THAT4305 Datasheet, PDF (16/20 Pages) List of Unclassifed Manufacturers – Pre-trimmed Analog Engine® IC

Page 16
THAT4305 Pre-trimmed Analog Engine^
Figure 19 tracks signal levels through the en-
coder and decoder of Figures 17 and 18. The en-
coder reduces the dynamic range at its input by a
factor of 2, compressing 120dB into 60dB. The de-
coder expands this dynamic range back to track that
of the encoder's input signal.
Compression
Process
Expansion
Process
20
0
20
0
-20
-20
-40
-40
-60
-80
-60
-100
-80
-100
In(Cmp)
Out Cmp
In(Exp)
Out Exp
Figure 19. 2:1 compander transfer characteristics
Hi-fi Compander
While the previous circuits perform adequately
in some applications, a few minor changes can re-
sult in substantially improved overall performance.
The following compander implementation adds pre-
and de-emphasis to the signal path. 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 be-
fore it passes through the transmission channel.
Matching signal-path de-emphasis in the decoder
brings the frequency response back to flat while si-
multaneously lowering the noise floor of the chan-
nel. This helps ensure that isolated low-frequency
signals mask the channel noise by reducing the per-
ception of high-frequency noise signals.
Of course, the drawback of signal-path
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. This
mitigates high-frequency overload by lowering the
level-match point to high-frequency signals. For a
given signal level, high-frequency signals are lowered
in level by the VCA more than low-frequency signals.
As an additional enhancement, we have included
a means to truncate the RMS detector’s low-level
response. This improves low level tracking between
different detectors by forcing each detector to "bot-
tom out" at a predetermined level, eliminating the ef-
fects of different low-level behavior from one
detector to the next.
System Performance
The compander shown in Figures 20 and 21 im-
plements all of the aforementioned improvements.
Assuming no change in VCA gain (G_dB), the
pre-emphasis network of R3 and C7 produces
~20 dB of signal-path pre-emphasis starting at
~2 kHz and stopping at ~19 kHz.
Note that R3 and C7 also compensate the input
to the VCA, so additional components are not re-
quired 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 imped-
ance of that source.
We have included ~10 dB of RMS pre-emphasis
(provided by R5 and C8 in the encoder, and R11 and
C18 in the decoder) for the detectors in both the en-
coder and the decoder. The center frequency of this
pre-emphasis circuit is aligned with the center fre-
quency of the signal path pre-emphasis when evalu-
ated 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 advantage of
the companding to avoid high-level high-frequency
overload in the transmission or storage channel.
R6 of the Hi-Fi encoder and R12 of the decoder
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 R3, and
when the current through R10 equals that of R12.
Since the input of the RMS detector is at virtual
ground, the current through R3 and R12 will be
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