Sounds like a dumb question but isn't...

[CaptainHawk1]

I'm a complete layman on this subject so bear with me...

I was thinking the other day how PS3 is constantly getting software updates and is able to do more with Blu Ray now than the day it came out.

So I was wondering, other than software and programming, what the hell is the difference between a CD optical drive and a DVD optical drive.

I'm probably missing something but it seems to me that they are both drives that use red lasers to read the information on said media.

So clarification from the experts is required.

 

[CuttingEdge100]

To the best of my knowledge DVD's use smaller and far more numerous data pits in the disc and as a result can store far more data than a CD.

CuttingEdge100

 

[Alpha_Geek]

Since the data is denser on the disk, one would presume that the laser focuses tighter to look at a smaller area, the stepper motor that moves the head assembly moves in smaller increments, and in the case of dual layer disks, there is some manner in which the focus point of the optics can be changed.

I'm sure there are other differences, but that's what comes to mind as far as hardware is concerned.

AG

 

[Verteron]

Since the data is denser on the disk, one would presume that the laser focuses tighter to look at a smaller area, the stepper motor that moves the head assembly moves in smaller increments, and in the case of dual layer disks, there is some manner in which the focus point of the optics can be changed.

The laser is indeed more focussed, but the main issue is that to read the much smaller pits on a BluRay, it needs to have a smaller wavelength. Blue light has a smaller wavelength than red light, which is why BluRay uses a blue-coloured laser instead of the red coloured laser you'll find in a DVD or CD player. Blue laser technology is significantly more expensive than red laser technology, hence why BD players are more expensive than DVD players right now (and would've been prohibitively expensive just a few years ago).

As far as the direct comparison between a DVD and a CD player goes, they also use different wavelengths of laser, CD using a 780nm laser, and DVD using a 650nm laser (BD for reference uses 450nm). Although they both look red, exactly the same issues as above are the case, the reds are different 'shades', it's just less obvious to a human eye and was never used as a part of marketing drive!

I mean, calling DVD "slightly lighter red-ROM" would've sounded pretty silly...

 

[Jadzia]

I was led to believe that CD used infra red lasers. Although I don't suppose it matters if one uses a red laser instead.

The physics problem is one of diffraction. Focussing light through a narrow channel (reflecting or not) causes the light to diffract. In layman's terms, this is where the light stops travelling with perfect geometry (in a straight line with perfect reflections) and instead will split into several fuzzy beams, each travelling at different angles.

Different colours of light diffract to different intensities. Blue light is tougher than red light and doesn't diffract as readily.

This is why CD/DVD's give rainbow reflections and appear slightly holographic. What you are seeing is the light being split into multiple fuzzy beams, and reflected at strange angles. since each colour diffracts a different amount, it separates it into the rainbow colours.

There's a nice picture of diffraction here.

Precisely this phenomenon is relevant when trying to read the data from the tiny pits on the disc. Blue light diffracts less than Red light, so it can "see" finer detail with greater clarity.

Blue-Ray uses finer detail than DVD; and DVD use finer detail than CD. The level of readable detail of each type of disc is dictated by the colour of the laser.

In the CD age, infra-red lasers were only cost effective diode laser we could make. So that alone dictated the maximum data space of the CD.

As we've got better at making diode lasers, the red and blue lasers used for DVD/BD have become cheap enough to make these higher data space discs possible.

In the future we will no doubt have ultra-violet ray discs offering upto a 1 TB capacity or something like that

 

[Verteron]

I was led to believe that CD used infra red lasers. Although I don't suppose it matters if one uses a red laser instead.

Technically you're right, CD drives use a 780nm laser which is just on the boundary of red/infra-red, I was just being a bit sloppy!

DVDs on the other hand are very definitely red, and Blu-Rays are blue/violet, I assume the next step may well be towards ultraviolet lasers for the next optical disk format, if indeed there ever is another optical disk format (it may well be the case that the industry moves to flash memory solutions or all-download online distribution channels).

 

[CaptainHawk1]

So the real difference is a difference in the shade of laser itself?

