Digital Feedback Reduction (Dfr); Function; Basic Dfr Setup - Shure Digital IntelliMix SCM820 Manual

Digital Feedback Reduction (DFR)

DFR uses Shure's patented Adaptive Notch Filter algorithm to discriminate between feedback and non-feedback sounds. It au-
tomatically detects feedback and deploys narrow-band notch filters at the feedback frequencies. DFR filters are narrow enough
to prevent any coloration of the audio program.
No sound system (the combination of microphones, mixing, signal processing, power amplifiers, loudspeakers and room
acoustics) produces an absolutely flat response. When the level of a sound system is increased, the frequencies at which
peaks occur will be the first to exceed the threshold of feedback. The DFR attenuates these frequencies, flattening the re-
sponse of the sound system and enabling operation at a higher overall level.
DFR is available for the SCM820 on firmware versions 1.1.0 and higher. For a unit running an earlier firmware version, use the
Shure Update Utility application to update the mixer to a new firmware. The application is available from www.shure.com/soft-
ware.

Function

Use DFR as the initial processor on any channel that receives signal from a live microphone where feedback is problematic.
When DFR detects feedback, it inserts a shallow, narrow filter into the audio path to reduce gain at the feedback frequency.
This filter is called a notch filter, due to the narrow section of the frequency spectrum it affects. The notch automatically deep-
ens if feedback continues on that frequency.

Basic DFR Setup

Digital Feedback Reducer will not enable you to increase system gain beyond the physical limits of the sound system. In most
cases, you reach a point of diminishing returns after five to eight notch filters are set. This is because there are usually only a
few dominant peaks in the response of the system. In most cases, you can expect a 6 dB to 9 dB improvement in gain-before-
feedback by using the DFR. When you are ringing out the system and notice that many frequencies feedback simultaneously,
you have reached the point of diminishing returns. If at this point the system still has insufficient gain before feedback, other
changes must be made to the sound system, such as changing the placement of the microphones and/or loudspeakers.
There are two basic ways in which to set-up the DFR to reduce feedback; the Ring-Out Method and the Insurance Policy
Method:
• Ring Out Method - With this method, you use the DFR as a preemptive measure against feedback for input channels that
operate near the feedback point and need an extra margin of stability. Using this method, you raise the channel's gain be-
yond its normal setting to deliberately make the system feed back. The DFR will then set the proper filters. Then, when
you reduce the input gain to an appropriate level, the system is stable and usable.
• Insurance Policy Method - With this method, you use the DFR as added insurance against unexpected feedback in an oth-
erwise stable system. Simply place the DFR processor in the signal path, without defining any settings. This method is
used for systems which already have sufficient gain-before-feedback, but need protection from occasional feedback oc-
currences due to non-stationary microphones or user-adjustable gain controls.
To ring out the system:
1. Remove any active filters by clicking the Clear All button.
2. Place microphones at their intended locations.
3. Set the mixer to Manual to open all mics.
4. Slowly raise the gain of the sound system while talking or clapping into the microphones.
5. Assign the DFR to the channel that begins to feed back. If multiple microphones feed back in the same frequency, as-
sign the DFR to the mix bus.
6. The DFR processor will deploy notch filters to attenuate the feedback frequencies.
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