lower resolution = larger files?

[Ghodmode]

I have some HD (720p & 1080p) children's movies I wanted to put on my daughter's tablet. Since the storage space is limited, I thought I could reduce the size of the videos to the native resolution of the tablet and save a few megabytes in the process. I'm using Handbrake and all I did was reduce the width of the video. I was very surprised to find that the new files were much larger. One was more than double the size of the original!

I clearly have a lot to learn about all those settings and what they mean. Can someone link some tutorials / documents / articles about video codecs, quality settings, etc. and how they affect file size?

If you're curious, my daughter's tablet is a Samsung GALAXY Tab 3 Kids edition with 1024x600 resolution.

Thank you.

[manono]

Can someone link some tutorials / documents / articles about video codecs, quality settings, etc. and how they affect file size?

Just about the only thing that affects the filesize is bitrate. If the reencode for the lower resolution wound up larger than you liked, lower the bitrate. Most people use a bitrate calculator to figure the filesize they want. If you used CRF encoding, raise the CRF number to get a lower filesize..

[Ghodmode]

Gotta love those acronyms Oh, well... Google to the rescue

I pretty much want the same settings as the original file at a smaller resolution. Looking at original file in VLC, the Statistics tab of the Media Information dialog says it has an input bitrate of 3160 kb/s and a Content bitrate of 2973 kb/s.

It looks like I'm in for a lot of trial and error here. Handbrake's default setting is Constant RF of 20, but I currently only have information about bitrate, so I guess I'll try setting it to Avg Bitrate of 2973 kb/s to match the original and see how that turns out.

There's an audio bitrate, too, but I haven't found any information on that yet so I'll just leave that setting alone.

This probably isn't worth all the effort just to get a few movies on my daughter's tablet, but the opportunity to learn and understand more makes it worth it.

Please let me know if I'm on the right track.

It's the middle of the night here right now, so I'm going to shut down my computer and try this tomorrow.

Thank you.

[Ghodmode]

I forgot to mention that I found a bunch of different bitrate calculators online. They don't all seem to agree, but I might need to look at them more closely.

[Detmek]

If you want to reduce the size don't use source file bitrate, use lower bitrate. In your example, don't use 3000kbs, use 1500kbs to reduce the size to 1/2 or use 1000kbs for 1\3 of original size.

[_Al_]

yes, you have no choice, you want to reduce the volume , say, half of the original, so trying CRF will not help much because your movies were already compressed and perhaps even selectively by scenes, so comparing some scenes will fail most likely a lot regarding quality, so you just lower your resolution and encode 2pass VBR, where average bitrate is half of the original.

Rough example:
Your movie has 90 minutes=5400 sec
Your movie has 5GB=41943040 kilo bits ... that link gets you to the calculator I used
41943040/5400=7700kbps that is you average so you'd choose half (if you want to reduce the size to half) about 3800kbps as your 2pass average bitrate

you could use CRF and reducing size, but it will be hit and miss, when you get hold of it, when you start to understand what bitrate you most likely get with what resolution, you might prefer using CRF because it is 1pass only, but even then you could be surpriced and be a way off etc., but just for rough reducing you perhaps do not mind and use CRF

[Ghodmode]

First attempt
I used an Avg Bitrate of 2973 kbps. I figured that the same video at the same bitrate with a slightly lower resolution (width 1024 vs original 1280x720) should result in a file that was at most the same size as the original... Boy was I wrong. The resulting file was even larger: more than 3x the original.

Apparently, I misunderstood the VLC statistics. That bitrate changes at different points in the video. I had originally paused it right after the beginning and opened the statistics. So I guess that bitrate information is useless for what I need.
Screenshot of VLC statistics: https://flic.kr/p/yxjsoW

Second attempt
Using one of the (many, conflicting) bitrate calculators (this one) I found on the web, I started with a target file size of 650Mb (the original file size) and an audio bitrate of 128 (1 step down from the default setting of 160). It came up with 751 kbps, so I used that. The resulting file is slightly smaller (632Mb) than the original. That's what I was looking for, but when I played the file I noticed the drop in audio quality right away. That's really saying something because I've never been an audiophile and I have slight hearing loss in one ear.

Third attempt
I discovered the "Auto Passthru" setting in Handbrake's Audio tab and I set the video bitrate at 751. I suspect this is going to give me exactly what I wanted. It's still encoding as I type this.

I really appreciate the replies to this thread. I got the desired end result and I learned a few things. I still want to learn more. If anyone wants to link to tutorials about video codecs, audio codecs, filters or anything else related, I'd love it. I might even eventually get to the point where I know enough to help others



Thank you.