Starting to build a 4k library. Hoping to get some help with what to look for in a good quality 4k video. Example below, which is better? Why? They are the same movie, one is 15GB and one is 30GB. Why is one twice the size? Thanks for any advice!
15gb
Format : HEVC
Format/Info : High Efficiency Video Coding
Format profile : Main 10@L5.1@High
Codec ID : V_MPEGH/ISO/HEVC
Duration : 2 h 17 min
Bit rate : 10.5 Mb/s
Width : 3 840 pixels
Height : 1 600 pixels
Display aspect ratio : 2.40:1
Frame rate mode : Constant
Frame rate : 23.976 (24000/1001) FPS
Color space : YUV
Chroma subsampling : 4:2:0 (Type 2)
Bit depth : 10 bits
Bits/(Pixel*Frame) : 0.072
Stream size : 10.1 GiB (67%)
30gb
Format : HEVC
Format/Info : High Efficiency Video Coding
Format profile : Main 10@L5.1@High
Codec ID : V_MPEGH/ISO/HEVC
Duration : 2 h 17 min
Bit rate : 24.4 Mb/s
Width : 3 840 pixels
Height : 1 612 pixels
Display aspect ratio : 2.40:1
Frame rate mode : Constant
Frame rate : 23.976 (24000/1001) FPS
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 10 bits
Bits/(Pixel*Frame) : 0.165
Stream size : 23.4 GiB (77%)
All modern video compression is âlossyâ, meaning that some amount of information (quality) is destroyed by compression. For all modern compression methods (in this case x.265 / HEVC) the amount of compression is adjustable as a trade-off between quality and encoding speed (and to a lesser extent CPU required to decode).
So the quick answer is that the large file contains more information, so itâs higher quality - BUT the big caveat is that when compressing the video, the amount of CPU effort expended to compress the video matters. Itâs possible to carefully analyze the video (at the expense of encode speed) and find a very efficient (small size) encoding with a very high quality, whereas if someone wants to shrink a file in a hurry, the CPU analysis isnât as good so either the file is bigger or the quality is worse (or both).
The only real answer is to watch them (at least a few scenes) and see if you can tell the difference. If you canât see the difference the smaller is fine, sometimes it will even be better quality to your eyes. If youâve got the HDD space, just store the video streams without re-encoding them, the quality will never improve by re-compressing. (Blu-ray are mostly x.264/AVC native and UHD is x.265/HEVC, with exceptions).
Also, a movie is more than video, itâs also audio. Pay attention to the audio quality, especially to extra audio-tracks. If you donât want all the languages, they just take up space.
Thanks for the tips. Iâll give each a watch. I wasnât sure if there was somethings in the stats that tells you âThis one is way better because of thisâ.
Visual quality is highly subjective, so no easy stat can tell you quality, but given the choice between two well-done encodes the higher the bitrate (large size) generally means higher quality. Remember though, that you can turn the bit-rate up but not get good quality, especially if you start with poor source material. If you encode, then re-encode, itâs like a photocopy of a copy, each time it gets worse, and nothing can make a source better than it was (for example, youâll never turn a DVD into a 4k UHD).
Also, Codec does make a big difference. In your example they were both the same format and codec (x.265/HEVC), but if theyâd been different that could be a tell toward quality. MPEG-2 (DVD format) is not very efficient (large files) but is very easy on the CPU, it was made when embedded CPUs for DVD players had minimal processing power. x.264 (AVC) was a big jump in efficiency, but requires more CPU power to encode and play-back. Nowadays most any CPU can handle it, but real-time transcoding on an embedded device (like a NAS) is problematic. But as an example, an MPEG-2 DVD is around 4-5 GB, whereas the same video at roughly the same quality is only around 1 GB when encoded with x.264. The same is true again for the new x.265. Even a decent modern processor will be pressed to do real-time transcoding of x.265 source material, especially 4k, but the advantage is smaller files - typically only about half the file size is needed to achieve similar quality to x.264.
