Best libx264 -vpre preset to use/is there a way to convert bit rate ONLY?

[ninethirty]

I've got an MKV file where the video is H264 (5500k bitrate) and the audio is DTS. Aside from the bit rate, the video is already perfect. I need to convert the bit rate, which I know how to do, but I can't seem to figure out a way to convert ONLY the bit rate and keep the rest of the video the same (maybe there is no way?)

So.. it seems I'm forced to do this: (ffmpeg -i video.mkv -vcodec libx264 -vpre ***)

What's the best preset to use. I'd rather not lose quality, other than changing the bit rate - hence why I asked if it's possible to convert just the bit rate.

I'd rather this didn't take hours to, but if it's got to, then I guess it is what it is.

Cheers in advance!

[guns1inger]

There is no way. If you reduce the bitrate, you reduce the amount of data available to present the image. Something has to give.

There are two important pieces missing from your post. The first if the target bitrate you are trying to reach. The second is your computer specs.

The first is important in order to try to put some context around your question and to get a sense of the scale of the reduction you are trying to achieve.

The second is import because H.264 encoding is very CPU intensive, your source is (at a guess) 720p, and even a quad core CPU can take 4 - 5 hours to process such a file.

You do have another option. You could take 1000kbps or more of the total bitrate by re-encoding the audio to AC3 and not touching the video at all.

[ninethirty]

Hey - thanks for the response.

The video bit rate right now is 5500 and I want to stream this to my AppleTV so I need to bring it down to 5000. The audio is DTS at 1509k. I was planning on changing the audio to AC3 anyway, so the last thing you mentioned would work (if I understand correctly).

My computer at work (where I can convert during the day) is a 2.66 Core2Duo MBP. At home I've got a 2.66 i7 MBP.

Cheers!

[guns1inger]

Apple TV - the low hanging fruit of the media playing tree. You have to re-encode. The 5000 kbps limit is on the video stream, not the total stream. Use a bitrate calculator to work out what bitrate you need, and do a 2-pass VBR encode. This will give you the best quality for a set size. Thankfully you don't have to take a lot off the top. Use the i7. The Core2Duo will take the better part of a day to do what the i7 will do it in a couple of hours.

[ninethirty]

AppleTV is easy for me.. we have a few macs in the house already

Thanks again for the help - just out of curiosity, I've used these settings for the first pass. Sound good?

ffmpeg -i inputb.mkv -an -pass 1 -vcodec libx264 -vpre slow_firstpass -b 5000k -threads 0 output.m4v

[guns1inger]

To be honest, I use XmediaRecode because I can't be bothered with commandline encoders any more. I'm sure someone else can tell you if those settings are appropriate.

[jagabo]

Try the veryfast preset.