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I recently had to burn a DVD for the first time in ages and I was wondering why CDs and DVDs start in the centre and go towards the edge.

Older rotating-disc media like vinyl records started from the edge and went towards the centre so it couldn't have been for historical reasons.

I am looking for good sources on the reasoning for this data structure.

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Why do CDs and DVDs fill up their data from the middle outwards?

Note the above assumption made in the question is incorrect.

Summary:

  • For historical reasons writing (and reading) from the inside makes sense (different size disks are possible as explained in other answers).

  • For read performance reasons modern disks may be written (and read) outside in or even in both directions (dual layer).

Notes:

  1. Most disks are a standard size.

  2. Commercially produced CDs and DVDs are not written but stamped on a press.

    Although all DVDs (both homemade and commercial) utilize "pits" and "bumps" physically created (the pits on the unreadable side, and the bumps are on the readable side) on the discs to store the video and audio information, there is a difference on how the "pits" and "bumps" are created on commercial DVDs vs the way they are made on a home recorded DVD.

    DVD movies you buy at the local video outlet are manufactured with a stamping process. This process is sort of like the way vinyl records are made - although the technology is obviously different (vinyl records are stamped with grooves versus DVD being stamped with pits and bumps).

    Source The Difference Between Commercial and Home-recorded DVDs

The rest of this answer concentrates on the performance aspect.

The exceptions to writing (and reading) from the inside are XBox games (and other games consoles games) and Dual Layer DVDs (movies).


Xbox Games

Xbox games have the data written from the outside in for performance reasons - since the outside is spinning faster than the inside the data can be read faster.

The Xbox games themselves are stored on DVD-9 (9GB single sided, double layer) formatted discs and are actually written in a very interesting fashion. The games are written from the outside of the DVD-9 discs to the inside, meaning that most discs will actually have the majority of their data stored around the outer perimeter of the disc.

Since the DVD drive in the Xbox is a Constant Angular Velocity (CAV) drive, it can read more data per second off of the outermost tracks making this a highly optimized way of storing data on the DVDs to reduce load times. Obviously the drive can also read regular CDs and DVDs as well.

Source Hardware Behind the Consoles - Part I: Microsoft's Xbox

Dual Layer DVDs (Movies)

Dual Layers DVDs can be written in either direction - there are two write modes.

Most movies are written as opposite track path. A movie will be split across across the layers, so no seek back to the inner edge is needed at the layer change.

There are two modes for dual layer orientation. With parallel track path (PTP), used on DVD-ROM, both layers start at the inside diameter (ID) and end at the outside diameter (OD) with the lead-out. With Opposite Track Path (OTP), used on DVD-Video, the lower layer starts at the ID and the upper layer starts at the OD, where the other layer ends, they share one lead-in and one lead-out.

Source DVD-R DL

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    I've accepted this answer as it has the most complete research and explains the different methodologies that I wasn't aware of when I originally asked the question. – Burgi Jan 3 '17 at 1:08
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    The thing about XBox games is simplified to the point of being less than accurate. An Xbox disc is a normal DVD written in the normal fashion (low sector numbers on the inside, high sector numbers on the outside). They simply use some nonstandard track layout (and nonstandard filesystem) to store stuff in the highest sectors available. Nothing is actually "written from the outside in" though, that would be physically incompatible with normal drives because the spiral direction would be wrong. – hobbs Jan 3 '17 at 7:31
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    The GameCube, as mentioned in another answer, is an example of a system that actually does run the spiral the other way, because that system doesn't care about reading normal DVDs. – hobbs Jan 3 '17 at 7:34
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    @hobbs I do not believe the Gamecube writes its discs with the spiral in the other direction. The Wii can read DVDs (though not officially), and plenty of DVD drive firmwares can be hacked to read Wii/GC discs. I don't believe there's any indication that the spiral itself is in an opposite direction; I seem to recall reading that it's a popular myth. If you can provide a good source (eg some technical analysis rather than someone on a forum asserting it) I'll gladly retract! – Muzer Jan 3 '17 at 15:01
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    @hobbs I don't see anything about GameCube discs being written outside-in in Anatomy of an Optical Medium Authentication. It's a proprietary file system, a physical sector whitener that's conceptually as different from standard DVD as CD-ROM XA is from mode 1, and some decryption keys in the Burst Cutting Area. – Damian Yerrick Jan 3 '17 at 17:26
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This is intended to allow for discs of different sizes.

