It's difficult to explain the basics of a highly-technical process like record playback without getting bogged down in eye-rolling minutiae. I'll try to stick to the big-picture trends and details, and avoid nerding out. But when you get right down to it---a major part of audiophilia is nerding out---no?
But I digress.
In the big picture, record playback is just the process of tracking the groove. If you've ever encountered a jukebox or a transcription turntable from the '40s or '50s, designed to play back 78s, you'll have seen the massive tonearms and bulky cartridges that often required ounces in tracking force---not grams. Remember, an ounce is 28.3495 grams--- let's say roughly 28 1/2 grams. The manual for the RCA 70-D transcription 'table, state of the art at its introduction in 1948, specifies tracking weight at 28 grams, plus or minus 1/2 gram---which is damned near close to an ounce. For comparison, depending on when they were minted---the metallic composition varied in density--- a US penny runs from 2.5 grams to 3.11 grams. A nickel runs about 5 grams---and as a kid, I saw phonographs with nickels taped to the tonearm's headshell, a brute-force method of improving tracking.
While those weights sound horrific in the context of a stereo microgroove vinyl record, keep in mind that 78s had much larger grooves, and thus the downforce is distributed over a much larger surface area than with a modern 45/45 microgroove record. Even today, with a modern tonearm and cartridge designed for 78 playback, optimal downforce is usually considered to be in the 4-5 gram range.
If you're looking for simple, straightforward, factual history---you need to stay away from the history of 78s. For starters, from the turn of the century through the '20s, record speeds were not standardized, and different labels had their own "house" speeds---which could vary anywhere from 70 to 100 rpm. To make matters even more confusing, groove sizes weren't standardized, either. Pre-1940, most mainstream labels had grooves playable by a conical or spherical stylus of around 3 mil in diameter(a mil being one one-thousandth of an inch---so 3 mil= 0.003"), but again, earlier and more obscure labels varied from that. Post-1940, most 78 grooves were playable with a 2.7 mil stylus.
Back in the day, many phono cartridge companies offered a variety of playback styli for archival work. Today, Esoteric Sound, maker of present-day Rek-O-Kut products, still offers kits with a wide range of styli size in both cartridges and replacement styli to assist in playback of early records.
Everything we've just discussed? Forget it all, when it comes to the 33 1/3 rpm LP (long-playing record) introduced by Columbia in 1948. The discs themselves were thinner than 78s, made from vinyl rather than shellac and clay, and the grooves were far finer. The LP thus had both longer playing time and (in general) lower surface noise than 78s. The LP was developed by a team at CBS Labs in Stamford, Connecticut, near New York City. The team leader was Dr. Peter Goldmark, a polymath who had previously led development of early rotating-disk color television systems, and subsequently helped develop videotape technology and scanning imaging systems for lunar landers. Goldmark's 1973 autobiography, Maverick Inventor: My Turbulent Years at CBS can still be found on Amazon and other outlets. It's an interesting read, though it does tend to be self-serving. But then: what autobiography isn't?
As if the change to the microgroove wasn't enough upheaval---stereo made matters even more complex. While stereo recordings had been made in the early 1930's by Alan Blumlein at EMI in England and by Keller and Fletcher at Bell Labs in the US---the famous Stokowski recordings---development of stereo discs didn't pick up momentum until the LP was on the market. The obvious question was: how do you record two separate channels of information to a disc, and then play it back? Several developers opted for two separate grooves; most famous of these was Emory Cook, whose Cook Laboratories released a number of binaural recordings, including demo discs featuring trains---soon to become a stereo cliche'. Cook's two-groove method required two separate cartridges, and clip-on cartridge assemblies were produced to adapt existing turntables.
The other contender was ultimately the victor: the 45/45 single groove method, which cut both channels into a single groove. The method had been explored by Bell Labs in the '30s, and was revived by Western Electric offshoot Westrex, in the '50s. As we've written about previously, the first single single-groove stereo discs were released in the US by the tiny Audio Fidelity label, using Westrex demo material. The warring formats and commercial competition can be read in contemporary accounts archived here, and it's pretty gripping stuff.
Somehow, a sidebar has grown into a full column. We'll get back on track with a look at the nuts and bolts of record playback in the next issue of Copper.