4305-DEMO THAT Corporation, 4305-DEMO Datasheet - Page 13

• 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

Document 600067 Rev 01
The Sidechain
is best not to linearly compress the entire dynamic
range of the audio signal.
sidechain of figure 16, we have added a threshold
amplifier with ±30 dB of threshold adjustment.
This restricts compression to signals above the
threshold, passing those below threshold without
any change. Additionally, we added a compression
ratio control adjustable from 1:1 to ∞:1.
we've added a means to apply up to ±20 dB of static,
or "make-up" gain.
through C1 and R1. Like the VCA, the input to the
detector is AC coupled.
current flowing into the detector's input from being
measured by the detector. If unchecked, such offsets
would limit resolution at low levels, producing an
artificial "floor" to the detector's low-level response.
the output of the detector is proportional to the log of
the RMS level of the input voltage. The output of the
detector will be approximately zero volts at the "zero
dB reference level" -- the point at which the RMS
value of the input current equals the timing current
(set at 7.5 μA for the 4305). We have chosen a value
of -10 dBu, (245 mV
level. The required input resistor can be calculated
as
of approximately -2 to the detector output signal
above threshold, and zero gain (A
below threshold.
plished by D1 and D2, which allow negative-going
output signals to pass but block positive-going ones.
Because U3 is configured to invert, positive-going
signals at the RMS output (indicating increasing ac
input levels) are passed onwards, while negative-
going RMS outputs are blocked. By feeding variable
dc into this stage via R7 and the threshold pot R10,
we can vary the point at which RMS output signals
begin to be passed through to the threshold amplifier
stage’s output (at the junction of D2 and R2.)
+6.2 mV/dB, but because R2 is approximately twice
R17, the threshold amplifier (U3) has a gain of -2, so
As noted earlier, for most effects compressors, it
The input signal is fed into the RMS detector
As previously discussed (in the theory section),
R =
Inverting threshold amplifier (U3) provides gain
The scaling at the output of the detector is
245mV
7.5 A
RMS
= 32.6k l 33k
The change in gain is accom-
RMS
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
) for the zero dB reference
THAT Corporation; 45 Sumner Street; Milford, MA 01757-1656; USA
This prevents any DC
To this end, in the
V
=0) to signals
.
Finally,
the scaling at its output is -12.4 mV/dB. To swing
the threshold over ±30 dB, we can calculate the
required value of R7 as follows:
buffer the VCA's control port, ensuring a low- imped-
ance drive at that point. (High impedances, even as
little as 50 to 100 ohms, will increase VCA distortion
at high signal levels.) Above threshold, when U4's
gain is -½, the net gain of the sidechain (from RMS
output to VCA control input) is unity, and the
compression ratio is ∞:1. The network of R3, R9,
and R11 in conjunction with R18, allows the gain of
U4 to vary from 0 to -½, and simultaneously shapes
the (linear) pot's response so that 50% rotation
results in 4:1 compression. 4:1 ratio at 50% rotation
is often considered a useful target.
adding static, or "make-up" gain. The control-voltage
sensitivity at the output of U4 is 6.2 mV/dB.
Therefore,
nearest 5% value.
U4, are generally relatively slow moving, so the
sidechain does not usually require wide bandwidth.
Furthermore, noise on the VCA control port can
modulate the VCA signal, thus adding noise to the
signal path. Accordingly, we added C5 in order
We can rearrange this to be
R7 = 403.2k
We chose 430 kΩ for R7.
U4 is a variable-gain inverter that serves to
Finally, R8 and R12 provide the means for
We can rearrange this to be
R8 =
We've chosen 620 kΩ for R8 since it is the
The signals in the sidechain, and at the output of
15V(
10k
15V(
5.1k
0.0124
0.0062
R7
R8
10k
5.1k
R7
R8
=
dB
=
dB
V
V
20dB
15V
)
)
30dB
15V
20dB
15V
= 30dB
= 20dB
$0.0062
5k
$ 0.0124
$ 0.0062
dB
V
.
.
= 625k
dB
V
dB
V
, and therefore,
, and therefore,
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