While high resolution audio technology continues blazing forward with lukewarm consumer interest (despite even the recent re-issue of Pink Floyd’s 1975 “Wish You Were Here” on SACD), another audio recording format appears to be coming on like gangbusters: vinyl. According to several industry sources, 2018 sales of vinyl recordings will be the strongest yet, at least since the death of vinyl was declared more than three decades ago as CDs were being introduced. Last year, in 2017, according to Nielson research, vinyl accounted for 14% of all physical album sales, and this year it’s expected to be even stronger, reflecting 12 years of continued growth for the once-moribund analog format.
Unlike SACDs, FLAC and other hi-res audio files, not to mention CDs, DVD audio, MP3s, WAV files, FM radio, and just about all other audio recording and transmission systems, vinyl recordings have a unique quirk: The quality of the recording varies as you play the disc through. That is to say, if you listen to one side of an LP, the recording quality of the first track on the LP is different from the quality of the last.
Inches Per Second
The reason for this disparity is basic physics: Since the LP (long playing) disc moves at a constant rate of 33-1/3 revolutions per minute, the amount of vinyl dedicated to each second, or minute, of recording time is greater at the outer edge of the disc compared with the inner tracks.
Here’s a thought experiment: Imagine “unwinding” the spiral audio track of a vinyl LP, so that it becomes one long straight line of vinyl track. One way of thinking about the quality of the recording would be by measuring how many inches of vinyl pass under the turntable’s stylus each second.
Recall from elementary school math that the ratio of a circle’s circumference to its diameter is expressed as pi, which is roughly 3.14. (But pi is actually an irrational number that goes on forever — check out this humorous video on the subject.)
Now consider how this principle applies to a 12-inch vinyl LP. Let’s say the first couple of tracks on the album have a diameter of roughly 11-inches (of course, the exact amount varies each time the disc turns, so the beginning of each song has a bigger diameter than the end.)
For the outer tracks then, the amount of vinyl per minute is roughly:
33.33 (revolutions per minute) x 11 inches (diameter) x 3.14 (pi) = 1151 inches per minute
Dividing that by 60 seconds in a minute, that works out to
1151 / 60 = 19.2 inches of vinyl per second
Now, consider the innermost tracks on the LP, near the end of the side, and let’s say they have a diameter of roughly 4-inches. Doing the same calculations,
33.33 x 11 x 3.14 = 419 inches per minute
419 / 60 = 7 inches of vinyl per second
That’s quite a difference, and it results in different sound quality from the beginning to the end of each side of an LP.
Impact on Sound Quality
The most obvious impact of this phenomenon on sound quality is on frequency response. The more vinyl per second, the better frequency response, since you can fit more waves into 19 inches of vinyl than you can fit into 7 inches.
But of course, the amount of recording medium that’s available is just one of many factors that affect the upper limit of an LPs frequency response range. The quality and mechanics of the stylus and magnetic cartridge built into the turntable also have a big impact, as does the pre-mastering process in the recording studio. While maximum frequencies as high as 100-kHz or even more can be recorded onto LPs at the outer tracks, it is very common for recording studios to limit the highest frequencies on their masters, just as they must do for audio CDs and SACDs due to the Nyquist theorem (the highest possible recorded frequency is half the sampling rate.)
Ironically, the better frequency response of the outer tracks on an LP may come at the expense of increased noise vulnerability. This depends partly on how clean is the environment where the LP is played. Imagine dust falling on our stretched out LP track. Imagine the dust is falling at a constant rate of so many particles per minute per square inch. Obviously more dust will accumulate on 19 inches of the vinyl track than on 7 inches in a given amount of time. If each speck of dust translates into an audible pop or click on playback, then we’ll hear more of these distractions on the outer tracks than the inner ones.
Retro Video Too
If you’re under 40 years old this may sound like ancient history, but back in the pre-digital video days before DVDs became popular there were several analog video disc formats too, the most popular of which was called the LaserDisc. An engineering miracle of the late 1970s and 80s, the LaserDisc was the first mass market optical disc format, and was vastly superior to the VHS and Betamax videotape systems of that era. LaserDisc had a loyal following among video and movie aficionados.
The engineers of that era were very aware of this issue of varying quality on audio discs, and set out to address it with the new video disc recording systems. As a result, those LaserDiscs were available in two different formats, called CAV and CLV.
CAV, for constant angular velocity, worked similarly to audio discs, and indeed the picture quality at the beginning of the movie (the outer tracks) was better than at the end (the inner tracks).
CLV, for constant linear velocity, addressed this issue head-on, by changing the speed of the disc as it played from beginning to end. So the number of revolutions per minute was lower while playing the outermost parts of the disc, and higher when playing the innermost parts of the disc.
The LaserDisc format never really caught on with the mass market, but the idea of varying the motor speed to provide consistent quality did, and has been incorporated into audio CDs, Super Audio CDs, DVDs, and all optical media formats ever since.
Although all current analog audio disc systems continue to use the age-old constant-rate turntable systems (33-1/3 rpm), it would be interesting to someday see the same principle used for video discs 30 years ago applied to analog audio.
Meanwhile, one distinct advantage of the high resolution digital audio formats of today is the consistency of recording quality, from beginning to end.
Cliff Roth teaches Communications Studies at the State University of NY at Cortland. He’s also a filmmaker, editor, and previously taught audio engineering at the Institute of Audio Research in New York City.