TAS5705
SLOS549–JUNE 2008...................................................................................................................................................................................................... www.ti.com
The threshold settings must be programmed as 32-bit (9.23 format) numbers.
Zero-valued and positive-valued threshold settings are not allowed, and cause unpredictable behavior if used.
•
•
Offsets O1 and O2 define, in dB, the attenuation (cut) or gain (boost) applied by the DRC-derived gain
coefficient at the threshold points T1 and T2, respectively. Positive offsets are defined as cuts, and thus boost
or gain selections are negative numbers.
Slopes k1 and k2 define the compression is to be performed within a given region, and the degree of
compression to be applied. Slopes are programmed as 26-bit (3.23 format) numbers.
DRC Implementation
Figure 4.13.1-1 shows the three elements comprising the DRC: (1) an RMS estimator, (2) a compression
coefficient computation engine, and (3) an attack/decay controller.
•
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 though Ch4 , the individual channel estimates are computed. The
outputs of the estimators are then compared, sample-by-sample, and the largest-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
though Ch4, the programmable parameters apply to both RMS estimators. The time window over which the
RMS estimation is computed can be determined by
1
twindow
=
f ln 1- ae
(
)
S
•
•
Compression 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. The programmable parameters, T1, T2, O1, O2, K1, and
K2, define the compression regions for both DRCs 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
signal amplitude boost (expansion). Parameters aa and (1 – aa) set the attack time constant to be used for
signal amplitude cuts. The transition-time constants can be determined by
1
ta
=
f ln 1- aa
(
)
S
1
td
=
f ln 1- ad
(
)
S
32
Audio Input
CH 1
RMS Voltage
Estimator
Compression/Offset
Cut
Attack/Decay Control
32
32
32
DRC-Derived
Gain Coefficient
K1
Audio Input
CH 2
RMS Voltage
Estimator
{
O1
Audio Out
Volume
Audio Input
CH 3
RMS Voltage
Estimator
td
ta
Audio In
T1
Audio Input
CH 4
RMS Voltage
Estimator
B0309-01
Figure 43. DRC Block Diagram
36
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