Scott Hull on Vinyl, Part Three

Header image for Scott Hull on Vinyl seriesAs disk cutting engineers we are always looking at sound. We have a microscope mounted on the record cutting lathe and we use this scope to determine the quality of the cut and to diagnose problems when they occur. We can also measure the groove width and separation between the grooves. The space between the grooves is called “land.”

Photo of quiet record grooves
Fig. 1
So, What do the squiggles mean? Lets look at the record groove closely. Very closely.

In the microscope a simple quiet groove looks like this. (Fig. 1)

There are four grooves in this picture. Each groove looks like three “lines.” The light from the scope lights up the bottom of the groove and the top edges. This is a picture of grooves cut in a fresh lacquer. It’s a very clean and quiet cut. This groove would make a very good sounding – albeit silent record.

When we add music to the picture this is what can happen to our cute little grooves. (Fig. 2)

We can notice the grooves move back and forth and they get fatter and skinnier.

Other things we can notice are that there are large sways in the groove that look a little like sine wave. These are the bass frequencies. Bass frequencies have large wavelengths and when cut they make the groove move in long sweeping curves. They’re so long I can barely get part of a wavelength in one slide.

Photo of record grooves with music content
Fig. 2
We can also see grooves that have tight little squiggles that look something like fish scales (center). These are the higher frequencies. Instruments like a cymbal or trumpet can make the very tight squiggles like those in the middle groove.

These sharp, high frequency squiggles are something we’re constantly dealing with. The sustained bright “S” sound is a particular challenge. In fact, there are so many reasons why “esses” are problematic I’ll devote a whole blog entry to just that.

The goal is to cut a “bright” groove that can still be played back by a standard quality needle and cartridge. If the movements of the groove are too sharp and bright, not all playback cartridges will be able to track the groove accurately. When a needle fails to track the groove you hear a fuzzy sounding distortion. A stiff DJ cartridge—one that is durable and can stand up to scratching and back cue-ing—will often be too stiff to accurately track all those sharp turns. “Hi-fi” cartridges are designed to have the flexibility to track those turns accurately. The trade-off is that they tend to be very delicate, and expensive.

Photo of record grooves
Fig. 3
In Fig. 3 we have cut some sine wave tones so that we can see more clearly the independent movement of the left and right channels.

This is a really interesting slide. It wasn’t easy to get all four grooves in one picture—and it wasn’t edited together in Photoshop, either!

The first groove on the left is a recording of a 4,000 cycle tone (4kHz) in both left and right channels in phase. Since the signal was in phase, the depth of the groove is constant, and you simply see the sine wave wiggles of the left and right walls. The left wall is the left channel; the right wall the right channel.

In the second groove you can notice that the left wall is straight. The left channel is silent and the right channel is playing the test tone on it’s own. Since the two channels are not identical in this example, the groove gets alternately deeper and shallower. This is because the channels are not in phase and it causes the playback needle to rise and fall. Remember that even though there are two channels of audio, there is only one point where the stylus touches the record. The movement of the groove, left and right, up and down, is completely analogous to the movement of the left and right speakers upon playback.

In the third groove both channels are off. This is our silent groove like the first photo above. And then the last groove has audio on the left channel and the right channel is silent. I love this slide—because it clearly displays what motion is shared by both channels and what is independent motion.

Next week we’ll look closely at the cutter head.

(Read all of the “Scott Hull on Vinyl” articles here.)

Fill out my online form.