This is from your DOD 866 series ii manual ~
Side Chain Input: Allows access to the signal detector circuit of the compressor, permitting control of the compressor with another signal for such applications as "ducking". When used with the Side Chain Output, the original input signal can be modified for applications such as "de-essing". Inserting a plug into this jack opens the internal side chain path so that the detector will only respond to the signal at this jack. In the stereo mode, both channels of the compressor react as one.
Side Chain Output: The Side Chain Output is the buffered output normally fed to the detector. It is used in conjunction with the Side Chain Input to modify the detector signal for special applications such as "ducking" and "de-essing". For these applications, the Side Chain Output signal is sent to a signal processor and returned through the Side Chain Input.
The uses for a compressor don't end with compression and protective limiting. Applications such as "ducking", "de-essing", and "de-thumping" can be achieved with equal ease, and their uses are many. The 866 provides Side Chain Inputs and Outputs, which allow direct access to each channel's detector circuits. Since the detectors control the compressing VCA (voltage-controlled amplifier), one can control the program material with a completely unrelated signal. This is done by inserting the control signal into the Side Chain Input. Ducking is a good example of this type of application. Ducking is simply gain reduction of a signal when another is present. This technique is widely used in sports broadcasting to reduce the level of the crowd background signal when the announcer is speaking. The preamplified voice of the announcer is sent to a side chain input to compress the noise of the crowd. The voice and crowd signals are then mixed together. For this type of application, the compression ratio is kept fairly low with long attack and release times. The Side Chain output is provided so that the controlling signal (not the program material) may be modified before reaching the detectors. The most common use of this technique is for de-essing. A de-esser reduces the high frequency sibilance in the "s"s and "t"s of speech to prevent tape saturation or high frequency driver damage. Connect the Side Chain Output to an equalizer whose output is connected to the Side Chain Input of the 866. The areas where most of the "ess" energy is located are between 2.5 kHz and 10 kHz. If these areas are boosted on the equalizer, the gain of the program material will be reduced more by the compressor because of the excess gain in that frequency range, thus reducing the sibilance of the program material. Attack and Release times should be set fairly short, and the compression ratio should be below 8:1.
Fender, Gallien-Krueger, D'Addario, StudioSlips.com, (Grumpy Old Fart #5)
Last edited by Joe Louvar : 12-29-2013 at 08:22 PM.