4305-DEMO THAT Corporation, 4305-DEMO Datasheet - Page 8

• Manufacturer Part Number: 4305-DEMO
• Description: Audio Modules & Development Tools Low-Cost Dynamics Processor Demo Board
• Manufacturer: THAT Corporation

Specifications of 4305-DEMO

• Description/function: Audio DSPs
• Operating Supply Voltage: 15 V
• Product: Audio Modules
• For Use With/related Products: THAT4305
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Page 8 of 20
compression requires infinite sidechain gain. Fortu-
nately, compression ratios of between 10 and 20
limit effectively enough that infinite gain is not
required.
Log-Based Expanders
in Figure 5, we can state that
in which gain GdB decreases as input signal level
IndB decreases. Combining these equations:
In
In
In this case, as mentioned earlier, infinite
Similarly, for the feedforward expander shown
Out
G
Note that the sign of k makes this an expander
Out
Out
Rearranging yields
Out
In
dB
dB
dB
dB
dB
Figure 5. Simplified feedforward expander,
Figure 4. Simplified feedback compressor,
dB
dB
dB
= k $ In
In
= 1 + k = C.R.
RMS
dB
= In
= In
= (1 + k)In
G
dB
dB
conceptual diagram.
dB
conceptual diagram
dB
k
+ G
+ k $ In
.
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
dB
dB
G
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
-k
, and that
.
dB
dB
, and
RMS
Out
dB
Out
Out
dB
dB
Adjusting the Level Match Point
implicit assumption that the decibel reference level
everywhere is that of the rms-level detector.
assumption simplifies the math, but it may not
correspond to real-world reference levels such as 1V
rms (0 dBV) or 0.775 V
is possible to offset the VCA's inherent behavior of
producing unity (0 dB) current gain at 0 mV control
voltage (E
current and current-to-voltage converting resistors
(R3 and R2, respectively, in Figure 2).
A
match" point of the RMS detector. Using similar “log
math” from AN101A, we can state for Figure 6:
VCA's control port gain in dB, and A
gain or attenuation (in dB) applied to the signal.
varying dc voltage intended to change the system’s
zero dB reference point (often referred to as level
match point) from that of the detector, and k is the
gain of the sidechain.
dB
In
, as well as an offset (LM
In the equations so far, we have made the
Figure 6 allows for a VCA voltage gain offset of
Out
We can also state that:
G
Substituting the second equation into the first,
Out
Out
OutdB =
dB
Out
This is the expansion ratio.
Figure 6. Feedback compressor with level match.
In
dB
dB
dB
dB
dB
dB
C+
= −k (Out
= 1 + k = E.R.
= In
= In
-E
= In
G
C-
THAT4305 Pre-trimmed Analog Engine®
In
) by selecting asymmetrical voltage-to-
dB
dB
dB
dB
dB
+k $ L.M.
+ k(L.M.
+ k $ L.M.
+ G
(1+k)
dB
dB
− L.M.
RMS
dB
+A
-k
dB
A
+ A
dB
(0 dBu). Additionally, it
dB
dB
.
− Out
− k $ Out
dB
dB
dB
) to vary the "level
, where G
)
L.M.
, where L.M. is a
RMS
dB
Σ
dB
) + A
Out
dB
+
is any static
-
dB
dB
+ A
Out
dB
dB
, or
dB
is the
This
, or
dB