4305-DEMO THAT Corporation, 4305-DEMO Datasheet - Page 7

• 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

Document 600067 Rev 01
times the noise floor will be masked by the high-level
signal.
of companding noise reduction systems. The RMS
detector responds accurately over a wide range of
levels; the VCA responds accurately to a wide range
of gain commands; and all the detector
inputs, outputs, and control ports are independently
accessible and fully configurable. All these features
mean that the 4305 will support a wide range of
compander designs, including simple 2:1 wide range
(level-independent) systems, level-dependent systems
with thresholds and varying compression slopes,
systems including noise gating and/or limiting, and
systems with varying degrees of pre-emphasis and
filtering in both the signal and detector paths.
Furthermore, much of this can be accomplished by
extensively conditioning the control voltage sidechain
rather than the audio signal itself. The audio signal
can pass through as little as one VCA and one
opamp, and still support multiple ratios, thresholds,
and time constants.
Analog Engines from THAT Corporation.
Analog Engines feature log-responding true-RMS
level detectors and exponentially controlled Black-
mer VCAs.
signals using the low-voltage, low-power 4315 or
4320 in a handheld, battery-operated device such as
a wireless microphone or instrument belt pack, and
expand (decode) that signal using the 4305 in a rack-
mount, line-operated receiver.
The Mathematics of Log-Based Dynamics
Processors
tor and the exponential control ports of the VCA can
be intimidating for designers unfamiliar with THAT
Corporation's offerings.
characteristics make developing audio processors
easy once a designer understands the concepts
involved. As noted earlier, AN101A: The Mathemat-
ics of Log-Based Dynamics Processors, discusses
these concepts in some detail. The following discus-
sion draws heavily from that application note.
The Feedforward Compressor
simple feedforward compressor.
The 4305 facilitates the design of a wide variety
Note that the 4305 is fully compatible with other
At first, the logarithmic output of the RMS detec-
Figure 3 shows a conceptual diagram of a very
It is possible to compress (encode)
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THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
However, in fact, these
Using the "log
and VCA
All our
math" principles explained in AN101A, we can state
that
in which gain G
increases. Combining these equations,
the compression ratio will be 1:1, and if k equals 1,
the compression ratio will be infinity:1.
can make a feedforward compressor/limiter by
having the gain of the sidechain vary from zero to
one.
negative. Negative compression results with 1<k<2.
The Feedback Compressor
compressor. Figure 4 shows a simplified theoretical
model of a feedback compressor. By inspection,
In
Out
Note that the sign of k makes this a compressor
Out = In
Rearranging yields
This is the compression ratio.
By inspection we can see that if k equals zero,
Note that if k>1, the compression ratio becomes
We can use the same approach for a feedback
Out
G
Out
Out
Out
G
In
dB
dB
dB
dB
Figure 3. Simplified feedforward compressor
dB
dB
dB
dB
dB
= −k $ Out
= −k $ In
=
In
= In
RMS
+ k $ Out
= In
= In
dB
(1−k)
dB
1
dB
dB
− k $ In
dB
dB
= C.R.
decreases as input signal level In
+ G
dB
-k
+ G
− k $ Out
.
dB
dB
dB
dB
dB
. Therefore,
= In
, and that
G
= In
, and
dB
dB
dB
dB
, and
(1 − k)
. As such,
.
Out
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Thus, we
dB
dB