The Swezey Digital Filter

Prior to the mid-1920's phonograph recordings were made mechanically without the use of any electronics.

One of the short comings of this method was that frequency response was not even across all frequencies.

Some frequencies would resonate in the equipment boosting them while others were dampened.

For example if you were recording a 1000 Hz tone and a 3000 Hz tone that were of equal volume one might record louder than the other and so seem louder on playback.

This unevenness is why so many early recordings sound like they were recorded inside a box.