BSS Audio DPR-504 Owner's Manual - Manual (Page 15)

15
7.2 Key listen
This momentary switch allows the operator to listen to the program material
after it has been processed by the key filters. This also applies to any external
equipment connected via the EXT key input socket.
In application it would be used during the setup of the side-chain key filter
and width controls, as it is often easier to more accurately set them whilst
listening to their effect.
The key listen signal appears at the output connector of the channel, and
overrides the normal program material. In this sense it must be used with
caution, as accidental operation will cause temporary loss of program signal
at the output connector.
Fig 7.1 Key side-chain
filter response