Transcription

I don't know if you've realized this, but vinyl records have gotten pretty popular. Analog is just so much more… real, man. 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 craaaaaaazy effects like that, ye olde LPs… they hold stereo sound. That's kinda weird, though, isn't it? Like, stereophonic means that we have two audio channels. And if you look to other old 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 one detecting the signal on a dedicated left and right audio track contained on the tape. 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… reasons. The analog soundtrack on motion picture film prints has two obvious channels, as well. And skipping ahead to the digital age, the data stream encoded on a compact disc contains two distinct audio channels that are decoded and reproduced by the CD player upon playback. Same goes for pretty much every digital sound format since. But a record like this has but the one groove that's played by a single stylus. Somehow, the action of dragging that single stylus through the single groove results in two audio channels that make the listening experience just that more real. What's going on here? Well, you might think it could be some sort of sound signal modulation trickery, but it's actually much simpler than that. You probably already know that the groove wiggles the stylus to make the sound signal. And it turns out, there's more than just one way to wiggle. 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. So I'm doing it now! Speaking of going way, way back, Thomas Edison's original phonograph pressed sound vibrations into a wax cylinder. A sound-collecting horn led to a diaphragm that would vibrate just like your eardrums do in response to changing sound pressure levels. That diaphragm was then attached to a sharp stylus, which would copy those vibrations as a rapid up-and-down motion. When this stylus was pressed into a rotating soft wax cylinder, it would carve a groove into the surface of the wax. Moving that stylus laterally across the rotating cylinder created a long spiral groove. And all the while it carved the groove, sound vibrations collected by the horn caused the stylus to move up and down, and the resulting variations in the groove's depth would become a record of those sound vibrations. That's why it's called a record! I'm skipping over plenty of details here, but a mold could be made of this soft wax record so that resilient copies, like this, could be mass-produced. Spin this record on a reproducing phonograph, and its stylus, as it floats over the bumpy groove, will move up and down in the same manner as the recording stylus did. That imparts vibrations in a second diaphragm which makes tiny little sound pressure waves. And with the help of an amplifying horn, the recorded sound could be reproduced. 🎵 Not terribly well, but it's the 19th century still. Don't expect too much. But then this guy Berliner was like, Cylinders? Really? And he thought of a better way to do this! Instead of wiggling a stylus up and down, you could do a side-to-side wiggle instead. That would allow you to carve a deep spiral groove into the surface of a flat disc, which would simplify everything a great deal. For starters, mass-producing hollow cylindrical records with fine details on its surface, although Edison did figure it out, was, uh, quite the process. A flat disc, meanwhile, could simply be stamped. Much cheaper, easier, and faster. Discs were also much easier to handle, less fragile than cylinders, and took up a fraction of the space in part because you could also, get this, put a second recording on the other side of the disc! Whoa. A side-to-side wiggly groove also greatly simplified the record players. Edison's design needs a worm gear to move the reproducing assembly along because any significant weight on the groove would destroy the recording. So the stylus just floats over the cylinder. But the reproducer of disc phonographs was free to rest all its weight directly on the record. It was the walls of the groove that held the sound, not the floor. So a steel needle could simply drag along the bottom without damaging the recording. Although plain, a record would wear down that needle and you needed to replace it quite frequently. Needles were cheap, though, and this arrangement meant the groove would advance the reproducer all by itself, eliminating the need for the worm gear mechanism and its associated complexity. The disc was obviously the better option, so much to Edison's chagrin, the cylinder was done for. As the years went on and we developed electronic sound amplification, thanks in no small part to radio, record players started using much smaller and lighter tone arms fitted with these newfangled phonograph pickups, which turned the groove's wiggling into an electrical signal that we would then amplify and pump into loudspeakers. There's a few different ways we can do that, including attaching the stylus to a magnet which moves within a coil of wire, therefore producing a teeny bit of voltage in that wire as it vibrates. But you could also attach the stylus to the wire instead of the magnet, or even use piezoelectric crystals. There were a ton of ways we tried doing this. Anyway, soon the tone arms and pickups would become so light that a tiny little sapphire or diamond stylus could be used. That allowed for the use of softer record materials with much smaller grooves that extended the runtime of recordings while also increasing audio fidelity. But the fundamental basics of the technology hadn't changed at all. Sound was still stored in the walls of the groove, wiggling the stylus left and right as it flew by. A bunch of refinements over 50 years or so improved the process and made it sound better than ever, but it was still the same concept as Berliner's original discs. All we've really accomplished is make the groove's wiggling more precise so it can better capture the nuances of a sound recording. And we made the thing that the groove wiggles more sensitive to its wiggling so it can reproduce it just as wiggly well. However, now that we've made the tone arms so light and the stylus so small, we don't have to worry about damage to the record nearly as much. And since this world we live in is three-dimensional, we could fit a second signal into the same groove if we revisit Edison's idea and add a little… depth. When playing a record, the groove's direction of motion takes up one of our three dimensions, but we still have two axes of motion available to the stylus. If we designed a pickup which could generate two independent signals from the stylus, one for its lateral movement and another for its vertical movement, we could recover a stereo signal from a single groove. So that's what we did. Sort of. What I just described is more than a little askew with reality. I'll explain 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 made starting in the late 1950s. Pretty wild. Stereo records do contain a combination of vertical and lateral movement in their grooves, but it's a bit more complicated than the left channel is vertical movement and the right channel is lateral movement. That would