WEBVTT Kind: captions Language: en 00:00:00.130 --> 00:00:04.920 If you’re watching something like, well like this, on a modern display, you probably 00:00:04.920 --> 00:00:09.510 don’t think too much about what your device is doing to make this happen. It’s not really 00:00:09.510 --> 00:00:14.130 that extraordinary in this modern world filled with computers and microprocessors. In fact, 00:00:14.130 --> 00:00:18.710 I’m willing to bet you have a rough idea of how this works. But I’ll explain anyway. 00:00:18.710 --> 00:00:22.410 If you get up close to a TV or monitor, you find there’s a grid, made of millions of 00:00:22.410 --> 00:00:26.849 little squares called pixels. From far enough away, these pixels blend into each other, 00:00:26.849 --> 00:00:31.589 and our eyes and brains build a coherent picture. To actually create the image on the screen, 00:00:31.589 --> 00:00:35.330 each pixel can have some instructions sent to it to tell it how much light it should 00:00:35.330 --> 00:00:39.430 emit. A series of controller circuits work with millions of transistors to methodically 00:00:39.430 --> 00:00:44.470 give every one of these squares a specific brightness value 60 or 120 times a second, 00:00:44.470 --> 00:00:48.680 sometimes more often than that. The instructions are further divided into three separate values 00:00:48.680 --> 00:00:52.800 for red, green, and blue which when combined together can create practically any color 00:00:52.800 --> 00:00:54.030 you can think of. 00:00:54.030 --> 00:00:57.990 But have you ever wondered how old-school TV worked? We’ve been sending video signals 00:00:57.990 --> 00:01:03.530 over the air for a very long time, in fact analog television predates World War II. There 00:01:03.530 --> 00:01:07.859 weren’t computer or logic circuits decoding number values then, in fact there weren’t 00:01:07.859 --> 00:01:14.630 even pixels. Yet, this somehow worked. How? What magic is going on to take the signal 00:01:14.630 --> 00:01:19.079 coming over this wire and turn it into a black and white image of me, all without a single 00:01:19.079 --> 00:01:20.380 digital circuit? 00:01:20.380 --> 00:01:25.350 To find out, let’s make things really simple. Instead of looking at millions of pixels, 00:01:25.350 --> 00:01:30.890 let’s look at just one. A single pixel is really just a point of light. Without using 00:01:30.890 --> 00:01:35.420 any digital circuitry, how can we tell the light what to do? Easy, by controlling how 00:01:35.420 --> 00:01:36.630 much power it gets. 00:01:36.630 --> 00:01:41.780 Here’s an LED hooked up to a power supply. By simply varying how much voltage it gets, 00:01:41.780 --> 00:01:46.569 you can change how much light it emits. Using radio technology, it’s really easy to build 00:01:46.569 --> 00:01:50.240 a circuit that can control the brightness of this LED or another light source based 00:01:50.240 --> 00:01:55.850 on the strength of a signal transmitted over the air. Of course, this is clearly not television, 00:01:55.850 --> 00:01:58.020 but it’s at the core of what makes it work. 00:01:58.020 --> 00:02:03.079 See, we really suck at seeing things that happen quickly. Our eyes and brains are just 00:02:03.079 --> 00:02:07.779 no good at processing fast visual information. Thus, it’s really easy to trick our eyes 00:02:07.779 --> 00:02:11.950 into seeing something that’s not really there. If you take a light source and move 00:02:11.950 --> 00:02:16.150 it very quickly, you no longer see a single light source, instead you see a continuous 00:02:16.150 --> 00:02:20.800 line that follow the path of the light. Our brains can’t process the light’s fast 00:02:20.800 --> 00:02:25.410 motion, and it just blurs together into a solid line. This phenomenon is called persistence 00:02:25.410 --> 00:02:29.990 of vision. Now, if you manipulate the voltage of the light while you move it, you can make 00:02:29.990 --> 00:02:31.430 patterns in the line. 00:02:31.430 --> 00:02:36.870 You can find a lot of toys that exploit persistence of vision. This old 20Q game works using this 00:02:36.870 --> 