Are GDs just dual layer CDs?

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Savlon
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Are GDs just dual layer CDs?

Post#1 » Mon Jan 25, 2021 2:14 pm

So, I don't really have much technical knowledge here but I have always been led to believe by information on the internet (which is generally all copied from the same source I get the impression) that GDs are just like CDs but the pits are just "packed more closely together", and I never really understood how there would be enoguh room to almost double the data capacity without extreme difficulty. Recently I have seen people talking about how Sega apparently could have made dual layer GDs that would work even on the drives not designed for it. I was talking about this to someone and I joked that mayeb it was already a dual layer CD becuase the percentage capacity increase is similar to what it is for DVD DL, and when I searched for "dual layer GD-ROM" I found this one site that says that the GD *is* a dual layer CD. Of course it gets some other minor details about the DC wrong (I think) like the American DC design also being based on SH4 as well as the Japanese one and makes a few other claims that probably aren't backed up anywhere else but I thought it was interesting anyway. I know the Wii's DVD DL drives apparently stop reading dual layer discs before single layer ones which I thoguht would line up with DCs failing to read GDs before they fail to read CDs and it would be a more realistic way to get the extra capacity.

Please don't ban me if you think this is a stupid post. I just thought I'd ask what everyone else thinks because it doesn't seem to have been discussed before and I thoguht it was plausible (thoguh someone will probably come in and tell me that the exact nature of GDs has been known for years and is no mystery). However there is this other site talking about a Sony "double-density compact disc" that does do it by simply increasing pit density rather than using two layers, just like how the GD has always been described as doing it: https://enacademic.com/dic.nsf/enwiki/11601771

edit: Actually I suppsoe it can't be dual layer if it can be read in an ordinary CD drive with only getting it to read a different "table of contents", unless no one has ever got that method to allow reading beyond the usual CD capacity?

edit again: Lol I don't know what i was thinking when I posted this. Actually I know perfectly well but now it seems less realistic.
Last edited by Savlon on Thu Jan 28, 2021 10:09 am, edited 1 time in total.

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dark
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Re: Are GDs just dual layer CDs?

Post#2 » Mon Jan 25, 2021 4:29 pm

Some stuff I've read in the past is that CDs are ordinarily written/pressed in a manner to provide some duplicate information on the disc, the point of this is to have some error protection built in - if there's a scuff preventing the laser from reading data in one spot on the cd, maybe the laser can pick up what it needs from the duplicate data that is stored in a physically different part of the CD. If you get rid of or reduce the amount of this ordinary CD error protection, you can fit more data on a typical disc - Atari did this with the Atari Jaguar CD, allowing for up to 790 megs on a disc with reduced data duplication/error protection. I suspect this might also be one way for how sega fit more data onto gdroms.

I also see the wikipedia entry for GDroms indicates the increased storage is related to the pits on the disc being packed more tightly together than a conventional CD.

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Aleron Ives
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Re: Are GDs just dual layer CDs?

Post#3 » Mon Jan 25, 2021 4:40 pm

No, GD-ROMs still have full error correction. If they didn't, one scratch would render your game unplayable. A GD-ROM is not a dual layer CD. The high density section of the disc just has pits packed closer together than on a normal CD. Remember, the original CD standard was for ~74 (72?) minutes, and that was replaced with 80 minutes by packing the pits closer together. You can buy 99 minute CD-Rs that violate the CD spec even more in order to fit more data on the disc. GD-ROM just takes it to the next level as far as using a CD laser is concerned.
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la-li-lu-le-lo
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Re: Are GDs just dual layer CDs?

Post#4 » Tue Jan 26, 2021 1:13 am

Savlon wrote:Of course it gets some other minor details about the DC wrong (I think) like the American DC design also being based on SH4 as well as the Japanese one and makes a few other claims that probably aren't backed up anywhere else but I thought it was interesting anyway.


