Space on a cd question

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Why can an audio cd only hold about 80 minutes or so of music,
while a DVD (which looks the same) can hold hours of both audio
and video????

Just wondering.

― peepee (peepee), Saturday, 25 September 2004 13:04 (twenty-one years ago)

DVDs have multiple layers of data, whereas CDs only have one layer, basically.

― James Mitchell (James Mitchell), Saturday, 25 September 2004 13:30 (twenty-one years ago)

CDs have smaller info-bladders, basically.

― latebloomer (latebloomer), Saturday, 25 September 2004 14:11 (twenty-one years ago)

...and on DVDs the grooves are closer together.

― James Mitchell (James Mitchell), Saturday, 25 September 2004 14:13 (twenty-one years ago)

My record CD burn is 81:57!!

― Spencer Chow (spencermfi), Saturday, 25 September 2004 17:33 (twenty-one years ago)

I'm always scared to break the 80 min barrier! Does your 81:57 cd play on all players, Spencer?

― Johnny Fever (johnny fever), Saturday, 25 September 2004 22:02 (twenty-one years ago)

"...and on DVDs the grooves are closer together."

This is the "groove cramming" Nick Lowe dismisses on the back cover of "Get Happy!".

― peepee (peepee), Saturday, 25 September 2004 23:22 (twenty-one years ago)

Okay, they may LOOK the same, but they're not; both technologies use little pits burned into the disc (aluminum with the ones you get at the store, but if you're using a recordable/rewriteable CD or DVD it's really a dye with a reflective backing layer), but the size of the pits is much much smaller when it comes to DVDs. This means that DVDs can cram on a hell of a lot more data: usually CDs top out at between 700 and 800 megabytes depending on how tightly they pack the data, and a single-layer DVD tops out at 4.7 gigabytes (4700 megabytes).

If you're moving into dual-layer DVDs, it's roughly the same technology, but instead of a purely-reflective backing layer (either on the ones you buy with movies pre-loaded or the ones that you burn yourself) there's one layer that's reflective, and another layer that's only partly reflective; when the player comes to the end of one layer it refocuses the laser on the DVD player right through the first layer onto the layer behind it (think of it like refocusing your eyes through a screen door and having the screen disappear...not quite 100% the same but you get the idea). This brings the size of a single-sided DVD up to 8.5 gigs, and if you have a dual-layer dual-sided DVD you can fit up to 17 gigs altogether.

― Sean Carruthers (SeanC), Sunday, 26 September 2004 00:09 (twenty-one years ago)

By the way, the new Super Audio CDs are really dual-layer discs, one of which is the SACD layer (same general size and type as DVD, but a different audio format), and the other being the standard CD layer.

― Sean Carruthers (SeanC), Sunday, 26 September 2004 00:10 (twenty-one years ago)

I'd bust out some clip art but this is probably already getting boring.

― Sean Carruthers (SeanC), Sunday, 26 September 2004 00:10 (twenty-one years ago)

DO IT

― s1ocki (slutsky), Sunday, 26 September 2004 00:11 (twenty-one years ago)

NO I BEG YOU PLEASE NO

― Sean Carruthers (SeanC), Sunday, 26 September 2004 00:14 (twenty-one years ago)

I went to a gig on Thursday, and another tomorrow. So Sean is saying I'd halfta go to 6 and a half more gigs to fill up a whole DVD!

― peepee (peepee), Sunday, 26 September 2004 02:07 (twenty-one years ago)

Seriously, could cds use the same technology, or would the sound quality suffer that much?

Or am I not getting it at all?
(ie don't DVDs have sound quality comparable to CDs?

― peepee (peepee), Sunday, 26 September 2004 02:08 (twenty-one years ago)

i don't understand what you mean!

― s1ocki (slutsky), Sunday, 26 September 2004 02:12 (twenty-one years ago)

what the hell are you on about? CDs are not the same as DVDs. They use the same technological concepts for data storage but are completely different beasts.