00:02:41.600 principle. A small circuit board with a few LEDs on it spins in a circle really fast, 00:02:41.600 --> 00:02:45.710 too fast for your eyes to keep track of. If the LEDs were lit up all the time, it would 00:02:45.710 --> 00:02:50.660 just appear to be a continuous circle. But the game uses sensors to track where the LEDs 00:02:50.660 --> 00:02:55.610 are, and turns each of them on and off at very specific times. By manipulating the brightness 00:02:55.610 --> 00:03:00.080 of the LEDs and timing it with their motion, it can draw simple text and graphics using 00:03:00.080 --> 00:03:01.810 just eight points of light. 00:03:01.810 --> 00:03:06.680 But a CRT television like this has only one point of light to work with. After all, this 00:03:06.680 --> 00:03:11.830 is 1920’s technology, and multitasking wasn’t really a thing yet. So first, what actually 00:03:11.830 --> 00:03:17.480 makes the light? Well, CRT stands for cathode ray tube. The name comes from cathode rays, 00:03:17.480 --> 00:03:22.690 which were discovered by Johann Hittorf in 1869. William Crookes had created these goofy 00:03:22.690 --> 00:03:27.120 tube things that were really important to early scientific discovery. He was able to 00:03:27.120 --> 00:03:31.650 evacuate nearly all the air from these tubes, which allowed electrons to move freely within 00:03:31.650 --> 00:03:35.600 them, though no one yet knew what electrons were. When electric current was sent through 00:03:35.600 --> 00:03:40.120 these tubes, something caused them to glow. Johann Hittorf was the first person to piece 00:03:40.120 --> 00:03:44.320 together that whatever was causing this phenomenon travelled in straight lines from the cathode, 00:03:44.320 --> 00:03:48.210 or negative electrode, observing how a stencil between the cathode and the surface of the 00:03:48.210 --> 00:03:54.460 tube cast a shadow. Eugen Goldstein gave them the name Cathode Rays, just like rays of sunlight. 00:03:54.460 --> 00:03:58.540 J J Thompson would later use these tubes to work out what these cathode rays were actually 00:03:58.540 --> 00:04:03.220 made of, and in doing so he happened to discover the electron. So good on him. 00:04:03.220 --> 00:04:08.220 Before we continue, SAFETY WARNING: Exploring the innards of a CRT television can be quite 00:04:08.220 --> 00:04:12.780 dangerous. A set as small as this can generate over a thousand volts through the flyback 00:04:12.780 --> 00:04:18.260 transformer, and the CRT’s glass can store a lethal charge. I know what is and what’s 00:04:18.260 --> 00:04:22.180 not OK to touch, and since you likely don’t, don’t try this at home. 00:04:22.180 --> 00:04:26.100 If you’ve ever messed around with antique radios you’ll have seen vacuum tubes, which 00:04:26.100 --> 00:04:30.880 are the precursors to transistors. These electronic components have the air evacuated from them 00:04:30.880 --> 00:04:35.330 so electrons can move freely, just like the crookes tubes. Using a heater filament to 00:04:35.330 --> 00:04:39.240 induce thermionic emission from a cathode, they can manipulate electric current in a 00:04:39.240 --> 00:04:44.300 bunch of ways. A television CRT is really a specialized vacuum tube that has had its 00:04:44.300 --> 00:04:49.750 top blown way up and out to form a screen. It’s then mounted sideways in a cabinet, 00:04:49.750 --> 00:04:53.420 and your eyes stare at the front of it. That’s what brought about the phrase, watching the 00:04:53.420 --> 00:04:56.900 tube. And it also explains the name of this site. 00:04:56.900 --> 00:05:00.380 Because it has no air inside, it has to be pretty strong to counteract the force of the 00:05:00.380 --> 00:05:05.790 atmosphere always trying to crush it. That’s why larger tube TV’s are so heavy--the glass 00:05:05.790 --> 00:05:11.190 needs to be quite thick on larger sets. Most of the tube is empty space with the meat and 00:05:11.190 --> 00:05:16.571 potatoes being at the very back. Here you’ll find the awesomely named electron gun. This 00:05:16.571 --> 00:05:20.300 component generates a stream of electrons and they