bitrate and quality when converting between mpeg and mp4


I used to grab/convert my video from my cam in mpeg2 format at 8000 KBs (previously in Ulead on windows and now in Kdenlive on Feisty) and not in DV. This enables me to use smaller files, use the quality that is getting on DVD and speeds up the render process enormously (It's already in MPEG2 and only addtional transitions/clips and so on need to be rendered). However, in Kdenlive now I would like to convert the massive DV files to mp4 files. When I convert them to mp4 I use 5000 KB/s which gives me perfect quality and even smaller files than MPEG2. However, when creating a "standard" DVD I want/need the bitrate for the MPEG2 back to 6000-8000 KB/s. I think this should work based on the assumption that 5000 KB/s mp4 contains much more "video information" than 6000-8000 MPEG2. Therefor, when uncompressing the 5000 KB/s mp4 stream and recompressing it to 6000-8000 KBs MPEG2, the uncompressed video stream/information should even be "down sampled" in information, therefor not losing any quality.
Is this assumption correct and does that work?

No the assumption is not strictly correct, but the difference in qualitymay be acceptable to you depending on what you are doing.

Whenever you re-encode an mpeg or any other lossy movie format, you will lose some amount of data. If quality is important to you, then you should aim to work with your original material as far through your workflow that you can before encoding to mpeg2 later.

However, depending on what level of quality you are after, the loss may be acceptable to you.

My advice :

Pick a fast moving dv file (explosions are great for this!), and a dv file that has high levels of fine detail - these are two worst-case scenarios.

Encode them directly to mpeg 2 at 6000-8000 KB/s
Also, encode them to mpeg 4, and then re-encode them to mpeg 2 at 6000-8000 KB/s

Play the two files (->mpeg2 and ->mpeg4->mpeg 2 versions) next to each other, and look closely for artifacts. In the fast moving file, you may see it looking blocky. In the fine detail file, see how well the detail is perserved.

Compare them to the original dv as well,

If your happy with the quality loss that you get, then go ahead :-)

Thanks for your reply. I am/was quite happy with the mpeg2 quality and also with the mp4 quality. As my question was dealing about my own video's (just plain family/holiday and so on movies), I have no "extreme" actions or details in it and on quick skipping and scanning my movies, all look well both in mpeg2, mp4 and mp4->mpeg2.
I just wanted to know.