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  1. Hey guys, I did a little reading on different forums and saw some posts from several years ago, but none with a definitive answer (maybe there isn't one?). I got MKVToolNix 31.0.0 thinking that I would be able to strip away subtitles and unwanted audio away from MKV files, without affecting video quality. I started noticing that when I organize my newly muxed MKVs (subtitles and audio stripped) in Plex, the video bitrate seems to go down. I read that MKVToolNix doesn't do any encoding/re-encoding so, in theory, the video quality should not be affected. But my tests, and a few other reports I've read on forums, leaves me with questions on this.

    I conducted about 5 experiments on the same file to test this out. I was hoping that if I gave you guys enough details, you guys might have a couple answers/theories as to why the video bitrate goes down if I'm simply stripping unwanted audio and subtitles. So here we go

    Computer

    MacBook Pro TB 2018
    macOS Mojave
    MKV ToolNix 31.0.0 (Recently updated from 30.0.0)


    Source MKV File Tested
    Image
    [Attachment 48250 - Click to enlarge]

    Image
    [Attachment 48257 - Click to enlarge]


    The Equalizer 2
    File Size = 16.17GB
    Video Tracks = 1
    Audio Tracks = 2
    • TrueHD Atmos
    • AC3
    Subtitle Tracks = 17
    • HDMV PGS = 16 (Various Languages)
    • SRT = 1



    So I tested muxing this 5 times. I muxed it twice without changing any settings to make absolutely sure video quality or size would not decrease... and it didn't, which is good. But the others, I had unexpected results, and here they are...



    First Mux Test
    Didn't uncheck any boxes. Left File as is.
    Result: No change in file size [16.17 GBs] or Video size [19135 kbps]
    Image
    [Attachment 48252 - Click to enlarge]



    Second Mix Test (Same)
    Again, didn't uncheck any boxes. Left File as is.
    Result: No change in file size [16.17 GBs] or Video size [19135 kbps]
    Image
    [Attachment 48253 - Click to enlarge]


    Third Mux Test (Same)
    Last time for sanity check, didn't uncheck any boxes. Left File as is.
    Result: No change in file size [16.17 GBs] or Video size [19135 kbps]
    Image
    [Attachment 48251 - Click to enlarge]


    Great, so the Muxes were quick, and no file size or video quality was reduced. Now I changed things...

    Fourth Mux Test
    Unchecked all foreign language subtitles. Unchecked AC3 Audio. Left only 2 HDMV PGS subtitles, and the TrueHD Audio track.
    Result: Smaller file size [15.5 GBs] (expected), and smaller video bitrate [18351 kbps] (unexpected).
    Image
    [Attachment 48255 - Click to enlarge]


    So I thought maybe the PGS Subtitles had something to do with it...

    Fifth Mux Test
    Unchecked only AC3 Audio. Left all HDMV PGS and SRT subtitles, and the TrueHD Audio track.
    Result: Smaller file size [15.63 GBs] (expected), and smaller video bitrate [18494 kbps] (unexpected).
    Image
    [Attachment 48256 - Click to enlarge]


    So to sum it up, when I strip anything away, it seems the video bitrate is going down. My question for your brilliant minds. Why is the Video Quality decreasing if I'm not re-encoding anything? Any help is appreciated guys. Thank you in advance!
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  2. Dinosaur Supervisor KarMa's Avatar
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    Originally Posted by safarian24 View Post
    My question for your brilliant minds. Why is the Video Quality decreasing if I'm not re-encoding anything?
    Using MKVToolnix to mux videos in MKV won't ever re-encode a video. It's simply a container, like a ZIP or a RAR but for Audio/Video playback. There is a lot going on in your post but I'm going to guess it's a simply problem of Plex counting the total bitrate of everything (Video, Audio, Subtitles), and then when you remove audio streams or subtitles streams then the total bitrate drops but the video bitrate stays the same.