are shot straight out to the front 00:05:20.300 --> 00:05:25.070 of the tube. The flyback transformer generates an extremely high voltage in the anode to 00:05:25.070 --> 00:05:29.320 attract the electrons to the front of the screen. Coating the inside surface of the 00:05:29.320 --> 00:05:34.230 tube is a special powder known as a phosphor. When the electrons sent from the gun hit the 00:05:34.230 --> 00:05:38.990 phosphor, it gets all excited and emits light, via fluorescence, but only in the spot the 00:05:38.990 --> 00:05:40.240 electrons are hitting it. 00:05:40.240 --> 00:05:45.200 Here’s a working CRT with one of the critical components to television removed because we 00:05:45.200 --> 00:05:48.940 haven’t gotten that far. Don’t worry, we’ll get there. So, the CRT is doing a 00:05:48.940 --> 00:05:52.310 bang-up job producing a stream of electrons and they’re going straight to the front 00:05:52.310 --> 00:06:00.310 of the screen, and colliding with it to make it glow. And, this is the result. So fascinating. 00:06:00.310 --> 00:06:04.310 But hang on, there’s more to it. The vast majority of the signal coming into this television 00:06:04.310 --> 00:06:09.380 is simply telling it how bright to make this point of light. Therefore, a signal that alternates 00:06:09.380 --> 00:06:15.730 between bright and dark will make this happen. Amazing. That doesn’t do that much good. 00:06:15.730 --> 00:06:19.389 Ah, but you see, the point of light can be moved. 00:06:19.389 --> 00:06:23.260 One of the things Crookes and others noticed when mucking about with his tubes was that 00:06:23.260 --> 00:06:28.570 a magnetic field can bend an electron beam. In other words, you can use a magnet to alter 00:06:28.570 --> 00:06:33.340 the path the beam makes through the tube. Watch. Here’s an ordinary strong magnet 00:06:33.340 --> 00:06:38.090 used for a nametag. When I move it around the neck of the tube, the point of light moves 00:06:38.090 --> 00:06:44.060 around the screen, also. Mind bending, more like beam bending, amiright? 00:06:44.060 --> 00:06:49.570 So then, here comes the other bit. This little bundle of wires is called the deflection yoke. 00:06:49.570 --> 00:06:54.241 This is responsible for moving the beam really really quickly, and fooling your eyes. The 00:06:54.241 --> 00:06:58.320 yoke is made of two electromagnets that surround the neck of the tube, and they work together 00:06:58.320 --> 00:07:03.080 to move the electron beam around in a set pattern. It does this by creating a fairly 00:07:03.080 --> 00:07:08.169 strong magnetic field which will deflect the beam’s path. First, I’ll turn on the horizontal 00:07:08.169 --> 00:07:14.949 deflector. Now, rather than a point of light, we see a line. This line is being drawn on 00:07:14.949 --> 00:07:20.250 the screen thousands of times a second, way too fast for your eyes to notice. Just like 00:07:20.250 --> 00:07:25.360 the POV effect from the toy, if we carefully control how bright this line is as it moves 00:07:25.360 --> 00:07:35.280 left and right we can create patterns in the line like this. 00:07:35.280 --> 00:07:39.740 But the yoke contains another magnet that can move the beam up and down. Let me switch 00:07:39.740 --> 00:07:45.759 to that one. We now have a vertical line being drawn on the screen, and we can control its 00:07:45.759 --> 00:07:50.190 intensity just like the horizontal line to draw patterns. This vertical movement happens 00:07:50.190 --> 00:07:54.870 much more slowly than the horizontal movement, with the line only being drawn 60 times in 00:07:54.870 --> 00:08:00.139 a second. Now, since we can point the beam from left to right, as well as up and down, 00:08:00.139 --> 00:08:04.650 we can point it anywhere we want on the screen. Let’s turn on both electromagnets at the 00:08:04.650 --> 00:08:13.630 same time. We now have an image on the whole screen. Pretty neat, huh? By carefully controlling 00:08:13.630 --> 00:08:18.009 the intensity of the beam over time, we can create a complete image. 00:08:18.009 --> 00:08:22.140 If you look really closely at a black and white television, you won’t find pixels. 