BSS Audio DPR-901II Owner's Manual - Manual (Page 16)

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DPR-901ii
Principle of Operation
The DPR-901ii is fundamentally different to ordinary de-essers, Dolby
systems and the like because it uses subtractive techniques to control the
degree of compression or expansion. This enables the VCA (gain control
element) to be kept out of the direct signal path. As a result, the through-
going signal is not subjected to the noise and distortion that’s intrinsic to
even the best VCAs. Moreover, the resultant filtered output exhibits far
less of the ‘time smearing’ that accompanies most conventional units and
has at times given equalisation a bad name. In fact, the subtractive VCA
technique can only operate correctly if the ‘passing through’ and ‘con-
trol’ signals at the subtractor are correctly aligned.
Figure 4.1 shows a simple block diagram that illustrates the principle of
operation of subtractive gain reduction and Fig 4.2 that with the addition
of filtering and invertor for gain addition or expansion.
Referring to figure 4.1, in operation the main signal path is simply
through the single inverting op-amp as the VCA is held at maximum
attenuation. When signal exceeds the set threshold level, the inverting
VCA is gradually opened, allowing a small ‘subtracting’ signal to pass to
the main inverting op-amp. Furthermore, this signal is also conditioned
by the filtering effect of the parametric filter. The resultant output is then
either reduced in level, or increased in level dependent on the ‘com-
press/expand’ switch, and it is only those frequencies passed by the filter,
that are so processed.
On the DPR-901ii, the threshold control is adjusting the voltage drive
level to the VCA, in conjunction with a fixed reference point, whilst the
‘compress/expand’ switch is replaced with a centre-off compress or
4.0 Principle of Operation
Fig 4.1 Subtractive
Gain Reduction
Fig 4.2 Subtractive
Gain Reduction or
Expansion