Datasheet

O1
No Discontinuity
+
|
T1 * T2
|
k1 ) O2 For (
|
T1
|
w
|
T2
|
)
t
a
+
*1
f
S
ȏn(1 * aa)
t
d
+
*1
f
S
ȏn(1 * ad)
t
window
+
* 1
f
S
ȏn(1 * ae)
TAS5548
SLES270 –NOVEMBER 2012 www.ti.com
• RMS estimator—This DRC element derives an estimate of the rms value of the audio data stream into
the DRC. For the DRC block shared by Ch1 and Ch2, two estimates are computed—an estimate of the
Ch1 audio data stream into the DRC, and an estimate of the Ch2 audio data stream into the DRC. The
outputs of the two estimators are then compared, sample-by-sample, and the larger-valued sample is
forwarded to the compression/expansion coefficient computation engine.
Two programmable parameters, ae and (1 – ae), set the effective time window over which the rms
estimate is made. For the DRC block shared by Ch1 and Ch2, the programmable parameters apply to
both rms estimators. The time window over which the rms estimation is computed can be determined
by:
A. Care should be taken when calculating the time window for 192kHz content. Please use 96kHz as the
sampling frequency for 96kHz AND 192kHz, as the TAS5548 uses a digital decimator to do all DAP
processing at 96kHz.
B. ae = energy time (1)
• Compression/expansion coefficient computation—This DRC element converts the output of the rms
estimator to a logarithmic number, determines the region where the input resides, and then computes
and outputs the appropriate coefficient to the attack/decay element. Seven programmable parameters,
T1, T2, O1, O2, k0, k1, and k2, define the three compression/expansion regions implemented by this
element.
• Attack/decay control—This DRC element controls the transition time of changes in the coefficient
computed in the compression/expansion coefficient computation element. Four programmable
parameters define the operation of this element. Parameters ad and (1 – ad) set the decay or release
time constant to be used for volume boost (expansion). Parameters aa and (1 – aa) set the attack time
constant to be used for volume cuts. The transition time constants can be determined by:
C. aa = attack time
D. ad - decay time (2)
3.8.2 Compression/Expansion Coefficient Computation Engine Parameters
Seven programmable parameters are assigned to each DRC block: two threshold parameters—T1 and
T2, two offset parameters—O1 and O2, and three slope parameters—k0, k1, and k2. The threshold
parameters establish the three regions of the DRC transfer curve, the offsets anchor the transfer curve by
establishing known gain settings at the threshold levels, and the slope parameters define whether a given
region is a compression or an expansion region.
T2 establishes the boundary between the high-volume region and the mid-volume region. T1 establishes
the boundary between the mid-volume region and the low-volume region. Both thresholds are set in
logarithmic space, and which region is active for any given rms estimator output sample is determined by
the logarithmic value of the sample.
Threshold T2 serves as the fulcrum or pivot point in the DRC transfer function. O2 defines the boost
(> 0 dB) or cut (< 0 dB) implemented by the DRC-derived gain coefficient for an rms input level of T2. If
O2 = 0 dB, the value of the derived gain coefficient is 1 (0x0080 0000 in 5.23 format). k2 is the slope of
the DRC transfer function for rms input levels above T2, and k1 is the slope of the DRC transfer function
for rms input levels below T2 (and above T1). The labeling of T2 as the fulcrum stems from the fact that
there cannot be a discontinuity in the transfer function at T2. The user can, however, set the DRC
parameters to realize a discontinuity in the transfer function at the boundary defined by T1. If no
discontinuity is desired at T1, the value for the offset term O1 must obey the following equation.
28 TAS5548 DAP Architecture Copyright © 2012, Texas Instruments Incorporated
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