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Yeah. It’s a great word, isn’t it?

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It may not seem like it, but even an antique
radio like this has to overcome a pretty significant

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problem before it can become…

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a radio.

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See, like any radio, I can choose to listen to

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different stations by just touching that dial,
and just like that the radio is able to zero

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in on a single frequency and allow me to listen
to…

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just that station.

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But have you ever stopped to 
think about how it’s doing that?

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Right now the radio is being bombarded by
electromagnetic radiation, the vast majority

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of which it can’t do anything with. Sure,
when it was made, it just had to deal with

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AM radio signals for the most part, but soon
came FM, Television, microwave transmissions,

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cell phones, WiFi, and the list goes on. And
yet, it still functions perfectly as an AM radio.

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[various stations tuning in and out]

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It’s able to toss out all of that nonsense

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and focus on just what I want it to focus
on.

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To do that - and to do that well - it’s
being a lot more clever than at first you

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might imagine. Obviously it has the ability
to tune into one frequency - and thus tune

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out the rest. But you might be surprised to
learn that it goes beyond being selective

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in the frequencies it receives. What it’s
actually doing is creating a second frequency

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of its own, mixing that with all the incoming
signals, and the resulting composite signal

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is then filtered and amplified.

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That’s what superheterodyne means -

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it’s not just a flowery marketing term like "Spectrohydramagnetic,"

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it actually comes from supersonic heterodyne.
Supersonic means that it’s above the human

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hearing range.

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Today we’d be more likely to call it ultrasonic

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but it was the 1900’s and

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radios were getting

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CRAZY!

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Hetero means different, and dyne means
frequency. So it’s a super (ultra) sonic

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frequency that’s different.

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OK, how does
that help?

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It helps through creating an intermediary
beat frequency. A beat frequency is what happens

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when two dissimilar frequencies cause interference
with one another. To demonstrate, I’ve placed

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these two speakers in front of me, facing
each other. This one is playing a 400 hz tone,

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and the other is playing a 401 hz tone.

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Notice how the sound appears to 
fade in and out once per second.

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[a solid tone with a throbbing undulation]

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On their own, they sound pretty

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much exactly the same. But together, that
slight misalignment of their frequencies is

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producing a beat.

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And the frequency of that beat is determined
by the difference between the two frequencies.

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The beat occurred once per second because
there was a discrepancy of one hertz.

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Hert?

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Increase the frequency from 401 to 404,

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and the frequency of this speaker could not be found.

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I’m just kidding,

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Internet humor!

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Now, since there’s a difference of four

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hertz, that beat frequency becomes four hertz.

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[the same tone with much faster undulation]

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To see what’s happening, I’ll make the
same tones in Audacity and put them next to

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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.

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If we were to add them together, at some points
they combine to make a larger amplitude. That’s

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constructive interference. At others they
combine to actually zero out the signal.

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That's destructive interference. When I actually
combine them, we end up with a new waveform

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that is pulsing 4 times per second.

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[the same throbbing tone]

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The real fun comes when we get into higher
frequencies. This speaker is playing a tone

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at 5,000 hertz or 5 kilohertz. This one will
play at 5,400 hertz, or 5.4 kilohertz.

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♫ low-fidelity guitar music ♫

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[ an ear splitting tone ]

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[ a lower tone appears ]

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The principle of constructive and destructive
interference producing a new signal at a different

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frequency also applies to radio waves. If
you inject a signal that is, say, 100 kilohertz

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offset from another, it will produce a beat
frequency of 100 kilohertz. Can you see how

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that might be useful? Let’s go back to the
radio.

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Say we want to tune to 780 AM, that’s 780 kilohertz.

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One way to do that would be
to create a filter which can block out every

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frequency that isn’t 780 kilohertz.

