BSS Audio DPR-504 Owner's Manual - Manual (Page 21)

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7.8 Link
Both channel 2 and channel 4 on the DPR 504 have a push switch which
alters the normal operational mode of the unit from four separate channels of
gates into one in which channel 1 plus 2 and channel 3 plus 4 work together.
The first group is operated by the controls of channel 1 only; the second group
is operated by the controls of channel 3 only. The two switches work
independently of each other so that the unit can be configured as two groups
of two channels, or one group of two channels and two independent channels.
The associated LED on channel one and three indicate that they are working
as the master control for the group.
When this mode is selected, the key side-chain signals from both the group
channels are summed together to provide a common mono signal. This is then
used by the master channel of the group for processing in the normal manner.
In addition, the VCA on the other group channel (slave channel) is disabled
from its own drive circuitry and connected in parallel with that of the master
channel, such that the action of the slave channel is now totally controlled by
the master.
If it is not required for the side-chain key signal of the slave channel to be
summed with that of the master channel, or visa versa, then the respective
EXT switch should be pressed. This then disconnects that channels signal -
provided nothing is externally connected to the rear panel external key input
jack socket. In application, it might be necessary to use the master/slave
linking facility when using two channels of the unit on a stereo program
source. It can also be useful when gating two microphone lines from similar
sources, such as backing vocals or orchestral overheads, or when gating stereo
effects returns or submixes from other program sources.
7.9 EXT
This switch selects the signal source for the key side-chain.
In the normal 'out' position, the signal path is continuous, and the side-chain
uses the normal program material appearing at the input connector as its
control.
If the EXT (external) position is chosen, then the key side-chain looks to the
rear mounted jack socket marked EXTernal key input for its source of signal.