VPI Classic Direct Drive Signature turntable The RIAA Curve
From the perspective of today's complex, high-tech world, a turntable seems to be a relatively simple, almost primitive device that uses 19th-century technology to make a platter rotate on a bearing at a specific speed.
From afar, a spinning platter is a spinning platter, and if a tonearm and cartridge can travel without a hiccup from lead-in to lead-out groove, well, mission accomplished. But if it's that simple, why do all turntables sound different from one another? Can we all agree that they do? That we don't need to "prove" this with double-blind A/B/X testing? Good. Then I'll proceed.
Although you can get sound from an LP by rotating it on a fingertip and holding a stiff bristle in the groove, the process of accurately extracting musical information from a record groove while suppressing extraneous noise has proven extremely complex. In fact, as some very talented and very smart people continue to apply to this process their knowledge, skills, and ideas, the goalposts keep moving. What was thought 20 years ago to be the maximum amount of information that could be extracted has been exceeded again and again. What could be inscribed in a record groove 50 years ago has taken the 50 years since to be accurately and fully retrieved. Even today, there's considerable doubt that we've heard it all.
The activity continues at both ends of the process, as mastering and pressing facilities are upgraded in the quest for more accurate groove inscription and greater retrieval of information, even as records produced half a century ago continue to yield significant new details as playback technology improves. The chances are good that what's locked in those old record grooves surpasses what information remains on the deteriorating master tapes from which they were made.
Today, the fun for analog devotees is to upgrade and hear their favorite records anew. Part of that fun is to hear, for instance, previously obscured "background singers" suddenly become recognizable"Oh, that's Linda Ronstadt!" "I didn't know that was James Taylor back there!"
Vinyl Playback: A Time Travel Technology: The information that follows is a synthesis of Keith Howard's "Cut and Thrust: RIAA LP Equalization," originally published in the March 2009 issue of Stereophile; and an excerpt from Gary A. Galo's "The Columbia LP Equalization Curve," originally published in the Spring 2009 issue, Vol.40 No.1, of the ARSC Journal, the publication of the Association for Recorded Sound Collections.
We all know that playing an LP at the wrong speed produces incorrect pitches. We also know that, in addition to shifting pitch, the sound gets brighter as the speed increases, and duller as it decreases. So it would seem that correct speed is essential to both correct pitch and flat frequency response.
However, vinyl playback is far more complex. The RIAA curve, often called "equalization," is made while cutting the lacquer from which an LP is to be pressed: the bass frequencies are attenuated and the treble frequencies are boosted, their full amplitudes to be restored at playback. It also involves a combination of constant-velocity and constant-amplitude record and playback characteristics. I broach the subject here because it's germane to my review of VPI Industries' Classic Direct turntable.
Magnetic phono cartridges, whether moving-magnet or moving-coil, and lathe cutter heads, are velocity-, not amplitude-sensitive transducers. The cutting head produces a velocity proportional to the input voltage, not a displacement proportional to the input voltage. The cartridge produces an output voltage proportional to the velocity, not the displacement, so its frequency response will be linear, or flat, only when it's fed a constant-velocity signal.
Here, velocity refers to the physical distance the stylus of the cutter or cartridge moves perpendicular to the groove in a given period of time. Records can be cut with constant velocity or with constant amplitude characteristics. Constant-velocity cutting would result in flat cartridge response, so equalization would not be needed at playback, but it's not practical. A theoretical constant-velocity cut would result in large groove excursions at low frequencies and ultra-small (ie, low-level) ones at higher frequencies. That is because a stylus tracing, for example, a 500 or 5000Hz signal moves a greater distance in a given period of time than it does when tracing a 50Hz signal, if both are of the same amplitude (fig.1). In order to keep the velocity constant at all frequencies over time, the amplitude must be halved as the relative frequency doubles.
Records can also be cut with constant amplitude. That is, regardless of frequency, the amplitude (level) remains constant. One benefit of this is that, as the frequency falls, the amplitude doesn't increase, so neither do the groove excursions, which allows much more music to fit on a record.
Because the cutter head, too, is a velocity-sensitive transducer, maintaining constant amplitude, instead of it halving as the frequency doubles (as in a constant-velocity cut), it requires a 6dB/octave rise in the signal fed to the cutter head, which would produce a far more linear line than the wavy upward line called the RIAA "pre-emphasis" recording curve, which you're probably familiar with (fig.2, taken from the Keith Howard article). Theoretically, your phono preamplifier produces an inverse of that 6dB/octave rise, to output a flat response. The reality is far more complex.
If constant amplitude were really maintained from low to high frequencies, the playback bass boost required below 50Hz would unacceptably amplify low-level noise along with the signal. And if constant amplitude were maintained into the very high frequencies, the wave radii would become so "spikey" that no playback stylus could accurately trace them.
The vinyl records we know and love are cut using both constant amplitude and constant velocity. The choices made at the beginning of the LP era, and later modified to produce the RIAA curve, were made to take into account noise and other factors that result from the slow playing speed of 331?3rpm.
Despite what's already been discussed, below 50.5Hz, cutting is actually constant velocitywhich results in flat response without equalization, but also somewhat larger groove excursions. The tradeoff was made to avoid noise problems resulting from having to boost the very low bass by 6dB/octave. If you look at the RIAA recording curve, you will see it flatten below 50.5Hz, which is why it is called the low-bass shelf.
Between 50.5 and 500.5Hz (the "bass turnover" point), cutting is constant amplitude. Between 500.5 and 2122Hz (the "treble transition"), cutting reverts to constant velocity; above 2122Hz, it is again constant amplitude. The rise in the curve above 2122Hz, though called "treble pre-emphasis," does not really depict increasing amplitude, but an increase in recorded velocity. Boosting the amplitude would make tracing the groove in playback more difficult.
The playback cartridge produces flat response for the constant-velocity portion of the curve, and for the constant-amplitude portion its output rises at 6dB/octave, which must be compensated for by the inverse RIAA curve built into phono preamplifiers. However according to Gary Galo's article, the RIAA "recording characteristic" is a graph of recorded velocity, not recorded amplitude. It becomes a graph of recorded amplitude when the record is played with a velocity-sensitive magnetic pickup. The complementary playback equalization results in flat frequency response.
My point is that the reasons why speed accuracy is critical in terms of both actual speed and microspeed consistency go far beyond perceived pitch. (Thanks to Gary A. Galo, Audio Engineer Emeritus, The Crane School of Music, SUNY Potsdam, for helping me through the technical concepts and for providing the graph of amplitude vs time.)Michael Fremer