I think you're referring to the Katana and Black Belt. The Black Belt initially used a PowerPC CPU, but Sega eventually asked the team to also use SH4.

Like Aleron Ives said, a GD-ROM has double the number of pits (in the high density section) compared to a CD. The other part is that the disc runs at half speed while the other components run at the same speed. That's how the drive is able to read double the number of pits in a single disc - however, it's not actually double because only part of the disc is high density (~1GB). It also has a section intended to run on a CD player or CD-ROM drive, and another section that stores text.

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WedgeStratos
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Re: Are GDs just dual layer CDs?

Post#5 » Tue Jan 26, 2021 12:30 pm

GD-ROM in particular is a proprietary standard created by Yamaha exclusively for Sega's set of hardware, covering Naomi, Triforce, Dreamcast, and Chihiro. If we take the word of the Wikipedia article regarding this...
There are three data areas on a GD-ROM disc. The first is in conventional CD format, and usually contains an audio track with a warning that the disc is for use on a Dreamcast, and can damage CD players. These vary by region. This section is 4 minutes long with the data size of 35 MB.

The CD section also contains a data segment, which is only readable in PCs. Although most discs include only text files identifying the game, its copyright and bibliography, some contain bonus material for home computer users (for example, Sonic Adventure contains images of Sonic characters to use on the desktop). There then follows a separator track which contains no data except for the text Produced by or under license from Sega Enterprises LTD Trademark Sega (Similar to the Sega Saturn, it was believed that the security key was stored in this area to prevent piracy). The final (outer) section of the disc contains the game data itself in a higher density format. This section is 112 minutes long (almost two hours), with a data size of 1.0 GB.


None of these have been given any superscript for their sources, but all of it sounds accurate to the design of the disc.

OGDCFAN99
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Re: Are GDs just dual layer CDs?

Post#6 » Tue Jan 26, 2021 2:25 pm

People always forget about the security ring which also takes up space on the disc. It works the same as the Saturn's copy protection where if the "wobble" isn't detected then the drive wont switch into GD mode.

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la-li-lu-le-lo
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Re: Are GDs just dual layer CDs?

Post#7 » Tue Jan 26, 2021 6:15 pm

OGDCFAN99 wrote:People always forget about the security ring which also takes up space on the disc. It works the same as the Saturn's copy protection where if the "wobble" isn't detected then the drive wont switch into GD mode.


Can you explain more about exactly how that works? I've heard about it before, but most information I've found about it is pretty vague.

Cass
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Re: Are GDs just dual layer CDs?

Post#8 » Wed Jan 27, 2021 8:21 am

I don't think I ever heard of the plans for dual layer GD's before. 2G disks would have been handy for a few of the multi disc titles alright. Imagine the noise the DC's ever angry GD-Rom drive would have made :lol:

Savlon
shadow
Posts: 12

Re: Are GDs just dual layer CDs?

Post#9 » Thu Jan 28, 2021 10:32 am

dark wrote:Some stuff I've read in the past is that CDs are ordinarily written/pressed in a manner to provide some duplicate information on the disc, the point of this is to have some error protection built in - if there's a scuff preventing the laser from reading data in one spot on the cd, maybe the laser can pick up what it needs from the duplicate data that is stored in a physically different part of the CD. If you get rid of or reduce the amount of this ordinary CD error protection, you can fit more data on a typical disc - Atari did this with the Atari Jaguar CD, allowing for up to 790 megs on a disc with reduced data duplication/error protection. I suspect this might also be one way for how sega fit more data onto gdroms.

I also see the wikipedia entry for GDroms indicates the increased storage is related to the pits on the disc being packed more tightly together than a conventional CD.


Yes I was thinking of Jag CD when you were talking about that. Thos are ridiculously fragile (scratch-wise) aren't they? I may be wrong but given what Atari at that time was like I get the impression that was just them not having time to implement error correction and then pulling a "its not a bug, its a feature".

