If you look at a blank CD-R, odds are you’ll find these two numbers.
80 minutes of audio, and 700 megabytes of data.
You may be surprised to learn that these two numbers don’t match.
See, let’s take the 80 minute figure.
We know that the Compact Disc Digital Audio standard consists of 2 16 bit streams
(2 because it’s stereo audio)
each consisting of 44,100 samples per second.
Let’s make it a bit easier and say that it’s one stream of 8 bit words or bytes,
so by doubling 44,100 twice
(once for stereo, once again to turn 16 bits into 8),
we find that it’s 176,400 bytes per second.
Multiply that figure by 60 for sixty seconds per minute,
and again by 80 for our eighty minute runtime, and you get…
846,720,000 bytes.
It would seem that an audio CD holds over 146 more megabytes than a data CD.
What gives?
We interrupt this program for a marginally important message.
In the computing world there is an unfortunate mixing of the terms megabyte and mebibyte.
You’ll almost never hear someone say mebibyte, because it’s a silly word, but the 700 on
this disc is actually referencing mebibytes.
What’s the difference?
A Megabyte is 1,000,000 bytes, based on powers of 10.
A mebibyte is based on powers of 2, so it is actually 2 to the 20th bytes.
So, a kibibyte is 1,024 bytes (rather than 1,000) and a mebibyte is 1,024 kibibytes.
This annoying debacle has been causing problems forever, for example hard drive manufacturers
quote sizes in gigabytes or terabytes, when your PC works in terms of gibibytes and tebibytes.
So this 2 terabyte portable drive only appears as 1.81 terabytes on my PC.
But my PC is actually misrepresenting this figure as terabytes when it is in fact tebibytes.
But CDs and DVDs are quoted in mebibytes and gibibytes, so even though this label shows
MB instead of the correct M lowercase i B, my computer does the same mislabeling, and
since they’re both wrong, they agree.
Anyway, all of this is to say, I’ll be using the word Megabyte, even though sometimes it’s
actually a mebibyte.
Please send complaints to:
It would seem that an audio CD holds over 146 more megabytes than a data CD.
What gives?
Well it’s actually more of a “what takes away” as the CD-ROM standard sacrifices
those bytes for more precise error correction.
As discussed in this channel’s first video on the compact disc, the CIRC error correction
used in the Compact Disc Digital Audio standard can perfectly correct errors up to 3,500 bits long.
But, it can also mask errors up to 12,000 bits long through interpolation.
That means that it’s essentially guessing what the audio samples should be for errors
between 3,500 bits and 12,000 bits.
Now, for audio samples, that’s fine.
12,000 bits is less than one one hundredth of a second of audio, so even though the error
correction might be fudging things, you’ll almost certainly not notice it.
But for data, that’s not gonna do it.
Data cannot be fudged without serious consequences.
In order for the compact disc to store data files reliably, it would either need to sacrifice
durability or come up with a new error correction scheme.
And it did neither.
But also yes.
CD-ROMs
(or would that be CDs-ROM?)
use the same CIRC error correction and basic data
structure of the CD-Audio disc, including the frame structure, subcode, and all that.
But more error correction was added within the frame (which we’ve now formally decided
to call a sector).
Of the 2,352 bytes available in a sector on an audio CD, 304 get sacrificed, mostly for
error correction, but also for some basic signalling, such as synchronization and distinguishing
between mode 1 and mode 2.
Combined with an additional layer of CIRC error correction, the CD-ROM standard maintained
the same durability with zero tolerance error correction.
We’ll get to Mode 2 later, but nearly all CD-Data applications use Mode 1 for its complete
error correction.
Now that the compact disc was being used for more than a simple linear stream of audio,
a true file system needed to be standardized.
Initially things were rather ad-hoc, but a group of 12 computer hardware manufacturers
met at what was then the High Sierra Hotel and Casino in Lake Tahoe, CA, where they settled
on a standard called the High Sierra Format.
Eventually this would become ISO 9660, the standard used to this day, however it has
been extended and improved over the years.
Now that the CD-ROM was officially a thing, whole new multimedia experiences could be created.
With the ability to store images, program files, sounds and even video, the possibilities
with CD-ROM were seemingly limitless.
No longer bound by the 1.44 megabytes of a 3.5 inch floppy, elaborate games with full-on
soundtracks, thousands of color images, and more complicated structure could be produced.
My greatest nostalgia factor comes from mid-nineties educational games, like the Magic School Bus
games or my personal favorite, JumpStart 3rd grade.
Seriously, this was a wicked awesome game and it’s holding up pretty well I’d say.
[In another day, the world will see the dawning of Polly Planet.]
*buzz sound*
[Whoops!]
[It seems there’s someone at the door.]
[Let’s take a little peek!]
Anyway, of course home computers weren’t the only beneficiaries of this sort of vast
data store.
Video game consoles soon appeared which could take advantage of it, in fact one of the earliest
was the Philips CD-i system.
