Neve 8816 - INS (Inserts); Sum and difference mode

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INS (Inserts)
This switches the main mix pre fade insert in and out of circuit via the input and
output connectors on the rear of the unit. It has two modes selectable by pressing the
mix level control. When a normal pre fade insert is selected the button will illuminate
yellow. If the mix level control is pressed then the illumination will change to red
indicating that the insert point has changed to a sum and difference insert.
In sum and difference mode the standard stereo signal is converted into a mono
common or sum signal on the left output and a mono difference signal on the right
output. This enables the engineer to apply different processes to the common (or
centre) image and the difference (or stereo) image. At the insert return the signal is
re-converted to a standard stereo signal. The technique is frequently used in
mastering recordings and is really handy for such things as:
Using an overall stereo EQ where the L & R Image will not shift with any tracking
errors within the EQ unit, or EQ the mono to subtlety change the mix balance post
mixing.
Compress and limit the sum and difference separately to negate tracking errors or
create a quite different stereo image.
Add a sub bass synth to the mono program for really deep mono bass that will not
make the cutter head jump if mastering to vinyl.
Listen to the mono and trim the DAW sample delays to get perfect L & R tracking and
save the results to an interleaved wav file.
A vocal could be brightened by EQing the Sum signal or Cymbals could be
brightened by EQing the Difference signal.
N.B. The underlying mode of the insert can be changed by pressing the mix level
control even if the insertion button is not selected.
2TR MIX (2 Track Mix)
When selected sends the 2 track return signal directly to the 2 track mix bus.
2TR Level Control
This adjusts the two track return level sent to the Cue Mix or 2 track mix bus and has
a range of ∞dB to +10dB. This is useful for sending 2 track mixes to the cue
headphones or as a reverb return to the cues or 2 track mix bus.