Aleron Ives wrote:No, GD-ROMs still have full error correction. If they didn't, one scratch would render your game unplayable. A GD-ROM is not a dual layer CD. The high density section of the disc just has pits packed closer together than on a normal CD. Remember, the original CD standard was for ~74 (72?) minutes, and that was replaced with 80 minutes by packing the pits closer together. You can buy 99 minute CD-Rs that violate the CD spec even more in order to fit more data on the disc. GD-ROM just takes it to the next level as far as using a CD laser is concerned.


Yeah that's probably more like it. I just thought maybe that was a chance it was false given most of the references to it either quote wikiepdia or were quoted by it, though of course this throwaway information by this one source is less reliable of course. I didn't know before you could actually buy higher density CD-Rs. Interesting.

la-li-lu-le-lo wrote:
Savlon wrote:Of course it gets some other minor details about the DC wrong (I think) like the American DC design also being based on SH4 as well as the Japanese one and makes a few other claims that probably aren't backed up anywhere else but I thought it was interesting anyway.


I think you're referring to the Katana and Black Belt. The Black Belt initially used a PowerPC CPU, but Sega eventually asked the team to also use SH4.

Like Aleron Ives said, a GD-ROM has double the number of pits (in the high density section) compared to a CD. The other part is that the disc runs at half speed while the other components run at the same speed. That's how the drive is able to read double the number of pits in a single disc - however, it's not actually double because only part of the disc is high density (~1GB). It also has a section intended to run on a CD player or CD-ROM drive, and another section that stores text.


Yeah a PPC 603 according to whatever I read. Oh OK, didn't know both were using a SH4 eventually. Thanks.
Yes that's what pretty much every source on the internet other than this one said. But what is the reason CDs weren't pushed further to begin with? I always thought that doubling the density would be unrealistic?

WedgeStratos wrote:GD-ROM in particular is a proprietary standard created by Yamaha exclusively for Sega's set of hardware, covering Naomi, Triforce, Dreamcast, and Chihiro. If we take the word of the Wikipedia article regarding this...
There are three data areas on a GD-ROM disc. The first is in conventional CD format, and usually contains an audio track with a warning that the disc is for use on a Dreamcast, and can damage CD players. These vary by region. This section is 4 minutes long with the data size of 35 MB.

The CD section also contains a data segment, which is only readable in PCs. Although most discs include only text files identifying the game, its copyright and bibliography, some contain bonus material for home computer users (for example, Sonic Adventure contains images of Sonic characters to use on the desktop). There then follows a separator track which contains no data except for the text Produced by or under license from Sega Enterprises LTD Trademark Sega (Similar to the Sega Saturn, it was believed that the security key was stored in this area to prevent piracy). The final (outer) section of the disc contains the game data itself in a higher density format. This section is 112 minutes long (almost two hours), with a data size of 1.0 GB.


None of these have been given any superscript for their sources, but all of it sounds accurate to the design of the disc.


Yes I've seen the wikipedia page but thanks for quoting it for anyone else that reads. I wonder if anyone who has contributed to it is on this forum? Also i don't really understand why it was used for TriForce and Chihiro. They wouldn't exactly be thinking about that level of cost cutting in an arcade machine. Am I wrong?
Cass wrote:I don't think I ever heard of the plans for dual layer GD's before. 2G disks would have been handy for a few of the multi disc titles alright. Imagine the noise the DC's ever angry GD-Rom drive would have made :lol:


I've never heard of any plans either, only fanboy's fantasies including them. I just heard them more than once close together so thoguht about it a little more. Yes they would be handy! And yes I would have to turn up the volume on my TV even louder lol.
Last edited by Savlon on Thu Jan 28, 2021 1:24 pm, edited 1 time in total.

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Ian Micheal
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Re: Are GDs just dual layer CDs?

Post#10 » Thu Jan 28, 2021 12:35 pm

people would be still using a gdemu and we can have now 2gb images on gdemu so job done

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