Equalisation is the process by which the amplitudes of different audio frequencies are varied to produce an overall flat (equalised) frequency response.
When the master record is made, the groove is cut by a magnetically operated cutter head. This moves the cutter rather like the cone of a loudspeaker is moved by the coil. The characteristics of a moving coil are such that signal amplitude is inversely proportional to frequency. In other words, the groove amplitude gets smaller and smaller as the frequency increases. Low frequencies thus make inefficient use of the groove space whilst high frequencies tend to get lost in the surface noise. To overcome these effects, the recording engineer attenuates the bass signals and boosts the treble signals before they are applied to the cutter. This evens out the range of signal amplitudes as cut into the vinyl and makes optimum use of the medium.
The playback equalisation applies the inverse response of that applied at the recording stage. Bass signals are now boosted and treble signals attenuated in order to restore the signal as originally recorded.
When micro groove records were introduced in the 1950’s, there were half a dozen or so different equalisation options. A few years later these were reduced to one by the RIAA (Recording Industry Association of America). Through tests, the RIAA decided on the best compromise combination of bass cut and treble lift. There are however some disadvantages to this strategy which are caused by having a large amount of bass boost in the playback chain. Mains hum was nearly always a problem especially with valve amplifiers and turntable rumble was made worse. However these were soluble problems and the overall improvement was very worthwhile.
Adoption of this standard meant that a 12 inch LP with a good dynamic range would play for about 18 minutes (this was a dramatic increase over the 3/4 minutes possible on a 78 rpm disc). In the 1960s ‘varigroove’ records were introduced and increased the playback time up to 32 minuets. Varigroove was a very simple idea; it varied the groove spacing to take account of the dynamic range of the music. This meant that the cutter head was told one revolution ahead of the forthcoming level, so if the level was low groove spacing could be significantly reduced and then quickly increased to accommodate music peaks. This gave the LP longer playing time and increased the peak playback level.
Another type of equalisation is applied to LP’s to take account of the falling high frequency output as the playback stylus moves toward the centre of the disc. An increasing level of high frequency lift is applied as the cutter head moves to the centre, the boost (usually at about 10kHz) rises to 4dB at the final groove. As you can see this is a minor change and is intended to give the LP a flat frequency response over its playing area.
In these days of digital recording the enormous dynamic range possible on a good LP is often forgotten, particularly when you consider that the CD has an overload margin of zero whilst the LP could manage any level that the cutting engineer thought practical.
Ganymede Test & Measurement
1st November 2001