6
English
DFR22 Theory
Feedback Reduction
No sound system (micro mixing/signal proc power amplifiers/loudsp room acoustics) has an absolutely flat frequency
response. When the level of a sound system is increased, the frequencies at which peaks occur will be the first to exceed the feedback threshold.
The DFR22 attenuates these frequencies, flattening the response of the sound system. The system can then operate at a higher overall level.
The DFR22 includes Shure's patented Adaptive Notch Filter algorithm that can discriminate between feedback and non-feedback sounds. When
this algorithm detects feedback, it inserts a shallow, narrow filter into the audio path, reducing gain at the frequency that is feeding back. (See
Figure 3.) This filter is called a notch filter, since it affects a narrow section of the audio spectrum. If the feedback does not stop, the filter depth is
increased. By default, filters in the DFR22 are deployed as HIGH Q filters. As a High Q filter gets deeper, its Q increases up to a Q of 101 (1/70th
of an octave). Using the DFR22 software, you can also set filters to deploy as LOW Q. Low Q filters affect a slightly wider range of frequencies by
maintaining a Q of 14.42 (1/10th of an octave) as they deepen. By default, the DFR22 can insert up to 16 notch filters per channel to reduce feed-
back.
The DFR22's feedback reduction algorithm can be used as a tool during system setup and for peace of mind against unexpected feedback. When
setting up a sound system with the DFR22, you can ring out the system by slowly raising the gain while talking into the microphones until the first
frequency begins to feed back. The DFR22 will automatically insert a notch filter to attenuate that frequency. Once the system stops feeding back,
you can further raise the level and repeat the process for additional frequencies. Typically, you can raise the gain 3 - 9 dB above the level at which
feedback first occurred. You should leave some filters available to catch feedback that might occur later, when the sound system is in use.
EFFECTS OF NOTCH FILTER ON FREQUENCY RESPONSE
FIGURE 3
DFR22 Limitations
The DFR22 (or any other feedback reduction processor) will not let you to increase gain beyond the physical limits of the sound system. In most
cases, the point of diminishing returns is reached once 5 to 8 notch filters have been set. This is because there are usually only a few dominant
peaks in a system's frequency response (Figure 4A). In most cases, you can expect a 6 to 9 dB improvement in gain-before-feedback. When you
are ringing out a system and notice that many frequencies feed back simultaneously, even when you increase the gain slowly, you have reached
the point of diminishing returns. If the system still has insufficient gain before feedback at this point, other changes must be made to the sound
system, such as changing the placement of the microphones and/or loudspeakers.
FREQUENCY RESPONSE OF AN UNEQUALIZED SOUND SYSTEM
FIGURE 4
RACK MOUNTING THE DFR22
Mount the DFR22 in any standard 19-inch audio equipment rack, using
the supplied screws. Refer to Figure 5.
NOTE
: Avoid mounting wireless equipment and power amplifiers directly
above or below the DFR22. Additional rack mount supports may be
necessary for mobile installations.
RACK MOUNTING THE DFR22
FIGURE 5
MEASURED FREQUENCY RESPONSE WITH PEAK
MEASURED FREQUENCY RESPONSE WITH NOTCH FILTER ADDED
= Feedback Threshold
Figure 4A
Figure 4B
Use notch filters to lower peaks
Lower overall system gain
Summary of Contents for DFR22AZ
Page 13: ...1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 G 17 18 19 20...
Page 24: ...1 0 16 G 66 0 16...
Page 35: ...1 2 2 3 3 1...