8169 Datasheet, PDF (24/30 Pages) STMicroelectronics – DIGITAL AUDIO PROCESSOR WITH MULTICHANNEL DDX™

STA304A
Table 2. DDX gain
DDX_GAIN_0 DDX_GAIN_1
0
0
0
1
1
0
1
1
DDXGain
1x
2x
2x
3x
DDX Compression
NO
NO
YES
YES
DDX Gain Compression
Since a full-scale output of the GC/Vol block is mapped to full output modulation, any signal exceeding 0 dBFS
at the output of the GC/Vol block will be clipped. The purpose of the compression algorithm is to reduce the
gain of the system when 0 dBFS has been exceeded such that clipping does not take place, thus performing an
output limiting function. This would yield a constant Vout once the gained input exceeds 0 dBFS.
With DDX_GAIN_0 = 1, the output of the GC/Vol block is compared to a threshold set just below 0 dBFS. When
the output of GC/Vol exceeds this threshold the gain of system is reduced following a set time-constant and
gain-reduction rate. This reduction in gain is stored as a variable. If subsequently the output of GC/Vol remains
below a separate lower threshold for a set amount of time the gain is then increased following another time-
constant and gain rate. Thus, seven different constants determine the attack, release, and limiting characteris-
tics of the compression algorithm. These constants have been tuned to sound as musical as possible when the
dynamic range of a recording is being reduced due to 0 dBFS being exceeded.
Figure 15. Compression response to 500 Hz sine at 0 dBFS with gain of +10 dB
24/30