In 1963, RCA introduced a set of enhancements to its LP record-making process under the trademark, “Dynagroove.” It was heralded as “adding brilliance and clarity, realistic presence, full-bodied tone and virtually eliminating surface noise and inner groove distortion.” However, it was criticized as early as 1963 by J. Gordon Holt in Stereophile magazine. He sniffed, “[It is] nothing more than a sophisticated way of bringing higher fi to record buyers who don’t care enough about hi-fi to invest in a decent playback system.” Although I recall a high public regard for Dynagroove in that era, today, one can find quite a few negative comments posted about it on the Net. I suspect that those stem in part, from the original criticisms by Holt. It could also be that some of the processing used in the system is no longer optimum, for the playback equipment used today (see below).

I was curious to see if the attacks of the audio elite were fully justified, so I turned to Dr. Harry Olson’s paper describing the Dynagroove system. It appeared in the April 1964 issue of the Journal of the Audio Engineering Society.

Dynagroove in a Nutshell

Let’s take a brief look at what Dynagroove really was. It was a comprehensive program to improve sound recording in all aspects from artist’s conception of the music to reproduction of it in the home. I know this might sound like so much marketing patter today. In the early sixties though, RCA Laboratories, under the direction of Dr. Olson, put together an interdisciplinary team of scientists, design engineers, recording engineers, musicians and music directors, to study the recording and record production process. The resulting system included the following improvements:

• New studios, with controlled attack and decay characteristics.
• Special selection of microphones. (Olson was an expert in mic design.)
• Microphone placement based on controlled, subjective testing.
• New, low-distortion microphone preamplifiers.
• A new type of master recording console, with special peak indicators to
  guard against overloading the master tape.
• New magnetic tape recorders with higher overload points.
• Recording at 30ips instead of the conventional 15ips.
• Development of a new magnetic tape formulation, with higher retentivity, lower noise level, higher coercivity and a heavier base material.
• A (controversial) Dynamic Spectrum Equalizer (DSE) intended to compensate for deficiencies of reproduction in a typical residential setting
• Sophisticated Recording Overload Indicator to guard against exceeding the limits allowable for cutting the master disk.
• Dynamic Styli Correlator (DSC), which canceled tracing distortion, due to a 0.7mil playing stylus. It was stronger at high frequencies and at the inner grooves.
• Optimization of vertical tracking angle in the disk cutter.
• New, higher-power disk cutting amplifiers to reduce distortion.
• Reduction of noise in the master lacquer disk by maintaining constant temperature of the cutting stylus.
• Improvements in the plating process used to produce final stampers from the master disk, resulting in higher signal-to-noise ratio.
• Development of a new, electrically-conducting plastic for the record disk,
  to dissipate static electricity, reducing dust pickup and the associated
  surface noise.

Controversy Over Dynamic Spectrum Equalization (DSE)

Perhaps the most controversial part of the Dynagroove system was the Dynamic Spectrum Equalizer (DSE). It was a variable filter which boosted low frequencies at low levels and boosted high frequencies at high levels. Another way to view it is: There was dynamic range compression at low frequencies and expansion at high frequencies. The processing was intended to compensate for the masking effect of room noise, for changes in the ear’s frequency sensitivity over level and for the reverberation characteristics of the average home.

It’s not surprising that this didn’t appeal to audio purists. I would agree with them, that I would prefer not to have my audio processed this way. Nevertheless, I recognize that there was real research which argued for it. I am sorry to have to admit that, comparing the CD and Dynagroove LP versions of one selection from 1963, the LP version sounded better. It was more dynamic and brighter than the CD. That prompted my interest, which led to this article. It was a brief comparison. If I were to compare more material, perhaps my opinion would change as to whether the LP sounds better. In any case, I definitely would prefer my music to be unprocessed, in that sense.

But there is another factor to consider: Most LP recordings needed some general compression to sound good. I have always (reluctantly) accepted that. The dynamic range of a symphony orchestra simply exceeds the usable dynamic range of an LP by too much. Given that the overall dynamic range had to be somewhat compressed, was there any way to restore some of the dynamics?

Have you ever noticed that after listening to loud music for a while, it no longer seems loud? The ear adjusts and much of what we perceive as dynamic range is what occurs over relatively short time periods (say several seconds) rather than minutes. The DSE was a dynamic range expander for the upper frequency range. It restored some of the short-term dynamics for that part of the spectrum, since that part didn’t dominate overall power level. The result was that the effects of necessary overall compression were somewhat mitigated.

