THAT4305 Datasheet, PDF (18/20 Pages) List of Unclassifed Manufacturers – Pre-trimmed Analog Engine® IC

Page 18
THAT4305 Pre-trimmed Analog Engine^
Encoder
In
(dBV)
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
Encoder
VCA Gain
(In dB)
-28
-23
-18
-13
-8
-3
2
7
12
17
22
27
32
Encoder
Out/
Decoder
In
(dBV)
4
-1
-6
-11
-16
-21
-26
-31
-36
-41
-46
-51
-56
IRMS _In
Decoder
VCA Gain
(mA)
0.1841
0.1035
0.0582
0.0327
0.0184
0.0104
0.0058
0.0033
0.0018
0.0010
0.0006
0.0003
0.0002
(In dB)
28
23
18
13
8
3
-2
-7
-12
-17
-22
-27
-32
Decoder
Out
(dBV)
20
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
Table 4. Hi-fi compander transfer characteristics
Table 4 shows the transfer characteristics of this
companding system (neglecting the effects of R6 and
R12). As before, the columns labeled Encoder VCA
Gain, Encoder Out, Decoder VCA Gain, and Decoder
Out use the equations derived previously in the sec-
tion titled "The Mathematics of Log Based
Companding Systems". The values in the column la-
beled RMS In are derived using the equation:
Encoder Out
(
)
I_{RMS In}
=
10
20
R_{RMS In}
Figure 22 tracks signal levels through the en-
coder and decoder of Figures 20 and 21. The com-
pression and expansion ratios here are the same as
those of the previous circuits, but the frequency
shaping afforded by signal pre- and de-emphasis
and detector pre-emphasis make this a superior
sounding system.
In this application, the VCA gain ranges over
about ±30 dB, which is well within specification, as
is the RMS detector input current.
Other Dynamics Processor Configurations
We have said before that the building blocks con-
tained within the 4305 are applicable to a very wide
range of dynamics processor configurations. These
include companding noise reduction systems with
ratios other (higher or lower) than 2:1:2, multi-band
companders, etc. The 4305 is versatile enough to be
used as the heart of a compressor, expander, noise
gate, AGC, de-esser, frequency-sensitive compres-
sor, and many other dynamics processors. It is be-
yond the scope of this data sheet to provide specific
advice about any of these functional classes. We re-
fer the interested reader to THAT's applications
notebooks volumes 1 and 2, which contain many cir-
cuits based on THAT's other VCAs and RMS level
detectors, but are largely applicable to the 4305 with
only minor variations. Of course, look for more ap-
plications information aimed specifically at the 4305
in the future.
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 22. Hi-fi compander transfer characteristics
Closing Thoughts
The design of dynamics processors and
companding systems is a very intricate art: witness
the proliferation of dynamics processors available in
the market today. Many of these are based on
THAT's VCAs and level detectors, yet they all have
individual sonic characteristics. In the applications
section of this data sheet, we have offered a few ex-
amples only as starting points. THAT Corporation's
applications engineering department is ready to as-
sist customers with suggestions for tailoring and ex-
tending these basic circuits to meet specific needs.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com