3d EQ, 4048A Parametric Filters | Altec Lansing 4948A instruction

Operating Manual-4048A digital signal processor

7.3d EQ

Each input and output of the Altec Lansing 4048A processor has its own EQ section offering full parametric EQ, four types of shelving filters, and an all-pass filter. Input channels have eight selectable EQ filters, while output chan- nels have six selectable EQ filters. On all inputs and outputs, each EQ filter is selectable between parametric (PEQ), 1st order Low Shelf (LS1), 2nd order Low Shelf (LS2), 1st order High Shelf (HS1), 2nd order High Shelf (HS2), and an All- Pass filter (AP).

Parametric EQ (PEQ): uses peak filters with the ability to control boost or cut, frequency center, and band- width. Think of one band of parametric EQ as a single graphic equalizer fader except that the frequency is variable, not fixed, and that the bandwidth, or how "wide" the filter affects the frequency spectrum at the center frequency, is completely variable. The smaller the bandwidth, the less the audio signal on either side of the frequency center is boost or cut, whereas a larger "wider" bandwidth produces an audible change to the overall tone of a signal. Paramet- ric filters are best used to hunt down and eliminate problem feedback frequencies, add or remove a characteristic "hot spot" from microphones, or clean up room resonance situations. It is well worth the time becoming proficient with parametric EQ filters, as they offer the best solution to many EQ problems.

Narrow (Low) Bandwidth

Wide (High) Bandwidth

+20dB
+10dB
0dB
-10dB
-20dB
-30dB
20Hz	200Hz	2KHz	20KHz

4048A Parametric Filters

4048A parametric filters have a boost/cut range of +15dB to -30dB. There is more cut than boost because one of the more common uses for parametric filters is to dramatically cut, or "notch out", very narrow frequencies (low bandwidth) in order to eliminate system feedback problems.

Every instance of a parametric EQ filter has a center frequency selected. The factory default is 1kHz, but each filter's center frequency is adjustable from 19.7Hz to 21.9kHz in 1/96 octave steps. Carefully sweeping a narrow bandwidth filter through a problem feedback area, with just a slight boost, is a quick way to find the exact frequency causing trouble. Once the offensive frequency has been found, cut the filter's level, and then adjust the bandwidth as narrow as possible while still eliminating the feedback problem. Bandwidth is adjustable from about 1/64 octave to four octaves, and the lower the bandwidth, the less audible the filter action will be. Finding the problem frequency is relatively easy, but finding the best combination of cut and bandwidth takes a little practice. Again, it is well worth the time getting comfortable with the notching procedure, so that problems can be quickly addressed with a sufficient but minimal amount of correction.