5.2.6 Replay equalisation
184.108.40.206 Equalisation became a possibility with the introduction of electrical recordings; it also became a necessity. Equalisation in recording is the application of a frequency dependant boost or cut to the signal before it is recorded, and the inverse cut or boost on replay. This became a possibility with electrical recordings because the recording and replay systems now included electrical circuitry which enabled a process which could not have been applied in the acoustic recording process. It became a necessity because the way sound is represented on a disc would not allow the dynamic range or frequency response that the electrical technology enabled, to be recorded otherwise.
220.127.116.11 Sound can be recorded on a disc in two different ways;”constant velocity” or “constant amplitude”. Constant velocity on a disc is when the transverse speed of the stylus remains constant regardless of the frequency. An ideal acoustic disc recording would display constant velocity characteristics throughout its recordable range. One of the implications of constant velocity is that the peak amplitude of the signal is inversely proportional to the frequency of that signal, which means that high frequencies are recorded with small amplitudes, and low frequencies are recorded with comparatively large amplitudes. The difference in amplitude can be very marked; across 8 octaves, for example, the ratio in amplitude between the lowest and highest frequency is 256:1. At low frequencies, constant velocity is unsuitable as the excursion of the groove becomes excessive, reducing the amount of available recording space, or causing cross over between tracks.
18.104.22.168 Constant amplitude, on the other hand, is when the amplitude remains constant regardless of the frequency. Constant amplitude, while most suitable for low frequencies, is unsuitable for higher frequencies as the transverse velocity of the recording or replay stylus could become so excessive as to cause distortion. To overcome the dilemma caused by both these approaches, disc manufacturers recorded electrical discs with more or less constant amplitude at the lower frequencies and constant velocity at the higher frequencies. The point of change between the two is described as the low frequency turnover (see table 5.2).
22.214.171.124 As the recording technology improved and increasingly higher frequencies could be captured, these higher frequencies resulted in correspondingly smaller amplitudes on the disc. A consequence of the very small amplitude of these high frequency components is that the ratio of the signal to the irregularity in the surface of the disc approaches equivalence. This would mean that the very high frequencies would be comparable in amplitude to the unwanted surface noise, otherwise known as a poor signal to noise ratio. To overcome this, the disc manufacturers began to boost the higher frequency signals so that these very high frequencies were often, though not always, constant amplitude recordings. The point at which the higher frequencies are switched from constant velocity to constant amplitude is called HF Roll-off Turnover (see table 5.2). The function of this higher frequency equalisation is improvement in the signal to noise ratio, and it is commonly termed pre-emphasis in recording and de-emphasis in replay.
126.96.36.199 The commonly used dynamic or magnetic pick-ups are velocity transducers, and their output can be directly fed into a standard preamplifier, if that is desired. Piezo-electrical and optical replay systems are amplitude transducers. In these cases a general 6dB/octave slope equalisation must be applied as the difference between a constant velocity and constant amplitude recording is 6dB per octave.
188.8.131.52 Acoustically recorded discs have no intentionally applied equalisation in recording (though engineers were known to adjust parts of the recording path). As a consequence of the recording process, the spectra of an acoustic disc would display resonant peaks in amplitude and related lows. Applying a standard equalisation to compensate for the acoustic recording process is not possible as resonances in the recording horn and the stylus diaphragm, not to mention other mechanical damping effects, can vary between recordings, even recordings from the same session. In such cases the recordings should be replayed flat, i.e. without equalisation, and equalisation should be applied after the transfer has been made.
184.108.40.206 With electrical recordings it is necessary to decide whether to apply an equalisation curve on replay, or to transfer flat.Where the curve is accurately known equalisation may be applied either at the preamplifier prior to making the copy, or applied digitally after making a flat copy.Where doubt remains as to the correct equalisation curve, a flat transfer should be made. Subsequent digital versions may employ whichever curve seems most appropriate, so long as the process is fully documented, and the flat transfer retained as the archival master file.Whether or not equalisation is applied during the initial transfer, it is imperative that noise and distortion from the analogue signal chain (everything between the stylus and analogue-to-digital converter) is kept to an absolute minimum.
