This exploded machine is capable of a marvelous feat.
These parts, when assembled correctly, are able to interact with this silver plastic thing
and make melodious sounds for you to enjoy in the highest of fidelities.
Am I being vague?
Maybe.
Well, more specifically, this is a compact disc player!
The innards of a Magnavox CDB 260, made in August of 1988.
From what I can gather this is a pretty basic CD player, even for the time.
But that’s fine, in fact it might even be better.
So, in the last video,
*ding*
we talked about how the data is stored on a compact disc.
Just below the top surface of the disc, millions of little bumps are found in the polycarbonate,
and the aluminum coating will reflect light like a mirror back from whence it came.
These bumps, from the bottom side, form pits and lands and they are what encodes the data.
So the first thing any CD player will need to do is to shine a light on the CD, a light
so finely focused that it’s shining mostly on only one track, and it needs a way to detect
the change in brightness caused by the pits.
That’s what this guy does, here.
This is the laser and lens assembly.
To understand what it does, it’s best to take a look at it from this angle.
Excellent.
OK, so notice that there are two circuit boards at right angles to each other.
This one to the side is where you will find the laser diode.
This is what creates the beam of light that will read the disc.
The light from the laser diode gets shot straight to the left, and a prism, right about here,
will cause the light to make a 90 degree turn, and end up going...
up through the lens, and
onto the disc.
Because the disc has a mirrored surface, that light is gonna come straight back down through
the lens.
And here’s where the other circuit board comes in.
Some of the light being reflected from the CD will go straight through the prism and
end up at the bottom.
Now you can’t see them because this is sealed and I don’t really want to break it but
inside, on top of the circuit board, there are 4 photodiodes arranged in a 2 x 2 grid.
Why are there 4, you ask?
Well, not only do the photodiodes need to react to the changes in brightness of the
reflected light in order to, for lack of a better word, detect the zeros and ones, but
they also forms the basis of the focus and tracking system.
A ha!
A new thing to unpack.
No CD is perfect.
All of them are going to have some deviations from perfectly flat, and they’re also probably
not going to be held exactly perfectly centered, so the track of pits will waver like this
off-center record.
To stay on the same track of pits and lands, some mechanism will need to be able to move
the lens closer to and farther away from the disc, in other words stay focused on it, and
there will also need to be a way to move the lens back and forth as the spiral track wobbles.
In other words, it needs to track it.
Philips’ solution to the problem of focus and tracking is the less common one, but it
is so much more elegant than Sony’s in my opinion.
I’ll show you Sony’s system later on, but if you didn’t see my last video, I showed
you the disc tray of this machine and suggested it hinted at the difference.
So, first the easy bit.
To maintain focus on the disc, the lens is floating, and you can kinda see that there’s
a coil of wire that can push it up or pull it down,
pretty much exactly like a loudspeaker driver.
The low mass of the lens allows it to react practically instantly to changes in the distance
between the CD and the lens, so even a badly warped CD won’t fool the CD player.
Alright, we have the Y axis accounted for.
But how do we move the lens left and right, in order to account for a wibbly wobbly track?
Well, notice that the lens is on an arm that’s surrounding a metal disc.
This allows the lens to rotate around the disc, and you’ll notice that this disc is
a magnet.
Flip this upside down and you'll find another coil of wire, surrounding the magnet.
This swing-arm design is remarkably similar to how hard drives move their read/write heads
with a voice coil, and it’s this that sets the Philips design far apart from Sony’s.
Because the lens can move vertically, and its platform can move horizontally, constant
focus and tracking can be maintained.
Now, I’ll reattach the laser mechanism to the spindle motor assembly.
It has ball bearings on the top and bottom, and when this bracket is in place the arm
essentially floats and moves nearly effortlessly.
This mechanism is fastened to this assembly, which also operates the disc tray.
The disc tray is made in two pieces, and it’s a rather unique design.
The bottom half sort of drops down when it is inserted into the player, and you can sort
of move it even while it’s open.
When it is holding a CD to be played, it is brought into the player and as it reaches
the spindle, the lower half drops down and this spring-loaded clamp will rest on it.
The reason why I said the tray might give a hint to the Philips pickup system, is that
it has a curved cut out to accommodate the path of the laser’s swing.
That’s a tell-tale sign that the CD player has a swing-arm laser mechanism.
Now with the disc held in place, the spindle motor spins the disc,
and the laser will attempt to read it.
And now we go back to the photodiodes, because this is the really genius bit.
If the disc is focused correctly and it’s being tracked correctly, all four photodiodes
will receive the same relative intensity of light.
This is because the spot of light that’s hitting the array is a perfect circle.
