WEBVTT Kind: captions Language: en 00:00:00.080 --> 00:00:01.910 Yeah. It’s a great word, isn’t it? 00:00:01.910 --> 00:00:06.730 It may not seem like it, but even an antique radio like this has to overcome a pretty significant 00:00:06.730 --> 00:00:08.920 problem before it can become… 00:00:08.920 --> 00:00:10.240 a radio. 00:00:10.240 --> 00:00:12.780 See, like any radio, I can choose to listen to 00:00:12.780 --> 00:00:17.800 different stations by just touching that dial, and just like that the radio is able to zero 00:00:17.800 --> 00:00:21.020 in on a single frequency and allow me to listen to… 00:00:21.020 --> 00:00:22.700 just that station. 00:00:22.700 --> 00:00:26.070 But have you ever stopped to think about how it’s doing that? 00:00:26.070 --> 00:00:30.350 Right now the radio is being bombarded by electromagnetic radiation, the vast majority 00:00:30.350 --> 00:00:34.760 of which it can’t do anything with. Sure, when it was made, it just had to deal with 00:00:34.760 --> 00:00:39.440 AM radio signals for the most part, but soon came FM, Television, microwave transmissions, 00:00:39.440 --> 00:00:46.620 cell phones, WiFi, and the list goes on. And yet, it still functions perfectly as an AM radio. 00:00:47.680 --> 00:00:53.960 [various stations tuning in and out] 00:00:55.680 --> 00:00:57.560 It’s able to toss out all of that nonsense 00:00:57.560 --> 00:01:00.690 and focus on just what I want it to focus on. 00:01:00.690 --> 00:01:04.540 To do that - and to do that well - it’s being a lot more clever than at first you 00:01:04.540 --> 00:01:10.010 might imagine. Obviously it has the ability to tune into one frequency - and thus tune 00:01:10.010 --> 00:01:15.010 out the rest. But you might be surprised to learn that it goes beyond being selective 00:01:15.010 --> 00:01:19.830 in the frequencies it receives. What it’s actually doing is creating a second frequency 00:01:19.830 --> 00:01:24.930 of its own, mixing that with all the incoming signals, and the resulting composite signal 00:01:24.930 --> 00:01:26.840 is then filtered and amplified. 00:01:26.840 --> 00:01:29.000 That’s what superheterodyne means - 00:01:29.000 --> 00:01:32.910 it’s not just a flowery marketing term like "Spectrohydramagnetic," 00:01:32.910 --> 00:01:37.700 it actually comes from supersonic heterodyne. Supersonic means that it’s above the human 00:01:37.700 --> 00:01:38.480 hearing range. 00:01:39.340 --> 00:01:41.740 Today we’d be more likely to call it ultrasonic 00:01:41.740 --> 00:01:42.920 but it was the 1900’s and 00:01:42.920 --> 00:01:43.560 radios were getting 00:01:43.560 --> 00:01:44.420 CRAZY! 00:01:44.780 --> 00:01:49.780 Hetero means different, and dyne means frequency. So it’s a super (ultra) sonic 00:01:49.780 --> 00:01:52.160 frequency that’s different. 00:01:52.160 --> 00:01:54.600 OK, how does that help? 00:01:54.610 --> 00:01:59.540 It helps through creating an intermediary beat frequency. A beat frequency is what happens 00:01:59.540 --> 00:02:04.960 when two dissimilar frequencies cause interference with one another. To demonstrate, I’ve placed 00:02:04.960 --> 00:02:10.530 these two speakers in front of me, facing each other. This one is playing a 400 hz tone, 00:02:10.530 --> 00:02:13.900 and the other is playing a 401 hz tone. 00:02:13.900 --> 00:02:17.740 Notice how the sound appears to fade in and out once per second. 00:02:18.320 --> 00:02:22.600 [a solid tone with a throbbing undulation] 00:02:23.560 --> 00:02:25.320 On their own, they sound pretty 00:02:25.330 --> 00:02:30.140 much exactly the same. But together, that slight misalignment of their frequencies is 00:02:30.140 --> 00:02:31.390 producing a beat. 00:02:31.390 --> 00:02:35.930 And the frequency of that beat is determined by the difference between the two frequencies. 00:02:35.930 --> 00:02:40.840 The beat occurred once per second because there was a discrepancy of one hertz. 00:02:40.840 --> 00:02:41.720 Hert? 00:02:42.100 --> 00:02:45.140 Increase the frequency from 401 to 404, 00:02:45.140 --> 00:02:48.240 and the frequency of this speaker could not be found. 00:02:48.240 --> 00:02:49.220 I’m just kidding, 00:02:49.220 --> 00:02:50.540 Internet humor! 00:02:50.780 --> 00:02:52.150 Now, since there’s a difference of four 00:02:52.150 --> 00:02:55.880 hertz, that beat frequency becomes four hertz. 