Once again, personally, I would prefer the unprocessed version. But I would bet that most listeners, audiophiles included, would choose the Dynagroove version, if it were not identified as such.

The Dynamic Styli Correlator (DSC)

By 1963, it was well-known that the 0.7-mil stylus which was used to play LP’s, would cause a significant amount of distortion as the ball-shaped tip attempted to trace the path cut by the chisel-shaped cutter stylus. This is called tracing distortion. You cannot use a chisel-shaped stylus to play records, because it would put too much stress on the plastic and damage the grooves. The Dynagroove solution was to model the distortion and introduce an equal but opposite signal into the recording. This would cancel the tracing distortion, upon playback. Since the amount of tracing distortion gets worse towards the inner grooves, the compensation had to vary with cutter head position. According to test data presented, the DSC worked very well.

However, DSC is now criticized for the fact that it was optimized for a 0.7-mil playback stylus and elliptical styli (typically 0.7 x 0.3-mil) are common today. Yet, it is not clear whether playback with the non-optimum elliptical stylus will incur more distortion than if the DSC were not used at all. That is an interesting topic for investigation.

In any case, the 1963 Allied Radio catalog did not list any cartridges with elliptical styli, though many big names such as Shure, Empire, Pickering, Stanton and others appear. The elliptical stylus had been invented by Edison around the turn of the century but it would be some time before it was rediscovered and became common in home stereo systems.

So the DSC part of Dynagroove was a real advance in the state of the art at the time. Whether it helps or hurts today, is an open question.

Other than DSE and DSC, all of the changes introduced in the Dynagroove system were unquestionably positive developments in producing high-quality LP records. Although DSE does cause controversy, it would seem to be unfair to ignore the many real enhancements that the system brought and to dismiss all of the excellent Dynagroove recordings which were made.

Reader Comments

Ed: This is posted (with permission) from an email exchange starting 7/2/23. The thread within this posting is in normal time order, whereas the individual posts in the Reader Comments section are top posted (in reverse time order). From Tore: Steve, I just saw your article on the Dynagroove system and the Dynamic Spectrum Equalizer, and it got my attention :) When I go through the original AES article by Olson, he presents a figure which is meant to illustrate the room gain in a typical living room. See below. If I understand the article correctly, the DSE filter (algorithm) would compensate for the typical room gain corresponding to the inverse of the figure above. In other words, the room compensation part of the algorithm would attenuate the bass at 50 Hz by about 5-6 dB, at 100 Hz by about 4 dB, etc. In other words, the room compensation part of Olson's DSE algorithm aimed for a flat room curve based on the assumption that a typical living room has a room gain of 5-6 dB in the lower frequencies. Do you agree with my interpretation of Olson's DSE? I look forward to hearing your thoughts! Best, Tore ------------------------------ From Steve L: Hi Tore, Thank you for your thoughtful and well-documented message. My interpretation is that "room gain" is one of the six criteria that needed to be considered to equalize home sound reproduction. Those criteria were listed on p.107 of the Olson paper. Room gain appears to be represented by Olson's term, "reverberation characteristic of the average room." I would focus on the family of curves in Figure 24 (below) for the DSE. At modest listening levels of 70dB, 100Hz-response is near 0dB relative to 1kHz. At 15dB lower level, 100Hz-response is about +3dB, while at 15dB higher level, it might be -0.5dB. So the other factors in the criteria seem to have moderated the curve shown in Figure 20, at moderate listening levels*. This tends to emphasize the impression I've had all along, that the secret to Dynagroove's success was that DSE had only a minor effect on most music. They restrained themselves from going very far with it, preferring to keep it as a finishing touch rather than something heavy-handed. I appreciate your interesting comments. Rest Regards, Steve * "At home the golden standard for how loud the music should be listened to is between 70-75dB" http://www.audiodrom.net/en/as-we-see-it-tips-thoughts/74-realistic-volume-levels ------------------------------ Stephen, Thanks for your response! What caught my eye in Olson’s DSE algorithm is how far ahead of his time he was. As I see it, Olson worked based on a certain set of first principles, where perfect reproduction and full resemblance with the sound source were his goal. His room compensation algorithm - based on measurements and the first principle that no frequencies are to be discriminated in the reproduction - is an early attempt at what is often called «room correction» today. However, Olson’s curve is quite different from the more modern room curves of Brüel & Kjær, Harman etc. Best, Tore PS: There are SPL recommendations for critical listening for professionals, and the recommended SPL levels are somewhere between 75 and 85 dB, say around 80 dB.

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