TAS5705
www.ti.com ...................................................................................................................................................................................................... SLOS549–JUNE 2008
Threshold Parameter Computation
For thresholds,
Tdb = –6.0206TINPUT = –6.0206TSUB_ADDRESS_ENTRY
If, for example, it is desired to set T1 = –64 dB, then the subaddress entry required to set T1 to –64 dB is
-64
Τ1SUB_ADDRESS_ENTRY
=
= 10.63
-6.0206
T1 is entered as a 32-bit number in 9.23 format. Therefore,
T1 = 10.63 = 0 1010.1010 0001 0100 0111 1010 111 = 0x0550 A3D7 in 9.23 format
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 has been 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, an n:1 compression is being performed.
For a 1:n expansion, the slope k can be found by
k = n – 1
For an n:1 compression, the slope k can be found by
1
k =
- 1
n
(1)
In compression (n:1), n is implied to be greater than 1. For compression, Equation 1 means –1 < k < 0 for n > 1.
Thus k must always lie in the range k > –1.
1
1
Compression equation: k = - 4 = -1 ® n = - ® -0.3333:1 compression
n
3
With k = –4 then, 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 signal amplitude, the output decreases in signal amplitude.
DRC Offset Calculation
The DRC offset is calculated by
10(g /20)
d
Goffset
=
15.5
where gd = desired gain (in dB) and 15.5 is the the fixed antilog normalization factor. A value of gd = 0 indicates
no offset.
Copyright © 2008, Texas Instruments Incorporated
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