FunctionalityThe structure of the Feedback Suppressor property dialog is similar to that of the PEQ block, including a graphical display of the filter settings and a table with specifics for Frequency, Gain, and Bandwidth/Q . Since you do not manually adjust the filter settings, the dialog does not provide any controls for the filters and the graphical elements are not adjustable as they are in the PEQ block.
During normal operation, the Feedback Suppressor block continuously evaluates the feedback characteristics of the audio system and adjusts the filters as necessary to suppress feedback.
note: All filters are floating – the DSP algorithm can change any filter to a new setting whenever and however it wants to. Initially, all filters start out as un-deployed with a gain of 0 dB.
note: As the block discovers a new feedback frequency, to suppress it chooses the next un-deployed filter, or, if all filters are deployed, it chooses the oldest deployed filter and sets a notch filter to suppress the feedback.
note: As the block changes filter settings, the Halogen property dialog updates the graphical display and filter table values.
The Feedback Suppressor block initially offers 5 filters. You may add up to 10 additional filters for a total of 15. When a new filter is added, it starts out un-deployed with a gain of 0 dB. The filter is automatically deployed and adjusted as required to suppress feedback.
The purpose of Auto Setup is to locate suspect feedback frequencies and determine the increased gain margin with the filters deployed. The more filters you use, the greater the margin you should be able to achieve, but the more you adversely affect audio quality. You should only run auto setup under the following conditions.
- Speakers and microphones are located as they will be under normal use
- Nominal gain structure has been properly adjusted and gain is low enough that there is no feedback occurring
When you click on the Auto Setup button, the feedback suppressor block enters auto setup mode. This is an automatic mode that attempts to characterize the audio system for feedback and to set available filters such that feedback is suppressed. The algorithm is as follows:
- Initially, all of the filters are set to flat (all un-deployed) and the block gain is set to 0 dB.
- The HAL begins to slowly increase the gain at a set rate.
- As the Feedback Suppressor block detects feedback, it deploys and adjusts filters to suppress it.
- The process continues until all filters are deployed or the gain reaches a maximum value of 12 dB or the user cancels auto setup mode by clicking on the Stop Setup button, or checking the Bypass All check box.
When Auto Setup mode is active, the Current Gain graph appears and shows the gain of the block, which increases with time as the auto setup process continues. The numeric gain value also appears in a label above the graph. Once auto setup mode is active, the Auto Setup button changes to a Stop Setup button. When the auto setup process is completed the Current Gain graph disappears and the dialog displays the Gain Margin box. This box shows the gain margin that the Feedback Suppressor block achieved by the auto setup process and remains visible until the user resets the filters by clicking on the Flatten All button or restarts the auto setup process.
note: The Gain Margin is the difference between the maximum gain achieved and the gain at which the first feedback frequency was detected during the Auto Setup process.
When auto setup mode is active, the following controls are disabled
- Add Filter
- Remove Filter
- Flatten All
When the auto setup process is complete, gain is returned to 0 dB and the block returns to normal operation.
note: The Auto Setup button is disabled when Bypass All is checked. If auto setup mode is active, checking Bypass All stops the auto setup process and returns the block to normal operation but with feedback sensing suspended as is always the case when Bypass All is checked.
tip: Once you have the feedback characterized and suppressed, you can copy the filter settings from the Feedback Suppressor block and paste them to an existing PEQ block to make a 'static' feedback suppressor block. This is one case where you can copy the settings of one type of block (Feedback Suppressor) and paste them to a different type (PEQ). See Cutting/Copying/Pasting Workspace Items. Note that this 'static' PEQ filters approach is only viable when all mics are in a fixed position, since physically moving mics around alters the feedback frequencies.
See Also