220.127.116.11 It is worth noting that a flat transfer will require around 20dB more headroom than one where an equalisation curve has been applied. However, as the potential dynamic range of a 24 bit digital to analogue convertor exceeds that of the original recording, the extra 20dB headroom can be accommodated.
18.104.22.168 Apart from the dynamic range limitations mentioned above, a drawback with transferring electrically recorded discs without de-emphasis is that stylus selection is primarily made through aural assessment of the effectiveness of each styli, and it is more difficult, though not impossible, to make reasonable assessment of the effect of different styli while listening to unequalised audio. An approach taken by some archives is to apply a standard, or house, curve to all recordings of a particular type in order to make stylus selection and other adjustments, and subsequently produce a simultaneous flat and equalised digital copy of the audio. As the exact equalisation is not always known, a flat1 copy has the advantage of allowing future users to apply equalisation as required, and is the preferred approach.
22.214.171.124 There is some debate as to whether noise reduction tools for the removal of audible clicks, hiss etc are more effective when used before an equalisation curve is applied rather than afterwards. The answer very likely varies according to the specific choice of tool and the nature of the job to which it is applied, and in any event will be subject to change as tools continue to evolve. The most important point in this regard is that noise reduction equipment, even fully automated tools with no user-definable parameters, ultimately employs subjective and irreversible processes, and so should not be used in the creation of archival master files.
126.96.36.199 A complete record of all decisions made, including choice of equipment, stylus, arm, and equalisation curve (or its absence) must be recorded and maintained in metadata.
188.8.131.52 The main equalisation curves for replay are listed below.
Equalisation Chart for Electrically
recorded coarse groove
(78 rpm) Discs
Turnover (-6 dB/octave,
except where marked)
|Roll-off @ 10 kHz|
|Brunswick||500 Hz (NAB)||0 dB|
|Capitol (1942)||400 Hz (AES)||2500 Hz||-12 dB|
|Columbia (1925)||200 Hz (250)||†5500 Hz (5200)||-7 dB (-8.5)|
|Columbia (1938)||300 Hz (250)||1590 Hz||-16 dB|
|Columbia (Eng.)||250 Hz||0 dB|
|Decca (1934)||400 Hz (AES)||2500 Hz||-12 dB|
|Decca FFRR (1949)||250 Hz||3000 Hz*||-5 dB|
|early 78s (mid-’30s)||500 Hz (NAB)||0 dB|
|EMI (1931)||250 Hz||0 dB|
|HMV (1931)||250 Hz||0 dB|
|London FFRR (1949)||250 Hz||3000 Hz*||-5 dB|
|Mercury||400 Hz (AES)||2500 Hz||-12 dB|
|MGM||500 Hz (RIAA)||2500 Hz||-12 dB|
|Parlophone||500 Hz (NAB)||0 dB|
|Victor (1925)||200–500 Hz||†5500 Hz (5200)||-7 dB (-8.5)|
|Victor (1938–47)||500 Hz (NAB)||†5500 Hz (5200)||-7 dB (-8.5)|
|Victor (1947–52)||500 Hz (NAB)||2120 Hz||-12 dB|
Table 1 Section 5.2 Equalisation Chart for Electrically Recorded Coarse Groove (78 rpm) Discs3.
* 3 dB/octave slope. N.B.A 6 dB/octave slope should not be used on these marked frequencies because though it may be adjusted to give the correct reading at 10kHz, rolloff would commence at the wrong frequency (6800 Hz) and be incorrect at all other frequencies.
† This only a recommended roll-off in order to achieve a more natural sound. The pronounced HF content is probably due to resonant peaks of the microphone and not due to the recording characteristic.
2 See Table 2, Section 5.3, footnote 5 for definitions of “Turnover” and “Rolloff”.
3 Ref: Heinz O. Graumann: Schallplatten-Schneidkennlinien und ihre Entzerrung, (Gramophone Disc-Recording Characteristics and their Equalizations) Funkschau 1958/Heft 15/705-707. The table does not include every curve ever used, and other reputable sources vary slightly in their description of some of those listed. Research in this area is ongoing, and readers may wish to compare with other findings, such as Powell & Stehle 1993 or Copeland 2008, Chapter 6 etc.