Technical Articles and Editorials
- Written by John E. Johnson, Jr.
- Published on 02 October 2009
When attending CES and Rocky Mountain Audio Fest over the past couple of years, I noticed that turntables are starting to be the majority of sources for exhibit rooms. We all have heard about the vinyl renaissance, but I just did not realize how strong it is. I mean, LPs are being played everywhere at hi-fi shows. So, we decided to compare Vinyl LPs to CDs in terms of the recording process, manufacturing, and the sound. Parts 1-5 (Part I: Introduction, Part 2: The Technology, Part 3: Turntables, Tonearms, and Cartridges, Part 4: The RIAA Curve, and Part 5: Setting up the Turntable) are published in a previous article. Here are Parts 6-9.
Part 6: In the Groove
The frequency response of an LP is about 10 Hz to 25 kHz, and it has a dynamic range of 75 dB. All the information is contained within a groove about 0.002" in width.
This groove is originally cut in a lacquer-coated aluminum disc using a cutting head with a sapphire stylus about 0.0002" in diameter (10 times smaller than the groove that is cut). Sapphire (a.k.a, corundum, which is the crystalline form of aluminum oxide) is actually better than diamond for cutting the original master, although diamond is better at playing the LPs at home. The cutting head is driven by stereo power amplifiers of about 400 watts each. So, cutting the master is the opposite of playing the recording once it is on the platter that you purchase. The diamond stylus that you play the records with is about 0.001" across.
A negative replica is made from the master, and then vinyl biscuits are melted on the negative, cooled, trimmed, and packaged for the consumer.
Why 33-1/3 RPM?
When the technology moved forward from cylinders to discs, it was calculated by Emil Berliner (inventor of the Gramophone) that 78 RPM would reproduce the frequency response using a steel needle. The 33-1/3 RPM speed was developed because the disc used for sound in early movies would hold 11 minutes of the soundtrack, which was the length of one movie reel. Then CBS applied that rotational speed for commercial music recordings and we had the LP with 30 minutes of music on each side of the disc.
The 45 RPM disc may seem to have been just for the popular music crowd, with two hit songs on each disc, but it was actually a technological advancement in sound quality as well, and in fact, current re-releases of LPs are sometimes in the 45 RPM format, with the music that was originally on two sides of one disc, now on two sides of two discs. I obtained a few of these and will discuss them later in comparison to the more conventional 33-1/3 RPM discs.
The LP was introduced in 1948, and stereo LPs soon followed. They were manufactured from polyvinyl chloride, which was sure a big improvement over the older 78's made from shellac (much lower surface noise). You could drop them without breaking them, but they were (are) just as sensitive to scratching, and of course, they didn't (don't) do well if left in the sun (they warp).
So, let's analyze the groove's capability.
The stylus tip follows the groove at 33-1/3 RPM, which means a linear velocity. The problem (Problem 1) is that the linear velocity changes, because the diameter of the spiral is smaller at the center of the disc than at the outer edge. This is one reason some high-end LP re-releases are at 45 RPM. With the smaller spiral diameter, the high frequencies are crowded into a smaller linear space, and they suffer.
The groove itself is like a "V", with the right channel being represented by sharp peaks and valleys on the outer edge of the V, and the left channel on the inner edge. So, the stylus, following the groove, is raised on the peaks, and lowered in the valleys. The stylus is connected to the cantilever, and the cantilever has either the coils (MC) or magnets (MM) attached to it, which moves, and induces electrical current in the coils, which is translated into the music through the amplifiers and speakers.
Stereo grooves could have used a lateral motion for one channel and vertical motion for the other, but it would have resulted in less compatibility with monophonic cartridges, and also, the lateral motion channel would have had lower fidelity than the vertical channel.
Problem 2: The stylus is attempting to translate both channels at the same time, and because the movement of the stylus shares some common direction (the stylus moves downward for both left and right channels), there is some bleeding of each channel into the other, called "crosstalk". In fact, channel separation is "only" about 30 dB. In other words, if you had a groove where there was a 1 kHz sine wave recorded only in the left channel, and you played it back at 100 dB from the left speaker, you would also hear that sine wave in the right channel at 70 dB.
That number doesn't sound very good at first glance, but with each 10 dB increase, it sounds (subjectively) twice as loud, and with each 10 dB decrease, it sounds 1/2 as loud. So, going from 100 dB to 90 dB cuts the loudness by 1/2, then to 80 dB is another 50% reduction, then to 70 dB is an additional 50% softer, for a total of 8 times softer at 70 dB. Nevertheless, there is substantial crosstalk with the stereo phonograph recording. CDs, by comparison, have no crosstalk generated by the medium itself. The two tracks are discrete. Any crosstalk is generated by the downstream electronics after the signal is converted from digital to analog. However, that crosstalk is still much less than the crosstalk from tracking an LP groove.
How do you like the problems with LPs so far? Here's another one. Problem 3: Because the translation of the sound from the groove to the stylus requires that the stylus be falling into the valleys or pushed up with the peaks in the groove, having a out of phase information in the left vs. right channel causes some issues with the laws of physics. In other words, if there is a valley in the left channel at the exact same time there is a peak in the right channel, the stylus finds it a little difficult to be going in opposite directions at the same time. The result? Inaccurate signal reproduction at that instant in time. So, LPs cannot handle a lot of material that is out of phase between the left and right channels.
There were some technology improvements made during the 1980's, such as the method of disc cutting (making the master disc that would be used to produce the vinyl copies that we purchased). Although the CD began to eclipse the LP in that same decade, we can certainly benefit from the new LP cutting technology that was developed at that time, and which has continued to improve, by going back and remastering (recutting) early classic recordings. Such remasterings and re-issues are now popping up all over the place. Some rock groups, whose musicians were only babies when the CD was introduced, insist in their contracts that their albums be released both as CDs and LPs.
Why, then, if there are these obviously serious problems, do so many people feel the LP sounds "better" than the CD?
Dynamics may be one reason. Along with digital music reproduction, came the ability to mix and edit the music digitally, and this offered incredible flexibility. Boom boxes proliferated, and these boxes didn't have much in the way of dynamic capability, so CD producers compressed the dynamic range, boosting the quiet parts of the music and attenuating the level of the loud passages. That way, consumers could hear every part of the music, with it all having about the same level of loudness. Although this worked for boom boxes, the music sounded terrible on good audio systems.
Not all companies compressed the dynamics on their CD releases. Telarc is notable here, producing early CDs that, even today, are just marvelous.
In my tests, comparing an LP version of a recording to its CD or SACD counterpart, I have not found dynamics to be an issue at all. In fact, the CD and SACD have the potential to record a higher dynamic range than the LP (LP has a dynamic range of 75 dB, CD has 96 dB, SACD has 120 dB, DVD-A has 144 dB). The problem is that CDs and SACDs have not truly exploited that potential.
Some have said that it is the extended frequency response that LPs have, but SACD has a very high frequency response, at least as good as LP. However, because of filters that are used in playing back digital recordings, there is phase shift that causes problems in the audible high frequency range, and my initial impression is that it is the high frequency area of an LP that has better definition than its digital counterpart.
Well, if the idea is that analog all the way gives the best sound, then why not just go back to playing analog recordings on cassette tapes? The problem there is that the tape speed is so low, the dynamic range is only about 60 dB. Plus, you have tape wear, tape hiss, and the inconvenience of having to rewind or fast foward to get to a particular song.