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  1. Member VideoNoobz's Avatar
    Join Date: Feb 2010
    Location: CyberSpace
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    Hello Everyone,

    As you can tell by my user name, I am relatively new to video conversion. I understand some of the basic concepts, and I have done some limited video conversions.

    My questions concern the huge file size difference between mkv files and mp4 files.

    I have always generally assumed that the larger that a video file is, the higher the quality will be, being as the video has undergone less compression. For example, a 1.37 MB file will generally be of better quality than a 700 MB file, a full DVD at 4.36 GB will be of better quality than a 1.37 MB file, and a high definition video will be even better than a DVD.

    Of course, there are other factors that we need to consider, such as the quality of the original file, the experience of the person encoding the file, the encoding parameters that said person chooses, etc.

    But what confuses me is this: I have noticed that I will sometimes encounter videos that are of the same resolution, but which are encoded in a different format, and which are of vastly different file sizes.

    For example, there may be an mkv version of a particular video that has a resolution of 1280x720, that is 7.0 GB in size, while at the same time, there may an mp4 version of the very same video, which also has a resolution of 1280x720, but it has a file size of 2.1 GB, or even smaller.

    I would assume that the mkv file is of better quality, being as it has a larger file size, even though both files are of the same screen resolution. Is this correct, or are the two files of the same quality?

    On a related note...

    Is the generally larger file size of mkv files due to something else? In other words, do mkv files and mp4 files use basically the same compression level, but mkv files are bigger for some other reason?

    Thanks in advance.
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  2. Member midders's Avatar
    Join Date: Sep 2008
    Location: United Kingdom
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    Both mp4 and mkv are "container" formats. This means that they can include video/audio/subtitle streams of different types, so the first thing that you should check is the video codec that is used in each container; for HD video this will likely be AVC, with AAC audio. The next thing to check is the video bitrate; this is a measure of the amount of data that is used to encode each frame of video. Given the same framesize (resolution) and duration, videos with a higher bitrate will tend to take up more space and generally (but not always) give higher quality. (Use MediaInfo or GSpot to get the above info).

    The last thing to note is that it is generally quick and simple to convert mp4 to mkv or vice versa, since the elementary streams (video/audio/subtitle) can be left intact and just placed in a new container with the right header.

    Slainte

    midders
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  3. Member VideoNoobz's Avatar
    Join Date: Feb 2010
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    Hello Midders. Thanks for your response. So I gather from what you are saying, that, as with audio files, one of the key factors affecting the size and quality of a video file is the actual bitrate. Thus, while two video files may have the same resolution, the one with a high bitrate will more than likely be of better quality, correct?
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  4. Always Watching guns1inger's Avatar
    Join Date: Apr 2004
    Location: Miskatonic U
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    If the 7GB file is a Bluray rip, chances are it will have the original HD DTS audio, which can be up to a third of the file size. If the MP4 version has two channel AAC then the difference can be substantial. Video bitrate is the other large factor. While it is a fair supposition to make regarding the larger file probably having higher quality, there are a number of factors that come into play when encoding that could mean the difference is not as large as you might think (the type of content being encoded), or factors with playback that mean the difference in quality is not as apparent (lower quality TV or upscaler etc.). So the actual difference will vary from file to file, and the difference it makes to the end viewer will be very subjective.
    Read my blog here.
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  5. Member VideoNoobz's Avatar
    Join Date: Feb 2010
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    Hello guns1inger,

    Thanks for the additional info. So it appears that this issue is a bit more complicated than what appears on the surface, and that video bitrate, audio bitrate, and other factors, all contribute to the overall size and quality of a file.

    With that in mind, I just downloaded the MediaInfo program that is recommended on this site. I think comparing files, and looking at the actual numbers, will help me to better grasp how this all works.

    BTW, that "pigasus" avatar worries me. I don't know how old you are, but are you familiar with Henry Gibson's famous old poem from "Laugh-In" regarding flying cows? Thus my concern with your avatar.
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  6. Always Watching guns1inger's Avatar
    Join Date: Apr 2004
    Location: Miskatonic U
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    Pigasus is the mascot for the JREF (James Randi Education Foundation). I may be getting older, but I am only old enough to know laugh-in from re-runs
    Read my blog here.
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  7. Member VideoNoobz's Avatar
    Join Date: Feb 2010
    Location: CyberSpace
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    Well, then, at the risk of being stoned for going a bit off-topic, allow me to share with you that short, but oh so magnificent poem by that bard of yesteryear:

    Little birdie in the sky,
    Why'd you do that in my eye?
    I'm no sissy, I don't cry,
    I'm just glad that cows don't fly.

    ...Now, just imagine what your JREF pigasus might do!
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