Lasers!
Rainbows!
The eighties!
Drum Machines!
Digital!
Big hair!
This is some righteous stuff!
No kidding, the compact disc was a radical departure from how,
well really how a lot of stuff worked.
You could argue that the CD, with its vast data capacity, relatively robust nature, and
with the further developments it spurred along, changed how the world did virtually all media.
That is at least until physical media became the seemingly undesirable thing it is today,
a time when streaming services and libraries on hard drives are all the rage.
Unless of course it’s vinyl, then by all means please
build your collections.
Wow the snark’s coming early today.
Well, this is the third video in a series on digital sound.
You can find a playlist to the previous two videos, in which we covered how digital sound
works (as well as the Nyquist-Shannon sampling theorem), but for now, sit back and relax
as you feast your eyes on the silver platter that is the compact disc.
No discussion of the Compact Disc is a good discussion unless it pays homage to its predecessor,
the LaserDisc.
Now, I’ve done a series on Laserdisc if you’d like to learn more, but in brief,
this was the very first commercial optical storage format.
First released in test markets in 1978, Laserdiscs were usually 12 inch or 30 centimeter discs,
and they were an analog video format, holding up to one hour of video per side.
Competing in a world where the videocassette recorder which could record from live TV already
existed, the Laserdisc failed to capture the hearts and minds of many individuals even
though it had steller video quality.
Throughout its life, it stayed a videophile-only format in most markets.
But, the development of the Laserdisc, which was done in large part by Philips, presented
an obvious solution to the problem of digital sound storage.
See, although Laserdisc is an analog format, the signals encoded on it are as a series
of pits and lands.
OK, optical disc fundamentals time!
You might already be aware of this, but optical discs are read by shining a laser up at a
reflective disc that is covered with little pits.
These pits are roughly one quarter as deep as the wavelength of laser light that will
hit them.
I ran into some inconsistencies regarding the exact depth of the pits, because the only
source I found which specifies it has the wrong wavelength of light listed for the CD,
but all you need to know is that when the focused laser hits a pit, the increased depth
causes the reflected light to destructively interfere with the projected light, which
reduces the overall intensity of the light reflected back.
That’s actually a pretty neat part that often gets overlooked.
Endless articles talk about the fact that the pits change how the light is reflected,
but very few mention the destructive interference aspect of it.
I’ve shown this diagram before, and it perfectly demonstrates what happens.
When the laser hits a not-pit, the light gets reflected right back down to the laser, and
a prism reflects some of this into the photocell.
But when it hits a pit, the destructive interference greatly reduces the intensity of the reflected
light, so very little light hits the photocell.
This is how a laser pickup system can tell the difference between a pit and a land.
In the LaserDisc system, these pits and lands were used to encode analog video and audio
signals via a weird hybrid of pulse-width modulation and frequency modulation--
Don't ask, it’s complicated
--and the result is a usable analog video signal from a shiny plastic disc.
So, having already invented a thing that used lasers to read information on a disc, when
it came time for a digital music format to hit the scene, Philips was poised to knock
it out of the park with a new disc, this one no less lasery, but quite a bit more compact.
They named the format along the lines of their previous compact invention, the compact cassette,
and in a non-coincidence, the diameter of the CD is roughly the same as the diagonal
length of the Compact Cassette.
Ah, but let’s not forget Sony’s role.
Sony, the people who seem to only make either runaway successes or disastrous failures,
had been working on digital audio for some time.
They were the ones that developed a PCM adapter for use with U-Matic videocassette recorders
as discussed in the previous video, and in fact they were working on an digital optical
audio disc before Philips released the Laserdisc.
One of their early prototypes from 1977 was the same size as one of these honkers and
held only an hour of digital audio, though at the same exact quality of the soon-to-be
Compact Disc-Digital Audio standard.
Philips and Sony were sort of working on the same thing at the same time, though Philips
had the notable advantage of having already developed and manufactured the Laserdisc by
the time things really heated up.
Still, Sony contributed a lot.
I don’t want to get too into the weeds of who did what, so let’s just jump to 1979,
the year that Sony and Philips first decided to for realsies collaborate on the project.
By this time, optical disc fundamentals had been established.
An optical disc seemed the perfect format for digital data, because you could just easily
call a pit a one and a land a zero.
But that’s not how it works!
No, the pits and lands are important, but it is the change between a pit and a land
that encodes a 1, and no change encodes a zero.
This is time-based, so a four-bit span of time which is comprised of either all pits
or all lands will produce 4 zero bits.
Start the sequence with a change and then continue as is, and you get 1000.
No change, change, no change, no change, and you get 0100.
And you get the idea.
This is called Non-Return-To-Zero inverted encoding.
Now, this has limitations because a long span of zeros requires an outside clock to keep
track of how many “dead” spaces have passed.
The longer you have between ones, or changes, the more ambiguous the number of zeroes becomes.
I know what you’re thinking.
Was that six zeros or only five?
