Equalization of (old) records
Updated: Jan 16, 2019
Summary
Vinyl (or older materials) records have been printed for one century now with almost the same basic process but according to different standards all these years. A number of sources on the web describe what the specific labels printed and the corresponding techniques they applied; a quick search can reveal how large this number is. The differences all lie in how the sound engineers coped with the limitations of the cutting process: generally speaking the lathe attenuated the content of bass frequencies and enhanced high frequencies following different equalization curves. RIAA (Recording Industries Association of America) established what became the worldwide standard for records cut in the mid-50’s, and all the labels then aligned to it. Phono preamplifiers must cope with those different standards to properly reproduce old records and restore correct tonal balances during playback.
Record and playback curves
During the playback, the original content of the record must be restored applying a reverse equalization that boosts low frequencies and attenuates highs.
Modern phono preamplifiers can equalize a record with an accuracy of 0.1dB or even better but almost all of them come with only the current RIAA standard. Unfortunately, if a record is played through a wrong equalization, accuracy will be lost. Playback errors may get very large at the extremes of the audio band and are usually limited to 1db or so in the 100Hz-10kHz band. The following example is the (theoretical) result of the RIAA equalization of an AES record: below 100Hz error increases in deep bass area while in the most fundamental 100Hz-10kHz band it stays within ± 1dB.
AES and RIAA are pretty similar standards and errors are limited (see fig. 2). Things can get much more worrisome if an old shellac (a 78 rpm) is played with the available RIAA equalization. An FFRR 78 rpm record played back through the RIAA curve will have a gross tonal unbalance (see fig. 3).
Equalization curves in practice
Equalization curves are based on a few parameters:
The bass turnover frequency indicates the point where the bass gets attenuated in recording and boosted in playback. This is the most important parameter. It is usually expressed as its corner frequency or the corresponding time constant.
The treble cut is the frequency at which the highs are boosted in recording and attenuated in playback. It can be expressed as the corner frequency, the associated time constant or the resulting attenuation at 10kHz (with reference to the flat mid-range band).
The bass shelf is the maximum amplification of bass frequencies allowed during playback; it can be expressed in terms of the frequency at which the deep bass attenuation starts or the maximum level in dB of deep bass with reference to the flat midrange area. It may also be identified by a literal code.
In reality, playback curves cannot reproduce sharp corners so practical RIAA and all the older playback curves are rounded at the nominal frequencies of 50.05Hz (bass shelf), 500.5Hz (bass turnover) and 2122Hz (treble cut). See figure 4 for a comparison of theoretical and practical curves and for an indication of curve parameters.
Let’s see how the single parameters affect the playback equalization curve and the effect they have on the tonal balance.
Bass Turnover
The bass turnover frequency sets the fundamental tonal balance of the playback. It can vary from 200Hz of some ancient Columbia records up to 1kHz; the most recent pre-RIAA records were cut in the 350Hz-800Hz range. A lower turnover frequency corresponds to a lower bass boost and, as a matter of fact, the most ancient records were cut at lower frequencies so errors in reproduction can get very large when the standard RIAA equalization is applied instead of the original and appropriate curve.
Bass Shelf
As discographic industry evolved, a bass shelf was introduced along with an increase of the turnover. Bass shelf is beneficial in preventing rumble due to warped media or due to turntable bearing and motor and it limits the max amplification of deep bass, safeguarding woofers.
Treble Cut
Treble cut effect in playback can start from 1592Hz for the Columbia 78 up to 6366Hz for the Decca FFRR (same age). RIAA set the standard at 2122Hz, limiting the midband flat area.
Subsonic Filter and Anti-Scratch Filter
Equalization of old records may, finally, require an adjustment to the playback curves to cope with excessive subsonic frequency noise or surface noise at higher frequency. The former is addressed with a 12dB/oct or steeper high pass filters at 50Hz or lower frequencies, the latter with milder 6dB/oct low pass filters from 6kHz and upward. In the late 70’s a more effective approach to bass rumble was proposed: the signal is split in low frequencies (below 150Hz or so) and converted to mono and hence summed back to the remaining stereo high frequency. The stereo- to-mono conversion nulls the rumble as it appears with opposite phases in the Right and Left channel. A typical correction is as in the following figure 8.
Reference Material
http://wiki.audacityteam.org/wiki/78rpm_playback_curves, and at the referenced sources, contains a very comprehensive list of equalization curves for old records. For the sake of clarity the following tables correlate the different parameters for equalization.
Download the complete PDF file (with all the pictures, graphics, tables) under the section "SUPPORT/DOWNLOAD".
https://docs.wixstatic.com/ugd/91e8f2_b1df9b867ba843c0b62fcd654306d3d0.pdf
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Author: Gianluca Sperti