THAT4305 Datasheet, PDF (8/20 Pages) List of Unclassifed Manufacturers – Pre-trimmed Analog Engine® IC

Page 8
THAT4305 Pre-trimmed Low-power Analog Engine
By inspection we can see that if k equals zero,
the compression ratio will be 1:1, and if k equals 1,
the compression ratio will be infinity:1. Thus, we
can make a feedforward compressor/limiter by hav-
ing the gain of the sidechain vary from zero to one.
Note that if k>1, the compression ratio becomes
negative. Negative compression results with 1<k<2.
The Feedback Compressor
We can use the same approach for a feedback
compressor. Figure 4 shows a simplified theoretical
model of a feedback compressor. By inspection,
Out_dB = In_dB + G_dB , and
G_dB = −k ⋅ Out_dB . Therefore,
Out_dB = In_dB − k ⋅ Out_dB , and
Out_dB + k ⋅ Out_dB = In_dB . As such,
In_dB
Out _dB
= 1+ k = C. R.
.
In_dB
G_dB
Out _dB
RMS
Out _dB
-k
Figure 4. Simplified feedback compressor,
conceptual diagram.
In this case, as mentioned earlier, infinite com-
pression requires infinite sidechain gain. Fortu-
nately, compression ratios of between 10 and 20
limit effectively enough that infinite gain is not re-
quired.
Log-Based Expanders
Similarly, for the feedforward expander shown in
Figure 5, we can state that
Out_dB = In_dB + G_dB , and that
G_dB = k ⋅ In_dB .
Note that the sign of k makes this an expander in
which gain GdB decreases as input signal level IndB
decreases. Combining these equations:
Out_dB = In_dB + k ⋅ In_dB , and
Out_dB = (1 + k)In_dB .
Rearranging yields
Out _dB
In_dB
= 1+ k = E. R.
This is the expansion ratio.
In_dB
RMS
In_dB
k
G_dB
Out _dB
Figure 5. Simplified feedforward expander,
conceptual diagram
Adjusting the Level Match Point
In the equations so far, we have made the im-
plicit assumption that the decibel reference level ev-
erywhere is that of the rms-level detector. This
assumption simplifies the math, but it may not cor-
respond to real-world reference levels such as 1V
rms (0 dBV) or 0.775V rms (0 dBu). Additionally, it
is possible to offset the VCA's inherent behavior of
producing unity (0 dB) current gain at 0 mV control
voltage (E_C+-E_C-) by selecting asymmetrical volt-
age-to-current and current-to-voltage converting re-
sistors (R3 and R2, respectively, in Figure 2).
Figure 6 allows for a VCA voltage gain offset of
A_dB, as well as an offset (LM_dB) to vary the "level
match" point of the RMS detector. Using similar “log
math” from AN101A, we can state for Figure 6:
Out_dB = In_dB + G_dB + A_dB , where GdB is the
VCA's control port gain in dB, and A_dB is any static
gain or attenuation (in dB) applied to the signal.
We can also state that:
G_dB = −k (Out_dB − L. M._dB ) , where L.M. is a vary-
ing dc voltage intended to change the system’s zero
dB reference point (often referred to as level match
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