The IC-756Pro2/3 microphone equaliser is described in Reference 1, p. 17. To paraphrase the text:
The microphone equaliser characteristics used for the IC-756PRO2/3 are based on the amplitude/frequency characteristics of a Baxandall-type audio tone control circuit which has been re-designed to be dedicated to the voice-frequency range. The transfer function of an analogue filter is simulated and converted into that of a digital filter to provide the microphone equaliser function.
A closer inspection of Fig. 1 reveals several interesting points:
The crossover point of the bass and treble equalisation curves is 650 Hz, rather than the more common 1 kHz.
The bass and treble boost/cut values are configurable in 1 dB steps.
Bass: Datum Line D1 indicates that each 1 dB step changes the response by 1 dB at 300 Hz.
Treble: Datum Line D2 indicates that each 1 dB step changes the response by 1 dB at 1 kHz.
As discussed here, the lower and upper cutoff frequencies for the WIDE, MID and NAR TOBW settings (respectively) are as follows.
Lower -6dB point: 100, 300 and 500 Hz.
Upper -6dB point: 2.5, 2.7 and 2.9 kHz.
A vertical line drawn at any one of these frequencies, intersecting the selected equalisation curve, will yield the actual boost or cut at the corresponding cutoff point.
Example: WIDE setting, 2.9 kHz cutoff, +2 dB Treble: at 2.9 kHz, treble boost is +5 dB.
"IC-756Pro II Technical Report", Icom Inc., 2003. (PDF) mirror site
Text copyright © 2006 A. Farson VA7OJ/AB4OJ. Image courtesy Icom Inc. Last updated: 18/06/2017.
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