![]() In the 3000 and 800 Hz recording above, the small bumps on the left edge bounce the needle rapidly in the upper-right direction, and the bigger bumps on the right edge bounce the needle slowly in the upper-left direction. As the needle is dragged through the groove, it contacts both edges at the same time. In stereo records, the two edges of the groove are different. The waveform of the higher-frequency 3000 Hz tone is on the left edge of the groove, and the waveform for the lower-frequency 800 Hz tone is on the right edge of the groove! Here’s a track that plays a 3000 Hz tone in the Left channel, and an 800 Hz tone in the Right channel:ĭo you see the two waveforms? If not, check out this version where I’ve highlighted the edges of one groove in red: Happily, my fancy test record has a couple tracks that are perfect for answering this question. In fact, a stereo record actually contains two totally separate recordings, but there’s still only one groove on the record, and one needle! How does that work?!? Let’s finish with a brain teaser: how do stereo records work? Remember that stereo recordings have two channels, a Left channel and a Right channel, and they can contain different sounds that’s how you might hear a song with the vocalist in your left ear and the rest of the band in your right ear. And by dragging a needle over that groove, we can recreate that pattern of pressure waves, send them through the air again, and into our ears, where the music lives again. ![]() Here’s Verdi again:īut the groove is still just a physical representation of the recorded sound wave, the pattern of pressure waves that traveled from the musicians’ instruments, through the air, and to the microphone that recorded this record. But for complex sounds like those in Verdi’s Aida, a lot of sine waves are combined together, and the resulting record groove looks a lot more complicated. These waveforms are simple because the sounds they encode are simple: pure tones with single frequencies. We see fewer complete wavelengths in the 440 Hz track because it’s a lower frequency than the 600 Hz tracks above. This 440 Hz note is commonly used by musicians to tune their instruments: The track below plays A-440, the A above Middle C on the piano. The grooves no longer wobble in sync the waves are out of phase with each other. Here’s a nearby track with a slightly different frequency: It means that the circumference of the record at this groove must be an exact multiple of the wavelength of the sine wave. I like how the grooves wobble in sync in the photo above. If you looked at that 600 Hz sound on an oscilloscope, you’d see a sinusoidal waveform that’s identical to the record groove shown above. The record groove is a physical manifestation of the waveform of the recorded sound. This signal is then amplified and sent to a speaker. A sensor on the needle detects this wobble and converts it to an electrical signal which oscillates 600 times per second. The groove for this simple tone is just a sine wave when the needle is being dragged through it, this groove wobbles the needle back and forth 600 times per second. Using this simple tone, we can see how the sound is encoded in the record. ![]() Here’s a closeup of a particularly riveting track that consists of nothing but a constant 600 Hz tone: And while it probably never made the Billboard Top 100, it can help us understand how records work. It contains a collection of standardized tones and clicks and other noises. This is a “test record” used to evaluate stereo equipment. Luckily, I have a special record that can help answer this question: How do those jagged lines capture all of Verdi’s opera? If you search Google for record grooves under a microscope, you’ll find a lot of similar images of record grooves with seemingly random jagged edges. You can see the grooves, the little paths that the needle follows. Here’s a photo I took of a phonograph record using a microscope in my lab:
0 Comments
Leave a Reply. |