00:08:22.140 --> 00:08:27.150 Rather, you’ll find lines. See, the image is made of lines, in fact there are roughly 00:08:27.150 --> 00:08:33.710 525 lines that make up the NTSC signal, and about 480 are visible on the screen. The deflection 00:08:33.710 --> 00:08:38.440 yoke is making a pattern on the screen called a raster, and in NTSC countries, it’s drawn 00:08:38.440 --> 00:08:42.390 on the screen 60 times a second. There’s a bit of a trick, though, because the screen 00:08:42.390 --> 00:08:47.899 is only COMPLETELY redrawn 30 times a second. See, as the raster is drawn, only every other 00:08:47.899 --> 00:08:52.460 line is filled in. This is called a field, and it’s the principle behind interlaced 00:08:52.460 --> 00:08:58.060 video. That’s the i in 1080i. Because not-a-lot of bandwidth is available, the whole screen 00:08:58.060 --> 00:09:02.050 can only reasonably be filled in 30 times a second, but this would be noticeable as 00:09:02.050 --> 00:09:06.360 flicker and could give many people headaches. By skipping every other line and then repeating 00:09:06.360 --> 00:09:10.870 the scan to fill in the rest, the screen is drawn from top to bottom 60 times a second, 00:09:10.870 --> 00:09:15.110 which was too fast for most people to perceive flickering. Also, it allowed for smoother 00:09:15.110 --> 00:09:19.450 motion, with the caveat that fast-moving objects would have less detail as only every-other 00:09:19.450 --> 00:09:23.590 line is filled in with each field (however that never proved to be a huge concern as 00:09:23.590 --> 00:09:26.390 it’s hard to see detail in fast moving objects, anyway.) 00:09:26.390 --> 00:09:30.580 Side-note: It’s no coincidence that the 60 hz refresh rate matches the frequency of 00:09:30.580 --> 00:09:35.680 the AC electricity sent into homes, as the 60 hz sine wave coming from the socket powering 00:09:35.680 --> 00:09:40.940 the TV made for a convenient timing reference for vertical scanning. PAL countries, which 00:09:40.940 --> 00:09:46.290 have 50 hz electricity, have a television frame rate of 25 frames per second interlaced, 00:09:46.290 --> 00:09:52.360 with a scanning refresh rate of 50 hz. So, tv framerates are what they are because convenience. 00:09:52.360 --> 00:09:56.480 So now that we have the means to generate this raster, well how does that make a picture? 00:09:56.480 --> 00:10:00.660 Well, it’s just like the POV effect from the toy, only it’s a helluva lot faster 00:10:00.660 --> 00:10:05.320 and the light moves in two dimensions. Let’s slow down time and see how the TV builds an 00:10:05.320 --> 00:10:10.950 image. Let’s say we want to show this on the screen. At the start of a field, the deflection 00:10:10.950 --> 00:10:15.130 yoke is pointing the electron beam at the top left of the screen. As it moves towards 00:10:15.130 --> 00:10:19.290 the right, the beam changes its intensity along with how bright the image should be, 00:10:19.290 --> 00:10:23.230 so at a point along the line that’s bright, it produces a lot of electrons, and thus that 00:10:23.230 --> 00:10:28.410 point on the screen glows brightly. Dark parts send little to no electrons. When the beam 00:10:28.410 --> 00:10:31.830 gets to the end of the line, the deflection yoke almost instantly pulls it back to the 00:10:31.830 --> 00:10:36.779 left-hand side and starts the next line. But remember, it skipped a line. This process 00:10:36.779 --> 00:10:40.700 repeats until it reaches the bottom of the screen. Then the yoke flings the beam back 00:10:40.700 --> 00:10:45.360 to the top, and we start again filling in the alternate lines. This happens way too 00:10:45.360 --> 00:10:49.550 fast for us to notice it, so it appears like a fully illuminated screen. 