why does ytdl always consider Default format spec: bestvideo+bestaudio/best to be incompatible merge?

[robbiedobbie1]

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Question

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why does ytdl always consider Default format spec: bestvideo+bestaudio/best to be webm? incompatible merge. i dont want .mkv i want mp4, but "best", by default appears to be webm for audio. which results in uneditable forced merge into .mkv
.mkv will play, but knackers any editing.

to get my mp4 i need to do this command every time:

youtube-dl -f bestvideo[ext=mp4]+bestaudio[ext=wav]/mp4

the ytdl defaults are causing the problem.

~~verbose log
C:\Users...\Desktop\resources>youtube-dl.exe --verbose
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['--verbose']
[debug] Encodings: locale cp1252, fs mbcs, out cp850, pref cp1252
[debug] youtube-dl version 2020.09.20
[debug] Python version 3.4.4 (CPython) - Windows-7-6.1.7601-SP1
[debug] exe versions: ffmpeg 4.3.1-2020-09-21-full_build-www.gyan.dev, ffprobe 4.3.1-2020-09-21-full_build-www.gyan.dev
[debug] Proxy map: {}
Usage: youtube-dl.exe [OPTIONS] URL [URL...]

youtube-dl.exe: error: You must provide at least one URL.
Type youtube-dl --help to see a list of all options.

C:\Users...\Desktop\resources>youtube-dl.exe https://www.youtube.com/watch?v=LvOBXIWHj-4 --verbose
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['https://www.youtube.com/watch?v=LvOBXIWHj-4', '--verbose']
[debug] Encodings: locale cp1252, fs mbcs, out cp850, pref cp1252
[debug] youtube-dl version 2020.09.20
[debug] Python version 3.4.4 (CPython) - Windows-7-6.1.7601-SP1
[debug] exe versions: ffmpeg 4.3.1-2020-09-21-full_build-www.gyan.dev, ffprobe 4.3.1-2020-09-21-full_build-www.gyan.dev
[debug] Proxy map: {}
[youtube] LvOBXIWHj-4: Downloading webpage
[debug] Default format spec: bestvideo+bestaudio/best
WARNING: Requested formats are incompatible for merge and will be merged into mkv.
[debug] Invoking downloader on 'https://r7---sn-8pgbpohxqp5-aigd.googlevideo.com/videoplayback?expire=1601882669&ei=zXV6X__ID
8HMWb6Cq9gG&ip=80.7.82.241&id=o-AJ4oELLur_BWxTYpdM-0ZErXg98lQPYc-A_bITSfxKXi&itag=135&aitags=133%2C134%2C135%2C160%2C242%2C24
3%2C244%2C278&source=youtube&requiressl=yes&mh=cN&mm=31%2C26&mn=sn-8pgbpohxqp5-aigd%2Csn-25ge7ns7&ms=au%2Conr&mv=m&mvi=7&pl=2
4&initcwndbps=2063750&vprv=1&mime=video%2Fmp4&gir=yes&clen=469175517&dur=4907.520&lmt=1573635280014386&mt=1601860958&fvip=3&k
eepalive=yes&fexp=23915654&c=WEB&txp=2316222&sparams=expire%2Cei%2Cip%2Cid%2Caitags%2Csource%2Crequiressl%2Cvprv%2Cmime%2Cgir
%2Cclen%2Cdur%2Clmt&sig=AOq0QJ8wRQIgWJ40_nmTIQfErwS1otzBn2P17Ut3wlLDlk3eTnLm2pMCIQD6MYBBkE60JbPqfJlH_B8hBAMsmuLpy4V0Bw3VNmAGW
w%3D%3D&lsparams=mh%2Cmm%2Cmn%2Cms%2Cmv%2Cmvi%2Cpl%2Cinitcwndbps&lsig=AG3C_xAwRQIgPUKH9P1VGO_6KGs1VZrEg57sNra-Tlsfq3dc16wQZOk
CIQCWCNAAog3ZYlYq4Sm-b-jO-4_P1ET0ubHwKs7mfwGSMg%3D%3D&ratebypass=yes'
[download] Destination: Witchfinder General 1968-LvOBXIWHj-4.f135.mp4
[download] 100% of 447.44MiB in 00:44
[debug] Invoking downloader on 'https://r7---sn-8pgbpohxqp5-aigd.googlevideo.com/videoplayback?expire=1601882669&ei=zXV6X__ID
8HMWb6Cq9gG&ip=80.7.82.241&id=o-AJ4oELLur_BWxTYpdM-0ZErXg98lQPYc-A_bITSfxKXi&itag=251&source=youtube&requiressl=yes&mh=cN&mm=
31%2C26&mn=sn-8pgbpohxqp5-aigd%2Csn-25ge7ns7&ms=au%2Conr&mv=m&mvi=7&pl=24&initcwndbps=2063750&vprv=1&mime=audio%2Fwebm&gir=ye
s&clen=70627230&dur=4907.541&lmt=1573644354271478&mt=1601860958&fvip=3&keepalive=yes&fexp=23915654&c=WEB&txp=2301222&sparams=
expire%2Cei%2Cip%2Cid%2Citag%2Csource%2Crequiressl%2Cvprv%2Cmime%2Cgir%2Cclen%2Cdur%2Clmt&sig=AOq0QJ8wRAIgOOLwjyMIX0iV-6l7d3C
PtmpWk8QKLM8e0ZZv0emSb5sCIBSWLPefvBYyI3N97AgluxitHJyOCwbt_j2EZeTeR8Le&lsparams=mh%2Cmm%2Cmn%2Cms%2Cmv%2Cmvi%2Cpl%2Cinitcwndbp
s&lsig=AG3C_xAwRQIgPUKH9P1VGO_6KGs1VZrEg57sNra-Tlsfq3dc16wQZOkCIQCWCNAAog3ZYlYq4Sm-b-jO-4_P1ET0ubHwKs7mfwGSMg%3D%3D&ratebypas
s=yes'
[download] Destination: Witchfinder General 1968-LvOBXIWHj-4.f251.webm
[download] 100% of 67.36MiB in 00:05
[ffmpeg] Merging formats into "Witchfinder General 1968-LvOBXIWHj-4.mkv"
[debug] ffmpeg command line: ffmpeg -y -loglevel "repeat+info" -i "file:Witchfinder General 1968-LvOBXIWHj-4.f135.mp4" -i "fi
le:Witchfinder General 1968-LvOBXIWHj-4.f251.webm" -c copy -map "0:v:0" -map "1:a:0" "file:Witchfinder General 1968-LvOBXIWHj
-4.temp.mkv"
Deleting original file Witchfinder General 1968-LvOBXIWHj-4.f135.mp4 (pass -k to keep)
Deleting original file Witchfinder General 1968-LvOBXIWHj-4.f251.webm (pass -k to keep)

C:\Users...\Desktop\resources>youtube-dl.exe -U
youtube-dl is up-to-date (2020.09.20)

C:\Users...\Desktop\resources>

[Feathered-Serpent]

My first guess: When I download the video, I get the following audio (in a mkv container):
Audio ID : 2 Format : Opus Codec ID : A_OPUS Duration : 1 h 21 min Channel(s) : 2 channels Channel layout : L R Sampling rate : 48.0 kHz Bit depth : 32 bits Compression mode : Lossy Delay relative to video : -7 ms Language : English Default : Yes Forced : No

And looking at https://en.wikipedia.org/wiki/MPEG-4_Part_14#Data_streams it doesn't seem that the mp4 container is by default able to carry such an audio format. Maybe yt-dl tries to take that into consideration.