The standard optical disc today is 12 cm in diameter. However, optical media have historically been produced in a wide variety of sizes. The LaserDiscs of the 1970s and 1980s were made in 30 cm, 20 cm, and 12 cm sizes; more recently, CDs, DVDs, and BDs are made in a 8 cm Mini form factor (storing 210 MB, 1.4 GB, and 7.8 GB, respectively) in addition to the standard 12 cm.

Unlike vinyl records, where the needle can easily be placed onto the record by hand, electronic optical disc players would need extra components (or at least extra firmware logic) to find the edge of the disc and locate the first track if they played from the outside in, which would make their design considerably more complex and expensive. The center of the disc provides a consistent start location for any disc inserted in the player, regardless of its size.

However, not all optical media are read from the inside out.

While most standard optical media are read this way, there are proprietary disc formats (sometimes seen in game consoles) that assume one or only a few specific sizes and are read from the outside in. There are a couple of reasons for this:

  • It is faster to read from the outside of the disc than from the inside. At the same rotational velocity (e.g. 10000 rpm), the linear velocity on the outside of the disc is higher than the linear velocity on the inside of the disc, resulting in higher performance towards the outside. Above 10000 rpm, optical discs can wobble excessively and even shatter, placing an upper limit on rotational velocity.
  • It makes illegal copying of discs more difficult. As mentioned above, most optical disc players work from the inside out. A disc that is read from the outside in cannot be read with ordinary optical disc readers.
  • This answer is incorrect. Not all DVDs are written this way :) – DavidPostill Jan 1 '17 at 18:46
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    @DavidPostill So it's the question that is incorrect. :) – Kamil Maciorowski Jan 1 '17 at 18:51
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    Agreed. This doesn't make the answer fundamentally wrong. The question (incorrectly) assumes that all optical media are read inside-out, and the answer is based on this assumption. I have edited the answer to reflect the fact that some proprietary optical media read outside-in. – bwDraco Jan 1 '17 at 18:56
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    The question does not say "all optical media" anywhere, it specifically refers to CDs and DVDs. – qasdfdsaq Jan 1 '17 at 19:14
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    It also allows to make different shaped discs. Although I'm not sure if it's intended or a consequence of writing first at the center. – Olivier Grégoire Jan 1 '17 at 21:55
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With vinyl it was easier to put the needle onto the outside than on the inside of the spinning record.

CDs can have different sizes and shapes. The disc can have any size beginning from the inside. Also most people will touch the cd on the border - so less fingerprints when the drive reads the first track.

BTW It has noting to do with the speed. The first CD drives had a constant speed reading the data.

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    Interesting point but they couldn't have known people would do weird and wonderful shapes to begin with. – Burgi Jan 1 '17 at 18:23
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    The size of a disk is probably pretty key here. If you assume you need to start at a fixed centre point and go outwards then your disk can, within reason, be any size you like. If you start at the outside and work inwards then you must have a set disk size. – Mokubai Jan 1 '17 at 18:23
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    @AndrewMorton Clarification: The first where derived from audio discs and had CLV. Laserdiscs as older system used CLV. Audio (or video) discs have no benefit from CAV, but from a constant data rate. -Audio CDs therefore had 495 to 212 rpm -> disc moves at 1.2m/s by read laser. For the constant data rate Audio CD, Video DVD and BluRay use CLV. First computer CD-ROMs had "single" or "1x" speed which was the equivalent to the audio CD's (~150 KiB/s). Later CD-ROMs should be faster, and to gain more speed (or have better marketing values) at some point CAV was introduced. – Offler Jan 1 '17 at 20:31
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    @burgi Shape is a side effect of the variable sizes. – Offler Jan 1 '17 at 20:32
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    @Burgi They knew, because it already happened decades ago with vinyl records. en.wikipedia.org/wiki/Unusual_types_of_gramophone_records – Agent_L Jan 2 '17 at 16:28
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Along with reasons that have already been cited, back when CDs were new, there was also some discussion of the fact that scratches from normal handling were a lot more likely to happen toward the edges of the disk than toward the center (especially those from misalignment when putting a disk into a disk tray).