Due to the optics of the lens and prism, an out-of-focus beam will morph into an ellipse,
and depending on which way it’s oriented, the focus is either near or far.
Additionally, tracking errors either left or right will push the spot...
left or right.
By comparing the relative intensities between these four photodiodes, the player can determine
what corrective action it needs to take.
So what does the comparing?
Well, when assembled, the ribbon cable which carries power to the laser unit and returns
signals from the diodes plugs in here on the circuit board.
Now if we flip it over, we find two rather large chips.
If we follow the traces from the plug where the ribbon cable went in, there are four that
go right to this chip, labeled TDA8808T.
What does this chip do?
Well, it’s a photodiode signal processor for compact disc players!
Its data sheet tells us that it is “a bipolar integrated circuit designed for use in compact
disc players with a single spot read-out system.
It amplifies the photo-diode signals and processes the error signals for the focus- and radial
control network.”
Well ain’t that handy!
So this chip not only handles the focus, but it also amplifies the actual signal coming
from the photodiodes.
Neat.
But it doesn’t do tracking.
But if you noticed this strikingly similar chip,
this one a TDA8809T,
you might have guessed that this guy does that.
But it only needs two inputs because it just wants the sums of these two diodes, which
handily the other chip has generated for it.
These two chips feed their calculated corrections up to this chip, which actually handles the
voltage to drive the deflection coils for the lens’s vertical movement and the arm’s
horizontal movement.
OK, there’s one last thing that the first chip handles.
It has some automatic gain control circuitry onboard to actually generate a binary stream
from what is the analog signal coming from the photodiodes.
Remember, the pits and lands represent ones and zeroes, but we’re still in the analog
realm, reading voltage from the four photodiodes.
So in essence, this chip is an analog-to-digital converter, creating an actual bit-stream for
the following components to utilize.
If we follow this trace, from the pin which is labeled “TO DECODER” on the datasheet,
we’ll find it heading over to this Motorola chip.
Now this one was a bit of a mystery for a while, in fact it tripped me up so much that
what you’re looking at now happened a day after what happened before.
Eh, what exactly the motorola chip does is not clear, as I couldn’t find a datasheet
for it, but it appears to be the main processor for the machine.
If you look at these pins here, you’ll see that they head to these connectors which connect
the main board to the front panel.
These are likely inputs for the buttons on the front of the machine, as well as the outputs
for driving the LEDs on the display, though there is probably some communication protocol
for that task, given there are definitely not enough pins to be driving each segment
and each indicator directly.
What is clear is that some of these traces head from the Motorola chip up to the tracking
chip, which would make sense because if the processor handles button inputs, it probably
also talks to the tracking chip for navigating around the disc.
But what’s odd is that the datasheet for the TDA8808T said that pin 10, which is definitely
this trace, I’ve checked with an ohmmeter, goes to the decoder.
And this other big chip, from Philips, is called the decoder chip.
Hmm, what’s going on?
To be honest, I’m not entirely sure.
I can make a few educated guesses, though.
First, if you haven’t already spotted it, you’ll see that the trace from pin 10 on
the focus chip branched off here, and it also connects to the decoder chip through a resistor
on the top side of the board.
So the decoder chip does have a connection to the laser diode’s output, but whether
or not this is where it’s getting the data it needs is uncertain.
There are direct connections between the processor chip and the decoder chip, so it could be
that this trace here is providing the decoder with some sort of redundancy or perhaps even
something as simple as a clock signal.
One thing that is certain is that the decoder chip has a number of connections to this chip
right next to it.
This chip is a RAM chip, which the player likely uses to undo the cross-interleaved
reed-solomon coding used in the disc.
Remember, the data is spread about on the disc in addition to having parity bytes mixed in,
so the player needs somewhere to dump the raw datastream in the short term for some
data reorganization and decoding.
Ah right, that’s probably why it’s called the decoder chip.
If I were to make a guess, which I am doing right now, I’d say that the processor contains
the lookup table and is demodulating the eight-to-fourteen modulation used on the disc.
Then it generates the raw stream of 8 bit words.
From that point, it sends the demodulated, but still CIRC encoded, data to the Philips
decoder chip.
The Philips chip dumps the raw data into RAM so it can reorganize and decode the CIRC error
correction and recover the actual audio samples from the datastream.
Then it can be sent to the DAC.
But the processor is probably also handling the logic of the disc.
There’s some low-level stuff in the datastream that can be seen before CIRC error correction
is decoded.
For example, the subcode.
Subcode?
Hmm, have we overlooked something?
Well kinda.
We’ve talked in the past about the table of contents and the timecode, but where are
these along this disc?
If the CD was just a raw stream of audio samples, how is the timecode being stored?
