Why do I end up with a smaller file when transcoding from H.264 to MPEG-2?

[attikiouzel]

I just transcoded a video encoded with x264 to MPEG-2 in order to create a video DVD. The thing is, the original AVI file was 4GB while the ISO I ended up with is less than 2GB. How is that possible? Isn't H.264 a superior standard? That has happened numerous times before. The quality doesn't seem to lower but I can't be sure since I'm not that experienced in understanding the difference.

I used DeVeDe and chose to adjust the output to the DVDR's size but that doesn't make any difference, I still end up with a much smaller ISO.

Thanks in advance.

[smok3]

the mistery is called bitrate -
" number of bits that are conveyed or processed per unit of time. "

(other than that: x264 is usually not found in avi files, use mediainfo and see what is actually inside that avi...)

[LoRd_MuldeR]

I just transcoded a video encoded with x264 to MPEG-2

x264 is an MPEG-4 AVC (H.264) encoder and, as such, it doesn't output MPEG-2. Or did I miss anything? :scared:

(There is the x262 project which tries to create a MPEG-2 encoder based on x264 code, but last time I looked at it, it was in a very early stage)

[attikiouzel]

LoRd_MuldeR: I transcoded an MPEG-4 (H.264) video to MPEG-2 in order to create a video DVD compatible with older DVD players. I just mentioned the software used for the original encoding (x264). I wasn't clear enough.

smok3: My mistake, it is an MKV file indeed, not an AVI. As for the bitrate, the chosen video bitrate was 8500Kbps. I used mediainfo as you recommended and got the following results:

Original MKV file:

Format: AVC
Bit rate: 4 899 Kbps

Resulting DVD-video:

Format : MPEG Video
Bit rate mode : Variable
Bit rate : 2 182 Kbps
Nominal bit rate : 8 500 Kbps

So I guess that changes my question to the following three:

1) Why isn't the bitrate I chose the actual video bitrate of the output MPEG files instead of being the "nominal" one?

2) Is there any meaning in choosing a bitrate higher than the bitrate of the source file when transcoding?

3) Does the above difference between the bitrate of the source and the output files mean there is a significant deterioration of image quality?

Thank you both for your help.

[LoRd_MuldeR]

Oups, I read over the word "...encoded..." in your first sentence :o

2) Is there any meaning in choosing a bitrate higher than the bitrate of the source file when transcoding?

When transcoding to a format that is MUCH less efficient than the format of the source (like H.264 -vs- MPEG-2), then yes.

(MPEG-2 will need a significant higher bitrate to retain the same amount of detail, compared to H.264)

[hello_hello]

What's the resolution of the original MKV video? Are you resizing down a fair bit when converting to DVD?

[manono]

1) The nominal bitrate is the max bitrate and not the average bitrate. As you can easily see, the average bitrate for the MPEG-2 video is less than half that of the 'source' H.264 encode. Thus the final size is also less than half.
2) Lord Mulder answered that one. But as hello_hello implies, it sort of depends on the 'source' and final resolutions also. If your 'source is hi-def and the final is DVD resolution, then such a bitrate drop might be acceptable. But I doubt it.
3) There's always a deterioration in quality when encoding to lossy codecs. In your case, I suspect there's a big deterioration because you're encoding to a much less efficient codec using less than half the bitrate. Thus, to answer the question posed in the thread title, it's all about the bitrate.

[qsdom9]

The format of the file is totally changed. First of all change the format of file then implement what you desire.
There are various options to settle it but first you are advised to change the format.