What is deemphasis? This used PS Audio DAC I'm demoing right now has a deemphasis LED on it, and I have no idea what it does.
The stereophile test cd 2 has several test tone tracks with pre-emphasis.
Gil - Preemphasis and deemphasis are complementary signal processing techniques which apply a treble boost during mastering and an inverse correction curve at the playback processor. The presence of preemphasis on a track is made known to the player by the setting of a bit in the digital stream ( within the channel status word), so your dac indicator just lets you know that the processor sees the bit and is carrying out the filtering.
The purpose of preemphasis is to reduce additive noise at higher frequencies. Since high frequencies usually have relatively low amplitudes, they can be boosted and later attenuated, with the attenuation applied not only to the signal but to any noise acquired along the way.
Preemphasis is no longer very common, due both to the headroom penalty in recording and to better equipment quality in general.