Datasheet

3−35
O
INPUT
+
O
DESIRED
) 24.0824 dB
6.0206
Gains or boosts are represented as negative numbers; cuts or attenuation are represented as positive numbers. For
example, to achieve a boost of 21 dB at threshold T1, the I
2
C coefficient value entered for O1 must be:
O1
INPUT
+
–21 dB ) 24.0824 dB
6.0206
+ 0.51197555
+ 0.1000_0011_0001_1101_0100
+ 0x00000041886A in 25.23 format
More examples of offset computations follow.
3.7.2.3 Slope Parameter Computation
In developing the equations used to determine the subaddress input value required to realize a given compression
or expansion within a given region of the DRC, the following convention is adopted.
DRC Transfer = Input Increase : Output Increase
If the DRC realizes an output increase of n dB for every dB increase in the rms value of the audio into the DRC, a
1:n expansion is being performed. If the DRC realizes a 1-dB increase in output level for every n-dB increase in the
rms value of the audio into the DRC, a n:1 compression is being performed.
For 1:n expansion, the slope k can be found by:
k = n − 1
For n:1 compression, the slope k can be found by: k +
1
n
–1
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, k +
1
n
–1 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.
0.5 Compression å k +
1
0.5
–1 + 1
1 : 2 Expansion å k + 2–1 + 1
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
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
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.