4305-DEMO THAT Corporation, 4305-DEMO Datasheet - Page 6

• 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

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through the VCA. As a result, the output signal level
is reduced, or compressed, relative to rising input
signal levels.
will vary the point at which compression begins.
Varying the gain between the RMS output and the
VCA control input varies the compression ratio.
especially versatile because they cannot become
unstable due to oscillation in the control loop. Unity
gain in the sidechain produces infinite compression
(where the output remains constant regardless of
increases in the input signal).
negative compression ratios are easily achievable.
(Negative compression occurs when the output signal
decreases as the input signal increases.) This
approximates the effect of playing music backwards,
since the attack is suppressed and the release is
increased in volume.
are possible. These include implementing more than
one threshold, different ratios, additional time
constants, ac-coupling of some (or all) of the detector
output signal, and many more. See AN101A, The
Mathematics of Log-Based Dynamic Processors, for
more details of how the sidechain gain determines
compression ratios.
Feedback Topologies
limiter design is to feed the output signal into the
RMS detector.
coupled) to the VCA's negative control port to reduce
signal levels.
designs, a threshold in the sidechain serves to stop
the compression action at low signal levels.
ently from feedforward.
compression requires infinite gain in the feedback
loop from RMS output to VCA control port. Of
course, infinite gain is impossible, so practical
feedback compressors are usually limited to ratios
no greater than 20 or so. Additionally, the gain in
the feedback loop alters the effective time constant of
the detector, shortening the attack as the ratio
becomes
appropriate, depending on the desired effect.
Varying the threshold setting of the sidechain
Feedforward
Many other variations of the feedforward concept
An alternative configuration for compressor/
The feedback topology behaves somewhat differ-
higher.
The RMS output is fed back (dc-
Similarly as with a feedforward
This
compressor
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THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
may
First, reaching infinite
or
With feedforward,
topologies
may
not
are
be
Expander (Gate) Configurations
feedforward compressor, it is possible to arrange
signal gain to decrease along with signal level, thus
producing an expander.
below a threshold (so, the threshold detector’s polar-
ity is reversed from that of a compressor) to reduce
noise or crosstalk during pauses in program
material.
"cleaning up" individual drum tracks to reduce rever-
beration, interference from microphones picking up
adjacent drum sounds, and alter the attack/decay
characteristic of individual drum sounds.
times, and carefully programmable release times. In
a 4305, this is best accomplished by using the RMS
detector as a log rectifier with very short time
constants, and following the detector output with a
time-constant stage that applies the desired attack
and release behavior. This alters the 4305 detector’s
natural response characteristics to peak, rather than
rms, time constants. We intend to produce an appli-
cation note showing examples of these circuits. Until
that is available, see DN 100, which shows a noise
gate application using THAT's 4301 Analog Engine®.
Noise Reduction (Compander) Configurations
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
compression/expansion ratio is modest (e.g. 2:1:2)
and is linearly applied across the entire signal
dynamic range.
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.
sor decreases signal levels, reducing them to fit
within the headroom limits of the channel.
expander increases the signal back to its original
level. While the channel noise may be increased by
this action, in a well-designed compander, at such
By changing the sign of the sidechain in a
Practical gates usually require very fast attack
During low-level audio passages, the compressor
During high-level audio passages, the compres-
An additional application of the 4305 is for
channel.
This technique has long been used for
THAT4305 Pre-trimmed Analog Engine®
Most
This is typically applied
commonly,
The
the