― Savin All My Love 4 u (Savin 4ll my (heart) 4u), Sunday, 26 September 2004 02:14 (twenty-one years ago)

If CDs used the same technology, they'd be DVDs.

As for the sound quality:

CDs: 44.1KHz @ 16-bit resolution
DVD movie audio: 48 KHz @ 16-bit (I think)
DVD Audio format: 192 KHz @ 24-bit resolution
Super Audio CD: 2.8 MHz @ 1-bit resolution

If you encoded a DVD-capacity device with CD-format you'd get something like 6 times the length, but you probably wouldn't be able to play it back on anything.

― Sean Carruthers (SeanC), Sunday, 26 September 2004 02:14 (twenty-one years ago)

In other words, if you tried to play back your 6-hour disc in a CD player, it wouldn't be able to read the disc because it would have the tracking of a DVD, and the CD player wouldn't track.

If you tried to play it on a DVD player, it probably wouldn't be able to decode the audio stream properly from a disc of that size because it would be expecting the audio in a different format.

I'd actually be interested to see what would happen if someone burned a "CD" with 4.7 gigs of CD audio. Would the DVD burning software even start the burn onto a DVD?

― Sean Carruthers (SeanC), Sunday, 26 September 2004 02:18 (twenty-one years ago)

or would it scream in agony?

― s1ocki (slutsky), Sunday, 26 September 2004 02:23 (twenty-one years ago)

My guess is no. I just tried this with Roxio and as soon as I broke 800 megs it told me to please stop with the nonsense. Killjoy.

― Sean Carruthers (SeanC), Sunday, 26 September 2004 02:24 (twenty-one years ago)

My record CD burn is 81:57!!
I've gotten 702.1MB of data on a CD, but occasionally, these CDs will not work on older systems with older CD drives.

Also, the main reason that a CD holds only ~700MB while a DVD holds ~8GB is that the DVD's pits/lands/grooves/sectors are smaller and closer together.
(There's a technical term for it, but I'm drawing a blank as to what it is right now.)

― Lord Custos Epsilon (Lord Custos Epsilon), Sunday, 26 September 2004 02:42 (twenty-one years ago)

Here at ILX labs, we call it "miniaturization"!

― Sean Carruthers (SeanC), Sunday, 26 September 2004 02:50 (twenty-one years ago)

Sean, put down that shrink ray before you hurt somebody.

― Lord Custos Epsilon (Lord Custos Epsilon), Sunday, 26 September 2004 03:06 (twenty-one years ago)

Wow. This explains why the "CD" I ordered containing several hours of Negativland's OTE radio show won't play in any of my CD players - only my computer. Now I'm wondering if it will work in my DVD player. I was really disappointed that it wouldn't work in my CD walkman.

― Bimble (bimble), Sunday, 26 September 2004 03:53 (twenty-one years ago)

Hey it works!! This actually greatly improves the portability issue. Thanks ILM!

― Bimble (bimble), Sunday, 26 September 2004 03:57 (twenty-one years ago)

Was it an MP3 CD perhaps? New DVD players know how to decode MP3 files and play it back as if it was a long CD.

― Sean Carruthers (SeanC), Sunday, 26 September 2004 04:07 (twenty-one years ago)

Well the files are in mp3 format.

― Bimble (bimble), Sunday, 26 September 2004 04:37 (twenty-one years ago)

I heard the analogy that if a CD was the size of Tokyo city, the pits in it would be the size of a soccer ball. In a DVD, they're the size of a golf ball.

Bimble, unless you have a CD player that can play mp3s, it won't work.

― Sasha (sgh), Sunday, 26 September 2004 23:19 (twenty-one years ago)

That's the reason, yeah; MP3 files are saved as straight data, not CD audio, which means the playback equipment has to be able to decode MP3 files. New DVD players do, as does your computer. There are a number of portable players (blasters and discman-type) that also play back MP3 discs. But old-school players are expecting Redbook audio and Redbook audio alone...give 'em a different format and they think the disc is blank.

― Sean Carruthers (SeanC), Sunday, 26 September 2004 23:28 (twenty-one years ago)

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