Mono compatibility is a feature that guarantees there are no substantial changes to the frequency response when a multi-channel recording is converted into a single channel recording. This is done by taking the sum of all input channels' signals.
Due to the mechanical nature of recording on vinyl, it is important that your supplied audio masters comply with mono compatibility.
Mono compatibility has to be taken into account throughout the recording, mixing and audio mastering stages. It can be checked by proof-listening and by a channel correlation meter. When proof-listening in single channel (MONO mode), we subjectively consider whether the recording has lost or gained any vital components of the sound, such as dynamics, volume, frequencies or even an entire instrument. Typically this is recognized most easily in the low frequencies.
Non-mono compatibility is usually caused by techniques employed during the original audio recording (such as poor placement of microphones and panning problems known as “comb filtering”) or by improper usage of the signal psychoacoustic processors.