Reduce resolution with ffmpeg (and keeping bitrate)

[krelian]

Hello

so I have this particular 1920x1080 file which is about 8GB with an average 15MB/s bitrate
I would like to make it much smaller in order to fit it on a single layer DVD (so I guess a little more than 4GB)
then build an AVCHD with tsmuxer

first thing I thought to reduce it to720p, as I really don't need it to be 1080p
so I used

ffmpeg -i input.mkv -vf "scale=1280:720" output.mkv

however the file is now WAY smaller, just about 800MB and the bit rate reduced all the way to 1 (ONE) MB/s, well 1376kb/s to be exact
so obviously I am missing something, how can I keep the current bitrate?

since we're at it
is there a way to scale it to 720 AND at the same time to shrink it to 4GB or so? (the audio part is about 150MB size)

thanks a lot

[blud7]

ffmpeg -i input.mkv -b:v 4M -c:a aac -ac 2 -b:a 128k output.mkv

No need to scale down to 720p, but you can include it if you wish since setting the bit rate will determine the size no matter the resolution.
-b:v 4M sets the bitrate to ~4 MB/s
-c:a aac sets the codec for the audio to the ffmpeg aac encoder which I think defaults to ogm
-ac 2 -b:a 128k sets the audio channels to 2 (stereo) and sets a decent bit rate for the audio stream

[krelian]

thank you!!!!!

well I already demuxed the file anyway but very good to know your tips anyway
I'll give it a try and report back

[Cornucopia]

Filesize = bitrate * running time
This is a Universal formula.

If you want the file smaller, you MUST either reduce the bitrate, or cut/edit the clip.
So if you don't edit it, and you say you want to keep the bitrate the same, the size of the file will remain the same, regardless of its resolution and regardless of what someone else tells you might be possible.
Btw, while your title said "keeping bitrate", blud7's suggestion does NOT do that. But then, maybe that isn't as much of a requirement for you?

Scott

[krelian]

Filesize = bitrate * running time
Btw, while your title said "keeping bitrate", blud7's suggestion does NOT do that. But then, maybe that isn't as much of a requirement for you?

you are right
very badly titled thread I suppose, sorry

anyway it worked very good
reduced to little more than 2GB, but of course quality much better than the mess I did before

thanks again

[johnnyquid]

I would recommend copying the audio rather than encoding it again.

"-c:a aac -ac 2 -b:a 128k" should be replaced with "-c:a copy" in the ffmpeg command.

The poster did not specify what format the audio is currently in. It could be surround sound rather than simple stereo.

The video codec could also be changed but it would need to be known what the file is intended to be played on and what codecs would be supported.

[krelian]

The poster did not specify what format the audio is currently in. It could be surround sound rather than simple stereo.

you are right, sorry
it's an AC3 track 2CH stereo, 192kbps

as mentioned, I made an AVCHD with tsmuxer. Disc will have to be played on a Blu Ray player

funny, my trusted burner crapped out just yesterday, after 20 years of great service so I didn't burn the disc yet
however the AVCHD folder (mounted with imgburn) played fine with VLC
but you never know, non-Sony/Panasonic blu players sometimes have issues with AVCHD
I'll replace the drive tomorrow

[krelian]

little update
I thought the drive was faulty, however it seems I was wrong
I have an insane amount of drives (Optiarc, Lite-On, pioneer) and only good media (TY, Verbatim, BBS)
tried any possible combination and always came out with a coaster
around 80% of burning imgburn stopped, claiming of a "medium error"

the nice folks over at imgburn pointed out while such error usually is due to bad media or faulty drive, it can also happen the file is corrupted
thinking of it I do remember I got a warning from ffmpeg about some frame errors, but I don't remember exactly

as I said the built BDROM folder plays perfectly with vlc on my PC

I guess I need to go back to roots, the original mkv file
is there a way to verify if there are errors of some sort, and in case how to fix them?

[rgr]

Make an image on disk first.