78rpm playback curves

Usage
These EQ curves are presented as a table of "Bass Turnover Frequencies" and "10 kHz Gain Rolloffs". EQ Curves for Audacity can be generated from these Frequency and Rolloff values using the experimental Nyquist plug-in "78 RPM EQ Curve Generator". This plug-in is obtainable from the top of this Forum topic and requires Audacity 1.3.13 or later. ''Please give feedback on this plug-in, or ask for help if you need it, by replying to that Forum topic.

  Extract 78EQCurveGen.ny from the zip file downloaded from the above Forum topic.  Place 78EQCurveGen.ny in the "Plug-ins" folder inside the Audacity installation folder, then launch or restart Audacity. For more help installing the .ny file to the correct location, click here.  Click. You can find help inside the plug-in by choosing one of the Help options in "Select Function or Help".  Choose the curve you want from the list below.  Enter the values for your chosen curve for  "Bass Turnover Frequency (Hz)"  "10 kHz Gain Rolloff (dB)"  "LF Shelving Frequency (Hz)" (if a value is given) in the equivalent boxes in the plug-in dialogue. Click "OK" in the plug-in to save the .xml file to your chosen location. Select some audio and choose.  Choose "Save/Manage Curves...".  Choose "Import...", navigate to the location where you saved the .xml file from 78EQCurveGen.ny, then "Open".  Click "OK". </ol>

Equalization (EQ) Curves Explained
When phonograph records are made, the sound being recorded is deliberately distorted by reducing the volume of the low frequencies and increasing the volume of the high frequencies. This process, known as 'pre-emphasis', allows the low frequencies to be accommodated in the limitations of the record groove and reduces the effect of high frequency surface noise. If pre-emphasis was not carried out, the bass notes in the music would create a groove in the record that oscillated so wildly that the stylus could jump out of it on playback, and the treble notes would be drowned out by the surface noise of the stylus in the groove.

On playback, the pre-emphasis must be reversed in order to restore the original sound. This is known as 'de-emphasis' or equalization (EQ).

Modern vinyl records use a method of pre-emphasis and de-emphasis adopted by the Record Industry Association of America (RIAA) in the 1950's, and the EQ curve used is known as the RIAA curve. However, before the RIAA curve was adopted, each record label used its own EQ curve for recording and, for these records (78rpm and early vinyl), the correct EQ curve must be used for playback.

Each 78rpm EQ curve is a combination of two filter characteristics; a bass boost curve, defined by a 'Bass Turnover' (or 3dB) frequency, and a treble cut curve, defined by a '10 kHz Gain Rolloff' parameter, i.e. a defined level of treble cut at 10 kHz.

As an example, Figure 1 below shows the characteristic of the bass boost curve defined by a 500Hz Bass Turnover, and Figure 2 shows the characteristic of the treble cut curve defined by a 10 kHz Gain Rolloff of -13.7dB. These two curves, when combined, give the characteristic shown in Figure 3.



Figure 1. Bass Boost curve: 3dB at 500 Hz



Figure 2. Treble Cut curve: -13.7 dB at 10 kHz



Figure 3. Combined Bass Boost and Treble Cut curve

The EQ curve may also include a Low Frequency Shelving filter, (although it is absent from the definition of most 78 rpm EQ curves). This addition reduces the effect of the bass boost at very low frequencies (typically 50 or 100 Hz) in order to attenuate low-frequency noise such as turntable rumble. Figure 4 shows the effect of a 50 Hz LF shelving filter being added to the curve in Figure 3. This is in fact the RIAA standard EQ curve.



Figure 4. Combined Bass Boost, Treble Cut and 50 Hz LF Shelving curve

Acoustically recorded (pre-electric) 78rpm records have a 'flat' EQ curve, i.e. with no bass boost or treble cut and, in some early electric EQ curves, while there is a bass boost curve, the 10 kHz Gain Rolloff is zero, i.e. there is no treble cut.

Note that in the combined EQ curve, the gains at the Bass Turnover frequency and at 10 kHz may be different from those specified by the parameters. This is not an error, but is due to fact that the gains of the bass boost and treble cut curves are simply added together.

Because 78 rpm EQ curves were non-standard and, in many cases, accurate records were not kept to show what EQ curves were used when recording 78s, there is a degree of uncertainty about what is the correct playback EQ for many record labels. The tables below have been obtained from websites which, in their turn, have compiled data from a number of sources and should prove reasonably accurate. However, the ear of the listener is the final arbiter - if it doesn't sound right, it isn't right!

According to NAB standards, the nominal speed of a 78 RPM record is precisely 78.26 RPM +/- 0.5%.

EQ Curve List
The curves in Table 1 and Table 2 below were obtained from the following publicly available sources:

1. Millennia 78 EQ chart (PDF).

2. Wilmut's World Wide Website

3. Wolverine Antique Music Society


 * According to the author of [2]:


 * Westrex (English Western Electric) should be used for HMV 78s with triangle matrix code and English Columbias with a W matrix code. Also for HMVs with diamond mark which are American Victor recordings.


 * Blumlein should be used for HMVs with a square by the matrix number, and English Columbias with a C, or in both cases with no code (post 1945) up to about 1953.


 * BSI 78 should be used for all post 1953 78s.

Table 1: 78rpm Playback Equalisation Curves


 * COL - Some recordings require a Bass Turnover of 300 Hz or 500 Hz, a 10 kHz rolloff of -16 dB and a LF Shelving filter at 100Hz.


 * This is often referred to as a "Columbia curve".

Table 2: Pre-1955 LP Playback Equalisation Curves