How do vinyl records hold stereo sound?

We answer that question!
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[TechnologyConnections]

A CLARIFICATION:
My script is missing two important words in describing the stereo groove: "behaves like." When discussing stereo-difference signals, I declared that that's what's on the record - which is arguably true but only when you consider lateral and vertical stylus movements in isolation. The raw L&R channels are still encoded as purely diagonal groove movement, but the phase difference in the the cutting head's actuators ensures that those movements, when combined and equal to one another, only result in lateral motion. The cartridge also has its pickups wired with the same phase arrangement to ensure the L&R channels are in phase with one another when playing a mono record (or the virtual center of a stereo record).
Is that a mono signal in the lateral motion with stereo-difference information in vertical motion? Well... you decide!

[captainchaos]

This technique is still a thing in digital recordings. In a "joint stereo" MP3 encoding, the sound is encoded as a mono channel and a stereo difference channel. The reason is that it compresses more efficiently and results in smaller files for the same quality.

[Nethershaw]

When you started discussing stereo difference channels using Audacity, it was very satisfying. I have a fun story about a radio station that did this by accident, causing me to make a curious detour on a job.

Back in 06, I was at university, and also working for the university's media department. They wanted to play the campus radio station over the ceiling speakers in the lobby of the student union, so we put a receiver in a closet and connected the signal. Only -- the ceiling speakers weren't in any kind of stereo configuration; the whole array was driven off the same mono signal, so we had to pass the radio station through a little downmixer. At first I didn't have the part with me, so I just shoved one channel through the speakers to test them, and then I went and got the downmixer to install.

When I did that, I found the audio almost totally went away.

I checked the wiring and I hadn't messed up; everything was connected the right way. The level indicator wasn't zero, though, so I went from the closet into the lobby with the volume turned up and it sounded like I was underwater. The signal was there, but coming out mud. I tried a different source signal, and it came out fine (after I almost blew out the speakers from nearly forgetting to reset the volume levels). That's when I had the thought, "oh crap, the radio station, they're broadcasting their stereo channels 180 degrees out of phase."

One of my roommates had a radio show and worked as a technician at the station so I told him about it. To prove what I was saying, I captured a segment of the station's live Internet stream, which I knew from my roommate was driven off the same mixer as the FM broadcast, and the software I used to capture it was Audacity. All I had to do was combine the left and right channels -- without having to invert one -- and play it back.

"I think you've got something wired backwards, " I said. We went into the station the next morning and fixed it. They were using analog audio equipment, not digital mixers, and at some point the polarity of one channel had been reversed in the feed.

That is the first time in my life I got to fix something by reversing the polarity, and it did satisfy a good chunk of the childhood engineering dreams I had since watching ST:TNG at age 3.

[derekloudon]

Here I am, retired and in my 70s with a lifetime (well almost) in electronics and it never occurred to me to question how stereo sound works on a vinyl record. Thanks for the ‘training session’.

[amdsemporn]

Fun fact: I often accidently heard only the stereo difference: When the headphone jack is not plugged in all the way, and both channels have contact but not the ground, you hear only the differences between the audio channels. Happened a lot in the good old days with mp3 players

[Noah-Lach]

I love how records are quite literally the physical embodiment of music. At a basic level, an LP is nothing more than a statue of a sound wave. That they can produce accurate sound with no electronics whatsoever feels so wonderfully primal.

[bradleybuyer]

Vinyl record trivia: Monty Python (back in the day) cut an album that had two sides on one side: Each 'side' was cut in one spiral grove right next to another spiral grove. Depending on how you set the needle down, you'd get one track or the other. They had no mention of this on the album notes, just left it for you to figure out. Of course, each track was half the length of a regular album side. Crazy!

[Gutsquasher]

Once you broke out the model everything clicked. Getting to see what the actual effect of the technique was really helped cement how they work at the same time, and how that translates to reading the groove. I remember watching your first video and, while I came away with both tracks being stored at a diagonal to each other, sharing a groove, I couldn't tell you why. I feel coming away from this that I could actually explain why they are at a diagonal and that feels fantastic.

The followup about the stereo difference was also really cool and, from a completely different perspective really helped me understand what the record is effectively encoding. "Two different tracks at 45 degree angle to eachother" is hard for me to understand how that translates to a record, but taking that extra step to explain how effectively it leads to a mono track being preserved in the lateral movement with stereo difference in the horizontal it really got it from a technical understanding to an intuitive one.

Thanks for another great video!

[rj-fkrf]

i've worked at a record store for 8 years now and the technology behind vinyl still blows my mind thank you for doing this

[christiang]

Your closed captions are just A++! Such a small detail that screams attention-to-detail! Love your content!

[nowster]

A tip for anyone doing restoration of stereo audio off records. Because dust and crud sits in the bottom of the groove, it proportionally affects the stereo difference signal more than the mono signal. Convert from LR to MS then do your de-clicking etc. on each separately. Be careful not to use anything that might affect the phase relationship between the two signals. Then convert back to LR after processing.

[ChrisGoosman]

What a great video, Alec! As a mastering engineer and former lacquer cutter, I think you did a fantastic job explaining the process. The construction paper model was a perfect visual aid! Bravo!

[MichaelSteeves]

My hearing is degrading and I usually have captions turned on (and hearing aids). I just about lost it with the captions on the Audacity playback! 🤣

[GuestInvitado-gdbi]

I grew up with LPs during the 80s. By then, fine audio equipment was available at a reasonable price. Speakers with fifteen inch kickers were very popular. Listening to the left and right audio was an experience. Even to this day, stereo is one of the greatest inventions of all time. Very interesting video, I enjoyed it. Thanks!

[CiscoWes]

15:15 that sounds like the “poor mans surround sound” setup. I had a 2nd pair of speakers hooked up from the two positive leads from left and right on my stereo when I was a kid and it gave this interesting surround sound effect from the rear speakers. That reminded me of my setup back in the day.

[davidmcgill]

That concept would also be used in FM radio broadcast. The combined channel is the main band while the difference channel is a side band. When the signal strength is weak the radio will swap to mono to retain quality.

[chaos.corner]

Stereo difference (along with some frequency filtering) is sometimes used to generate Karaoke tracks as the singer is typically placed in the center of the stereo stage while instruments are distributed. There are some Karaoke machine chips with this functionality built-in.

[nedhedrick]

Fun stuff. I'm old enough to remember the coolness of the original stereo records. But honestly, what really WOW'ed me was "quadrophonic" records. That could also be an interesting follow-on to this...

[PassengerPigeonsLE]

I was actually listening to a record when I realized this is how it worked! The snares on Todd Rundgren’s A Wizard A True Star are intentionally put out of phase, but panned hard left and right. Listening back to the cd, it sounds like normal out of phase, and in mono it disappears. But on the vinyl, it gets a really weird, unique spatial quality to it as the needle fights with that huge Side channel! I love weird Mid-Side tricks like that, and in other music where listening to it in mono will ‘erase’ certain vocal lines

[perrybarton]

Nice touch with the stereo difference version of the closing music! 🤓 Back in the dark ages (‘70s) there was a device called the Thompson Vocal Eliminator which employed this very technique. Since the lead vocal was/is usually panned center, shifting the phase of one channel by 90° would cancel out the vocal, leaving only any reverb triggered by said vocal. I never actually operated one of those, so I don’t know if anything was done to allow the low-frequency sounds of kick drum and bass (also usually panned center) to come through unscathed.