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Now, we can do that, but to design a filter
that can shift which frequency it lets through

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(and thus allows you to tune the radio) is
tricky. It was done before, and until the

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invention of the superheterodyne radio receiver
in 1917/18/20, that was essentially how all

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radios worked. But the trouble was that these
filters weren’t that precise, and because

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of that the radio wouldn’t be all that sensitive.
Competing signals at similar frequencies might

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blend together, and the signals themselves
would have to be pretty strong in order for

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the radio to pick them up. It was always a
tradeoff between the precision of the filter,

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and the strength of the received signal.

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Further complicating things, these early radios
also had trouble processing the signals themselves.

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The amplification circuitry of the day didn’t
like dealing with the (at the time) very high

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frequency of radio signals, as it was hard
to design a vacuum tube that could effectively

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amplify these signals in order to produce
audible sound.

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But what if instead of trying to design a
radio that works in radio’s frequency range,

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we take a completely different approach? What
if we design a filter that looks for just

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one specific frequency, AND we put that frequency
outside the normal broadcast range? That would

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solve the interference and signal strength
problems, and if we were able to lower the

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00:06:04,749 --> 00:06:10,080
frequency of the signal, the radio could amplify
it and make sound more easily. Additionally,

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we could make the filter more precise because
it is set to a fixed frequency rather than

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needing to move around as the radio is tuned.

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And that’s what superheterodyne radios do.
Take a look at the schematic for this radio,

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a Philco 42-PT-7, and you’ll notice it says
intermediate frequency: 455 K. C. This stood

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for kilocycles, which is another word for
kilohertz. I like these old schematics because

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they often contain outdated terms, like how
what today we call capacitors were then called

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condensers.

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Anyway, what this means is that, in a way,
this radio is always tuned to 455 kilohertz.

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Now that may seem pretty useless, but it’s
actually genius. See, this tube here, which

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is a pentagrid converter tube, is able to
generate its own sine wave output at whatever

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frequency we like. This component here, called
the tuning capacitor, works with the tube

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to change the frequency it generates as the
plates move in and out (which therefore changes

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00:07:06,619 --> 00:07:12,089
its capacitance value). By turning the tuning
knob, the capacitance value of this component

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changes, and this in turn increases or decreases
the frequency being generated by the oscillator tube.

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If we want to tune to 780 AM, we can take
all of the incoming signals, mix them together,

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and inject a sine wave at 1,235 kilohertz.
The interference between our desired frequency,

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780, and the frequency we’re injecting,
1,235, will produce a beat frequency at 455

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kilohertz. And, since this tube here, the
intermediate frequency amplifier, is tuned

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to only pass through signals at 455 kilohertz,
this newly created composite signal will pass

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right through it. It then gets rectified by
the detector tube, amplified by the output

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tube, and with the help of some other smoothing
capacitors, the resulting output gets sent

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to the loudspeaker, and we get audible sound.

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See how cool this is? We don’t have to change
the frequency we’re looking for in order

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to change which frequency we receive. By modifying
the frequency that we inject into the mix,

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we can single one out because the combined
signals will produce beat interference which

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will pass right through our intermediary filter.

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It’s science!

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It’s math!

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It’s fantastic!

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But, this isn’t to say that we can just
inject a signal and everything is fine and dandy.

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There’s one slight problem.

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See, if we inject our 1,235 kilohertz signal without

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doing any other filtering, we’ll actually
get beat frequencies from two incoming frequencies.

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One from 780 kilohertz, and the other from
1,690 kilohertz. Both of these are 455 kilohertz

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away from the frequency we’re injecting.
So, the radio will typically need some other filtering

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on its input to block out one of these frequencies,

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and thus prevent the second signal from coming through.

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That second signal is often called
an image, because it’s analogous to a mirror

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image of the intended frequency - flipped
across the axis of the injected frequency.

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This first filter doesn’t need to be precise,
but it does need to move with the tuning dial.

