De-emphasis
FM de-emphasis is the tuner's treble-reducing response that complements the treble boost applied when an FM broadcast is transmitted.
Its setting is expressed as a time constant in microseconds (µs), commonly 50 or 75 µs. Matching pre-emphasis and de-emphasis restores the intended frequency balance while reducing high-frequency noise added during reception.
De-emphasis — Tuner
Use the setting specified for the broadcast system where the tuner operates. The wrong time constant changes the balance of high and low frequencies; a larger number is not an upgrade. A switchable tuner offers a choice, while a fixed setting must be checked for the relevant regional version.
Compare the available settings with their stated purpose. The 25 µs Dolby FM option belongs to that particular noise-reduction arrangement, not ordinary 50/75 µs operation. De-emphasis is separate from tuning range and channel spacing, so neither a familiar frequency band nor a fine tuning step establishes the correct response.