:06:39 | 15:07:39 |
| sixth of minute  | 0        | 4        | 10       |
| seconds counter  | 0        | 30       | 24       |


  


  

  


  So you can see the vlc clock keeps pace with the original clock from the source movie.. even when it is only able to produce frames at 2/3 of the rate. However, the it is taking 50% long to get through the whole source movie I guess ?

  


  I am having trouble coming up with a theory that can explain exactly how this happens.

  


  Does anybody know how I can "correct" this ? (make it so that the movie is played at the same rate that it is recorded)

  


  I am thinking of using a lower frame rate and size as the input video.. but it would be nice to have something that will always work as expected, just with a lower frame rate, no matter how busy the processor is.