How do vinyl records hold stereo sound?

I don't know if you've noticed... but

vinyl

records

have become very popular! Analog is so much more...real, man. And just like those newfangled music streaming services that can put a different audio signal into each of your airpods so the music can create amazing effects like those old LPs... they contain

stereo

sound

. That's a little strange, isn't it? Stereo means that we have two audio channels. And if you're looking for other older

sound

formats, you'll find two separate audio tracks to carry those two channels. The audio head of a compact cassette deck has two distinct pickups, each of which detects the signal on a dedicated left and right audio track contained on the tape.

The same goes for reel-to-reel tape recorders, although here the heads are separate and the left and right channels are interlaced on the tape for... reasons. The analog soundtrack of the film prints also has two obvious channels. And jumping into the digital age, the data stream encoded on a compact disc contains two distinct audio channels that the CD player decodes and plays during playback. The same goes for almost all digital sound formats since then. But a record like this only has one rhythm that is played with a single pencil. Somehow, the action of dragging that single stylus across the single groove results in two channels of audio that make the listening experience even more real.

What's going on here? Well, you might think it might be some kind of sound signal modulation trick, but it's actually much simpler than that. You probably already know that the groove moves the stylus to generate the sound signal. And it turns out there's more than one way to get around. I covered this topic when this channel was just a baby, but I didn't really give it the time it deserves. So I'm doing it now! Speaking of way back, Thomas Edison's original phonograph pressed sound vibrations into a wax cylinder. A horn that collected sound led to a diaphragm that vibrated like eardrums do in response to changes in sound pressure levels.

That diaphragm was then attached to a sharp pencil that would copy those vibrations as a rapid up and down motion. When this pencil was pressed into a rotating cylinder of soft wax, it carved a groove into the surface of the wax. Moving that pencil laterally across the rotating barrel created a long spiral groove. And as it carved the groove, the sound vibrations picked up by the horn would cause the needle to move up and down, and the resulting variations in the depth of the groove would become a record of those sound vibrations. That's why it's called a disk!

I'm skipping a lot of details here, but a mold could be made from this soft wax disc so sturdy copies like this could be mass-produced. Spin this record under a phonograph player and your stylus, as it floats over the bumpy groove, will move up and down in the same way that the recording stylus did. This transmits vibrations into a second diaphragm which produces small sound pressure waves and, with the help of an amplifier horn, the recorded sound can be reproduced. Not very well, but we are still in the 19th century. Do not expect too much. But of course, this guy from Berlin said: “Cylinders?

Really?" and thought of a better way to do this. Instead of moving the stylus up and down, you could move it from side to side. That would allow you to carve a deep spiral groove into the surface of a flat disc , which would make everything much simpler. To begin with, mass-producing hollow cylindrical discs with fine details on their surface, although Edison discovered it, was a process. Meanwhile, a flat disc could simply be stamped. faster. The discs were also much easier to handle, less fragile than cylinders and took up a fraction of the space, partly because you could also (get this) put a second recording on the other side of the disc.

Wavy side-to-side also greatly simplified record players. Edison's design requires a worm gear to drive the playback assembly because any significant weight in the groove would destroy the recording, so the stylus simply floats on the cylinder. But the disc phonograph player was free to rest its entire weight directly on the record. It was the walls of the groove that retained the sound, not the floor, so a steel needle could simply be dragged along the bottom without damaging the recording (although playing a record would wear out that needle and it needed replacing quite frequently). However, needles were cheap and this arrangement meant that the slot would advance the player on its own, eliminating the need for the worm gear mechanism and its associated complexity.

The disc was obviously the best option, so, much to Edison's chagrin, the cylinder was finished. As the years went by and we developed electronic sound amplification (thanks in large part to the radio), record players began to use much smaller, lighter arms equipped with these novel phonograph pickups that converted the movement of the rhythm into a signal. electric which then amplify and pump into the speakers. There are several ways to do this, including connecting the stylus to a magnet that moves inside a coil of wire, thus producing a small tension in that wire as it vibrates. But you can also connect the stylus to the wire instead of the magnet, or even use piezoelectric crystals.

