[FFmpeg-user] Why does the 'detelecine' filter exist?

[Huspnet]

> On May 29, 2025, at 4:18 PM, Mark Filipak <markfilipak.imdb at gmail.com> wrote:
> 
>> Sorry, I should have made it clear that I'm a hobbyist (in case my post didn't adequately do that for me). Also, I probably shouldn't have included "-r 24" in my example without further explanation. I have had better audio sync by using either "-r 24", "-r 23.976" to specify output frame rate, ...
> 
> Specifying output frame rate is necessary only when you want to change frame rate and don't care about results. I believe you are packaging as MKV? MKV has known problems preserving audio sync. There are many reasons why I recommend MP4.

I provided with MKV example for the purpose of simplicity. Most of what I do using ffmpeg is almost exclusively MP4 output. 

> 
>> ... but that only helps when I add a frame rate input specification to the beginning of the filter chain, such as "fps=30000/1001" ("29.97" may be accepted there instead, but I think it has not always been so). I've also tried ""-fps_mode vfr" instead of the "-r" specification so I could try "mpdecimate" after "decimate" to clean up remaining duplicates. I am becoming convinced the the duplicates that remain after applying "decimate" result from telecine errors in the source video. If that is true (that I'm transcoding video that contains flaws), then, for my most commonly transcoded sources, my best audio sync has been achieved using "-vf fps=30000/1001,fieldmatch,decimate" in the filter chain, then "-r 23.976" to specify output frame rate. But that's just my experience.
> 
> '-r' works by dropping and-or duplicating frames, only -- nothing fancy. That may be your entire problem.

Thanks. I'm testing "-fps_mode vfr" instead. I would try "vsync", but last time I did, ffmpeg complained. I also remain suspicious of errors in my source video. For short passages, it seems there are fewer duplicate frames when I follow "fieldmatch, decimate" with "mpdecimate=hi=100:lo=100:frac=1:max=0", followed by my crop values (my source videos commonly have black bars). I need the variable frame rate to give "mpdecimate" any chance of helping.

>>  for any of you who may be interested in adapting for your own scripting systems, so I'll be happy to share on request.
> 
> Please do. Sometimes a script is just a script, but sometimes, it's a good cigar.

I'm not sure what the list may do to this, here it is. (I have a blog, but it might as well be a journal for all the traffic it gets. I've copied the script from there hoping that it will be the same when posted here. I'm including the URL in case the AppleScript is damaged too much through posting to be useful:
https://prehensileblog.wordpress.com/2025/05/29/applescript-for-simplifying-small-audio-duration-changes-using-ffmpeg/
Here's an AppleScript droplet I use to make small changes in the duration of AC3 audio files so I can fine tune a/v sync for video I transcode. It makes use of AppleScript "do shell script" construct to send the command line task to the Terminal. It asks the user for the duration in milliseconds by which the audio needs to be adjusted by means of the ffmpeg "atempo" command. The script also allows the user to specify whether the audio is to be lengthened or shortened through the use of the "-" (minus) character. I used to have a pipe method that fed the audio from ffmpeg to SoX and back to ffmpeg, which employed a cute little animation showing progress, but I've long since abandoned that for the sake of efficiency. I'm sure that the script could be used for a lot of different audio codecs with very little modification, but the one I've use here is AC3. The script is for use on a Silicon Mac where a recent version of ffmpeg has been installed natively (the ffmpeg executable should exist in "/opt/homebrew/bin/"). There are quite a few notification dialogs in the script that may be removed if you prefer. The basic usage for me is to 1) demux to separate audio and video component files, 2) convert the audio to AC3 (or modify the script to accommodate your audio file type), 3) use the script by dropping the audio file onto it's icon or by running the script in Script Editor and selecting the file, and 4) remuxing the original video component file with the newly created (and hopefully fixed) audio file.
## Begin AppleScript

property newline : ASCII character 10
property tmpfile : "/tmp/execme.command"

on open the_items
    my runtime(the_items)
end open

on runtime(the_items)
    set afile to item 1 of the_items
    set targetstring to (quoted form of POSIX path of afile)
    set targetinfo to (do shell script "/opt/homebrew/bin/ffmpeg -i" & space & targetstring & space & "2>&1 | grep \"Duration\"") as text
    set hrs to word 2 of targetinfo
    set mns to word 3 of targetinfo
    set secnds to word 4 of targetinfo
    set hrs2secnds to (hrs * 3600)
    set mns2secnds to (mns * 60)
    set calctime to (hrs2secnds + mns2secnds + secnds)
    
    #set initval to ((calctime as text) & "0")
    
    set initval to (calctime as text)
    
    display dialog "The existing duration of the audio is" & space & initval & space & "seconds."
    
    set adjustval to text returned of (display dialog "How much should the original audio be changed in seconds? IMPORTANT: If original audio is too short, use \"-\"(minus) before the number. Include all three decimal places as shown." default answer "5.000")
    
    set tryval to my roundthis((initval - adjustval), 2)
    
    display dialog "The adjusted duration of the audio will be" & space & tryval & space & "seconds."
    
    set calcval to (initval / (initval - adjustval) as Unicode text)
    
    display dialog "The tempo will be changed to" & space & calcval & space & "of the current tempo (greater than 1 indicates its duration will be decreased, less than 1 indicates it will be increased)."
    
    set i to afile as alias
    tell application "Finder" to set P to (i's container as Unicode text)
    set n to (info for i)'s name
    my replace_chars(n, ".ac3", "_adjusted.ac3")
    set output_file to POSIX path of (P & result)
    set outputfile to (quoted form of output_file)
    
    set theShellScript to ("sleep 1" & ";/opt/homebrew/bin/ffmpeg -i" & space & targetstring & space & "-af atempo=" & calcval & space & "-acodec ac3 -ab 640k -y" & space & outputfile & ";mv" & space & (quoted form of tmpfile) & space & (quoted form of (POSIX path of (path totrash)))) as Unicode text
    delay 1
    do shell script "echo " & quoted form of theShellScript & " > " & tmpfile
    
    repeat
        try
            do shell script "chmod +x " & tmpfile
            do shell script "open -a Terminal.app" & space & tmpfile
            exit repeat
        end try
    end repeat
end runtime

on replace_chars(this_text, _bad, _good)
    set AppleScript's text item delimiters to the _bad
    set the item_list to every text item of this_text
    set AppleScript's text item delimiters to the _good as text
    set this_text to the item_list as text
    set AppleScript's text item delimiters to ""
    return this_text
end replace_chars

on roundthis(n, numDecimals)
    set x to 10 ^ numDecimals
    (((n * x) + 0.5) div 1) / x
end roundthis

on run
    try
        runtime(the_items)
    on error
        set the_items to ((choose file with prompt "Please choose the AC3 file for which you want to change duration:") as list)
        runtime(the_items)
    end try
end run

## End AppleScript



L. Lee