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Otherwise, the radio couldn’t receive the
full frequency range. If you take a look at

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the schematic, you can see that the tuning
condenser (slash capacitor) takes a role in

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two places - it functions both as a filter
on the antenna and as a generator of sorts

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for the oscillator tube. But to be clear - that
first filter is really sloppy. It just has

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to block out frequencies that are some 400
kilohertz or further away from the local oscillator’s

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frequency.

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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

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affecting the sound? Well, that has to do
with the way the signal is actually encoded.

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Remember, the frequency of the radio signal
is simply a carrier. For amplitude modulation,

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it’s the overall intensity of the signal
over time that encodes sound. So we can shift

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the carrier frequency up and down, without
actually changing the information that it

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00:09:54,920 --> 00:09:59,800
carries. The beat frequency generated by the
mixing of the signals will follow the same

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00:09:59,800 --> 00:10:02,430
exact pattern as the original carrier wave.

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00:10:02,430 --> 00:10:06,640
The superheterodyne radio receiver solved
one of the fundamental problems of radio in

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00:10:06,640 --> 00:10:11,720
a very clever way. It proved to be such a
good method of isolating frequencies that

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it became the de facto tuner design for many
decades, continuing into the television age

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and through the transistorization of radio,

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though these days tuning is largely done with software.

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It wasn’t perfect though. The presence of
a local oscillator in the radio meant that

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it created a bit of electromagnetic radiation
of its own, which could interfere with other

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radios nearby. To be fair, nearby usually
meant within the same room, but it’s only

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fair to say that there were downsides to this
approach. Still, thanks to the Superhet as

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00:10:42,600 --> 00:10:47,400
it's sometimes called, tuning to your favorite
station became a piece of cake.

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Thanks for watching, and I hope you enjoyed
this video. I’ve been sitting on it

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00:10:50,700 --> 00:10:54,940
for a long time, as my earliest videos
dealt with the history of artificial sound.

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You can watch them if you like, but be warned--they
are pretty cringetastic.

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And that’s my assessment!

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As always, thank you to everyone who supports
the channel on Patreon, especially the fine

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00:11:04,820 --> 00:11:08,980
folks that are scrolling up your screen. With
the generous support of people like you,

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00:11:08,980 --> 00:11:12,640
Technology Connections has gone
 from my hobby to my job!

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00:11:12,640 --> 00:11:14,860
And I’m very grateful for your support.

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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

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00:11:18,710 --> 00:11:22,630
footage, as well as the inside scoop on the
latest projects, please check out my Patreon

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00:11:22,630 --> 00:11:25,880
page. Thank you for your consideration, and
I’ll see you next time!

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♫ incomprehensibly smooth jazz ♫

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It was always a trade-off betreen...

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See you can’t think that things are going well

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00:11:35,640 --> 00:11:38,160
because as soon as you think that, it
falls apart.

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00:11:38,160 --> 00:11:39,820
Nope. I missed a “The”

168
00:11:39,820 --> 00:11:41,870
I love these old schematics because they often

169
00:11:41,870 --> 00:11:44,840
contain outdated terms, like how we today
call

170
00:11:44,840 --> 00:11:47,600
bweeeehhh, bweeeehhh

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00:11:47,600 --> 00:11:48,880
Take a look at the schamat…

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00:11:49,640 --> 00:11:50,800
mouth noises!

173
00:11:50,800 --> 00:11:52,440
We need to restart.

174
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.

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00:11:58,280 --> 00:12:01,780
The wires aren’t hooked up yet so you’re gonna know
I’m faking this!

176
00:12:02,420 --> 00:12:03,680
Eurghhh!

177
00:12:03,680 --> 00:12:06,000
Because there was a discrepen….

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00:12:06,820 --> 00:12:08,880
I mixed
my tenses

179
00:12:09,600 --> 00:12:12,380
[clears throat, inhales as if about to start
speaking]

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00:12:12,380 --> 00:12:14,940
Oh yeah, let’s move this over.