Feedback Suppressor (FBS) Operation

FBS is used when there is a danger of feedback in live situations. The FBS detects feedback on a live microphone channel, and uses a set of up to five (5) fixed and fifteen (15) dynamic filters to counteract the frequency peaks at the detected feedback frequencies. It may be possible to achieve an additional 3dB to 9dB of gain on mic channels where feedback would have otherwise prevented these levels.

See topic, FBS Dialog Overview, for more information.

Dynamic Filters

Dynamic filters auto-detect feedback frequencies when FBS is switched on (unbypassed). The dynamic filters are notch filters with a user-defined Q setting and a variable cut, providing gain reduction up to 30dB at a narrow frequency band. The default Q setting can be set in the Tools | Options menu, and further defined on the Settings tab of the FBS dialog prior to engaging the FBS dynamic filters.

NOTE: Changing the Q setting after dynamic filters have been generated will clear all dynamic filters.

Fixed Filters

Fixed filters are notch filters with an adjustable center frequency and Q, a possible cut of up to 30dB, and a recommended cut of ≤24dB. The fixed filters are typically set by converting the dynamic filters to fixed, and not manually set, though further adjustments are possible from the Fixed Filters tab.

Using FBS

The two approaches to utilizing the FBS are as follows:

• Load and Go. The FBS may be simply loaded into the processor block and engaged, so that the dynamic filters are active and will detect and counteract feedback if and when the need arises. In this case, only the dynamic filters come into play. No operator setup is required, though some settings are recommended. This approach will work fine for a system that does not operate near the point of feedback but needs protection from a moving microphone, for example, that may come too close to a speaker and potentially cause feedback.

• Ring Out. Using this approach during system setup, feedback is forced by raising mic gain to the point of feedback while testing the microphone (simulating live operation). The FBS detects the feedback, and the dynamic filters are automatically set to reduce level at the feedback frequencies. Once set, the dynamic filters may then be converted to fixed filters by pressing the Move to Fixed button for each filter. The user may choose any of the dynamic filters for this purpose. Filters with a larger amount of cut, or those filters generated first, could be considered the most likely to feed back, and are good candidates for conversion to fixed filters. The dynamic filter will automatically clear when it is moved to a fixed filter.

Dynamic filters can also be cleared by clicking the Clear button for each filter, and can then either be used for feedback that may occur during a live event (such as with moving microphones), or for additional feedback suppression by further pushing the mic gain. For the latter, mic gain should be reduced slightly when feedback can no longer be suppressed.

To begin the FBS setup:

  1. Set Mic/Line Input and mic mix-point levels (see Optimizing Audio Levels for more information).

  2. Insert an FBS processor into the desired Mic/Line Input channel. For information on Q settings and other FBS parameter and display options, see FBS Dialog Overview.

To use the Load and Go method:

  1. Open the FBS dialog and click Bypass to disengage (Bypass is engaged by default). The FBS processor is now engaged for Load and Go operation.

  2. Optionally, engage and set the Hold Time for the dynamic filters.

To use the Ring Out method:

  1. Open the FBS dialog and click Bypass to disengage (Bypass is engaged by default).

  2. Raise the Mic/Line Input gain until feedback starts to occur. Allow the FBS to detect one or more frequencies, until feedback is suppressed.

  3. If desired, repeat Step 2. This process may continue until all dynamic filters have detected feedback frequencies. Once this has occurred, attenuate the mic level by 2dB to 3dB.

  4. Engage the Lock button (optional). Doing so locks the current set of dynamic filters, and prevents dynamic filters from being detected during the "move to fixed" procedure in the next step. Lock is a temporary setting, and will be disengaged when the FBS dialog is closed.

  5. Move up to five dynamic filters to Fixed Filters by clicking the Move to Fixed button for each dynamic filter. It is the user's discretion which filters are moved. Logical choices, for example, would be the last five dynamic filters, or the five filters with the largest cut value.

  6. Choose one of the following actions to complete the FBS setup.

    • Click the Lock button to disengage (if engaged in Step 4). Close the FBS dialog. This will leave five dynamic filters open if spontaneous feedback occurs.

    • Click Bypass, which bypasses only the dynamic filters. Some feedback may occur, so attenuate the mic level as needed. This procedure retains the detected filters, if needed, but leaves only the fixed filters in play.