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I don’t know if you’ve realized this…

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but vinyl records have gotten pretty popular!

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Analog is just so much more… real, man.

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And just like those newfangled streaming music services that can put
a different audio signal into each of your airpods so the music can do

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cahraaazy effects like that
[pans rapidly left and right]

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ye old LPs...

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they hold stereo sound.

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That’s kinda weird, though, isn’t it?

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Like, stereophonic means we have two audio channels.

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And if you look to other old sound formats,
you’ll find two separate audio tracks to carry those two channels.

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The audio head of a compact cassette deck has two distinct pickups,

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each one detecting the signal on a dedicated left and right audio track
contained on the tape.

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Same goes for reel-to-reel tape recorders, though here the heads are separated and the left and right channels are interleaved on the tape for...

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reasons.

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The analog soundtrack on motion picture film prints
has two obvious channels, as well.

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And skipping ahead to the digital age, the datastream encoded on a 
compact disc contains two distinct audio channels

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that are decoded and reproduced
by the CD player upon playback.

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Same goes for pretty much every digital sound format since.

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But a record like this has but the one groove that's played by a single stylus.

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Somehow, the action of dragging that single stylus through the single groove

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results in two audio channels that make the listening experience 
just that more real.

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What’s going on, here?

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Well, you might think it could be some sort of sound signal
modulation trickery, but it’s actually much simpler than that.

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You probably already know that the groove wiggles
the stylus to make the sound signal.

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And it turns out there’s more than just one way to wiggle.

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I covered this way way back when this channel was just a baby,
 but I didn’t really give it the time it’s worthy of.

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So I’m doing it now!

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Speaking of going way way back,

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Thomas Edison’s original phonograph
pressed sound vibrations into a wax cylinder.

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A sound-collecting horn led to a diaphragm
that would vibrate just like your eardrums do

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in response to changing sound pressure levels.

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That diaphragm was then attached to a sharp stylus
which would copy those vibrations as a rapid up-and-down motion.

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When this stylus was pressed into a rotating soft wax cylinder,
it would carve a groove into the surface of the wax.

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Moving that stylus laterally across the rotating cylinder
created a long, spiral groove.

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And all the while it carved the groove, sound vibrations collected by the horn
caused the stylus to move up and down,

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and the resulting variations in the groove’s depth
would become a record of those sound vibrations.

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That’s why it’s called a record!

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I’m skipping over plenty of details here,

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but a mold could be made of this soft wax record so that resilient copies,
like this, could be mass produced.

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Spin this record below a reproducing phonograph and its stylus,
as it floats over the bumpy groove,

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will move up and down in the same manner as the recording stylus did.

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That imparts vibrations in a second diaphragm
which makes tiny little sound pressure waves,

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and with the help of an amplifying horn the recorded sound could be reproduced.

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[very warbly, low-fidelity violin playing]

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Not terribly well, but it’s the 19th century still.

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Don’t expect too much.

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But then, this guy Berliner was like

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“Cylinders? Really?”

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and he thought of a better way to do this.

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Instead of wiggling a stylus up and down,
you could do a side-to-side wiggle instead.

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That would allow you to carve a deep, spiral groove into the surface of a flat disc which would simplify everything a great deal.

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For starters, mass-producing hollow, cylindrical records
with fine details on its surface,

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although Edison did figure it out, was, uh

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quite the process.

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A flat disc, meanwhile, could simply be stamped.

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Much cheaper, easier, and faster.

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Discs were also much easier to handle,
less fragile than cylinders, and took up a fraction of the space

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in part because you could also - get this -

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put a second recording on the other side of the disc!

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Whoah.

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A side-to-side wiggly groove also greatly simplified the record players.

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Edison’s design needs a worm gear to move the reproducing assembly along

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because any significant weight on the groove would destroy the recording - 
so the stylus just floats over the cylinder.

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But the reproducer of disc phonographs was
free to rest all its weight directly on the record.

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It was the walls of the groove that held the sound, not the floor,

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so a steel needle could simply drag along the bottom without damaging the recording

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(although playing a record would wear down that needle
and you needed to replace it quite frequently).

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Needles were cheap, though, and this arrangement
meant the groove would advance the reproducer all by itself,

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eliminating the need for the worm gear mechanism and its associated complexity.

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The disc was obviously the better option so
much to Edison’s chagrin the cylinder was done for.

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As the years went on and we developed electronic sound amplification
(thanks in no small part to radio),

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record players started using much smaller and lighter tone arms
fitted with these newfangled phonograph pickups

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which turned the groove’s wiggling into an electrical signal
that we would then amplify and pump into loudspeakers.

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There’s a few different ways we can do that,

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including attaching the stylus to a magnet which moves within a coil of wire,

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therefore producing a teeny bit of voltage in that wire as it vibrates.