    Use something like Mediainfo to get a better understanding of your file instead of relying on Plex. Also MKV is not very good at always reporting all the bitrates of video and audio, sometimes only keeping track of total bitrate.
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  3. I agree mostly with KarMa except that MediaInfo isn't perfect either. The only way mkvmerge reduces video bitrate is by removing empty filler NALUs (which doesn't reduce quality whatsoever, they are fillers after all).
    Often bitrate calculations are more estimates than perfect. For example if you have multiple variable bitrate tracks (in your case you have H.264 + Atmos). After muxing with mkvmerge the bitrate info can be more exact because it takes exact statistics and writes them as tags.
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  4. Originally Posted by KarMa View Post
    Using MKVToolnix to mux videos in MKV won't ever re-encode a video. It's simply a container, like a ZIP or a RAR but for Audio/Video playback. There is a lot going on in your post but I'm going to guess it's a simply problem of Plex counting the total bitrate of everything (Video, Audio, Subtitles), and then when you remove audio streams or subtitles streams then the total bitrate drops but the video bitrate stays the same.
    Karma, thanks for the reply! From what I understand on Plex forums it's simply calculating the Video bitrate, not the total bitrate. But then again, it's Plex, not some professional video info tool, so I'm not sure. The screen shots I posted show the Video bitrate on both the right and left sides of the window going down... which I'm not understanding, since I never touched anything with video in the Muxing process.

    Originally Posted by KarMa View Post
    Use something like Mediainfo to get a better understanding of your file instead of relying on Plex. Also MKV is not very good at always reporting all the bitrates of video and audio, sometimes only keeping track of total bitrate.
    I tried MediaInfo and it seems to give me a rounded answer possibly. But I pulled both the original Mux, and the 4th (most changed) Mux, and the video bit rate seems to be the same. However Plex is showing higher bitrate than MediaInfo too

    Original Mux
    Image
    [Attachment 48262 - Click to enlarge]


    Fourth Mux (Most Changed)
    Image
    [Attachment 48263 - Click to enlarge]


    Both show 14000 kbps, lower than the ~19000 kbps on Plex for both files. I guess another fair question to ask is, should I be worried?
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  5. Originally Posted by sneaker View Post
    I agree mostly with KarMa except that MediaInfo isn't perfect either. The only way mkvmerge reduces video bitrate is by removing empty filler NALUs (which doesn't reduce quality whatsoever, they are fillers after all).
    Often bitrate calculations are more estimates than perfect. For example if you have multiple variable bitrate tracks (in your case you have H.264 + Atmos). After muxing with mkvmerge the bitrate info can be more exact because it takes exact statistics and writes them as tags.
    Thanks for the reply sneaker! Just so I understand better, the video track is separate from the audio and subtitle tracks in my example, correct? I guess what I'm still having trouble wrapping my head around is the fact that I'm not touching anything with the video track, so why should the bitrate go down?

    In my first two tests where I changed nothing, video bitrate didn't go down. On my 4th and 5th tests, video bit rate went down however the 4th and 5th tests also varied in video bitrate reduction based on how much I stripped from the original MKV. So there's some relation between how much I strip, and the video bit rate... which is confusing the heck out me
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  6. The actual video bitrate didn't go down. Plex/MediaInfo erroneously told you it went down. To exactly calculate the bitrate software would have to parse the complete 16 GB file (unless there are mkv statistic tags in the file like written by newer mkvmerge versions). So they use various methods to make a best guess. Some good, some bad. Also depends on the file how accurate it will be.

    For testing take both the original file as well as the remux and extract the raw video bitstream:
    eac3to "original.mkv" "original.264"
    eac3to "remux.mkv" "remux.264"

    Then check if both "original.264" and "remux.264" are the same size. I'm pretty confident they will be.
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  7. Originally Posted by sneaker View Post
    The actual video bitrate didn't go down. Plex/MediaInfo erroneously told you it went down. To exactly calculate the bitrate software would have to parse the complete 16 GB file (unless there are mkv statistic tags in the file like written by newer mkvmerge versions). So they use various methods to make a best guess. Some good, some bad. Also depends on the file how accurate it will be.

    For testing take both the original file as well as the remux and extract the raw video bitstream:
    eac3to "original.mkv" "original.264"
    eac3to "remux.mkv" "remux.264"

    Then check if both "original.264" and "remux.264" are the same size. I'm pretty confident they will be.
    Do you know what program or app I could use to test this? Thanks for the info!
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