Empress Effects ECM-519 compressor - Manual (Page 10)

Auto Modes
The auto modes react to the audio coming in and modify the
attack and release in real-time depending on what the music's
doing. In each auto mode, there are 2 detectors that run in
parallel. One with really fast time constants at a higher
threshold that reacts primarily to peaks, and a slower detector,
with a lower threshold to smooth the overall signal. There is
also some frequency compensation in these detectors which
changes the ratio slightly depending on frequency. This allows
the auto modes to provide extremely transparent compression
that isn't possible with traditional attack and release controls.
Auto mode 1 : This mode is great for general dynamic control. It
can be used while tracking vocals to control peaks and smooth
out the overall level, without nasty artifacts. Also, it works great
for sitting things in a dense mix, but still allowing them to
breathe.
Auto mode 2 : This is great on acoustic guitar. The attack is
more open and it makes things sound big and present without
squashing the transients too much. On individual sources it's
good for bringing them forward in a mix and even works well on
a mix buss to add some overall punch.