There were many ways we tried to do this. Anyway, soon the arms and pickups would become so light that a small sapphire or diamond stylus could be used. That allowed the use of softer recording materials with much smaller beats that extended the duration of the recordings while increasing the fidelity of the audio. But the basic fundamentals of the technology had not changed at all. The sound was still stored in the walls of the groove, moving the stylus from left to right as it flew past. A lot of improvements over fifty years improved the process and made it sound better than ever, but it was still the same concept as the original Berliner

records

.

All we've really accomplished is making the beat movement more precise so it can better capture the nuances of a sound recording, and we made the rhythm movement more sensitive to your movement so it can play it just as well. However, now that we've made the tonearm so light and the stylus so small, we don't have to worry as much about damaging the record. And since this world we live in is three-dimensional, we could fit a second signal into the same groove by revisiting Edison's idea and adding a little... depth. When playing a record, the direction of movement of the beat occupies one of our three dimensions, but we still have two axes of movement available to the stylus.

If we designed a pickup that could generate two independent signals from the stylus: one for its lateral movement and one for its vertical movement, we could recover a

stereo

signal from a single groove. So that's what we did. Something like. What I just described is more than twisted with reality. I'll explain it to you in a moment. It turns out that English engineer Alan Blumlein, who basically created the concept of stereo sound, would describe in a 1931 patent the exact method by which stereo records would eventually be produced starting in the late 1950s. Pretty wild. Stereo records contain a combination of vertical and lateral motion in their beats, but it's a little more complicated than "left channel is vertical motion and right channel is lateral motion." That would work, but not very well.

For starters, the two channels would sound very different from each other due to the different distortion profiles, so you wouldn't be able to get a very pleasant recording. And secondly, a record made this way would not work with a conventional phonograph. Until now, the groove only moved from left to right, so that is the only direction of movement that a conventional phonograph cartridge can detect. If you put a left-handed and right-handed uppy-downy stereo record on an old turntable, only the left-handed and right-handed channel would be heard, and no one wants that. So instead, we take that up-down-left-right concept and do it like this. *wrooup* Each of the two channels is now recorded to disk as a diagonal motion, and the walls of the grove will move the stylus in all directions.

To detect this two-dimensional motion, a stereo phonograph cartridge has two pickups placed at right angles to each other. Both pickups share a single needle, and this entire contraption is positioned at a 45-degree offset from the groove. With this arrangement, each pickup is affected by both vertical and horizontal movement. And when you hear that, you might wonder how this works. See, when we want to generate a signal only on the left channel, the pen must move back and forth in a downward and leftward motion. Doing that will only cause movement in one of the pickup coils, and the other will not realize what is happening.

Then, of course, the stylus needs to vibrate along the opposite diagonal, in a downward and clockwise motion, to produce a sound only in the correct channel. This may seem crazy and overly complicated, but this arrangement has several advantages. First of all, it works perfectly with old mono recordings. When playing a mono record, its purely lateral vibrations will affect both pickups equally and produce an essentially identical signal in the left and right channels. You won't have to deal with a dead channel that no one likes; Both channels will play the same thing. Better still, the 45 degree offset means that, as long as we are careful in the mastering process, we can produce stereo records that are actually compatible with mono phonographs.

Just as in the days of the acoustic phonograph, the process of creating a record begins by carving a spiral groove into a blank record with a cutting stylus, but now that stylus is connected to high-speed actuators that make it vibrate with accuracy based on an audio signal. . For stereo recordings, the cutter stylus is connected to two actuators, one for each channel, which are arranged at the same 45-degree offset as the playback cartridge. The left channel actuator vibrates the cutting needle diagonally, downward and to the left, while the right channel actuator cuts on the opposite diagonal. And here's why this is so important: As long as we ensure the correct polarity of the signal, when the left and right channel signals are identical, that will cancel out the vertical movement of the cutting pen and we will only get lateral movement.