Well, earlier I had said that 192 bits of audio samples are stored in a frame,
consisting of 24 bytes.
That’s not strictly true, though.
The frames are actually 33 bytes.
Now, if you’ve been keeping track, reed-solomon code produces a parity byte for every 3 audio bytes,
which would mean that in 24 bytes of audio, there should be 8 redundant parity bytes.
That would bring the frame to 32 total bytes.
But there’s an extra one.
Ooh, the plot thickens!
That is the subcode byte, of which there is one in each frame.
98 frames comprise the timecode frame, sometimes also called a sector, though the use of the
term sector was added in the CD-ROM days and is not part of the original Red Book standard.
However I’m going to continue to use the word Sector
because “Frame” and “Timecode Frame” are too similar.
These combine to create 1/75th of a second of audio, so each second contains 75 timecode
frames, or sectors, which comprise the most specific time references there are on the CD.
The way the subcode is handled is pretty ingenious.
Though there is only one byte per frame, a total of 96 frames have a usable byte per sector.
The first two in a 98 frame sector are used as a synchronization word.
Now, you might have realized that 96 is divisible by 8.
So rather than just create one bitstream, each subcode byte actually represents one
bit of 8 subcode channels.
Depending on which channel the player wants to read, it will be looking for the nth bit
in each subcode byte, and stringing them together.
And the crucial one for the timecode is the Q bit.
And even within just that datastream, there’s some extra specificity.
The first bits of the Q channel are actually signifying specific information about the
disc, and it’s the 72 bits before the last 16 that we need to look into.
But even then, there are different modes that can be selected within the datastream of the
Q channel, so there was a lot of thinking going into this.
I’ve linked the Wiki article on compact disc subcode as it goes over this much better
than I can.
This explains the rather elegant way in which a CD player can find tracks on a disc.
The lead-in contained the table-of-contents (which incidentally is sent in the Q channel
subcode as well) and the CD player stored in memory.
It now knows where each of the tracks are by their time code.
So when you select Track 10, which the Table of Contents told the player can be found
at this time code, it will first send its laser to around where that timecode probably is
(after all the physical characteristics are nearly identical from CD to CD)
and then it simply mutes the audio while it looks.
By reading the Q channel datastream it knows exactly what it’s looking at, and it will
move inward or outward in fine steps until it sees exactly the timecode that it’s looking for.
Then it will unmute the audio, and play the song.
In a rather smart move, Sony and Philips minded their P’s and Q’s by making these the
only two subcode channels used in the Red Book standard.
6 entire data streams were left unused for future extensions of the standard.
And each of these streams, when discounting the two synchronization bits, can produce
7.2 kilobits per second.
Now of course that’s not a lot, but that’s 900 characters of text per second.
Multiply that by 6 channels and you have 5.4 kilobytes per second of untapped data
available, or about 24 megabytes total for a 74 minute CD.
In later years this would be used to produce some nifty things.
Which we’ll get to, don’t worry.
But just imagine for a moment how crazy forward thinking that was.
In any CD, there were more than 16 3.5 inch floppy diskettes worth of data
sitting completely unused.
And in 1982, these things weren’t even in commercial production yet, plus they wouldn’t
hit 1.44 megabytes in size until 1986.
In a time when hard drives for computers were around 10 megabytes, if you even had one in
your PC, the idea of having 24 megabytes in one place, let alone 24 megabytes of what
amounted to unneeded and unused data on a sound storage medium, was quite remarkable.
But remember, no one was thinking of that at all.
The CD-ROM was some years away, though it does strike me as a bit of a waste that no
one developed computer peripherals that could interface with the subcode-out jacks found
in some CD players to get programs or data off of audio CDs.
That would have been kinda nifty.
Now of course, this just an educated guess, but the decoder chip probably isn’t even
looking at the subcode.
The motorola chip is probably the only thing paying attention to it, and in fact it might
even strip that out before sending the CIRC encoded audio data to the decoder chip.
But don’t assume the decoder chip has an easy job; it’s got a lot of math to work
out to undo the reed-solomon code.
Plus, keep in mind that there are 16 bit audio samples here, but we’re dealing with 8 bit
words, so it needs to decode what exactly each sample is and send this and this alone
to the DAC.
Which is definitely the role of this chip.
Flipping the board back over, if we follow the traces we find we’ve got a couple of
connections to this little 8 pin chip.
And believe it or not, that is the DAC.
This diminutive TDA1543 is all that we need to turn ones and zeros into music.
Just provide it with a clock signal, your data stream, a power and ground, and you’ll
get left and right channel outputs on pins 6 and 8.
Now the datasheet says that pin 3 is the data input, and if we follow the trace back, sure
enough this heads to the Philips chip.
