THAT4305 Datasheet, PDF (7/20 Pages) List of Unclassifed Manufacturers – Pre-trimmed Analog Engine® IC

Document 600067 Rev 00
Page 7
Noise Reduction (Compander) Configurations
An additional application of the 4305 is for
noise reduction systems. In these applications, one
Analog Engine is configured for use as a compressor
to condition audio signals before feeding them into a
noisy channel. A second Analog Engine, configured
as an expander, is located at the receiver end of the
noisy channel. Most commonly, the compression/ex-
pansion ratio is modest (e.g. 2:1:2) and is linearly
applied across the entire signal dynamic range.
During low-level audio passages, the compressor
increases signal levels, bringing them up above the
noise floor of the channel. At the receiving end, the
expander reduces the signal back to its original
level, in the process attenuating channel noise.
During high-level audio passages, the compres-
sor decreases signal levels, reducing them to fit
within the headroom limits of the channel. The ex-
pander increases the signal back to its original level.
While the channel noise may be increased by this ac-
tion, in a well-designed compander, at such times
the noise floor will be masked by the high-level sig-
nal.
The 4305 facilitates the design of a wide variety
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 and VCA in-
puts, 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 sys-
tems with thresholds and varying compression
slopes, systems including noise gating and/or limit-
ing, and systems with varying degrees of
pre-emphasis and filtering in both the signal and de-
tector paths. Furthermore, much of this can be ac-
complished 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 ra-
tios, thresholds, and time constants.
Note that the 4305 is fully compatible with other
Analog Engines from THAT Corporation. All our An-
alog Engines feature log-responding true-RMS level
detectors and exponentially controlled Blackmer
VCAs. It is possible to compress (encode) 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
At first, the logarithmic output of the RMS detec-
tor and the exponential control ports of the VCA can
be intimidating for designers unfamiliar with THAT
Corporation's offerings. However, in fact, these
characteristics make developing audio processors
easy once a designer understands the concepts in-
volved. As noted earlier, AN101A: The Mathematics
of Log-Based Dynamics Processors, discusses these
concepts in some detail. The following discussion
draws heavily from that application note.
The Feedforward Compressor
Figure 3 shows a conceptual diagram of a very
simple feedforward compressor. Using the "log
math" principles explained in AN101A, 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 a compressor
in which gain G_dB decreases as input signal level
In_dB increases. Combining these equations,
Out = In_dB − k ⋅ In_dB = In_dB(1 − k) .
Rearranging yields
In_dB
Out _dB
=
1
(1− k)
= C. R.
This is the compression ratio.
In_dB
RMS
In_dB
-k
G_dB
Out _dB
Figure 3. Simplified feedforward compressor,
conceptual diagram.
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