With the data starting from the center, and scratches more likely toward the edges, scratches are more likely to happen on part of the disk that contains no data, so the scratch has no effect on reading the data.

  • Except that, at least in some cases, identification blocks and write-protect stuff were in the outermost rings. – Carl Witthoft Jan 5 '17 at 16:23
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    @CarlWitthoft: True--but those were things hacked on after the fact, not part of the original design. – Jerry Coffin Jan 5 '17 at 16:26
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Note also that CDs originally anticipated portability as a feature, e.g., Sony Walkman, which already existed as a cassette player. Those early portable players slowed down their rotation as the read head moved farther from the centre.

In practical terms, that meant that as your batteries depleted, you actually needed less mechanical power to turn the disk to get the same surface data rate.

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    That sort of implies the battery run time was limited to playing one CD. – fixer1234 Jan 5 '17 at 20:55
  • It was. Well, after a while it was. :) – Stijn de Witt Jan 7 '17 at 23:01
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Other answers are good. I read somewhere though that damage to disks happen most often and more severe on the outer part than on the inner part.

If this is true, that would have meant for CDs that they will have better durability. Rarely commercial CDs are written full. Damage to the unwritten part of the CD would be irrelevant.

Also the TOC or filesystem on a disk, if written in the most inner part of CD/DVD would be better protected and allow at least reading partial data from the rest of the CD. But I haven't checked exact position of this data on the different disk types and writing modes so I don't know if it is written like that.

  • Could you provide some sources for this? This has already been speculated by @jerrycoffin (also without sources). – Burgi Jan 9 '17 at 12:26
  • @Burgi, ops, missed his answer. I don't have a reference. Was long ago. I can only say though that some burning software has the priority setting, where one can choose where particular files are written. Files with higher prio get written before those with lower prio. k3b exposes that, as well I guess the lower level tools used by it (probably mkisofs -sort). – akostadinov Jan 9 '17 at 13:23
  • On the other hand I don't see mkisofs readme mention anything about scratches: fifi.org/doc/mkisofs/README.sort – akostadinov Jan 9 '17 at 13:29
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Most CDs/DVDs are not fully filled up. It is faster for the computer to store or find the required data in the inner portion due to smaller diameter, the distance traveled by the reading head is less, as compared to the outer periphery of the disc, where diameter is maximum. Saving and searching in the innermost circles / cylinders saves both time and energy.

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    Wouldn't it be the opposite? The outer portion holds more so less lateral travel is required. – fixer1234 Jan 2 '17 at 5:42
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    Actually, AFAIK, the reading rate is constant, the drive simply has to spin faster when reading data from inner most cylinders. – Salman A Jan 2 '17 at 8:00
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    @SalmanA: Music CD's are Constant Angular Velocity. Modern computer drives are indeed Constant Linear Velocity, rotating faster for the inside tracks. This requires smarter drive electronics. – MSalters Jan 2 '17 at 8:46
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    @MSalters music CDs are CLV. All CD disks have constant linear bit density - so music is played with CLV; variable spin rate was done by adjusting rpm to keep a FIFO half full. same was true with early CDRom drives. Newer ones are agile to read a wide range of bit rates so they can just spin fast and adapt to whatever bitrate results. – greggo Jan 3 '17 at 23:51

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