It technically wasn’t using CD-ROMs, though, as CD-i discs were in fact their very own
category, published in the Green Book.
But unless you’re either very young or were living under a rock through the 1990’s and
early 2000’s, I’m sure you already knew what CD-ROMs could do.
But the OM, in that CD-ROM, man that’s a bummer.
Sure, having 700 megabytes at your disposal is fantastic, but no one has a CD manufacturing
and duplication facility just at home.
But wait, they do!
The development of the CD-R, for Compact Disc Recordable, meant that no longer would you
be forced to write up a contract with a commercial CD pressing facility for at least 10,000 discs
just to make a mixtape, instead all you needed was a $35,000 machine and a computer!
It would take until the mid-to-late 90’s for CD burners to come down in price, and
even then there were annoyances, but now we're really talking.
A CD-R, which incidentally is a pirate’s favorite kind of CD, is constructed much the
same as a normal CD, but with two important changes.
First, the pits and lands molded into the polycarbonate layer are replaced with a continuous
spiral pregroove.
This spiral will guide the laser of a drive writing to a disc, and it also contains the
ATIP which tells the drive the properties of the disc, such as its maximum write speed.
Right on top of the pregroove is a thin layer of an organic dye.
That’s why CD-Rs are sometimes a strange color, or even just a little bit off from
the normal silver.
Then on top of the dye, a partially transparent thin layer of metal, usually aluminum, is placed.
This allows the laser light to be reflected back while still being influenced by the dye.
To burn data on the disc, the laser switches from it’s everyday gentle glow
to an all-out LIGHT CANNON
of FIRE LASER,
and the high intensity heats up the dye so much that its optical
properties change.
That’s right, CD burners are literally burning the equivalent of pits onto the disc.
Once it’s been written, almost any CD drive, going back to the earliest CD players, can
read these discs.
The parts of the dye that were made into a “pit” by the writing laser will dim the
reading laser’s reflection, which is much the same as how real pits cause destructive
interference.
So long as the CD reader can tell the difference between a burned spot and an untouched spot,
it will read the data just fine.
Being able to store data files onto a compact disc at home meant that just one of these
guys could replace hundreds of floppy disks.
But now we had a new problem.
Remember how in a normal CD, there are three parts?
The Lead-in, program area, and lead-out?
Well, if we stuck to this limitation for the CD-R, then if you were to write as little
as a Word document on a blank CD,
you’ll have written the lead-out,
and now the disc is done.
The Orange Book introduced the standard of multi-session writing.
Now, after the lead-out of the disc, another lead-in can be made.
Each time a new session is made, the lead-in is updated to include the location of all
the files.
On a multi-session disc, the drive will look for all of the lead-ins, and once it finds
the last one, it basically just ignores the rest.
This is also how a file can be “deleted” from a CD-R, as the latest lead-in may simply
not include it in the table of contents.
But the data is still physically there, and there are ways of getting to it.
Of course, the central limitation to the CD-R is that is a write-once, read many format.
In fact, its original name was going to be CD-Write Once, which I really wish had been
stuck with because we could have called them CD-WOs!
Woah!
But, we wouldn’t be stuck with write-only for long.
Interestingly, the Orange Book detailed a rewritable CD based on magneto-optical technology
in 1990 (the technology behind the MiniDisc).
This never made it to commercial production, but the CD-MO would have been the perfect
companion to the CD-WO.
Instead, we got the CD-RW in 1997.
This is functionally identical to a CD-R, but the organic dye layer is replaced with
a Silver-Indium-Antimony-Tellurium alloy, also called by at least one person
AgInSbTe.
This alloy has a unique property which alters its reflectivity based on phase changes.
In an unburned disc, the alloy is in a polycrystalline structure.
But when the laser heats it to somewhere in the neighborhood of 500-700 degrees C, it
melts it.
And when it solidifies, it no longer has the same crystalline structure, and thus it reflects
light about 15-25% less intensely than the crystalline parts.
So, spots that are melted by the laser turn into sorta pits, and spots that aren’t remain
sorta lands.
What allows the disc to be re-written is that when the alloy is heated to around 200 degrees C,
it doesn’t melt, but any parts that have lost their polycrystalline structure will
actually reform.
This effectively erases all of the burned pits, and allows the disc to be written again.
CD-RWs have several severe limitations, though.
Unlike a CD-R, these discs generally cannot be read in standard drives.
Because the change in reflectivity between a pit and land is so slight, a CD reader needs
to be much more sensitive to pick up the data.
After the introduction of the CD-RW, many standard CD readers were made more sensitive
to accommodate them, sometimes branded as Multi-Read, but it certainly wasn’t universal.
Another big problem of the CD-RW was that the discs weren’t always backwards compatible.
Because of that phase-changing alloy, there is actually a minimum speed to write the disc.
This basic 1-4X disc will work in pretty much any drive, so long as you can force it to
burn slowly, but later, higher-speed discs might need a minimum speed of 8x or more,
so if you only had a 4x drive, you’re SOL.