00:10:49.550 --> 00:10:53.649 One thing to note is that the vertical deflection isn’t happening in steps. Rather it’s 00:10:53.649 --> 00:10:58.100 a constant downward motion. This means that the horizontal lines are actually slightly 00:10:58.100 --> 00:11:03.050 slanted downward to the right. To counteract this, the deflection yoke is mounted to the 00:11:03.050 --> 00:11:07.721 tube ever so slightly crooked, so the lines drawn on the screen are actually level. The 00:11:07.721 --> 00:11:12.649 constant downward travel is also how the interlacing is accomplished. The next line will start 00:11:12.649 --> 00:11:16.410 at the same height as the end of the first, which creates a gap. 00:11:16.410 --> 00:11:20.220 You may remember an extremely high-pitched noise coming from a TV set whenever it was 00:11:20.220 --> 00:11:25.149 turned on. This noise actually came from the deflection yoke and the electronics that drive 00:11:25.149 --> 00:11:31.820 it. In NTSC televisions, the horizontal deflection occurred 525 times per frame, and there are 00:11:31.820 --> 00:11:37.500 30 frames in a second, which means the electron beam is being deflected left-and-right 15,750 00:11:37.500 --> 00:11:43.240 times per second. In PAL countries, the framerate is only 25 frames per second, but 625 lines 00:11:43.240 --> 00:11:48.700 are drawn with each frame, which works out to 15,625 deflection per second. The yoke 00:11:48.700 --> 00:11:52.580 and the flyback transformer, along with some other components, actually vibrate at this 00:11:52.580 --> 00:11:57.990 frequency ever so slightly, which produces audible noise. This is what it sounds like. 00:11:57.990 --> 00:12:01.959 Adults over the age of 25 or so can’t hear this sound, as it’s at the upper limit of 00:12:01.959 --> 00:12:07.420 human hearing range, which gradually diminishes with age. So for those viewers, sorry. 00:12:07.420 --> 00:12:10.970 When it comes to actually producing an image, the trickiest part is matching that raster 00:12:10.970 --> 00:12:15.630 to an incoming television signal. To help with this, the TV signal contains triggers 00:12:15.630 --> 00:12:20.330 which assist the TV in grabbing hold of the image and keeping it in one place. 00:12:20.330 --> 00:12:23.920 This Sony TV is correctly tuned to channel 3, which is currently displaying this video 00:12:23.920 --> 00:12:29.330 that you’re hearing right now. But there’s absolute nonsense on the screen. What gives? 00:12:29.330 --> 00:12:33.640 Well, the TV is generating its own raster, and right now it’s not synchronized with 00:12:33.640 --> 00:12:37.980 the raster coming into the TV. You’re seeing all of the image, but each part is in the 00:12:37.980 --> 00:12:42.670 wrong place because it’s not lined up. Here, to show you what the TV’s looking for, let’s 00:12:42.670 --> 00:12:46.910 fade to white. You’ll notice that there are a ton of black gaps swirling around what 00:12:46.910 --> 00:12:51.700 should be an entirely white screen. These gaps are the horizontal blanking intervals 00:12:51.700 --> 00:12:57.160 between individual scanlines. When horizontal hold is properly adjusted, electronics in 00:12:57.160 --> 00:12:59.950 the TV can see these gaps and line them up. 00:12:59.950 --> 00:13:03.660 Hold up, how can the set tell the difference between the blanking intervals and a black 00:13:03.660 --> 00:13:08.560 spot on the screen? Well, it can tell them apart because the blanking intervals are actually 00:13:08.560 --> 00:13:14.420 BLACKER THAN BLACK. No, really. Here’s a one line of a television signal drawn on a 00:13:14.420 --> 00:13:18.620 graph. These parts at the ends are the blanking intervals between scan lines. They are the 00:13:18.620 --> 00:13:23.459 lowest parts of the graph because their amplitude is near zero. Here is the actual start of 00:13:23.459 --> 00:13:27.590 the scan line. The higher the line goes, the brighter that part of the scan line will be 00:13:27.590 --> 00:13:33.190 drawn on the screen. Makes sense, but black is all the way up here. Television sets are 00:13:33.190 --> 00:13:38.120 calibrated to not fire the electron gun at amplitudes at or below this amount, so to 00:13:38.120 --> 00:13:42.910 they eye, any amplitude below this point won’t be visible, but the electronics can clearly 00:13:42.910 --> 00:13:47.110 tell blanking intervals from signals. The blanking interval isn’t there just to provide 