Equalization – issues & a solution

For most people, equalization will not be an issue because most LP recordings are to the RIAA standard, and that is built in to most record playing equipment. This page is only for situations in which the equalization and/or speed of the record does not match that of your equipment.

As an example, the Percy Grainger sample on this site was captured at 45rpm via a preamplifier intended for RIAA records; the original record was manufactured to play at 80rpm using the Westrex system. Equalization was done with the Equalizer software from this site, together with an audio editor for necessary resampling. The whole process can be somewhat complicated, and is not recommended for a beginner.

Some typical issues that I addressed in writing the Equalizer application are:

Phase issues

Electronic equipment deals with audio as a continuous electical signal in real time, and is subject to the law of cause and effect – the output at any instant can depend only on the input up to that instant. A consequence of causality is this: the phase response is completely determined by the amplitude response. Digital Signal Processing (DSP) can easily circumvent (not violate) this law by reading ahead. As a consequence, a DSP equalizer which reproduces a particular frequency-dependent amplitude response, can have almost any phase response whatever. There is no doubt that phase is important in the spatial perception of stereo audio material.

Equalizer performs high-quality emulation of electronics in both its amplitude and phase response. This is particularly useful if you are removing an existing RIAA equalization and replacing it with one which is correct for the actual record – even something as simple as equalizing a Decca ffrr LP after it has been captured using RIAA equalization.

Speed issues

If a 78rpm record is captured using playback at 45rpm, then the pitch will have to be adjusted using your audio editor to perform the correct resampling. A problem arises if the original capture also applied RIAA equalization. Such equalization will be affected by changing the pitch. Equalizer is able to correct for such problems using pre-set inverse equalizers which take account of speed changes. For further information consult the manual.

Mono recordings

Older records were monophonic, but it is still useful to capture them as a stereo file, so as to have separate access to the information from the two walls of the record groove. Equalizer has the ability to mix these files down to mono and the balance may be selected to achieve the best result. This is often much better than a simple mono mix, performed by some form of hard wiring. For collectors of historical records who have vertical disks (Edison DD, etc) in their collections, Equalizer will also invert one channel during mix-down to mono.

Equalizer – a software solution

Equalizer is an application to assist in capturing and restoring audio from gramophone records whose equalization and/or speed does not match your equipment. The ideal solution is to buy a special-purpose preamplifier and other equipment. Another is to capture the audio using the equipment at hand, then re-equalize the digital file. That is what the Equalizer application is designed to do. It is free software.