Handbrake can convert a DVD clip which is about 30 minutes to a 300MB .mkv file. For experiment, I tried also 1GB file size and 2GB, and they both came out as 1GB or 2GB.

But I estimate that the original video on the DVD should be only about 600MB, so how can it make the file "bigger than it was before"? (the mkv, mp4 format should allow better compression that the DVD's mpeg2).

I also remember I tried experimenting with divx or xvid before, and there was a limit to file size: for example, I can specify a 1.5 hour video to have 3GB or to have 8Mbit/s, but the content simply didn't have so much "bits" for it to store (because it can highly compress the video), so the final file could not exceed 1.8GB or something like that, no matter how high the bit rate or desired file size I gave it. But with Handbrake, it seems like I can create a 1GB or 2GB file with such a short clip. Why is that?

3 Answers

It has to do with upsampling/downsampling.

Imagine having to write this software: You have to read a compressed file that could have been compressed in a variety of formats, and then recompress it in one of another variety of formats.

Wait!! Why can't you go directly to another format??

Well, how would you? The compression is lossy, and in order to understand how to compress it, you first have to decompress it, to see what it's saying!

Furthermore, you don't want your algorithms to explode; i.e. if you're converting between 10 formats, you don't want to have 10 * 9 = 90 algorithms for compression and 90 for decompression to convert from every format to every other format, but you'd rather have 10 algorithms to go to a decompressed format, and 10 more to go down to another compressed format.

So you have to decompress it first. And after that, it's your choice how to recompress -- and no matter what you choose, you'll lose a lot more than if you hadn't done anything at all.

i.e. This is a bit naive of a calculation, but if something preserves 50% of the original, and you decompress it and recompress it, you got 50% of that -- i.e. 25%. This doesn't capture some of the other aspects, but it does explain why recompression (aka transcoding) is rarely recommended; if you need it, try to recompress the original instead, if possible.

Hope this makes sense.

4

Media compression isn't like zip and similar compression algorithms, which opportunistically exploit exact repetition. At its simplest, it's more like finding a representation of the signal which can have detail removed (or added) with minimal visible change to the decoded signal. That being the case, it's very easy to "compress" a small file into a larger file. You derive that transformed representation, then pad it out with extra neutral data.

That's pretty much what you get when you demand a conversion from a low-bitrate video file to a high-bitrate video file.

Strictly speaking, you probably are gaining something by having 2GB rather than 1GB, even for a video that started at 600MB - though it's probably not something that you want at any price.

The original video was compressed with MPEG2 (for DVD) rather than using MKV. Different compression systems have different "artifacts". Each conversion of the file can at best preserve the available detail. With the change in compression scheme, it is inevitable that at least something is lost. With a larger file, there's less loss - even with files much larger than the original. That means you're less likely to lose all the unwanted artifacts of the previous compression scheme.

Another way to look at it...

The set of videos that can be perfectly represented using any compression scheme is very limited, in comparison with uncompressed video. Reduce the file size by one byte, and in principle you have divided the number of possible videos that can be represented by 256.

The set of videos that can be perfectly represented in MPEG2 is very different to the set of videos that can be perfectly represented in MKV. Even if you had perfect codecs, you're unlikely to perfectly preserve your video converting from one compression method to another - even converting to a much higher bitrate.

It's because Handbrake is free and the compression algorithms only go so far. The ones the pros use is, well, more expensive.

Mehrdad's answer nails it on the head. In order to go from MPEG2 (DVD) to H.264(I assume that's what you are using) directly, you have to write the program specifically to convert that. Which brings on what Mehrdad was saying. Basically have to write jack of all trades compression algorithm or write a bunch of smallers ones. Neither ones sounds like a winning solutions to me.

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