00:13:47.110 --> 00:13:51.459 a reference for the beginning and end of a scan line, it’s also there prevent anything 00:13:51.459 --> 00:13:55.660 from being drawn on the screen as the deflection yoke sweeps the electron beam back to the 00:13:55.660 --> 00:14:01.079 left-hand side before the start of the next line. The TV just has to line these low points 00:14:01.079 --> 00:14:04.930 up by catching them at the beginning of each scan line, and then they’ll fall into the 00:14:04.930 --> 00:14:08.019 TV’s own raster. Everything is hunky dory. 00:14:08.019 --> 00:14:12.080 So then, when I adjust the horizontal hold, you can see that this moves the blanking intervals 00:14:12.080 --> 00:14:17.831 closer to each other, and eventually, the image snaps into place. well, sort of. Now 00:14:17.831 --> 00:14:22.920 the image is rolling, it’s continually moving downwards. Ah, see, we have only synchronized 00:14:22.920 --> 00:14:27.279 the television's raster with the horizontal components of the signal. Without a reference 00:14:27.279 --> 00:14:32.540 as to what starts a field scan, the pictures just gonna roll around like this. See that 00:14:32.540 --> 00:14:36.589 hunk of black between my head and my waist? That’s the vertical blanking interval, which 00:14:36.589 --> 00:14:41.290 is little more than a bunch of empty scan lines. Just like the horizontal intervals, 00:14:41.290 --> 00:14:45.431 it allows the deflection yoke time to get back to the top of the field. Again, this 00:14:45.431 --> 00:14:50.470 is BLACKER THAN BLACK, and it allows the television to hold onto the start of each field and keep 00:14:50.470 --> 00:14:55.570 them in one place. The vertical blanking interval also contains some special pulses to differentiate 00:14:55.570 --> 00:14:57.260 between the odd and even numbered fields. 00:14:57.260 --> 00:15:01.230 So, i’ll adjust the vertical hold, and eventually, the frame snaps into place, and you get a 00:15:01.230 --> 00:15:06.649 truly stable image. Very intentionally, the CRT is scanning outside the borders of the 00:15:06.649 --> 00:15:11.100 face of the tube. This is called overscan, and it’s done to hide the blanking intervals 00:15:11.100 --> 00:15:15.400 as well as just ensure the whole screen is being used. On this set, you can see how the 00:15:15.400 --> 00:15:20.940 scan extends beyond the tube itself when looking from behind. This unseen overscan area was 00:15:20.940 --> 00:15:25.760 used later to add closed captioning into television broadcasts. On one of the lines that make 00:15:25.760 --> 00:15:31.040 up the VBI, alternating white-black bits created a barcode of sorts that contained digital 00:15:31.040 --> 00:15:36.290 text information. A decoder inside the television set could read this data from that line, and 00:15:36.290 --> 00:15:40.529 when enabled place text graphics on top of the image. I think that’s pretty friggin 00:15:40.529 --> 00:15:41.529 nifty. 00:15:41.529 --> 00:15:45.380 As far as audio, well that’s really simple. That’s nothing more than simple FM radio 00:15:45.380 --> 00:15:50.139 built into the TV, and each channel has an audio signal being transmitted at a set offset 00:15:50.139 --> 00:15:54.450 frequency from the video source. Since the signals are transmitted together, they are 00:15:54.450 --> 00:15:55.450 always in sync. 00:15:55.450 --> 00:16:00.060 So, that’s how these old things work. But there’s a lot more to explore. For one, 00:16:00.060 --> 00:16:04.459 how did television cameras actually create the signal that drives this TV? And who were 00:16:04.459 --> 00:16:10.860 the people responsible for inventing it? What about color? We’ll explore that in a later episode, along with 00:16:10.860 --> 00:16:15.640 the precursor to CRT television, mechanical television, so be sure to subscribe to Technology 00:16:15.640 --> 00:16:19.709 Connections. If you liked this video, I humbly ask that you hit that like button and maybe 00:16:19.709 --> 00:16:23.920 leave a comment. I’m doing my best to keep videos like this headed your way. Thanks for 00:16:23.920 --> 00:16:26.240 watching!