Datasheet

Compression equation: k + *4 +
1
n
*1 å n + *
1
3
å *0.3333 : 1 compression
Expansion equation: k + *4 + n * 1 å n + *3 å 1 : *3 expansion
0.5 : 1 compression å k +
1
0.5
* 1 + 1
1 : 2 expansion å k + 2 * 1 + 1
k +
1
n
* 1
k +
1
n
* 1
TAS5548
SLES270 –NOVEMBER 2012 www.ti.com
For n : 1 compression, the slope k can be found by:
In both expansion (1 : n) and compression (n : 1), n is implied to be greater than 1. Thus, for expansion:
k = n – 1 means k > 0 for n > 1. Likewise, for compression, means –1 < k < 0 for n > 1. Thus, it
appears that k must always lie in the range k > –1.
The DRC imposes no such restriction and k can be programmed to values as negative as –15.999. To
determine what results when such values of k are entered, it is first helpful to note that the compression
and expansion equations for k are actually the same equation. For example, a 1 : 2 expansion is also a
0.5 : 1 compression.
As can be seen, the same value for k is obtained either way. The ability to choose values of k less than –1
allows the DRC to implement negative-slope transfer curves within a given region. Negative-slope transfer
curves are usually not associated with compression and expansion operations, but the definition of these
operations can be expanded to include negative-slope transfer functions. For example, if k = –4
With k = –4, the output decreases 3 dB for every 1 dB increase in the rms value of the audio into the
DRC. As the input increases in volume, the output decreases in volume.
3.9 THD Manager
The THD manager is designed to set the max output level target after all processing has been completed.
The Audio clip engages at +24dB between (pre) and (post) stage. 10% distortion occurs when audio is
clipping approx +2.4 to 3dB over full scale. There is amplitude loss when clipping, so THD(post) might
allow slight gain through THD manager. 10% distortion clipping will account for approx -1dB of output level
loss. This is accounted for as seen with +1dB in step 2 to set output level +0dB
Example setup to modify 10% THD output level: * note that coefficient calculations are approximate for
simplicity
1. Signal path settings
– Input -10dBFS
– Volume 0xD9 0000 000C +15dB
– THD Manager (pre) 0xE9 0650 0000 +22dB
– THD Manager (post) 0xEA 0006 7000 -26dB
2. resulting output
– output clipping at 10% distortion with output level +0dB
– input -10 vol +15 THD(pre) +22 THD(post) -26
– -10 +5 +27(clip) +1
3. Begin clipping at -12dBFS input with +0dB output level
– THD Manager (pre) 0xE9 07FF FFFF +24dB (previous setting +22dB + 2dB)
– result: input -12dBFS output clipping at 10% distortion with output level +0dB
– input -12 vol +15 THD(pre) +24 THD(post) -26
– -12 +3 +27(clip) +1
4. Begin clipping at -12dBFS input with -10dB output
– THD Manager (post) 0xEA 0002 0000 -36dB (previous setting -26dB -10dB)
– result: input -12dBFS output clipping at 10% distortion with output level +0dB
– input -12 vol +15 THD(pre) +24 THD(post) -36
30 TAS5548 DAP Architecture Copyright © 2012, Texas Instruments Incorporated
Submit Documentation Feedback
Product Folder Links: TAS5548