So let’s put this entirely back together.
The circuit board is held down with only one screw, as it also gets support from screws
on the rear.
The CD mechanism rests above the circuit board here, and we need to plug in the ribbon cable
for the laser mechanism, as well as the spindle motor and the disc tray motor.
The disc tray slides back in place like this, though you have to kind of hold up the moveable
piece as you do so, and now we’re ready to put the faceplate back on.
Simply plug in the connectors to its circuit board for the display and buttons, and don’t
forget the headphone jack, and now we just have to put on this plastic thing which
connects the power button to the actual power switch, which is way back here for some reason,
and now it’s more or less assembled.
Well, except for the cover.
So now let’s play a CD.
I turn the player on, and you’ll see that it performs a check of whether or not a CD
is in it.
It’s empty, so it enters a standby mode.
I press the open/close button and the motor pushes the tray open.
Now I deposit my disc of choice, and press the open/close button again.
Once the tray has closed, the focus chip performs its initial focus routine, where it sweeps
the lens from its lowest to highest position in order to detect whether or not a disc is there.
Once it sees the disc it locks focus on it, and as the disc is spun by the spindle motor,
it begins analyzing the ratio of stimulation between the two pairs of photodiodes to maintain
focus even on badly warped discs.
Now, the tracking chip kicks in, nudging the laser left and right at the request of the
Motorola chip, which is working for the Philips chip.
The tracking chip is helping to look for the table of contents.
Once it’s seen it, it locks onto it and the processing chips load the TOC into memory.
At this point the player re-enters a standby mode.
If I had selected play, or pushed the tray in rather than hitting the close button, it
would have started playing immediately after loading the TOC.
In any case, it’s ready to play the disc.
I hit PLAY, and the disc begins spinning while the focus chip maintains focus and the tracking
chip helps to look for the beginning of the program area.
After finding it, the tracking chip simply locks on to the datastream, and the raw data
is processed by these two chips.
Again, exact specifics unknown here.
After undoing the eight-to-fourteen modulation, as well as reordering the data and filtering
out the parity bytes from the CIRC error correction, the player has now extracted the raw datastream
of audio samples.
These are sent to the DAC, which recreates the original analog sound wave based on the
principles of the Nyquist-Shannon Sampling Theorem.
If I want to select a later track using the NEXT button, the Motorola chip will take over
the tracking chip once more, nudging the laser farther out while it looks in the Q channel subcode
for the timecode of the next track.
Once it sees it, it locks onto the datastream again, and the audio is processed as normal.
If I select a track that’s far from the start, the laser flings itself out there,
still looking in the Q channel, for the appropriate track.
The CD player will happily decode the audio for you until it reaches the end of the last track.
Now, it’s reading the lead-out, in which the subcode will tell the player it’s reached
the end of the disc, and it stops.
Unless of course you’ve got repeat enabled, in which case it’s back to the beginning
to start the whole thing over again!
Hooft, that was intense.
Like the circus!
Oh no, you’re seeing the CD player all disassembled again!
The continuity errors!
The horrors!
OK, I’ll cut back on the silliness.
This was my best attempt at explaining how this CD player works in the most comprehensive
way possible, while trying not to get too technical.
Already I can tell that this is a longer-than-normal video, so if you made it here, thanks!
And congratulations!
Soon, I’ll be taking this back apart--
after I reassemble it--
and adding some test wires
to the board so that we can take a look at the waveforms produced by these components
with an oscilloscope.
That will probably be the next video on the channel, so be sure to subscribe so you won’t
miss it!
Thanks for watching, and I really hope you enjoyed this video, it was a lot of fun to
make, especially going through the circuit board, finding the datasheets, and all that jazz.
As always, a great big thank you to everyone who supports the channel on Patreon.
One of the greatest things about the support I have from Patreon is that it allows me to
keep the channel going without resorting to outside sponsorships.
I’d much rather this channel stay entirely community driven, and thanks to people like
you that’s remained the case!
If you’re interested in pledging some support to the channel to help it grow, please check
out my Patreon page!
Thank you for your consideration, and I’ll see you next time!
♫ uncomfortably smooth jazz ♫
...binary stream from what is the analog
(long pause)
No, that was right.
OK.
That it Reepresent…Reepresents.
Reepresents?
Reepresents.
These continue to create 1/75th....
Oh man…
c’mon!
But even then, there are different nnmmmmmMMMMMMmmm.
“modes!”
...which the CD player stored in memory.
It now knows whe…
(glances at mechanism he just bumped up against)
or that
...plus they wouldn’t be the 1.44 megabytes in size until 1986…
they wouldn’t… hit…
dangit.