equalization curves applied in the vinyl pressing process

[Greta]

I'd like to bring a question to the community.

In short,
when a vinyl record is being recorded, some problems must be solved:
since low frequencies tend to affect wide and deep grooves, and high-frequency signals tend to have small amplitudes, an equalization process is applied before sending the signal to the engraver lathe.
The low frequencies are attenuated to reduce the dynamics and the high frequencies are amplified.
Obviously, the reverse equalization process must be applied to the output signal coming from the stylus while reading the record: low frequencies must be re-amplified and the high frequencies re-attenuated.
And this reverse equalization is done by the pre/phono or the AD converter.

From the mid 50s more or less, the standard equalization curve adopted by "almost" all the record companies is the RIAA curve/equalization.
Before the RIAA standard curve was introduced, each record company adopted own “secret” curve (such as AES, Audiophile, Columbia, Capitol, MGM, NAB, RCA1, RCA2, RCAO, HMV, Decca/London's FFRR and many others).

Now the point of the question:
since to listen to a record as it was conceived (the master), the same equalization adopted in the pressing phase must be applied reversely, it is necessary to know which curve was originally adopted by the record company.
Currently all records are engraved using the RIAA curve, which is proposed as the standard curve since 1954. The modern pre/phono/AD converters have that equalization built-in, so there are no problems.
But the transition from the many private curves to the RIAA standard was gradual, and is very easy to find records engraved with non-RIAA equalization even from the 60's or 70's.

I don't own vinyls and record player, but I've read of people owning a preamplifier with the selection of equalization curves, and they assure that if the wrong equalization is applied, the listening experience can change a lot.
I was guessing if it's possible to find around a list of the curves used by the various companies before 1954 but most of all in the years of the gradual transition to RIAA standard.
You guys owning vinyls collections, can confirm I suppose, that no mention is made about the used equalization on the covers or labels.
The bigger companies switched to the RIAA standard early on, probably, who knows. But I was guessing what the smaller ones did, what curves of equalization they used back than.
Obviously, I'm referring to the library/production world here, with a particular and very personal interest in Italian labels.

Any thoughts/suggestions?

[Upgrade]

A preamp with multiple EQ curves is much more common with 78s than ones meant for LPs.

Audacity has a brief section on early 33 LPs w/o RIAA which are dwarfed by the number of 78 labels.

plugins.audacityteam.org/additional-resources/eq-curves/playback-equalization-for-78-rpm-shellacs-and-early-33-lps

Some of the early Billboard issues mention that radio station turntables were hooked up with specifically labeled resistors for Columbia, RCA Victor etc. to get the correct EQ curve.

But generally speaking, mono LP recordings would be more likely to get nonstandard EQ curves than stereo LP  ones.

_____

Granted the older 78s were made of shellac, but later ones, especially very late 1960s library music 78s were made of vinyl.

If it’s for that window of time for pre-RIAA vinyl LPs, those physical preamps tend to be more expensive since there is less demand than for 78s.

If you had such a recording in question made from a record that ran through a RIAA preamp, it’s easy to determine if you could hear the difference between RIAA and a different curve.

You’d ideally want to record the record flat without RIAA in the first place and then apply the different curve in audacity.

However, if you have the former, you can invert the RIAA curve in audacity and apply it to the waveform to remove the RIAA. The sound will be very hissy with no bass and more or less resembles what is actually on the record. Then apply the new EQ curve and compare the original track with the new version and see if you can hear the difference and if it’s significant enough to pursue further.

Then there’s another can of worms of using the correct sized and shaped stylus since groove widths weren’t always standard as well.

[nidostar]

One of my other musical interests aside from library music is collecting 78 rpm records (shellac). More particularly I enjoy ripping the cleaned records and adjusting them such that the RIAA curve is correctly applied and as much surface noise and clicks are eliminated. I've been pleasantly surprised with the results. I have gathered a fair amount of info regarding applying modified RIAA curves and have a number of them as presets in Audacity. Unfortunately, I'm away from my main computer at the moment so cannot access it. However, you may find the two link below a good place to start.

h**p://www.shellac.org/slumber/equal.html
ht**s://github.com/audacity/audacity-support/blob/plugins/additional-resources/eq-curves/playback-equalization-for-78-rpm-shellacs-and-early-33-lps.md

[Greta]

Interesting readings.

But one of my main goals by raising this question was to know if there's any chance to find a list of the EQ curves all the labels applied.
With a specific attention to "our" beloved English, French, German, Italian (and others) library labels.

Or we have to suppose they all used the RIAA standard?

[nidostar]

As I mentioned I'm afraid I don't have all my research available to me at the moment. I've not seen a list that goes into the detail you're looking for. I seem to remember the list I have shows only the curves used by the major commercial record labels (Decca, Vocalion, EMI etc) though it is quite extensive. I suspect that a library would have used one of the commercial labels to press their records for them as it's unlikely they would have had their own pressing plants. So my guess is that we would need to know who handled which library's pressing to know which curve might have been applied.

[stackjackson]

I can't really follow much of the technicalities here, but it may be worthwhile to keep in mind that most library labels produced vinyl records merely to serve as demos for potential and existing clients, and not to be used as the final product. I don't imagine, then, that these labels were all that concerned with insuring the highest quality reproduction, just high enough sound quality to be properly previewed. I would assume that highest (or most professional) level of quality would have been reserved for the tapes of the pieces selected by the client and sent to them for professional use.

But again, I know nothing about any of this...

[Greta]

It's not a matter of quality Stack.

Every single vinyl press must go through an equalization before being printed, and that's because the low frequencies would generate too large grooves in the vinyl and just one track would probably fill in a whole side.
It's an unavoidable process in making vinyl records, not something that you can choose to do or not.
Other problems would occur without a pre-equalization, but we would get into complicated topics that I myself don't understand that much.

The point was to try understand what equalization the various labels used, the way to use the exact reverse equalization while playing the record.
As you must use a reverse equalization to play the record, it's much better to use the right one.

[Greta]

I suspect that a library would have used one of the commercial labels to press their records for them as it's unlikely they would have had their own pressing plants. So my guess is that we would need to know who handled which library's pressing to know which curve might have been applied.

That's a good point. I din't think about it..