Equalisation

Acoustic recording and playback


The waves cut into the cylinder surface of earliest phonograph records were perpendicular to the surface so this is usually referred to as "hill-and-dale" recording. The inventor credited with the idea of the flat disc was Emile Berliner who considered that the variable resistance offered by a soft recording surface to a stylus vibrating perpendicularly to the surface would reduce the bass response, so his discs were "lateral cut" recordings. This remained the standard process when electrical recording was introduced.
Acoustic recordings were made by playing or singing loudly into a horn connected to the cutting stylus. Pressure variations in the air inside the horn would make the stylus vibrate laterally or vertically to cut the groove. Ideally for high quality recordings the amplitude of stylus vibration would be directly proportional to the amplitude of the music being recorded. In practice however there were so many resonances introduced by the recording apparatus that the characteristic would be very non-linear, as shown by the red line on on the third graph.
Playback of acoustic discs was also mechanical, with a needle in the pick up or "sound box" following the groove undulations which in most designs would make a diaphragm oscillate to generate sound waves which would be channelled to the listener through an external or internal horn. As with the recording apparatus, there would be much resonance and damping and the typical sound box response would also be highly non-linear, as shown in the graph on the left. For both recoding and playback, the highest energy response would be in the frequency range from about 500 Hz to about 3kHz with very little sound below 150 Hz or above 5 kHz.


Electrical recording and playback
The magnetic pickup generates an electro-motive force when the needle tip is displaced sideways. In accordance with Faraday's law of electromagnetic induction, the size of the e.m.f. is proportional to the lateral velocity of the needle. For a pure (simple harmonic) tone of frequency f and amplitude a, the energy of vibration is proportional to both f² and a². Therefore, if the intensity of the recorded note is to be independent of frequency, the product af must be constant. The maximum velocity during simple harmonic motion is 2πfa so if the maximum velocity of the needle tip in the groove is designed to be constant, the input and output energies are the same for all frequencies. This condition is referred to as constant velocity recording.
Since af=consant the amplitude a of equal intensity sounds is inversely proportional to the frequency f. The amplitude of the grooves for bass notes is consequently larger than that for high frequency tones of the same intensity. A one-octave rise in pitch means a doubling of frequency which, for constant amplitude recording, would lead to a factor of 2² increase in energy and conversely for constant velocity recording to a decrease in energy by the same amount. The decibel scale is logarithmic and an increase in energy of 2² would be equivalent to 2log₁₀2 ≈ 2 x 0.3 or 0.6 bel or 6 decibel. For constant velocity recording, the amplitude therefore decreases as the frequency rises at the rate of 6dB/octave.
A constant amplitude recording would result from the application of frequency dependent amplification of the electrical signal produced from the sound wave vibrations of the microphone. Higher frequencies would then result in faster velocity of the stylus which, during subsequent playback or the recording, would produce louder sounds.

The early electrical recording curve
The difference in groove radius between the inside and outside of an early electrically recorded 78 RPM record is about 2½ inches or 65 mm. For a 3-minute performance there are 3 x 78 = 234 grooves. Therefore there are about 100 grooves to the inch. A stylus of tip radius 3 mil (diameter 0.006 inches) fits into the groove, so the groove width is around 6 mil. The thickness of the wall between adjacent grooves is therefore about 4 mil or 0.004 inches. As there are so many grooves per inch, with constant velocity recording there would be encroachment of the stylus from one groove to the next for loud bass notes. Larger inter-groove spacing would prevent overlap, but this would reduce the playing time too much. The bass recording characteristic in electrical recording was therefore designed to be attenuated progressively below a few hundred Hz. When a needle of tip radius 3 mil is on the outer groove of a 10 inch record revolving at 78 rpm, the wavelength of a recorded sound of frequency 5000 Hz is about 5 mil or 0.005 inches. This decreases towards the end of the recording. The needle tip has little space to follow such small undulations, so the treble recording characteristic continuously attenuates above 5 kHz towards the end of the record. A typical recording characteristic of early electrically recorded 78s is shown in blue on the graph.

The rising portion is recorded on the constant velocity system but it becomes constant amplitude along the horizontal section. The right hand diagram shows a constant amplitude recording. In the early days of electrical recording most British companies used constant velocity at lower frequency by cutting the bass amplitude but constant amplitude at higher frequency. Note how the blue curve in the graph would match the constant velocity/constant amplitude line below the frequency axis of the upper graph.

Some time after the introduction of electrical recording, amplified electrical reproduction began to replace the acoustic wind-up gramophone. Records could then be made with a pre-emphasis or boost in amplitude when recording frequencies above a few kilohertz. Compensation for this pre-emphasis was applied during playback, as was compensation for the de-emphasis applied during recording to the bass frequencies. In other words, in order to achieve achieve a flat output when replaying later electrically recorded 78s, and also for vinyl long playing records, the treble was cut and the bass was boosted during reproduction. The amplifier frequency response should be the inverse of the recording characteristic. An additional advantage is that, since most surface noise is of high frequency, it too is reduced on listening.

A chart defining the general properties that have to be specified to define the various recording characteristics is shown.

Roll-off defines the cut in high frequency reproduction needed to compensate for pre-emphasis during recording.
Turn-over refers to the frequency at which bass frequencies should first be boosted to compensate for de-emphasis during recording.
The expressions "roll-off" and "turn-over" are rather unfortunate terms.
A perfect constant velocity recording characteristic would be a straight line of gradient 6db/octave.

No universal agreement on the best practical shape of recording curve was achieved until the autumn of 1954 when the RIAA standard was adopted for 78s and vinyl discs. Before that, record companies introduced and modified their own standards at various times, using different amounts of boost and cut and switching them in and out at different frequencies. It is therefore not easy to judge the amount of equalisation that must be applied to a given recording.