What's up with Blu-ray layer capacities?

[roxics]

I've known the sizes of BD layers for years, but never really thought about it. When looking at the specs again it hit me, the technology (or maybe it's the labeling) that's strange.

Actual BD specs:
BD-R Single layer 25GB (23.3GiB)
BD-R Dual layer 50GB (46.6GiB)
BD-XL Triple layer 100GB (93.1GiB)
BD-XL Quadruple layer 128GB (119.2GiB)

Single layer is 25GBs, ok cool.
Dual layer is twice that. Makes sense. 25GBs per layer. I see a pattern here.
Triple layer is 100GBs? That's not 25GBs per layer anymore. We just doubled the dual layer capacity. But maybe it just doubles every time.
Quad layer is 28GBs more than triple layer???? What? Where did that come from?

One would think that each layer would be 25GBs, resulting in:
25GBs, 50GBs, 75GBs, and 100GBs

Or maybe they would just double every time, resulting in:
25GBs, 50GBs, 100GBs, and 200GBs

So what is actually going on here?

[Vidd]

BD-XL is a different technology/specification than BD-R. That is why most standard Blu-ray drives and standalone players can't read BD-XL discs.

More specifically:

BD-R is 25GB per layer with a Minimum Mark length of 0.149 um
SL Discs = 25GB
DL Discs = 50GB

BD-XL Dual Layer/Triple Layer is 33.4GB per layer with a Minimum Mark length of 0.112 um
DL Discs = 66GB
TL Discs = 100GB

BD-XL Quad Layer is 32GB per layer with a Minimum Mark length of 0.117 um
QL Discs = 128GB

[Cornucopia]

The limits of technology at the time of spec definition.

Dvd had SL and could do that easily, but they needed to stretch it to cover movie length, so they added DL. But DL at the time pushed the limits, so as you know, DL is NOT fully twice what SL is.

By the time of BD, technology had advanced enough that DL was fully incorporated from the start and was no longer "pushing the boundaries", so it IS fully 2x BD's SL. And at that time they knew they were going to try for further, but were not yet ready to...

With BDXL, they were now ready, and so added a much more efficient upper layer scheme, but found they ran out of reliable space after 128 (pushing the limits again), so they cut it off at that.

Since the creation of the BDXL spec, they have advanced technology further, and are incorporating those advances in the "Archival Disc" format, but once you create a consumer spec, it is basically set in stone for backward compatibility reasons, so they can only incorporate those advances in supplemental specs. Plus, they're not revising the spec just to make some numbers fit evenly.

Scott

[roxics]

You guys are awesome! I'm happy to have learned this today. It makes a lot more sense now.
Cornucopia, I'm glad you threw in the dual layer DVD info as well. I thought about bringing that up, how it hadn't quite reached double capacity. But like you said I figured the technology just wasn't fully figured out yet or because it was red laser there was some kind of limitation. Wasn't sure. So figured I'd stick to BD.

Off topic (sort of) I've been thinking about Minidiscs lately. Not the 80mm open discs but the actual audio format invented by Sony in the early 1990s, the 64mm enclosed rerecordable Minidisc format. Just for fun I'm trying to figure out what the capacity of a Minidisc would be if it were using BD and BDXL technology. What each layer size would be. Now that you have me introduced to it, I'm curious what the capacity would be using Archival disc technology as well.

I don't know if any of you are better at math than me, but we know that a single layer 120mm disc is 25GB and 50GBs dual layer. 80mm disc is 7.5GBs single layer and 15GB for dual layer.
So what would a 64mm disc be for single dual, triple, quad and archival?

Just to throw a wrench in the works I also found this. https://www.engadget.com/2006/10/05/tdk-unveils-16-5gb-mini-blu-ray-discs-at-ceatec/

Any help on this would be great.

[Cornucopia]

You can get a better handle on all these optical disc measurings if you look at it by sector size & sector count. Then the difference between mini (8cm) and full (12cm) discs, as well as AudioCD vs CDROM vs VideoCD become apparent and calculable.

Scott