Just found a strange switch on a CD burning app (Burn-OSX) which stated: "Pre-emphasis of 50/15μs". Turned out to be a historical hidden gem, aimed at "boosting the signal (especially the higher frequencies) in the recording before it was put onto CD, and embedding flags in the disc's subcode to tell a CD player to apply de-emphasis on playback. Some CD players even had a de-emphasis button that could be used to manually apply de-emphasis".