work, but not very well. For a start, the two channels would sound quite different from one another due to differing distortion profiles. So you couldn't get a very pleasant recording. And secondly, a record made this way wouldn't work with a conventional phonograph. Up till now, the groove only ever wiggled left and right, so that's the only direction of movement a conventional phonograph cartridge can detect. If an upy-downy lefty-righty stereo record was placed on an old record player, you'd only hear whichever channel was the lefty-righty one. And nobody wants that. So instead, we take that up-down left-right concept and go like this. Ruh! Now each of the two channels is recorded on the disc as a diagonal movement, and the walls of the groove will sort of move the stylus around every which way. To detect this two-dimensional movement, a stereo phonograph cartridge has two pickups placed at right angles to one another. Both pickups share the single stylus, and this whole contraption is set at a 45 degree offset with respect to 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 at all. See, when we want to generate a signal on only the left channel, the stylus needs to move back and forth in a downward and leftward motion. Doing that will only cause movement in one of the pickup coils, with the other one oblivious to what's happening. Then of course the stylus needs to vibrate along the opposite diagonal in a downward and rightward motion to produce a sound on only the right channel. This might sound bonkers and overly complicated, but this arrangement has several advantages. First, 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 one dead channel, which nobody likes. Both channels will play the same thing. Better yet, the 45 degree offset means that so long as we're careful in the mastering process, we can produce stereo records that are actually backward-compatible with mono phonographs. Just like in the acoustic phonograph days, the process of making a record starts by carving a spiral groove in a blank disc with a cutting stylus. But now that stylus is hooked up to high-speed actuators that precisely vibrate it based on an audio signal. For stereo recordings, the cutting stylus is attached to two actuators, one for each channel, that are arranged with the same 45 degree offset of the playback cartridge. The actuator for the left channel vibrates the cutting stylus diagonally, downward and leftward, with the right channel actuator cutting in the opposite diagonal. And here's why this is such a big deal. So long as we ensure the correct signal polarity, when the left and right channel signals are identical, that will cancel out vertical movement of the cutting stylus and we will only get lateral movement. See, if this actuator is pushing while the other is pulling, both of those actions result in the stylus moving to the left, but the downward motion imparted by this actuator is perfectly canceled out by the upward motion of the other. The cutting head will only move vertically when there is a difference in intensity between the two signals the actuators receive, as that causes an imbalance in the pulling force between them. See, stereo sound could be made from two completely independent signals like we see in magnetic tape, but it can also come from a monosignal combined with a stereo difference signal. And that's what's actually contained on a stereo record. It's a monosignal feeding both loudspeakers equally when the groove moves side to side, but when there's some vertical movement going on, the signal gets pushed towards the left or right channel. Now of course, this is happening instantaneously, so it's not like we're just panning a signal left and right. Every individual movement in the stylus, meaning every sound detail the record contains, can be pushed to the left or the right channel, and to virtually any degree. Just gotta do a little extra wiggling. And I should say, it's not like there's any signal processing going on here, that's just how the signals will get generated in a stereo cartridge when this more complex groove moves the stylus around in two dimensions, rather than just one. But the vertical component arguably doesn't contain any sound information. It just pushes the sound towards the left or the right channel. I'm sure semantics discussions in the comments will be fantastic. Anyway, aside from this method doing a fine job of making a stereo signal possible, if you play one of these new stereo records on an old monophonograph, well you won't get stereo sound, obviously, but you won't be missing anything either. A sound that's only supposed to be on the left channel still vibrates the stylus left and right. And the same goes for right channel sounds. So old monophonographs will still let you hear everything on the record, meaning these new stereo records are backward compatible. The vertical component in their grooves just… doesn't do anything because an older phonograph doesn't know what that is. It has no means to detect it. Now, if we flipped the polarity of one of the actuators in the cutting process, then equal signal intensity on both channels would cancel out lateral movement and would be recorded purely as changes in depth. We'd still get a kind of functional stereo record out of this, but it would sound extremely weird, especially when played on a monophonograph. Monorecord players would only pick up sounds that are panned left or right, and anything in the virtual center would be silent. That's effectively what listening to the raw stereo difference channel would sound like. And here, I can simulate this in Audacity. Let's see, just split out the track, invert one of them, smush it back together into mono… OK, here's what the recording normally sounds like. And here's a simulated stereo difference channel. So, that's what's going on with stereo records. In a way, it's a clever combination of Edison's original idea and Berliner's improvement. I don't think Alan Blumlein was thinking of that at all when he first concocted this idea. But that won't stop me from making the connection. Now, this method is not perfect. Complete stereo channel separation just isn't possible when your sound is coming from physical movements. Even if you nailed the angles just right and the record was mastered perfectly, some vibrations from one of the pickups are liable to make their way through the cartridge and into the other. So, no matter how staunchly someone defends the sound of vinyl, records are objectively pretty bad at isolating the two channels. But it's still really clever. And the effect of having two channels is just that. An effect. It doesn't need to be perfect to make a huge difference to the listening experience. Ya like jazz? 🎵Jazz music🎵 And secondly, a record made this way wouldn't work at all with conve… oh I shouldn't have added that. Don't add words! Signal possible. If you play one of these new stereo records on a old mono phonograph… oh crap. Spin this sound record below a reproducing phonograph and its stylus as it… can do craaaazy effects like that! Ye olde LPs hold… dang it. Craaaaaaazy effects like that. Ye olde LPs… I just did it again.