TAS5504 Datasheet, PDF (32/96 Pages) Texas Instruments – 4 Channel Digital Audio PWM Processor

Introduction
• Thresholds T1 and T2 define, in dB, the boundaries of the three regions of the DRC, as referenced to the
rms value of the data into the DRC. Zero valued threshold settings reference the maximum valued rms
input into the DRC and negative valued thresholds reference all other rms input levels. Positive valued
thresholds have no physical meaning and are not allowed. In addition, zero valued threshold settings are
not allowed.
Although the DRC input is limited to 32-bit words, the DRC itself operates using the 48-bit word format of the
DAP. The 32-bit samples input into the DRC are placed in the upper 32 bits of this 48-bit word space. This
means that the threshold settings must be programmed as 48-bit (25.23 format) numbers.
CAUTION: 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. Offsets must be programmed as 48-bit (25.23 format)
numbers.
• Slopes k0, k1, and k2 define whether compression or expansion is to be performed within a given region,
and the degree of compression or expansion to be applied. Slopes are programmed as 28-bit (5.23 format)
numbers.
1.10.1 DRC Implementation
The three elements comprising the DRC: (1) an rms estimator, (2) a compression/expansion coefficient
computation engine, and (3) an attack/decay controller. DRC1 applies to channels 1 −3 and DRC2 applies
only to channel 4. DRC1 uses I²C register 0x98 − 0x9C and DRC2 uses I²C registers 0x9D − 0xA1.
• RMS estimator—This DRC element derives an estimate of the rms value of the audio data stream into
the DRC. For DRC1 (Ch 1−3), the individual channel estimates are computed. The outputs of the
estimators are then compared, sample-by-sample, and the larger valued sample is forwarded to the
compression/expansion coefficient computation engine.
Two programmable parameters (I²C 0x98), E and (1 – E), set the effective time window over which the rms
estimate is made. For the DRC1 block, the programmable parameters apply to all rms estimators. The
time window over which the rms estimation is computed can be determined by:
twindow
+
FS
*1
ȏn(1 *
E)
• Compression/expansion coefficient computation—This DRC element converts the output of the rms
estimator to a logarithmic number, determines the region that 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 D and 1 − D set the decay or release time
constant to be used for volume boost (expansion). Parameters A and 1 − A set the attack time constant
to be used for volume cuts. The transition time constants can be determined by:
ta
+
FS
*1
ȏn(1 *
A)
td
+
FS
*1
ȏn(1 *
D)
Figure 1−19 shows how the TAS5504 attack and decay are defined. Note that this is opposite of some
definitions of attack and decay.
24 TAS5504
SLES123 − October 2004