And before we put the CD to rest, let’s look at some of the other stuff that those
crafty engineers at Sony and Philips jammed into here.
Remember the subcode channels R through W?
Eventually these would get used.
CD+G, for CD Graphics, was used in karaoke machines.
The first CD+G discs were released in 1985.
This was further refined into the CD+EG, standing for Extended Graphics.
In what seems backwards, it would take 11 years for Sony to introduce the CD Text format,
in 1996.
Also using the R through W subcode channels, this was essentially a way to add metadata
such as track titles, artists, album, and all that jazz.
Oddly, I can’t find references to it being used for lyrics, which seems a wasted opportunity
as there definitely is room.
Remember the Mode 2 in CD-ROM I talked about earlier?
That’s right, I’m putting it in the same video this time.
CD-ROM Mode 2 sacrificed some of the error correction of mode 1 for a larger data capacity,
about 800 megabytes.
You wouldn’t want a computer program or documents stored in Mode 2, but for things
like video files or large collections of images where a small error will simply result in
a glitch or artifact, it was OK to use.
Video CDs, defined in the White book, used Mode 2, and now that I know that, VCDs make
a lot more sense to me.
The video compression of VCD is ridiculous, but knowing that it’s actually playing with
800 megabytes makes it seem a little less crazy.
And there were other odds and ends.
The delightful
beige
book defined a standard for photo CDs in 1992.
The standard was a little ahead of its time though, as in 1992 CD-ROM drives really weren’t
that common yet, and it also stored photos in a proprietary format.
Kodak developed the Photo CD in a time when digital cameras weren’t really a thing at
all for consumers, so in a sense it was a very clever product.
Get your film developed and receive a disc with very high-resolution scans for future use.
But by the time CD-ROM drives became widespread, digital cameras were starting to appear.
And, once CD burners came down in price, well you could just burn JPEGs onto a CD (which,
you might recall, many DVD players can show as a slideshow on your TV).
You look back at Kodak’s history and you just can’t help but feel sorry for them.
And to round out the rainbow, the Blue Book of 1995 defined the Enhanced CD, which functioned
both as an Audio CD and a CD-ROM, with two sessions on a stamped disc.
This was kinda neat, as a CD player would treat it as a normal audio CD, but you could
store photos or interactive material in the data portion which a computer could access.
The Scarlet book of 1999 defined the standards of the Super Audio CD, a high-res audio format,
which is pretty much just a DVD but with audio only.
It also had significantly different audio encoding, which I won’t go into right now,
and multi-channel support.
And the last of the rainbow books was the Purple Book of 2000, which if you thought
the Super Audio CD was obscure, well you haven’t read the Purple Book.
All this was was a reduction in pit size and narrower spacing to double the data capacity
of a CD.
And, it was only for use as data storage, not audio.
But, when DVDs already existed, what was the point?
That’s pretty much what everyone asked, and it was practically dead on arrival.
But an interesting thing about the purple book.
In a way, it had been implemented (although much more subtly) long before it was published.
See, the original Compact Disc standard was 74 minutes or 650 megabytes of data.
But, the vast majority of CD-Rs and CD-RWs you can buy are 80 minutes, 700 megabytes.
It was discovered not long into the life of the CD that the tolerances were generous enough
to space the tracks just a little bit tighter together without violating the Red Book specs.
The purple book just did that to a much more radical extent, but many years too late.
And finally, one thing you should know about pretty much all writable optical media.
The dyes and alloys aren’t terribly stable.
Many of the earliest CD-Rs are failing, as the dye is degrading or the disc is delaminating.
If you have any precious data stored on optical media, you might want to back that up pretty soon.
Thanks for watching, I hope you enjoyed the video!
This just about wraps up the saga on the Compact Disc.
I think I hit all the bases, but I certainly didn’t go into detail on everything.
If there’s something more specific you’d like me to look into, let me know in the comments
and I’ll see what I can do!
As always, thank you to everyone who supports this channel on Patreon.
I say this every time, but seriously.
Thank you.
Your support makes this channel possible, and you are awesome for it.
If you would like to join the Patreon crew to support the channel as well as get perks
like early video access, some occasional behind the scenes stuff, and the inside scoop on
the latest projects, please check out my Patreon page.
Thanks for your consideration, and I’ll see you next time!
♪ exceptionally smooth jazz ♪
So long as the CD reader can tell the difference between a burned spot and an untouched spot,
it will read the data just fine.
I need to record that line because I screwed it up.
The development of the CD-R, for Compact Disc Recordable, meant that no longer… wait…
…
No that was right.
[clears throat] Let’s try again!
...was replaced with a silver, indium, antinomy…
antiMOny, right?
It’s Antimony?
Yeah.
Antimony Cricket!
And it also stored photos in a proprietary format Kodak developed….
AHH!
It technically wasn’t using CD-ROMs, though, as CD-i discs were in fact their very own
category, published…
…
...in the green book.