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What is inside a DVD player? (4 of 5)

Video transcript
- [Instructor] Okay, so we're gonna talk about the CD drive itself or the DVD drive itself, which is right here. I've returned the power board back to its normal location just so you can see how the wires are routed and I went ahead and removed the mounting screws for this drive, just to reduce the amount of time it takes to get it out so we can explain it. So you can see the power for the motors comes through these two wires here. This chip controls the electrical impulses through these wires and tells the motors to drive the tray out, spin the spindle or move the laser back and forth, so there's three functions there and that requires three motors so we should see three motors on the back. Let's see if we can remove these components here or these wires. And this wire here is the ribbon cable that takes signals from the laser and brings it back to this board, sends it back to the board as electrical impulses, which are encoded as ones or zeroes, and then this board converts those ones and zeroes into an analog signal that your TV can read. It may also have digital signals as well, but I believe this one just has analog out and so we're gonna remove this ribbon cable if we can. There we go, and there's some neat things about this ribbon cable. One is that it has a lot of different really fine wires in it, so it allows for a lot of different signals to be sent and it has this stiff piece of plastic at the end, which allows it to be easily plugged into the very narrow slot there. Alright, so now we're gonna take a look at the DVD drive itself, so first thing we're gonna look at is the structure and the mechanical components, and then in the next video, we'll look at the laser and how it works, so in this part, let's flip it over here. We've got a number of different components, so we have three motors, as expected. Remember, the one motor here causes the spindle to turn around, that's this thing. This motor actually drives these gears and causes the laser assembly to move. You can see it's shifting there, moving back and forth, and this motor causes the tray to slide out and to move out from the actual player so that you can put your disk in and have it spun around and read, so there are a number of really fascinating engineering things on this. One of the things they have to do with a board like this is it's super important that everything remains very accurate and that there's a large degree of precision because the disk is reading a very small, or the laser is reading a very small place on this disk and so in order for it to do that, everything has to line up just right, so there's a number of mechanical and electromechanical devices that make that possible. So one of the things that is done there that makes possible is that this has two closed loop control switches. So an open loop control would be to, say, have this device move backwards and forwards and have it stop based on time, so maybe our time over here on the computer says, okay, I'm gonna let this go for five seconds in this direction and then when it gets to the end there, it'll go five seconds in this direction. Well, that's great except for the fact that the gears have a little bit of slack in them, just a little bit, and over time, walking backwards and forwards, the laser could get out of alignment or get off of where it should be, out of its position. So the engineers have come up with an ingenious solution and that is that they have a closed loop control, which means that there's a switch that gets hit every time that laser gets to the end, so when this rack here, and this is called a rack and pinion when you have a round gear and teeth on a straight edge like that, that's a rack and pinion and it converts rotational motion from the motor into linear motion, and this small gear to big gear slows the motor's motions way down, so you get a nice, smooth slow motion. It also increases the torque or the power of the motor, the twisting force, so having all these gear ratios the way they are enable this to move more smoothly and slowly and with a little bit more control and power, and so when this gets to the end of its cycle, it hits this switch, pushes that switch down and resets the entire device so that it knows, oh, hey, I'm at zero, and so it resets every time and you don't have to worry about it getting out of alignment. There's another closed loop control switch right here and so it's very, very important that everything remain in true alignment, and so that's what these switches are there for. So this resets every time the CD tray opens and closes, so whenever the tray opens and closes, it resets and lets the player know, lets the motor controller and control board know over here, hey, I'm at zero again, and so then it knows how to run through a sequence again to operate in exactly the way it's supposed to. Again, if you use time or some other way of controlling the motor, it may pull the device, it may pull the train too far, it may pull it hard against the other plastic and wear the motor out, but when you have that switch in there, you can reset it every time. Keeps things running smoothly. Okay, and so we can also take a look at this metal bracket here that is holding these two motors. This motor is the motor that turns the spindle and this is, again, the motor that moves the laser, and so they're mounted on this metal bracket which means that they're very stable and solid and the bracket has been made rigid by having these bends in it, so it gives it a really strong, sturdy... It's very strong and sturdy and rigid because of the bends. They add stiffness to the metal, and there's these two little rubber pieces here and these screws, and those rubber pieces actually allow the drive plate, this metal plate, to drop down, so the plate actually pivots down like this when the DVD comes out, and that's important because you can see that the spinner head is actually above the tray, so if the DVD came in like this, it would hit right there, it wouldn't work, so the tray has to swing down, and it does that. There's grooves along the side that allow the tray to swing down and out of the way, so when this tray, when the CD stops spinning, this drops down and the DVD can move out of the way and when the DVD gets put in, it goes into right here and then this bracket moves up and grabs the DVD and allows it to spin freely, so it's not rubbing on the plastic. So that's how that part works. Again, the metal's really critical because it provides that rigid platform and helps to maintain that dimensional stability which is required to keep everything just locked in place, and you can see that these two metal bars here are tightly connected to this metal bracket, this stiffening bracket, and they allow for the laser to track very smoothly across this edge and you can see there's one contact point here and then there's a long edge that's on that point and that helps to maintain very consistent accuracy and smooth motion. These rods are smooth and polished and again, allow the laser to track in a very smooth pattern, and you can see that these are injection molded parts here and the ribs here add stiffening and strength and complete the DVD drive and allow you to also fasten all the different components together.