Blu-Ray & Their GigaBytes

[cmcasper]

This is a question I've been meaning to bring up for quite a while. I have tons of movies, but I don't claim to know much about the "inner workings" of a blu-ray disc.

I'll try to clear this up. I just bought Transcendence on Blu-Ray, and looked at the size of the data on the blu-ray disc. It takes up roughly 21.4 GB of space. There are a decent amount of special features on the disc, so my question is why is that number so low? Does that mean the quality of the main feature will be lower than a movie with, say 45 GB?

[nmycon]

There are other things you have to take into account.

For example a movie with a single audio track will take up much less space than one with 4 or 5 audio tracks, especially if that includes multiple lossless tracks.

Also, a 5.1 lossless audio track will be larger than a 2.0 lossless track.

It also goes without saying that the length of the film and amount (and resolution) of special features should be taken into account as well.

It's almost impossible to compare blu-rays using their filesize because pretty much all discs will have differing audio options and extras.

I'd say a better indication of quality is the bitrate.

[TripleHBK]

Here's some facts in a nut shell...

-There are 2 types of Blu Ray Disks. BD-25 which and BD-50. The 25/50 stands for gigabytes. However actual functional space on these disks is actually less than 25/50. From my experience a BD-25 can hold right around 23.3gb of data, while a BD-50 can hold right around 46.8gb of data.

-An uncompressed audio track for a 2 hour movie usually takes up around 2-2.5gb of space per track. So if there are multiple language options all in uncompressed format, take that 2gb x the number of language tracks. (Dolby Digital or standard DTS usually clock in right around 1gb per 2 hours of audio.

-Movie size has very little to do with the quality of the picture. While it certainly helps to have a larger file size, as another user pointed out, having a high bit rate tends to help the most common problem with Blu rays (ie. compression problems) however picture quality is largely up to the encode of the film itself and whether it has had a fresh scan/remaster. Proof of this can be seen in the various HD digital files available on the internet. There are plenty of great examples of films that look great in 720p with a file size of only 1.5gb.

-Lastly, special features are what fill out the rest of the disk as the other user stated. So the more features on the disk, the smaller the movie file will need to be to accommodate the other parts.

[MOONPHASE]

Originally Posted by cmcasper

This is a question I've been meaning to bring up for quite a while. I have tons of movies, but I don't claim to know much about the "inner workings" of a blu-ray disc.

I'll try to clear this up. I just bought Transcendence on Blu-Ray, and looked at the size of the data on the blu-ray disc. It takes up roughly 21.4 GB of space. There are a decent amount of special features on the disc, so my question is why is that number so low? Does that mean the quality of the main feature will be lower than a movie with, say 45 GB?

Transcendence has really short Extras. So most of the GBs on it are just for the movie and audio itself. But if it had like some 20 minute Interview or something like that with it being in HD then yes it would need a BD-50.

TV shows are more likely to BD-50s. Like i've even seen ones that like 8-10 episodes per disc. Movies are more likely to use BD-25s unless it's a long movie with a bunch of Extras or has longer Extras.

[cmcasper]

Originally Posted by nmycon

There are other things you have to take into account.

For example a movie with a single audio track will take up much less space than one with 4 or 5 audio tracks, especially if that includes multiple lossless tracks.

Also, a 5.1 lossless audio track will be larger than a 2.0 lossless track.

It also goes without saying that the length of the film and amount (and resolution) of special features should be taken into account as well.

It's almost impossible to compare blu-rays using their filesize because pretty much all discs will have differing audio options and extras.

I'd say a better indication of quality is the bitrate.

Thanks guys, great info! How would I go about checking bitrate for each blu-ray disc, if that' something I wanted to do?

[nmycon]

Originally Posted by cmcasper

Thanks guys, great info! How would I go about checking bitrate for each blu-ray disc, if that' something I wanted to do?

Most players have some sort of "Display" function. If you press that button it will bring up the timeline and show other information such as the video/audio compression formats (AVC for video, DTS-HD for audio, etc.), as well as the current bitrate.

Look at it this way, a two hour movie with several lossless audio tracks (for the record, Transcendence only has one) and a substantial amount of extras is capable of fitting on a BD-50 with wiggle room, although if the final size of the disk image is less than 25 GB, they will use BD-25s to save money.

There's no evidence that studios are applying unnecessary compression to get larger files to fit on a 25 GB disc, so I wouldn't worry about quality depending on file size.