Well, to tell you the truth in all this excitement I kinda lost track myself.
Which is why you need an outside timing source keeping track of how many shots, I mean bits,
have passed.
This isn’t somewhere you should just feel lucky.
But that’s not a problem.
What is a problem is the nature of everything.
How likely do you think it is that a pressed CD is absolutely perfect, has no scratches
at all, and that the CD player reading the disc will perfectly, without any errors, reproduce
the correct sequence of ones and zeros?
If you answered “not likely at all” you’re a winner!
To help make the data less susceptible to stupid little things, two robust error-fighting
mechanisms are built into the data stream.
The Red-Book standard, the first of the Rainbow Books that defines the physical, digital,
and other various standards surrounding each type of Compact Disc, not only specifies the
sample rate and bit depth of the Compact Disc Digital Audio format, which as previously
discussed is 44.1 kilohertz, 16 bit, but it also defines how the data is encoded on the disc.
Now this is rather technical but I think interesting, so bear with me.
So, each audio sample is a signed, which means it can be positive or negative,
16 bit two’s complement,
which is a mathematical operation on binary numbers that we’re just gonna not worry about right now because it hurts my brain,
integer.
I mean, we knew we’re dealing with 16 bit audio so each sample is gonna be 16 bits.
Now, on the disc, 12 samples, 6 each for the left and right channels, are stored together
in a frame of 192 bits (that’s 24 bytes, for those playing along at home).
These frames are then encoded using an error correction scheme called CIRC, which stands
for Cross-interleaved Reed-Solomon coding.
In addition to adding one data parity byte to every three raw data bytes, the effect
of CIRC is basically to spread the data out over a longer distance.
That’s where the interleaving part of the name comes from.
By jumbling up multiple frames and adding parity bytes, CIRC can correct up to 3,500
bits of error-filled or even missing data, and can compensate for up to 12,000 bits by
masking errors via interpolation.
This translates to up to 2.4mm gaps in data being completely corrected for, and up to
an 8.5mm scrambling of data, either through a scratch or some other damage, being reasonably
interpolated and masked.
The end result is that the error is either completely corrected,
or it’s fudged well enough that you won’t hear it.
Now we’re gonna go on a minor tangent here.
I apologize.
I’ve linked in the description the source material from the Wikipedia article on CIRC.
This source material is from a book written by Kees Schouhamer Immink, one of the central
engineers involved in the development of the Compact Disc.
He even won an Emmy for his work on coding technology for optical recording formats.
I’m bringing this up because for every person that tells you Wikipedia is a worthless starting
point for research, I want you to show them the references section at the bottom of articles.
Yes, you would be very unwise to cite “Wikipedia” in any research, but let me tell you,
you may be amazed at the quality of the source material, and you’d be a fool to not at
least look at these references when doing any preliminary research of your own.
Rant over.
In addition to CIRC, the use of eight-to-fourteen modulation limits the total number of possible
combinations of ones and zeros.
EFM translates each 8 bit word into a 14 bit word.
These 14 bit words are translated back into their original 8 bit words using a lookup table.
The point of doing this is to reduce the possibility of errors.
The use of EFM makes it such that binary ones are always separated by at least two zeros,
and a maximum of 10 zeros.
This means that every pit and land is at least 3 clock cycles long.
It also means that if the CD player reads one, zero, one, it knows it must have made
an error because that’s not a possible sequence.
Plus, with a maximum of 10 zeroes, it reduces the accuracy required in the clock of the
CD player for worst-case clock recovery, as there must be a one after every eleventh bit,
and this will in effect synchronize and restart the zero-counting clock.
Keep in mind that with simple 8 bit encoding, in theory you could have 14 zeros in a row.
When you realize all of the processing that has to be done by the CD player before it
can even extract the data it needs to send to the DAC and play the dang music, you gain
a newfound respect for its achievements of the time.
Granted, using a look-up table and performing some basic arithmetic is easy for a computer,
but the fact that this was being done at a consumer hardware level in 1982, with a data
throughput faster than what any contemporary microcomputer would reasonably be expected
to process, impresses me.
To help provide logical access to the contents of the disc, the disc’s data stream is divided
into three parts.
The lead-in, the program area, and lead-out.
The lead-in contains the disc’s Table of Contents, which is basically an announcement
to the player of how long it is, how many tracks it contains, and what the timecode
is for each of the tracks.
The CD player can, through reading the table of contents, determine where each track is
for accessing it nearly instantly.
A CD can have up to 99 tracks, which themselves can be divided further into 100 indexes, though
this feature was rarely ever used and few CD players could access the index information.
One thing that computers have made a little confusing about the Compact Disc-Digital Audio
standard is that audio CDs do not contain files.
If you pop one into a PC, it’s gonna show you each track as its own object.
But the computer is interpreting that for you.
In raw form, the data on a CD is just one continuous stream, and the table-of-contents
in the lead-in simply defines where along the stream each track is.
The concept of using a CD for computer data storage
just wasn’t really in the cards yet.