00:02:57.100 --> 00:03:02.880 [the same tone with much faster undulation] 00:03:03.540 --> 00:03:07.739 To see what’s happening, I’ll make the same tones in Audacity and put them next to 00:03:07.739 --> 00:03:13.610 each other. If we zoom in on the waveforms, we can see that they’re not quite aligned. 00:03:13.610 --> 00:03:18.900 If we were to add them together, at some points they combine to make a larger amplitude. That’s 00:03:18.900 --> 00:03:23.640 constructive interference. At others they combine to actually zero out the signal. 00:03:23.640 --> 00:03:28.760 That's destructive interference. When I actually combine them, we end up with a new waveform 00:03:28.760 --> 00:03:31.319 that is pulsing 4 times per second. 00:03:31.940 --> 00:03:34.240 [the same throbbing tone] 00:03:34.880 --> 00:03:39.220 The real fun comes when we get into higher frequencies. This speaker is playing a tone 00:03:39.220 --> 00:03:46.140 at 5,000 hertz or 5 kilohertz. This one will play at 5,400 hertz, or 5.4 kilohertz. 00:03:46.780 --> 00:04:04.620 ♫ low-fidelity guitar music ♫ 00:04:08.120 --> 00:04:10.560 [ an ear splitting tone ] 00:04:10.560 --> 00:04:15.740 [ a lower tone appears ] 00:04:19.960 --> 00:04:23.999 The principle of constructive and destructive interference producing a new signal at a different 00:04:23.999 --> 00:04:29.779 frequency also applies to radio waves. If you inject a signal that is, say, 100 kilohertz 00:04:29.779 --> 00:04:34.979 offset from another, it will produce a beat frequency of 100 kilohertz. Can you see how 00:04:34.979 --> 00:04:38.260 that might be useful? Let’s go back to the radio. 00:04:38.260 --> 00:04:42.900 Say we want to tune to 780 AM, that’s 780 kilohertz. 00:04:42.900 --> 00:04:46.490 One way to do that would be to create a filter which can block out every 00:04:46.490 --> 00:04:49.199 frequency that isn’t 780 kilohertz. 00:04:49.199 --> 00:04:54.129 Now, we can do that, but to design a filter that can shift which frequency it lets through 00:04:54.129 --> 00:04:59.400 (and thus allows you to tune the radio) is tricky. It was done before, and until the 00:04:59.400 --> 00:05:04.610 invention of the superheterodyne radio receiver in 1917/18/20, that was essentially how all 00:05:04.610 --> 00:05:09.280 radios worked. But the trouble was that these filters weren’t that precise, and because 00:05:09.280 --> 00:05:14.039 of that the radio wouldn’t be all that sensitive. Competing signals at similar frequencies might 00:05:14.039 --> 00:05:18.409 blend together, and the signals themselves would have to be pretty strong in order for 00:05:18.409 --> 00:05:23.309 the radio to pick them up. It was always a tradeoff between the precision of the filter, 00:05:23.309 --> 00:05:25.449 and the strength of the received signal. 00:05:25.449 --> 00:05:30.520 Further complicating things, these early radios also had trouble processing the signals themselves. 00:05:30.520 --> 00:05:35.360 The amplification circuitry of the day didn’t like dealing with the (at the time) very high 00:05:35.360 --> 00:05:40.229 frequency of radio signals, as it was hard to design a vacuum tube that could effectively 00:05:40.229 --> 00:05:43.759 amplify these signals in order to produce audible sound. 00:05:43.759 --> 00:05:48.669 But what if instead of trying to design a radio that works in radio’s frequency range, 00:05:48.669 --> 00:05:53.409 we take a completely different approach? What if we design a filter that looks for just 00:05:53.409 --> 00:05:59.909 one specific frequency, AND we put that frequency outside the normal broadcast range? That would 00:05:59.909 --> 00:06:04.749 solve the interference and signal strength problems, and if we were able to lower the 00:06:04.749 --> 00:06:10.080 frequency of the signal, the radio could amplify it and make sound more easily. Additionally, 00:06:10.080 --> 00:06:14.499 we could make the filter more precise because it is set to a fixed frequency rather than 00:06:14.499 --> 00:06:17.360 needing to move around as the radio is tuned. 