[ The comments being read snarkily ]
...the rest of that comment was really insightful,
thanks for sharing.
OK, could I have been a little more clear?
“I’ll show you Sony’s system later on, but if you…”
Yes.
Yes I could have.
But, this is in fact “later on”, so perhaps maybe we shouldn’t jump to conclusions so quickly.
Tell you what, I’ll work on being a little more precise in my choice of words.
And perhaps you, dear viewer, could refrain from jumping to conclusions quite so quickly.
If we put our minds to it, together, we can enhance our understanding through complex
thought both on the part of the speaker, and on the part of the listener.
♫ And that’s how the world gets better ♫
Alright, this video is coming at you in three parts.
As we discussed in the last video--
NO WAIT, wait wait wait.
The ding was way too loud, let’s tone it down.
[ Ding sound from Jeopardy! ]
Much better.
So as we discussed in the last video, we’re gonna take a look at some of the waveforms
coming from the components of this Magnavox (which is really a rebadged Philips because
Philips couldn’t use that name in the US due to it’s passing similarity to Philco,
I guess, but that’s a stretch) CD player.
That will be part one.
In part 2, we’ll take a look at a Sony laser pickup and discuss the differences.
Plus I will tear this one apart to expose the laser diode and photodiodes.
And in Part 3, we’ll discuss why Sony’s pickup solution would become the default standard,
used to this day in optical drives.
We’ll begin with Part 1.
And Part 1 will be quite a bit different from most other videos.
That’s because much of it is unscripted.
Off the cuff.
Just gonna wing it.
Yeah.
But first, a few corrections.
As usual, the Internet has provided answers that I did not find, and has revealed some
mistakes and false assumption from the last video.
Of course.
But that’s OK, we all make mistakes, and now I’m gonna tell you what we learned thanks
to the diligent research of more knowledgeable people than me.
Remember it’s OK to not know everything.
And it’s OK to ask for advice from professionals.
♫ And that’s h--♫
Let me reveal the most dumb mistake, and it was right on screen.
The best kind of mistake.
I had been assuming that pin 10 was the output signal from the focus chip, and that it would
be a binary stream from the pits and lands.
Boy was that wrong on many counts.
First, it’s not pin 10 that does that, it’s pin 3.
And it’s labeled right here on the datasheet.
Pin 3 is also labeled to decoder, which I missed, and most importantly it’s a sum
of the outputs from all 4 photodiodes as illustrated right here.
Yes pin 10 also gets the sum of the 4 diodes, but unlike pin 10, pin three has an amplifier,
equalizer, and another amplifier to boost the output.
Hm hmmm.
I want to give a big thanks and shoutout to Zim0256 who not only created the helpful diagram
that I used on a few occasions but who also gave more complete info on what is going on.
The Motorola processor is a custom chip based around an HC08 microcontroller, and it is
in fact the decoder that is doing most of the work.
I couldn’t find its datasheet because the P on the end was limiting my search results.
And to think, we just learned about minding our p’s and q’s.
Anyway, in fact the decoder is handling all the raw data; undoing the eight-to-fourteen
modulation, isolating the subcode, and all that.
It even digitizes the output from pin 3, which at this point is still analog which we’ll
see in a moment.
It sends the extracted subcode to the processor so it knows how to handle the disc and respond
to user requests.
Really, the processor is just managing the living and breathing tasks of the machine,
such as the user interface, display, and other goodies like popping the disc tray in and
out and getting the whole process going.
Speaking of the disc tray, a few people had asked how the machine can tell that the disc
tray has been pushed in manually as opposed to someone hitting the open/close button.
Some other people noticed this strange button switch near the decoder.
These two groups of people are discussing the same thing.
To the right of the disc tray is a little series of levers that rest on top of this
button when it’s assembled.
Little nubs on the bottom of the tray will cause this to push down on the button in either
the open or closed position.
This not only tells the processor that the tray has reached the end of its travel in
either direction, but it also means that it can detect if the tray has been pushed in,
as the button will be released (and thus the switch opened) as soon as the tray is moved
out of its open position.
OK, it’s time for the script to be put to the side.
I’ve got a sort of odd setup here, because I need to see both the oscilloscope screen
and what I’m poking on the board, so we’re gonna do some picture-in-picture editing magic
and throw the scope screen up high.
Excellent!
And you know what, maybe we’ll throw another camera up here so you can see what I’m doing.
Even better.
All the cameras!
All the tripods!
Well this is… awkward.
It’ll have to do.
As promised, we’re gonna poke around on some of the things here!
Now I brought my laptop here so I can actually go and look and see what I’m looking at,
and you’ll notice these, uh, wires that I’ve soldered onto chips underneath.