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But you could also attach the stylus to the wire instead of the magnet,

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or even use piezoelectric crystals.

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There were a ton of ways we tried doing this.

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Anyway, soon, the tone arms and pickups would become so light
that a tiny little sapphire or diamond stylus could be used.

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That allowed for the use of softer record
materials with much smaller grooves

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that extended the runtime of recordings while also increasing audio fidelity.

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But the fundamental basics of the technology hadn’t changed at all.

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Sound was still stored in the walls of the groove,
wiggling the stylus left and right as it flew by.

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A bunch of refinements over fifty years or
so improved the process and made it sound better than ever

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but it was still the same concept as Berliner’s original discs.

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All we’ve really accomplished is make the groove’s wiggling more precise
so it can better capture the nuances of a sound recording,

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and we made the thing that the groove wiggles more sensitive to its wiggling
so it can reproduce it just as wiggly-well.

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However, now that we’ve made the tone arm
so light and the stylus so small,

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we don’t have to worry about damage to the record nearly as much.

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And since this world we live in is three-dimensional,

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we could fit a second signal into the same groove
if we revisit Edison’s idea and add a little…

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depth.

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When playing a record, the groove’s direction
of motion takes up one of our three dimensions,

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but we still have two axes of motion available to the stylus.

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If we designed a pickup which could generate
two independent signals from the stylus:

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one for its lateral movement
and another for its vertical movement,

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we could recover a stereo signal from a single groove.

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So that’s what we did.

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Sort of.

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What I just described is more than a little askew with reality.

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I’ll explain in a moment.

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It turns out that English engineer Alan Blumlein,

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who basically created the concept of stereo sound,

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would describe in a 1931 patent the exact method by which stereo records would eventually be made starting in the late 1950’s.

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Pretty wild.

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Stereo records do contain a combination of
vertical and lateral movement in their grooves,

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but it’s a bit more complicated than

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“the left channel is vertical movement and the right channel is lateral movement.”

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That would work, but not very well.

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For a start, the two channels would sound quite different from one another
due to differing distortion profiles,

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so you couldn’t get a very pleasant recording.

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And secondly, a record made this way wouldn’t
work with a conventional phonograph.

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Up ‘til now, the groove only ever wiggled left and right, so that’s the only direction of movement a conventional phonograph cartridge can detect.

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If an uppy-downy lefty-righty stereo record was placed on an old record player,

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you’d only hear whichever channel was the lefty-righty one - and nobody wants that.

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So instead, we take that up-down-left-right concept and go like this.

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*wrooup*

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Now, each of the two channels is recorded on the disc as a diagonal movement,

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and the walls of the grove will sort of move the stylus around every which way.

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To detect this two-dimensional movement, 
a stereo phonograph cartridge has two pickups placed at right-angles to one another.

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Both pickups share the single stylus, and this whole contraption
is set at a 45 degree offset with respect to the groove.

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With this arrangement,

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each pickup is affected by both vertical and horizontal movement.

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And when you hear that, you might wonder how this works at all.

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See, when we want to generate a signal on only the left channel,

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the stylus needs to move back and forth in a downward and leftward motion.

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Doing that will only cause movement in one of the pickup coils,

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with the other one oblivious to what’s happening.

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Then, of course, the stylus needs to vibrate along the opposite diagonal,

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in a downward and rightward motion,

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to produce a sound on only the right channel.

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This might sound bonkers and overly complicated,
but this arrangement has several advantages.

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First, it works perfectly with old, mono recordings.

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When playing a mono record, its purely lateral
vibrations will affect both pickups equally

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and produce an essentially identical signal in the left and right channels.

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You won’t have to deal with one dead channel which nobody likes;
both channels will play the same thing.

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Better yet, the 45 degree offset means that
so long as we are careful in the mastering process,

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we can produce stereo records that
are actually backward-compatible with mono phonographs.

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Just like in the acoustic phonograph days,

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the process of making a record starts by carving a spiral groove in a blank disc with a cutting stylus,

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but now that stylus is hooked up to high-speed actuators
that precisely vibrate it based on an audio signal.

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For stereo recordings, the cutting stylus
is attached to two actuators,

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one for each channel,

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that are arranged with the same 45 degree offset of the playback cartridge.

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The actuator for the left channel vibrates
the cutting stylus diagonally, downward and leftward,

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with the right channel actuator cutting in the opposite diagonal.

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And here’s why this is such a big deal:

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So long as we ensure the correct signal polarity, 
when the left and right channel signals are identical,

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that will cancel out vertical movement of the cutting stylus
and we will only get lateral movement.

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See, if this actuator is pushing while the other is pulling,

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both of those actions result in the stylus moving to the left,

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but the downward motion imparted by this actuator is perfectly canceled out by the upward motion of the other.