00:06:17.360 --> 00:06:21.909 And that’s what superheterodyne radios do. Take a look at the schematic for this radio, 00:06:21.909 --> 00:06:29.419 a Philco 42-PT-7, and you’ll notice it says intermediate frequency: 455 K. C. This stood 00:06:29.419 --> 00:06:33.839 for kilocycles, which is another word for kilohertz. I like these old schematics because 00:06:33.839 --> 00:06:38.719 they often contain outdated terms, like how what today we call capacitors were then called 00:06:38.719 --> 00:06:39.719 condensers. 00:06:39.719 --> 00:06:46.699 Anyway, what this means is that, in a way, this radio is always tuned to 455 kilohertz. 00:06:46.699 --> 00:06:51.779 Now that may seem pretty useless, but it’s actually genius. See, this tube here, which 00:06:51.779 --> 00:06:56.789 is a pentagrid converter tube, is able to generate its own sine wave output at whatever 00:06:56.789 --> 00:07:01.939 frequency we like. This component here, called the tuning capacitor, works with the tube 00:07:01.939 --> 00:07:06.619 to change the frequency it generates as the plates move in and out (which therefore changes 00:07:06.619 --> 00:07:12.089 its capacitance value). By turning the tuning knob, the capacitance value of this component 00:07:12.089 --> 00:07:17.600 changes, and this in turn increases or decreases the frequency being generated by the oscillator tube. 00:07:17.600 --> 00:07:23.379 If we want to tune to 780 AM, we can take all of the incoming signals, mix them together, 00:07:23.380 --> 00:07:30.280 and inject a sine wave at 1,235 kilohertz. The interference between our desired frequency, 00:07:30.280 --> 00:07:36.980 780, and the frequency we’re injecting, 1,235, will produce a beat frequency at 455 00:07:36.990 --> 00:07:42.389 kilohertz. And, since this tube here, the intermediate frequency amplifier, is tuned 00:07:42.389 --> 00:07:48.580 to only pass through signals at 455 kilohertz, this newly created composite signal will pass 00:07:48.580 --> 00:07:53.240 right through it. It then gets rectified by the detector tube, amplified by the output 00:07:53.240 --> 00:07:57.389 tube, and with the help of some other smoothing capacitors, the resulting output gets sent 00:07:57.389 --> 00:08:00.389 to the loudspeaker, and we get audible sound. 00:08:00.389 --> 00:08:04.639 See how cool this is? We don’t have to change the frequency we’re looking for in order 00:08:04.639 --> 00:08:10.309 to change which frequency we receive. By modifying the frequency that we inject into the mix, 00:08:10.309 --> 00:08:14.809 we can single one out because the combined signals will produce beat interference which 00:08:14.809 --> 00:08:17.700 will pass right through our intermediary filter. 00:08:17.700 --> 00:08:18.660 It’s science! 00:08:18.660 --> 00:08:19.620 It’s math! 00:08:19.620 --> 00:08:20.820 It’s fantastic! 00:08:20.820 --> 00:08:25.340 But, this isn’t to say that we can just inject a signal and everything is fine and dandy. 00:08:25.340 --> 00:08:27.040 There’s one slight problem. 00:08:27.040 --> 00:08:30.189 See, if we inject our 1,235 kilohertz signal without 00:08:30.189 --> 00:08:35.630 doing any other filtering, we’ll actually get beat frequencies from two incoming frequencies. 00:08:35.630 --> 00:08:41.669 One from 780 kilohertz, and the other from 1,690 kilohertz. Both of these are 455 kilohertz 00:08:41.669 --> 00:08:46.240 away from the frequency we’re injecting. So, the radio will typically need some other filtering 00:08:46.240 --> 00:08:48.960 on its input to block out one of these frequencies, 00:08:48.960 --> 00:08:51.740 and thus prevent the second signal from coming through. 00:08:51.740 --> 00:08:55.550 That second signal is often called an image, because it’s analogous to a mirror 00:08:55.550 --> 00:09:00.950 image of the intended frequency - flipped across the axis of the injected frequency. 00:09:00.950 --> 00:09:05.900 This first filter doesn’t need to be precise, but it does need to move with the tuning dial. 00:09:05.900 --> 00:09:10.170 Otherwise, the radio couldn’t receive the full frequency range. If you take a look at 00:09:10.170 --> 00:09:15.010 the schematic, you can see that the tuning condenser (slash capacitor) takes a role in 00:09:15.010 --> 00:09:20.710 two places - it functions both as a filter on the antenna and as a generator of sorts 00:09:20.710 --> 00:09:25.950 for the oscillator tube. But to be clear - that first filter is really sloppy. It just has 00:09:25.950 --> 00:09:31.210 to block out frequencies that are some 400 kilohertz or further away from the local oscillator’s 00:09:31.210 --> 00:09:32.210 frequency. 