Lowering the bitrate on a .mkv/mp4 file?

[TECK]

I was wondering if you could suggest a tool that will allow me to lower the bitrate on a .mkv/mp4 file. Right now, I re-encode the movie with StaxRip.

The ideal solution for me would be to have a tool that will lower the video bitrate in a MKV container. Technically, I want to lower the bitrate from 4125+ to 2450-2950. I tried uncropMKV but it fails constantly in a Windows 7 x64 environment. Is StaxRip the best solution for me (video quality wise)?

Thanks for your help.

[Blue_MiSfit]

You're asking for "compressed domain transcoding", which (while technically possible AFAIK) has never been implemented for MPEG-4 AVC / H.264. I'm guessing this is the type of video stored in your MKV / MP4 files?

The best solution is to simply encode at the proper bitrate to begin with. I'd suggest going back to your source material and encoding a new version. How did you make these MKV or MP4 files?

Derek

[TECK]

My friend has several personal videos related to his wedding. They were done by his photographer in .mp4, with a hi-def cam. I'm not sure if MKV container is better that MP4, the important part is to lower the bitrate so all videos could fit on one Bluray disk. Since StaxRip does not support AC3 448kb by default in a MP4 container (at least I don't know how to do it), I'm converting the files to a MKV container. I was wondering if there is an easier way for me to re-encode everything beside using StaxRip. So far it works great but is very time consuming.

[CarlEdman]

@Blue_Misfit is right.

Compressed domain transcoding can be done and my experience is pretty fast and highly accurate with good results (if the additional compression ratio is not too high). In the days that people backed up their 9 GByte dual-layer DVDs to 4.5 GByte writable DVDs, several programs did a pretty good job on the MPEG-2 video streams contained therein.

But, as @Blue_Misfit says, there does not appear to be a general purpose, generally available h264 compressed domain transcoder available and writing one would require deep and broad understanding of a more complex compression format. That said, if anybody wrote one, it would be much appreciated.

Also, I'd think that you are right. 448 kBps AC3 and 4125+ kBps video seems likely to be quite excessive, even for a professionally shot high-def home video. It may reflect the fact that a small or handheld device needing to do real-time compression to a demanding format may compensate for its time/CPU limitations by just throwing excess bits into the stream to conserve quality.

x264 running on a much more powerful processor in a non-time constrained environment may do a much, much better job with no visible loss of quality. Reencoding with it (I assume that is what StaxRip uses) would be slow, but it likely would be quite effective.

By the way, AC3 in MP4 is legal and supported by most software video players, but the consensus is that many hardware players do not support it.

[Ghitulescu]

Why don't you author the blu-ray with say multiavchd, and let it do the whole thing automatically (or pick up another tool like BDreauthor). 448kbps AC-3 is overkill for amateur videos (even if using external mics). Remember there are thousands of music videos (concerts and so on) in 224kbps or 384, and nobody complained about the quality. Better save those precious bits for video.

[Guest]

Compressed domain transcoding can be done "Compressed domain transcoding" is a nonsense term. Maybe "recoding" or "requanting" would be acceptable, but transcoding implies changing the video coding standard, e.g., AVC to MPEG2. To transcode from AVC to MPEG2 would require decoding the frames, which means it is not a compressed domain process. Simply reducing the bitrate in the compressed domain is not transcoding.

Sure, it's a bit picky but using precise language is important IMHO.

[TECK]

Why don't you author the blu-ray with say multiavchd, and let it do the whole thing automatically (or pick up another tool like BDreauthor). 448kbps AC-3 is overkill for amateur videos (even if using external mics). Remember there are thousands of music videos (concerts and so on) in 224kbps or 384, and nobody complained about the quality. Better save those precious bits for video.

As I said in the previous post, those are .mp4 wedding files.
Maybe I was not clear what I need... My question guys is: Do you recommend another tool similar to uncropMKV that works in Windows 7 x64 and allows me to simply change the bitrate to a lower value? As I mentioned before, StaxRip works great. I want to preserve the AC3 448kbs and only lower the bitrate, that way the files will be half their size.

The way I do it now is: Convert the MP4 container to a Xvid 720p .avi (it preserves the audio at 448kbs) then I change the container to MP4 by simply copying the video and audio.

Thanks. :)

[Guest]

You cannot lower the bitrate without re-encoding. Re-quanting is theoretically possible but I do not know of any tools that do it for AVC video.

[TECK]

As a side note, that is one of the reasons why I purchased a DGIndexNV license. It does help a lot with the processing, while the CPU stays at 30%. Thanks neuron2. :)

I guess I will stick with the StaxRip processing, as to myself is the most complete package.

[Blue_MiSfit]

@neuron2: Thank you for the clarification. I always thought "compressed domain transcoding" was in fact the proper techincal term. Now I know!

@TECk: Your only option is to re-encode. I'd suggest doing this with x264, which is quite possible with StaxRip or any number of other encoding applications. @Ghitulescu brings up a good point of using a tool like MultiAVCHD as this is quite literally what it's designed for.

Derek

[TECK]

I thought so. Thank you for the useful information, guys.