I’ll bring up the image of that.
Part of the issue with this player is that it has to be--the ribbon cable between the
drive and the board is very very short, and there’s pretty much no way to operate this
unless it’s assembled.
I really don’t know how people would service these things, ‘cause it, ya know… a lot
of the… a lot of the stuff that I wanna see is underneath the CD reader like the,
uh, the tracking and focus chips are basically directly under here.
That’s not helpful.
But we can see what else we can take a look at.
I did find, just from poking--at one point I found the raw datastream.
I think it was one of these guys.
Unfortun… what is happening here?
Oh.
Well this is kind of interesting, what’s… uh I wonder what’s on this line, let’s
see if I can figure it out.
Notice that we’re on a really long time scale.
I think this is the subcode.
If I can hopefully… get the... trigger…
I’m...eurgh…
I’m really...have no experience with a scope.
So, just.
Just so ya know.
Let’s try…
Oops that’s what I meant.
That’s what I meant!
The other way…
Oh!
It’s probably you, OK.
Yep, OK.
Ha ha!
I found it!
So this here is the waveform that’s coming from the amplifier chip from the au, from
uh the TDA8808T augh that one, that one.
So this, this is the amplified output of the… oops, of the pits and lands.
So this is the raw analog signal coming from the disc.
And if we look a little closer… you can see if I touch the disc… see how that’s…
Oh. Well I’ve killed it.
Great.
Great job!
So if I kinda just touch it you can see the--it slows down a bit.
Let me zoom out a bit.
Oop--zoom out a bit.
See that?
It just completely--it gave up there.
That click sound you heard I believe is the laser kicking up all the way.
But yeah so this--this isn’t the raw output from the photodiodes but this is the amplified
output coming from pin 3, going to the decoder.
The output from the photodiodes is very very small, this is… if we take a look and compare
the voltage level there’s almost nothing there.
And it’s so noisy I can’t see anything.
These all should be the same.
There’s nothing there.
Which is a shame because I really wanted to show you that!
Oh well.
Wanted to show you some interesting stuff, and I think that we saw some interesting things
but not as many as I was hoping.
Well, with that disappointing segment out of the way, let’s move on to the part that
I didn’t forget about.
To see how Sony handled the optical system differently, we need to get our hands on a
Sony CD player, hopefully of similar vintage to this Philips one.
Oh perfect, a Sony CD changer from 1992!
Now it doesn’t really matter that it’s a changer, there’s still a standard CD mechanism
in here, it’s just on a pivot and will happily pop up and fall down to greet or say farewell
to the disc it plays.
It works in tandem with the disc tray, which rather than just being an inny-outty operation,
it’s an inny-outty with rotational flair.
Now I embarked on the frightening task of getting this mechanism out of this player.
Armed only with an ordinary screwdriver, I went hamfisted and started unscrewing stuff,
before I realized I hadn’t filmed the close up segments of the laser tracking a disc.
After undoing what I had begun, I began once more.
The screws are the enemy!
Be gone!
Or, be twisted counterclockwise sufficiently until such time that they can be gone!
Having gained unrestricted access to our prize, I loosened the four screws holding it in place.
But it wouldn’t budge.
Yet more screws needed liberation, until finally I had successfully removed the crown jewel
from the temple of forbidden technology.
And, uh, here it is.
One of the big differences you’ll see between this and the Philips mechanism is everything
is much more integrated.
In fact this one chip here, if the datasheet I found is at all correct (link below) has
32 k of RAM built in, does the EFM demodulation as well as the CIRC error correction, handles
the focus, tracking, and spindle motor (using this chip as a driver to handle the power),
extracts the subcode, provides a data output for the DAC, and can even do such exotic things
as double-speed playback and variable pitch playback (assuming the player it’s inside
has those functions enabled).
Did I mention it’s bilingual?
It’s interesting how just 4 years time managed to integrate functions that are spread out
via multiple chips on this Philips machine into just this one chip on the board of the
laser pickup.
But that makes a lot of sense.
See, this Philips board only works with one mechanism.
Because its driver components are part of the main board design, anything but a Philips
CDM4 pickup won’t work without at least some modification.
But for the Sony machine, this ribbon cable is essentially just proving a power, ground,
a communication bus to the CPU, and a datastream for the DAC.
This component can be completely redesigned, and so long as it accepts the same inputs
and produce the same outputs, the main board really won’t care.
This allows Sony to make production improvements and even radical changes to this mechanism
and how it works without disturbing the main board.
You can see how Sony played a modular approach via the markings on the board.
This one board was used in many different players, with different components marked
“FOR X player only”.
By my count it was used in at least 5 players.
Really, Sony’s markings are just a lot more helpful.
Wondering which one of these is the DAC?
