unnecessary compressionwhen converting to dvd?

[elbarthos]

I have a couple of mkv files I want to convert to dvd format and burn onto dvd. Both files are about 1.4 GB in size, so that makes a total of 2.8 GB. A standard dvd can contain 4.7 GB. Now I know that converting mkv files to dvd files somehow makes them bigger, so that some compression may be necessary in order to make them fit onto dvd. But no matter what program I use (Freemake Video Converter, Winx DVD Converter, ImToo, etc), they all seem to use way too much compression.

Because when I look at the finished dvd, I can see that there's still an enormous amount of blank space. When I put the dvd in my computer drive and click on properties, the amount of space that's still available on the dvd is about 3 GB.

How is this possible? When I use, for instance, ImToo DVD Creator, and I add those 2 files I have to set the video quality to "normal". The program then tells me that this will take up about 3.5 GB of disc space. When I try to set the video quality to a higher setting, I get the message that it won't fit.

But then I burn the dvd with video quality set to "standard", and it turns out that in reality only 1.29 GB of disc space it used, while the program told me it would take 3.5 GB. So it seems to me that there is a lot of unnecessary compression, that I somehow can't avoid.

How is this possible?

[hech54]

Both files are about 1.4 GB in size,

Yes but WHAT are they?

[elbarthos]

Both files are about 1.4 GB in size,

Yes but WHAT are they?

What do you mean? In the first sentence I already told they're mkv files

[davexnet]

Some DVD creation programs maximize the bitrate (up to DVD specs, eg. approx 9mbps video) to fill the disk as much as possible some do not.
But the running time of the source files is important, which has not been mentioned.

The size in GB of the source is irrelevant.

[hech54]

Both files are about 1.4 GB in size,

Yes but WHAT are they?

What do you mean? In the first sentence I already told they're mkv files

MKV is a container that can contain any number of video and audio codec combinations.
https://en.wikipedia.org/wiki/Matroska

[elbarthos]

Some DVD creation programs maximize the bitrate (up to DVD specs, eg. approx 9mbps video) to fill the disk as much as possible some do not.
But the running time of the source files is important, which has not been mentioned.

The size in GB of the source is irrelevant.

The total running time of both files is 90 minutes

[elbarthos]

Both files are about 1.4 GB in size,

Yes but WHAT are they?

What do you mean? In the first sentence I already told they're mkv files

MKV is a container that can contain any number of video and audio codec combinations.
https://en.wikipedia.org/wiki/Matroska

All I can see is that the name of the files end in mkv. How can I find out the codecs?

[hech54]

If you have MPC to play files, there is an option to show the file details.
Or you can get MediaInfo as a lone tool.

Either way, if your MKV files were not made from a DVD, you'll be re-encoding.
And even if the MKV files were made from DVD, but two different DVDs.....you'll be re-encoding.

[johnmeyer]

Converting to DVD does NOT automatically make them larger, nor does it make them smaller. Instead, size is determined by only one thing, and nothing else. That one thing is the bitrate used for encoding. This is true for any video encoder, whether h.264, MPEG-2 or anything else.

DVDs use MPEG-2 encoding, and nothing else. Any competent DVD encoder will give you the choice of encoding using either variable bitrate or constant bitrate. Some encoders offer the additional option, for variable bitrate, of two-pass (or "n"-pass) encoding, in order to more intelligently allocate where the bits are used, with more bits used for fast-moving action, and fewer bits for slow-moving action. The more minutes of video you try to squeeze onto one disc, the more important it becomes to use multi-pass variable bitrate. My own rule of thumb is to always use 2-pass variable bitrate for any encoding under 6,500,000 bits per second. 2-pass takes twice as long to encode, which is why many people don't use it unless it is needed. If you don't care about the encoding time, then use 2-pass variable bitrate for all of your MPEG-2 encoding.

There is an upper limit on the bitrate that can be used. This limit is for the total bitrate, including the bits used for audio and subtitles. I cannot remember, off the top of my head, the exact number, but I think it is around 9,800,000 bits per second.

To answer your specific question, you need to use a "bitrate calculator." You can find lots of these online. Just Google it. The bitrate calculator will tell you what bitrate to use in order to just fit your video onto a 4.3 GB single-layer DVD with the highest possible visual quality. As one example, using the bitrate calculator I created for Excel over a decade ago, if you want to fit 90 minutes (the total running time for both of your MKV files, according to what you said), and if you encode the audio using AC-3 at 192 kbps (a typical, average quality setting), then you need to set your MPEG-2 encoder to use 6,580,000 bits per second.

[videobruger]

Some DVD creation programs maximize the bitrate (up to DVD specs, eg. approx 9mbps video) to fill the disk as much as possible

Which is the case for AVStoDVD and DVDStyler.

some do not.

Which apparently is the case for ImToo DVD Creator.

[manono]

Which is the case for AVStoDVD and DVDStyler.

Yes, save yourself more headaches and just use AvsToDVD.

[Cornucopia]

@johnmeyer,
The max total bitrate for all streams combined is 10.08Mbps. The max bitrate of any individual video stream is 9.8Mbps (exceptions during multiangle). And for (LPCM) audio it is 6144kbps (~6Mbps), and much less for the compressed formats.

@elbarthos,
The universal equation for figuring out filesize with multimedia is: Filesize = Bitrate * Running time. That bitrate works for constant or variable, as long as you take the variable's average for your calculations.
Doesn't matter the format, quality, resolution, framerate, generation. Nor, as in your case, the source format or codec or size. Why?
Because whenever re-encoding, the source is always decoded back to its closest approximation to the original uncompressed master first. The encoding portion then only looks at that approximation's features when analyzing. Notice your original file's format and codec is inconsequential at that point, as they don't exist with that intermediate approximation.
So let's say you have 2 files of unknown codec/s or quality (remember, mkv is just the container not the codec streams inside) that total to 90 minutes. Or 5400 seconds. You can see how a 1Mbps bitrate would equal 675MB (remembering to change bits to Bytes), and 8Mbps = 5400MB. Since a standard single sided DVD is 4.37GB, the bitrate used would have to be lower than that 8 in order to fit the disc's capacity. And you also cannot take advantage of the max possible dvd bitrate for the same reason.
If you are encoding and creating a dvd that seems to have plenty of room left on it (visually), clearly your chosen bitrate is not taking advantage of the full capacity of the disc, so you would need to up the bitrate choice to whatever as closely matches your size goal as you are comfortable with, quality-wise. If SL discs are your media, in order to nearly exactly fit those 2 titles, you would do: 4.37 * 8 * 1024 / 5400 = 6.6Mbps. Of course you must also account for audio, subtitle(s)? and VOB container overhead, so that figure will be less. Probably 6Mbps.
That's how you should approach it every time.

Scott