Aphex 320D Compellor - Manual (Page 22)

320D
Compellor
Page 22 Page 23
8.1 Model Evolution
The model 320D Compellor is a dual channel audio processor capable of being stereo linked.
It is essentially a Model 320A with a digital audio interface added. In order to make room for
the necessary additional connectors on the back panel, we eliminated the analog input imped-
ance selectors and the remote control jack of the Model 320A. We also limited the operating
level selectors to +4dBu/-10dBV selections.
8.2 Signal Flow
The Compellor contains an input stage, an intermediate VCA stage, and an output stage. The
input audio signal undergoes all processing in the VCA stage and is subsequently sent out
through the output stage. A side chain system produces the control signals which change the
VCA gain according to the signal processing requirements. When using a digital audio input,
the digital input is first converted to analog by a high grade codec and the recovered analog
is sent through the analog input path. The analog input jack is disabled. The digital output is
generated by the codec from the analog VCA output signal. The digital and analog outputs are
available simultaneously. You can have an analog input and digital output or vice-versa.
8.3 Processing Functions
A Compellor provides automatic gain control and excess peak control through the three prin-
cipal functions of leveling, compression, and peak limiting. In addition, a dynamic verification
gate (DVG), silence gate, and dynamic release computer (DRC) are incorporated as propri-
etary support functions. A very subtle stereo enhancement technique is also included as a
user selectable feature.
8.4 Leveling Function
The leveler is a slow acting automatic gain controller. This means that it responds to the aver-
age power level of the audio signal much as the ear hears the loudness, or relative volume
level. It constantly but slowly adjusts the VCA gain, attempting to keep the average volume
level of the output signal constant. The compression ratio of the leveler is about 20:1 which
means that if the input signal changes by 20dB, the leveler could keep the change of output
level down to only 1dB. The actual range of leveling depends on how much gain reduction
the user chooses.
The leveler has two operating speeds which can be selected by a front panel switch: “fast”
and “slow”. The slow speed will not affect musical dynamics but will act fast enough to follow
the general density trends of a program mix. The fast speed is better for plain voice work
as it can follow the faster and more unpredictable voice changes of announcers and singers
as they weave around the mic or use expression. The frequency discriminate leveling in the
series “A” Compellors has so improved the leveling function that fast leveling is now feasible
for use with musical programs to materially increase the loudness density of a mix without
causing objectionable bass pullback and bass pumping.
The leveler is influenced by both the silence gate and dynamic verification gate whereby the
gain control generated by the leveler can be frozen by either of the two gates. This means
that either gate can make the leveler stop changing the VCA gain and hold the most recent
leveling value.
The frequency discriminate leveler, as opposed to the previous leveler, responds more slowly
to low frequencies than to higher frequencies. This represents a significant improvement
whereby the ear can perceive much less effect of the bass signals controlling the Compellor
gain. The prior leveler responds to all frequencies at the same rate.
8. System Description