Hey look, it’s labeled right here.
And you’ll find a PULSE chip on the bottom.
I’m pretty sure these are 1-bit DACs, based on some vague research.
I’m sure someone will correct me if I’m wrong, and I’m counting on it.
And the fact that they’ve labeled these jumper connections on the top of the board
for what they are sure is helpful.
That would’ve been nice,
PHILIPS.
Anyway, back to the laser because that’s the real difference.
First, gone is the swing-arm.
Instead, the laser is mounted on a sled, which moves linearly with the help of this rack
and pinion drive.
Now, you might suspect that this drive arrangement does not have the precision to track the disc.
The floating arm of the Philips system means it can move subtly and precisely to track
an off-center disc.
But this brutal, inelegant plastic gear train can’t possibly be used for the tracking.
And it isn’t.
Just like the Philips system, this lens is floating and can move up and down.
But unlike the philips system, this lens can also move left and right.
With the help of a diode I have laying around, you can see that the laser can move quite
a bit to either side.
Let’s take off this plastic shroud to get a better look.
Ah this one is perfect for showing this.
Yes.
OK, I’m sure you can see these rectangular coils surrounding a vertical structure.
These coils will push the lens upward if a voltage is passed through them.
But if you look very carefully, you’ll also see a pair of circular coils on the outer edges.
When a voltage is applied to these, depending on the polarity it will attract itself to
these handy magnets placed on the sides, or it will push itself away.
This causes the lens to pivot either left or right, depending on which way you slice it.
The best part of this setup is that the lens is completely independent of the sled that
it’s riding on.
If the sled moves a bit, it doesn’t care--it will just react to that movement as though
the track of pits it’s... tracking has moved slightly.
That means that the sled only has to move in coarse steps, and the lens will take care
of the rest.
But that’s just one significant difference.
The other is in the photodiode arrangement.
You may have seen the term
THREE-BEAM LASER TRACKING
before. Or something like that.
In these systems, rather than using only four photodiodes like in the Philips system, we
use six.
In the middle are four cells arranged…
actually exactly like the Philips system.
These are used in an identical fashion for focus--an elliptical reflection indicates
an out-of-focus beam, and its orientation indicates if focus is near or far.
But to correct tracking errors, two additional diodes to either side are monitoring for an
off-center beam.
In these systems, a diffraction grating splits the laser beam into three.
In reality it splits it into infinite beams of increasingly weak intensity as they deviate
from the center but we only care about the center beam and the two immediately surrounding it.
Now, when tracked correctly, the outside beams shouldn’t land on the stream of pits.
They should instead land just outside of them, which will thus cause them to reflect the
featureless boundaries.
The reason this works to track the stream is that if the track deviates, suddenly one
of the tracking diodes will start seeing the datastream.
That’s not supposed to happen, so the player will react by nudging the lens in the direction
of the activated tracking diode.
This system is certainly more complex on the hardware side, but given how simple the trigger
is--no need to compare ratios, simply see if a single thing is happening--it’s probably
a lot easier to implement.
However, this make me wonder if this system is fundamentally worse at tracking discs.
See, I’ve heard anecdotal evidence that the original Philips single-beam tracking
is superior at tracking scratched or damaged discs.
And with the knowledge of how the three-beam tracking works, that kind of makes sense.
Imagine a scratch appears just next to the datastream.
If that scratch is bad enough, it might bend one of the tracking beams towards the raw
datastream, causing the lens to deflect for a defect that isn’t really there.
With the Philips system, it wouldn’t see this scratch because it only has the one scanning beam.
It would ignore it and thus be unaffected.
Now of course, these conventional systems aren’t remarkably inferior.
They can tolerate scratches pretty well, too.
But it makes me wonder if there is truth to the anecdote that the swing-arm servo, Philips
single beam tracking whatever is indeed superior.
One last thing before we answer why this became the standard.
I’m not afraid to tear this apart and find what inside.
So first, you can see the actual laser diode here as opposed to the Philips machine where
its housed in this plastic.
This, like in the Philips setup, projects the light sideways where it hits a prism,
get shot up at the disc surface (which the floating lens helps to keep tracked and focused)
and it gets reflected back, goes straight through the prism, and lands on this little
chip here.
Now this is interesting, because on this machine we can actually see the backside of the photodiode
array, unlike on the Philips machine where we could only see the board.
If I keep going to remove the lens and get everything apart, you can see the prism in here.
Take a look, it does a great job of reflecting light at a 90 degree angle.
And now, let’s see what we can see of the photodiodes.
Well, don’t get your hopes up--they are pretty much microscopic.
To be fair, we are dealing with laser light focused on microscopic pits, so its
stands to reason that these are probably pretty small.