See, if this actuator pushes while the other pulls, both actions result in the pencil moving to the left, but the downward motion imparted by this actuator cancels perfectly with the upward motion of the other one. The cutting head will only move vertically when there is a difference in intensity between the two signals received by the actuators, as this causes an imbalance in the pulling force between them. You see, stereo sound could be produced from two completely independent signals, as we see on magnetic tape, but it can also come from a mono signal combined with a differential stereo signal.

And that's what a stereo record really contains: it's a mono signal that feeds both speakers equally when the beat moves from side to side, but when there's some vertical movement, the signal is pushed to the left or right channel. Now, of course, this happens instantaneously, so it's not like we're just scrolling a signal from left to right; Each individual movement of the stylus, that is, each sound detail contained on the record, can be pushed to the left or right channel and to virtually any degree. I just have to move a little more. And I have to say, it's not like there's any signal processing here, that's how signals are generated on a stereo cartridge when this more complex groove moves the stylus in two dimensions instead of just one.

But it could be said that the vertical component does not contain any sound information; it simply pushes the sound to the left or right channel. I'm sure the semantic discussions in the comments will be fantastic. Anyway, aside from this method doing a good job of making a stereo signal possible, if you play one of these new stereo records on an old mono phonograph, you obviously won't get stereo sound, but you won't miss out. anything either. A sound that's only supposed to be in the left channel still vibrates the stylus left and right, and the same goes for sounds in the right channel.

Therefore, the old mono phonographs will still allow you to listen to the entire contents of the record, which means that these new stereo records are compatiblewith previous versions. The vertical component in your beats just doesn't do anything because an older phonograph doesn't know what that is. You have no means to detect it. Now, if we reverse the polarity of one of the actuators in the cutting process, then the same signal intensity in both channels would cancel the lateral movement and would be recorded only as changes in depth. We would still get some sort of functional stereo record with this, but it would sound extremely strange, especially when played on a mono phonograph.

Mono turntables would only pick up sounds moving left or right, and anything in the virtual center would be silent. This is what it would sound like to listen to the raw stereo difference channel, and here I can simulate this in Audacity! Let's see, just split the track, reverse one of them, join it back in mono, OK, this is what the recording normally sounds like: ♫ some percussion ♫ oh, now there's a wonderful bass line! cymbal construction, I've got some congas, MM, I've got some HORNS! ba da ba da ba da ba da ba dum... And here's a simulated stereo difference channel. ♫ the same percussion, but now it's strangely muffled and sounds like it's coming from a distant bathroom ♫ hey, wait, where did the bass line go? cymbaaalllLLLLSSSS, HORNS So it's the same song, sure, but it's like... haunted.

So, that's what's happening with stereo records. In a way, it is a clever combination of Edison's original idea and Berliner's improvement. I don't think Alan Blumlein was thinking about that when he first invented this idea, but that won't stop me from making the connection. Now, this method is not perfect. Complete stereo channel separation is simply not possible when the sound comes from physical movements. Even if you got the angles right and the record was mastered perfectly, some vibrations from one of the pickups can travel through the cartridge and into the other. So no matter how strongly someone defends the sound of

vinyl

, the records are objectively pretty bad at isolating the two channels.

But it's still very smart and the effect of having two channels is just that: an effect. It doesn't have to be perfect to make a big difference to the listening experience. Do you like jazz? ♫ smooth, distorted jazz ♫ And secondly, a record made this way wouldn't work at all with conven-- I shouldn't add that, don't add words! ...possible signal. If you play one of these new stereo records on an old mono phonograph - aw, shit. Spin this sound record under a phonograph player and your stylus like fff... ...and you make cahraaaaazy effects like that, old LPs contain cahraaaaazy effects like that. , old LPs… I just did it again!

Normally, I'd put some kind of joke in here, but this time, I think we should do it IN STEREO STEREO LIKE THIS, TWO LIKE THIS, TWO SUBTITLES, ONE FOR EACH EYE SUBTITLES, ONE FOR EACH EYE! HOLD YOUR PHONE OVER YOUR FACE AND IT'S IN 3D! EXPENSIVE AND IT'S IN 3D! Just kidding, don't look stupid!