Then when you have combo drives does the color of the laser change? For that matter, how does my BD PS3 also play DVD's and CD's or is it a matter of you can step down but you can't step up?

 

[Verteron]

So the real difference is a difference in the shade of laser itself?

Then when you have combo drives does the color of the laser change? For that matter, how does my BD PS3 also play DVD's and CD's or is it a matter of you can step down but you can't step up?

I'm not sure about DVD/CD combo drives, but BD/DVD combo drives definitely include a separate red laser for reading older formats, at least I recall there being a lot of talk about including one for just this reason when BD was being designed.

If you think about what Jadzia was saying, it's possible I suppose that a DVD laser might be able to read the larger pits on a CD, given they're quite close together, and they may do this rather than having something separate. The problem with a blue laser reading a disk designed for red might be down to the reflectivity as well as the wavelength, although I don't know for sure.

 

[Jadzia]

So the real difference is a difference in the shade of laser itself?

Then when you have combo drives does the color of the laser change? For that matter, how does my BD PS3 also play DVD's and CD's or is it a matter of you can step down but you can't step up?

The colour of the laser light is determined by the chemicals in the diode, which can't be changed, so the wavelength (colour) is fixed.

The combo drive will use two diode lasers. These may or may not be packaged into the same physical component, comparable to a multicolour LED.

 

[Alpha_Geek]

So the real difference is a difference in the shade of laser itself?

Then when you have combo drives does the color of the laser change? For that matter, how does my BD PS3 also play DVD's and CD's or is it a matter of you can step down but you can't step up?

Well if you're looking for a difference you can maybe see with your eyes and no tools, then yeah... That's the "real" difference.

The mechanical differences I cited already are also true.

Most DVD drives were built with backwards compatibility with CD-ROM in mind (If I had to guess, they put multiple LED laser emitters in the lens assembly, one at the correct wavelength for each, or made an emitter with a broad bandwidth), so I see no reason this trend would not continue with the bulk of leter and higher density optical disk technology.

 

[CaptainHawk1]

Thanks for all of the information folks, it's really helped a lot and thanks for not treating me like a complete idiot for not knowing this stuff.

 

[Meredith]

CD tracks are spiral tracks and DVD tracks are more like a hard drive in pie shaped sectors.


* Edit, I think i was thinking of Blu-Ray disks.

 

[Verteron]

CD tracks are spiral tracks and DVD tracks are more like a hard drive in pie shaped sectors.


* Edit, I think i was thinking of Blu-Ray disks.

I have to call you on this one as I'm pretty certain it's not true. What leads you to think this, and do you have a source?

 

[Jadzia]

CD tracks are spiral tracks and DVD tracks are more like a hard drive in pie shaped sectors.


* Edit, I think i was thinking of Blu-Ray disks.

I have to call you on this one as I'm pretty certain it's not true. What leads you to think this, and do you have a source?

I've always thought all optical discs were concentric rings and not spirals too. But I'd be happy to accept that I'm wrong on that.

Sectors I think is more about formatting standards (orange book vs yellow book and all that). Since an audio CD has no need for a file system, it has no need for sectors.

DVD (and the type of video data intended for it) is of the computer age, so it uses a file system, and sectors in it's formatting standard.

 

[hyzmarca]

All optical discs use spirals, with one exception. It's more efficient. The only technology to use concentric tracks is DVD-RAM, which is both expensive and unpopular.
You can actually calculate the length of a disc's spiral, and someone has already done so.
http://www.lightbyte.com/SpiralLength.htm

Yes, a dual layer DVD does have a track length of over 15 miles.

Combo drives don't use multiple diodes and don't need them. Higher resolution lasers will reflect lower resolution data just fine, all they need is firmware to interpret it and to move the laser in the proper increments to follow the track.

 

[Verteron]

Combo drives don't use multiple diodes and don't need them. Higher resolution lasers will reflect lower resolution data just fine, all they need is firmware to interpret it and to move the laser in the proper increments to follow the track.

A quick Google seems to disagree with you.

Here is a press release from a company that makes one of these 'triple diodes'.

Here is some news about such a product, a bluray combo drive that specifically mentions how it includes red and blue diodes to read both disk formats.