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The cutting head will only move vertically
when there is a difference in intensity between the two signals the actuators receive,

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as that causes an imbalance in pulling force between them.

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See, stereo sound could be made from two completely
independent signals like we see in magnetic tape,

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but it can also come from a mono signal
combined with a stereo-difference signal.

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And that’s what’s actually contained on a stereo record — it’s a mono signal feeding both loudspeakers equally when the groove moves side-to-side,

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but when there’s some vertical movement goin’ on,
the signal gets pushed towards the left or the right channel.

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Now, of course, this is happening instantaneously,

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so it’s not like we’re just panning a signal left and right;

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every individual movement in the stylus —
meaning every sound detail the record contains

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— can be pushed to the left or the right channel and to virtually any degree.

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Just gotta do a little extra wiggling.

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And I should say it’s not like there’s
any signal processing going on here,

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that’s just how the signals get generated in a stereo cartridge

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when this more complex groove moves the stylus around
in two dimensions rather than just one.

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But the vertical component arguably doesn’t contain any sound information;
 it just pushes the sound towards the left or the right channel.

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I’m sure semantics discussions in the comments will be fantastic.

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Anyway, aside from this method doing a fine
job of making a stereo signal possible,

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if you play one of these new stereo records on an old mono phonograph,

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well you won’t get stereo sound, obviously,
but you won’t be missing anything either.

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A sound that’s only supposed to be on the left channel
still vibrates the stylus left and right,

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and the same goes for right-channel sounds.

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So old mono phonographs will still let you hear everything on the record,

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meaning these new stereo records are backward compatible.

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The vertical component in their grooves just
doesn’t do anything because an older phonograph doesn’t know what that is.

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It has no means to detect it.

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Now, if we flipped the polarity of one of
the actuators in the cutting process,

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then equal signal intensity on both channels
would cancel out lateral movement

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and would be recorded purely as changes in depth.

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We’d still get a kind-of functional stereo record out of this,

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but it would sound extremely weird,
especially when played on a mono phonograph.

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Mono record players would only pick up sounds
that are panned left or right,

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and anything in the virtual center would be silent.

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That’s effectively what listening to the raw stereo-difference channel would sound like -

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and here, I can simulate this in Audacity!

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Let’s see, just split out the track,

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invert one of em, smoosh it back together into mono,

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OK, here’s what the recording normally sounds like:

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♫ some percussion ♫

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ooh now there's a groovy bass line!

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cymbals build, got some congas,

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MM, got some HORNS!

195
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ba da ba da

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ba da ba da

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ba dum...

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And here’s a simulated stereo-difference channel.

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♫ same percussion, but now it's weirdly muffled and sounds like it's coming from a distant bathroom ♫

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hey wait where'd the bass line go?

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cymbaaalllLLLLSSSS,

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HORNS

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So it's the same song, for sure, but it's, like...

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haunted.

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So, that’s what’s going on with stereo records.

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In a way, it’s a clever combination of Edison’s original idea
and Berliner’s improvement.

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I don’t think Alan Blumlein was thinking
of that at all when he first concocted this idea,

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but that won’t stop me from making the connection.

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Now, this method is not perfect.

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Complete stereo channel separation just isn’t possible when your sound is coming from physical movements.

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Even if you nailed the angles just right and the record was mastered perfectly,

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some vibrations from one of the pickups are liable to make
their way through the cartridge and into the other.

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So no matter how staunchly someone defends the sound of vinyl,

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records are objectively pretty bad at isolating the two channels.

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But it’s still really clever, and the effect of having two channels is just that - an effect.

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It doesn’t need to be perfect to make a huge difference to the listening experience.

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Ya like jazz?

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♫ distortedly smooth jazz ♫

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And secondly, a record made this way wouldn’t work at all with conven--

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I shouldn’t add that,

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Don’t add words!

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...signal possible.

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If you play one of these new stereo records on a old mono phonogr -

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aw, crap

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Spin this sound record below a reproducing phonograph and its stylus as it fff…

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[defeated]

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…and do cahraaaaaazy effects like that,

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ye old LPs hold [clank]

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dangit

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cahraaaaaazy effects like that, ye old LPs…

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[clank]

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I just did it again!

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So normally, I'd put some sort of gag here, but this time, I think we should do it

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IN                             IN
   STEREO                   STEREO

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THAT'S RIGHT, TWO                           THAT'S RIGHT, TWO
CAPTIONS, ONE FOR EACH EYE     CAPTIONS, ONE FOR EACH EYE!

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HOLD YOUR PHONE UP TO YOUR        HOLD YOUR PHONE UP TO YOUR
FACE AND IT'S IN 3D!                           FACE AND IT'S IN 3D!

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just kidding, don't you look silly!