I mean, a single audio track would easily fill up entire hard drives of the time, so
the idea of creating files to contain the audio data was just absurd.
The later CD-ROM, following the Yellow-book standard, would allow for file structures
on discs like a computer is used to accessing.
But we’ll get to that later.
OK, so let’s look a little bit closer at the disc itself.
First, I want you to see if you can spot a big difference between a Laserdisc and a CD.
Looking at the edge of a Laserdisc, can you see that seam in the middle?
Laserdiscs are obviously double sided, so that seam is the join between the two halves
of the disc, and the data is sandwiched between two sides.
But if you look at the CD, there is no seam.
One of the weirder things about the CD is that the data layer is actually at the top
of the disc, just beneath the label.
I’ve linked to a How It’s Made episode on the Compact Disc that does a pretty good
job of demonstrating how mass-produced discs are made.
Their manufacture is surprisingly similar to that of conventional vinyl records, as
they are stamped (or rather molded) from a master disc.
The video does a great job of showing this process, but it skips over the actual etching
of the data onto the glass master.
In short, the master is covered in a solution that will either evaporate or harden when
exposed to laser light, depending on the mastering process used.
By using the laser to etch pits into the coating, which is then hardened with a development
process, the glass master is now covered in bumps that will represent the lands in the
molded discs.
The master disc is metallized to harden these bumps, and now polycarbonate discs are molded
from it.
And that’s something that I find really neat about the Compact Disc.
The data in stamped discs is actually in the plastic itself.
When the disc comes out of the mold, it’s completely transparent.
But it has all of the data on its surface.
To make the disc readable, it’s then covered in a thin film of aluminum via a vapor metallization
process, and now the disc could theoretically be read by a CD player.
But remember, those pits and lands are on the top of the disc, exposed.
If they could be touched, the data would be destroyed.
So before the disc can be handled, it needs a thin coat of varnish to be spread along
the top, and now the data is protected from damage.
Putting the data layer on the top of the disc meant that the disc was even more tolerant
of scratches, as these scratches are out of focus to the laser reading the disc from below.
This was a pretty smart move.
The laser would read through almost the entire 1.2mm thickness of the CD.
As a consequence, double-sided CDs were never a legitimate thing.
But there were some weird shenanigans tried with making double sided hybrid discs.
One of these I have somewhere in my collection but I couldn’t find it because I’m disorganized,
(sorry) is the DualDisc.
“Weird Al” Yankovic’s twelfth studio album, Straight Outta Lynwood, was released
in 2006 as a DualDisc, with one side being a fully compliant DVD containing music videos
and other goodies, and the other side being a…
almost CD.
DualDiscs were .3mm thicker than a standard CD or DVD, comprising a total thickness of
1.5mm rather than the standard 1.2.
The CD layer was placed .9 mm into the disc, with the DVD layer .6 into the disc from the
other side, which is correct for a DVD.
But it’s off by at least .2mm according to the Red Book standard.
Because of this, DualDiscs did not contain the Compact Disc Digital Audio logo on them
because they technically weren’t CDs.
They featured language saying that they were intended to work in standard CD players, and
almost certainly they would given the tolerances a standard CD player is designed to deal with,
but they could not actually be officially referred to as a Compact Disc.
So there’s some fun trivia for you.
AAAND, that’s where we’ll hit pause.
There’s a lot of neat stuff to uncover about the CD, even though it’s teetering on the
edge of obsolete and insignificant.
OK, it is pretty much obsolete.
This dime-sized SD card holds more data than this entire column of CDs.
That’s kinda sad, but also amazing.
In my next video on this subject, we’ll explore more of the innards of the compact
disc, such as the optical pickups mechanisms and the rather major difference between how
Philips designed its laser pickup compared to most others.
If you take a look at the disc tray on this Magnavox unit, you might get a hint.
Of course we’ll also touch on the other various uses of the CD, such as CD-ROM, CD-R
and CD-RW, and other stuff.
And then, we’ll relish in the fact that everything is on the Internet now.
Thanks for watching, I hope you enjoyed the video!
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Don’t forget there’s a Technology Connections subreddit now, so if you’re a redditer you
can go over there and watch me not know how to use it, and I’m getting better a Twitter
so if that’s something you do, you can follow me @TechConnectify.
Anyway, that’s it for now.
I’ll see you next time!
Cue obnoxious music!
♫ uncomfortably smooth jazz ♫
This line read better before…
With its vast data capacity, relatively lobrust…
this line.
This line is going to kill me!
When it came time for digital music formats to hit the scene, Philips was poised to knock
it out of the par--poised?
(stares into camera doubting himself)
No, that’s what I wrote.
That’s what I wrote!
I’m regretting it now…
The Red Book Standard, the first of the rainbow books that define the physical, dizhidal,
and other… dizhidal dizidal diblelr….
(clears throat)
...reduces the accuracy required in the clock of the CD player for weush…
[exasperated sigh]
The master disc is metAL….
Metalized.