I don’t have a microscope handy
--yet--
but I do have my ridiculous macro lens setup so
I could get these extreme closeups.
You can definitely see that these are three clusters, with one used for data capture and
focus control, and the two to either side for tracking.
To show how small this is, I’ve placed a dime next to it for scale.
For a more International perspective, how about a micro SD card?
And just for fun, here’s the point of a safety pin.
These really are tiny.
So now, why did Sony’s floating lens and stepping platform become the de facto method
of reading optical discs?
With more photodiodes and greater mechanical complexity, you might think it to be inefficient.
Well it may be inefficient in some ways, but in most ways it’s in fact much more efficient.
First, let’s take size.
If we look at the Philips system, you’ll notice that the entire laser pickup is essentially
behind the disc.
The pivot point for the swing arm has to be farther to the rear of the player, otherwise
it would simply rotate around the circumference of the disc, and that wouldn’t be very helpful.
This off the bat limited its potential to miniaturize.
And boy, did this get smaller quickly.
Portable CD players burst on the scene in 1984.
And they would only continue to get smaller, with one CD player featured by Techmoan actually
being smaller than a CD.
And that was in 1988, the same year as this Philips machine was made!
In fact, this very mechanism is larger than it needs to be.
If these parts were shifted downward, it could be no longer than the distance here.
And this laser platform could get smaller.
And smaller.
Until eventually you’re into modern slim drives for PCs which have the tiniest of lenses
and thinnest of assemblies.
And the PC market would only continue to make this Philips system harder to justify.
In fact, one fatal flaw of the Philips reader would make sure it fizzled into obscurity:
its mass.
Imagine in a portable player, which can be set in any position, or even moved around
while playing, that it had this swinging laser pickup.
It’s so loosey-goosey that it’s doubtful it will track very well.
And its mass also limits how quickly it can move.
When CD-ROM drives appeared, and getting faster and faster and faster, the laser would have
to be able to wiggle itself back and forth at frequencies in excess of 10 kilohertz.
That’s easy if you’re just wiggling at small piece of plastic back and forth, but
a lot harder if you’re wiggling this big swinging thing.
And so, Sony’s three-beam laser tracking would be miniaturized and improved over the years.
Developments in anti-skip functionality meant that CD walkmen could be tossed around without
fear of skipping or glitches, or getting stuck repeating a bit of the track.
That was accomplished simply by reading the disc at faster than normal speeds, creating
an intermediary data buffer between the disc and the processor.
The DAC could have upwards of 40 seconds at its disposal for the laser to get back on
track and resume the datastream.
Which, thanks to the timecode, is easy to piece together if a problem does arise, thus
eliminating the effects of skipping.
And of course, later on the wavelength of light would change to red, packing the pits
closer together and creating the MUSE high definition laserdisc.
And then later the DVD, an obscure digital video format you might have heard of.
Later we’d say, enough with the red, in with the blue and we’d be packing upwards
of 25 gigabytes onto a slightly different silver plastic thing, after we briefly reenacted
the videotape format wars of the 1980’s.
Because why not.
But we’re getting ahead of ourselves.
There’s lots more to talk about.
When we next check in on optical disc technology, we’ll discuss the Yellow Book, the standard
published in 1988 that defines the specifications of the CD-ROM.
Then we’ll talk about some writable disc technologies like CD-R and CD-RW, and then,
if interests persists, we might dig a little deeper into the DVD and some of the follies
of its development.
Such as how this logo mysteriously features a disc for a solid state memory format.
Hmm.
Thanks for watching, I hope you enjoyed the video!
If you missed the first two videos on the Compact Disc, you can check them out in the
playlist on Digital Sound that will quietly pop up above me.
So serene.
As always, a great big thank you to every who supports this channel on Patreon, especially
the fine folks that are scrolling up your screen.
If you’re interested in pledging some support to the channel to help it grow, please check
out my Patreon page.
Thanks for your consideration, and I’ll see you next time!
[ everyone’s favorite, gloriously tacky music plays ]
Hey one thing, my most viral video to date was about how Sony used the tracking servos
in a CD player to create the copy protection scheme in the original Playstation.
You might want to check that video out, but be warned that there are a number of annoying
things about the flow of information over there.
It goes into a lot of detail about the PlayStation and some of its history, and the comments
on there (if you dare to look at them) indicate that many people were… not happy about it.
So if you haven’t seen it, check it out, and apologies in advance.
Ha, and another thing!
So if you didn’t know, there’s a second channel - Technology Connections 2 - where
I upload weird stuff from time to time.
And I just uploaded a video of a couple of CD players that are very similar but have
some unique differences.
So if you want to check if out, please feel free to click the link down below or up above.