5.3.6 Replay Equalisation
5.3.6.1 The need for equalisation and the manner in which it was developed is explained in Section 5.2.6. Equalisation is also applied to microgroove recordings and primarily involves reducing the level of frequencies below about 500 Hz which is the LF turnover below which the recording is constant amplitude, and boosting those above about 2 kHz. Between 500 Hz and 2 kHz the recording is characterised by constant velocity (see 5.2.6). The application of equalisation in the recording process has to be compensated for in the replay chain. Many companies had their own, usually minor, variations on this theme, and for accurate reproduction, exact replay equalisation needs to be applied (see Table 1 Section 5.3 below).
5.3.6.2 Records made after about 1955 complied with what is now known as the RIAA (Record Industry Association of America) curve which became a well observed standard throughout the industry. RIAA replay characteristics are defined by a replay cut of 6 dB/octave from 20 Hz to 500 Hz, a flat shelf between 500 Hz and 2.12 kHz (318 µs and 75 µs respectively) and a 6 dB/Octave treble cut from 2.12 kHz. The flat shelf is approximately 19.3 dB below zero.
5.3.6.3 The Equalisation curves for replay are listed below.
|
Equalisation Curves by Name |
LF Roll-off | LF Turnover |
HF Roll-off Turnover (-6 dB/octave, except where marked) |
Roll-off @ 10 kHz |
|---|---|---|---|---|
| AES | 50 Hz | 400 Hz (375) | 2500 Hz | -12 dB |
| FFRR (1949) | 40 Hz | 250 Hz | 3000 Hz* | -5 dB |
| FFRR (1951) | 300 Hz (250) | 2120 Hz | -14 dB | |
| FFRR (1953) | 100 Hz | 450 Hz (500) | 3180 Hz (5200) | -11 dB (-8.5) |
| LP/COL | 100 Hz | 500 Hz3 | 1590 Hz | -16 dB |
| NAB | 500 Hz | 1590 Hz | -16 dB | |
| Orthophonic (RCA) | 50 Hz | 500 Hz | 3180 Hz (5200) | -11 dB (-8.5) |
| 629 | 629 Hz (750) | |||
| RIAA | 50 Hz | 500 Hz4 | 2500 Hz | -13.7 dB |
Table 1 Section 5.3 Equalisation Curves by Name
|
Equalisation Chart for Pre-1955 LP Records5 |
LF Roll-off | LF Turnover |
HF Roll-off Turnover (-6 dB/octave, except where marked) |
Roll-off @ 10 kHz |
|---|---|---|---|---|
| Audio Fidelity | 500 Hz (NAB) | 1590 Hz | -16 dB | |
| Capitol | 400 Hz (AES) | 2500 Hz | -12 dB | |
| Capitol-Cetra | 400 Hz (AES) | 2500 Hz | -12 dB | |
| Columbia | 500 Hz (COL) | 1590 Hz | -16 dB | |
| Decca | 400 Hz (AES) | 2500 | -12 dB | |
| Decca (until 11/55) | 100 Hz | 500 Hz (COL) | 1590 Hz (1600) | -16 dB |
| Decca FFRR (1951) 3dB slope | 300 Hz (250) | 2120 Hz | -14 dB | |
| Decca FFRR (1953) 3dB slope | 450 Hz (500) | 2800 Hz | -11 dB(-8.5) | |
| Ducretet-Thomson | 450 Hz (500) | 2800 Hz | -11 dB(-8.5) | |
| EMS | 375 Hz | 2500 Hz | -12 dB | |
| Epic (until 1954) | 500 Hz (COL) | 1590 Hz | -16 dB | |
| Esoteric | 400 Hz (AES) | 2500 Hz | -12 dB | |
| Folkways | 500 Hz (COL) | 1590 Hz | -16 dB | |
| HMV | 500 Hz (COL) | 1590 Hz | -16 dB | |
| London (up to LL-846) | 100 Hz | 450 Hz (500) | 2800 Hz | -11 dB(-8.5) |
| London International | 100 Hz | 450 Hz (500) | 2800 Hz | -11 dB(-8.5) |
| Mercury (until 10/54) | 400 Hz (AES) | 2800 Hz | -11 dB | |
| MGM | 500 Hz (NAB) | 2800 Hz | -11 dB | |
| RCA Victor (until 8/52) | 50 Hz | 500 Hz (NAB) | 2120 Hz | -12 dB |
| Vox (until 1954) | 500 Hz (COL) | 1590 Hz | -16 dB | |
|
Westminster (pre-1956) or |
500 Hz (NAB) 400 Hz (AES) |
1590 Hz 2800 Hz |
-16 dB -11 dB |
Table 2 Section 5.3 Equalisation Chart for Pre-1955 LP Records
3. modified from NAB: less bass below 150 Hz, requiring about 3 dB boost.
4. RIAA and NAB are very similar.
5. This information is taken from several sources: the “DialYour Discs” chart which appeared in High Fidelity magazine during the early 1950s, the chart compiled by James R. Powell, Jr. and published in the ARSC Journal, and the jackets of various early LPs. “Turnover” (col. 2) is the frequency below which the record manufacturer diminished the bass when mastering the disc, requiring a corresponding boost during playback. In the chart, turnover is stated using the name of the recording curve, as given on most older pre-amps; a list of these curves and their turnover frequencies is at the end of the chart. ”Roll-off”(col.3) is the amount of treble cut at 10kHz required during playback to compensate for pre-emphasis added during disc mastering. In the chart, roll-off is stated in dB.
