4305-DEMO THAT Corporation, 4305-DEMO Datasheet - Page 16

• 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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encoder and decoder of Figures 17 and 18.
encoder reduces the dynamic range at its input by a
factor of 2, compressing 120 dB into 60 dB. The
decoder expands this dynamic range back to track
that of the encoder's input signal.
Hi-fi Compander
in some applications, a few minor changes can result
in substantially improved overall performance. The
following compander implementation adds pre- and
de-emphasis to the signal path.
pre-emphasis helps overcome the rising noise level
with frequency of an FM RF channel by raising the
level of the high frequency portions of the signal
before it passes through the transmission channel.
Matching signal-path de-emphasis in the decoder
brings the frequency response back to flat while
simultaneously lowering the noise floor of the
channel.
low-frequency signals mask the channel noise by
reducing the perception of high-frequency noise
signals.
pre-emphasis is that it can cause overload in the
channel when high-level, high-frequency signals are
present. To guard against this problem, we have
added RMS pre-emphasis to both detectors.
mitigates high-frequency overload by lowering the
level-match point to high-frequency signals.
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-60
-80
Figure 19 tracks signal levels through the
While the previous circuits perform adequately
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Figure 19. 2:1 compander transfer characteristics
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THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
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given signal level, high-frequency signals are lowered
in level by the VCA more than low-frequency signals.
a means to truncate the RMS detector’s low-level
response. This improves low level tracking between
different detectors by forcing each detector to
"bottom out" at a predetermined level, eliminating
the effects of different low-level behavior from one
detector to the next.
System Performance
implements all of the aforementioned improvements.
Assuming no change in VCA gain (G
pre-emphasis network of R3 and C7 produces
~20 dB of signal-path pre-emphasis starting at
~2 kHz and stopping at ~19 kHz.
to the VCA, so additional components are not
required to implement this feature. Signal fed to this
network is buffered by U2; while this buffer is not
always necessary, the pre-emphasis network must be
driven from a low source impedance to ensure
proper tracking between the encoder pre-emphasis
and the decoder de-emphasis. If driven from an
unbuffered source, the pre-emphasis network should
be adjusted to take into account the impedance of
that source.
(provided by R5 and C8 in the encoder, and R11 and
C18 in the decoder) for the detectors in both the
encoder and the decoder. The center frequency of
this pre-emphasis circuit is aligned with the center
frequency of the signal path pre-emphasis when
evaluated on a logarithmic frequency scale. This
shifts the level match of the encoder symmetrically
about the mid-point of the signal-path pre-emphasis,
which configures the system to take the best advan-
tage of the companding to avoid high-level high-
frequency overload in the transmission or storage
channel.
are intended to force each of the detectors to stop
responding to low level signals at the same point in
order to improve tracking. This floor occurs when
the RMS current through R1 equals that of R6, and
when the current through R10 equals that of R12.
Since the input of the RMS detector is at virtual
ground, the current through R6 and R12 will be
As an additional enhancement, we have included
The compander shown in Figures 20 and 21
Note that R3 and C7 also compensate the input
We have included ~10 dB of RMS pre-emphasis
R6 of the Hi-Fi encoder and R12 of the decoder
THAT4305 Pre-trimmed Analog Engine®
dB
), the