00:09:32.210 --> 00:09:35.780 Now some of you might be asking, how can we change the frequency of the signal without 00:09:35.780 --> 00:09:40.530 affecting the sound? Well, that has to do with the way the signal is actually encoded. 00:09:40.530 --> 00:09:45.280 Remember, the frequency of the radio signal is simply a carrier. For amplitude modulation, 00:09:45.280 --> 00:09:50.720 it’s the overall intensity of the signal over time that encodes sound. So we can shift 00:09:50.720 --> 00:09:54.920 the carrier frequency up and down, without actually changing the information that it 00:09:54.920 --> 00:09:59.800 carries. The beat frequency generated by the mixing of the signals will follow the same 00:09:59.800 --> 00:10:02.430 exact pattern as the original carrier wave. 00:10:02.430 --> 00:10:06.640 The superheterodyne radio receiver solved one of the fundamental problems of radio in 00:10:06.640 --> 00:10:11.720 a very clever way. It proved to be such a good method of isolating frequencies that 00:10:11.720 --> 00:10:16.760 it became the de facto tuner design for many decades, continuing into the television age 00:10:16.760 --> 00:10:19.240 and through the transistorization of radio, 00:10:19.240 --> 00:10:22.400 though these days tuning is largely done with software. 00:10:22.400 --> 00:10:26.580 It wasn’t perfect though. The presence of a local oscillator in the radio meant that 00:10:26.580 --> 00:10:31.190 it created a bit of electromagnetic radiation of its own, which could interfere with other 00:10:31.190 --> 00:10:37.380 radios nearby. To be fair, nearby usually meant within the same room, but it’s only 00:10:37.380 --> 00:10:42.600 fair to say that there were downsides to this approach. Still, thanks to the Superhet as 00:10:42.600 --> 00:10:47.400 it's sometimes called, tuning to your favorite station became a piece of cake. 00:10:47.400 --> 00:10:50.700 Thanks for watching, and I hope you enjoyed this video. I’ve been sitting on it 00:10:50.700 --> 00:10:54.940 for a long time, as my earliest videos dealt with the history of artificial sound. 00:10:54.940 --> 00:10:59.600 You can watch them if you like, but be warned--they are pretty cringetastic. 00:10:59.600 --> 00:11:00.990 And that’s my assessment! 00:11:00.990 --> 00:11:04.820 As always, thank you to everyone who supports the channel on Patreon, especially the fine 00:11:04.820 --> 00:11:08.980 folks that are scrolling up your screen. With the generous support of people like you, 00:11:08.980 --> 00:11:12.640 Technology Connections has gone from my hobby to my job! 00:11:12.640 --> 00:11:14.860 And I’m very grateful for your support. 00:11:14.870 --> 00:11:18.710 If you would like to support the channel and get perks like early video access, behind-the-scenes 00:11:18.710 --> 00:11:22.630 footage, as well as the inside scoop on the latest projects, please check out my Patreon 00:11:22.630 --> 00:11:25.880 page. Thank you for your consideration, and I’ll see you next time! 00:11:26.460 --> 00:11:29.000 ♫ incomprehensibly smooth jazz ♫ 00:11:29.980 --> 00:11:32.900 It was always a trade-off betreen... 00:11:34.080 --> 00:11:35.640 See you can’t think that things are going well 00:11:35.640 --> 00:11:38.160 because as soon as you think that, it falls apart. 00:11:38.160 --> 00:11:39.820 Nope. I missed a “The” 00:11:39.820 --> 00:11:41.870 I love these old schematics because they often 00:11:41.870 --> 00:11:44.840 contain outdated terms, like how we today call 00:11:44.840 --> 00:11:47.600 bweeeehhh, bweeeehhh 00:11:47.600 --> 00:11:48.880 Take a look at the schamat… 00:11:49.640 --> 00:11:50.800 mouth noises! 00:11:50.800 --> 00:11:52.440 We need to restart. 00:11:52.440 --> 00:11:58.280 ...and one is playing a 400 Hz tone, and the other is playing a 401 Hz tone. 00:11:58.280 --> 00:12:01.780 The wires aren’t hooked up yet so you’re gonna know I’m faking this! 00:12:02.420 --> 00:12:03.680 Eurghhh! 00:12:03.680 --> 00:12:06.000 Because there was a discrepen…. 00:12:06.820 --> 00:12:08.880 I mixed my tenses 00:12:09.600 --> 00:12:12.380 [clears throat, inhales as if about to start speaking] 00:12:12.380 --> 00:12:14.940 Oh yeah, let’s move this over.