One BIG Punch is based upon a widely used microphone
conditioner that includes a microphone preamplifier with variable
compression and noise gating. The SSM2165 integrated circuit utilized
is manufactured by Analog Devices and was designed for improving
voice clarity in communications and public address systems. This
device features low noise and distortion, variable compression ratio,
automatic limiting to prevent overload, adjustable release time, and
a noise gate to prevent amplification of noise or hum. In order to
avoid artifacts that result from hard clipping techniques, this
processor uses three different signal transformation regions. W4RT
Electronics selected the several adjustments to provide the
best general talk power and articulation performance for
users of the FT-817.
Figure 1. Input/Output Characteristics of the SSM2165
(ref. Fig. 2, Analog Devices SSM2165
Application Note).
Figure 1 illustrates the general input/output
characteristics of the One BIG Punch. The input is shown along the
abscissa in dB and the output is shown on the ordinate in dB. As is
evident by examination of the figure, there are three signal
transformation regions, viz., limiting, compression, and downward
expansion regions. The SSM2165 has several fixed parameters. These
are the rotation point (Vrp),
downward expansion threshold (VDE),
and the gains associated with the limiting and downward expansion
regions. With no processing, the input/output transformation is
linear with unity gain and is illustrated in Fig. 1 by the dashed
line having a slope of 1:1 or unity. The compression region is where
your voice is normally processed and has a compression ratio (or
slope) of r:1 as illustrated in Fig. 1. This roughly 40 dB region
extends between VDE
and Vrp. The
action of the integral voltage-controlled amplifier in the SSM2165 is
shown in Fig. 1 as the distance between the normal line (1:1 slope)
and the rotation point (rp). This is one of the W4RT Electronics
selected parameters. The compression ratio in the limiting region is
fixed by Analog Devices at approximately 10:1 (this means that a -10
dB change in the input results in a -1 dB change in the output). If
any of your voice peaks reach into this limiting region, they are
rather strongly compressed, NOT clipped. Any input signals below the
downward expansion threshold VDE
are expanded at a ratio of about 1:3, i.e., a change in the input of
-1 dB results in change in the output of -3 dB. This feature is very
important for FT-817 users since it helps to mitigate the pickup of
background and circuit noises.
