Consoles 973value, or about 1.00005. “Slightly” says it all. This
dB-per-time gain trajectory works quite well in audio,
emulating the dynamic response of many good analog
systems.
The last things that happen are that the newly modi-
fied GR value is saved for use next sample and also
used to multiply with the present input sample to create
the gain-reduced output sample.
All in all, it is almost an exact parallel to a simple
analog feedback-style limiter; complexity concessions
exist for operating in a sampled-time system (such as
the initial input/last GR premultiply), as opposed to
relying on the always existent signals in the continuum
of analog. On the other hand, one effortlessly achieves
true dB/time gain rates for attack and release, usually a
feature of posher analog designs and only ever approxi-
mated in simple systems.
25.22.2.2 Feedback-Style Limiting and Compression
Unlike most analog GR elements, the “MPY” in a DSP
is directly linear in operation—i.e., a gain-reduction
value of say 0.3 will cause the signal through the multi-
plier to be reduced some 10 dB. It is not linear-by-dB,
like a VCA, or a mangled exponential/logarithmic like
thing such as from using a raw semiconductor element
such as a transistor or FET. Yet, as has been shown
above, linear-by-dB results can be achieved fairly
simply. Emulating other laws can get rather interesting,
but are certainly attainable, in pursuit of a sound. Simi-
larly, the determination of the amount of instantaneous
feedback in a feedback limiter depends on many things,
not least the attack and release time constants neces-
sarily applied to it. Another is whether the control signal
is being generated all the time and only applied when
the threshold is exceeded or alternatively if the
control-signal determination is only woken up when the
threshold is exceeded. Both can work well, and both
sound utterly different.
Feedback-style compressors can use basic limiters as
above as a starting point. The limiter (using the required
attack characteristics of the compressor as its
attack/release time constants) creates an overage signal
implicit in its own control signal, representing the
amount the input signal is exceeding the limiter
threshold at a given moment. By manipulating this
overage so as to create a control signal more in accord
with a chosen compression ratio rather than the hard
limiting, a suitable release time constant applied, the
doctored control signal is used in a second multiply on
the untrammeled input signal, outside of the feedback
loop. As an approach, this combines the edge and sound
of a feedback-style dynamics unit with a sane determin-
istic compression ratio. Fig. 25-143 shows a family of
deceptively analog-looking input output curves from a
digital soft-knee compressor using the described tech-
nique.25.22.2.3 GatingThe purpose of a gate is to attenuate completely or
partly a signal that falls below a given threshold. Typi-
cally they should wake up (open) quickly, hang open for
a while if the signal goes away just in case it really
hasn’t, and then close at a gentler rate. Also, to prevent
“falsing,” there are two thresholds, one for opening and
the other, slightly lower in level to determine closure.
Written as described above, it is about as “digital,”
yes/no, a set of conditions as one can ever hope to meet
in audio and is a complete natural for the literal
approach.
The absolute value of the input signal is compared to
the open threshold; if tripped, a target control signal of 1
(unattenuated) is applied to a short low-pass filter
bearing the attack (open) time constants feeding the
attenuator multiplier (this will quickly ramp up the gate
to open); at the same time a counter is initialized. The
counter is the hang-time counter. It is reinitialized at
every sample that the close threshold is exceeded such
that it doesn’t get a chance to start counting down unless
the signal really has gone away. If that occurs, and the
counter does count down to zero, a control signal value
of zero (for off) or some other value representing an
amount of off attenuation (depth) is applied to a longer
release time constant low-pass filter, the output of
which is applied to the attenuator multiplier.
Fig. 25-144 shows the dynamic transfer characteris-
tics of a microphone input using a combination gate/soft
limiter; this combination is used extensively, in this case
for a stage backup vocalist. If the singer is makingFigure 25-143. A family of compression curves from a digi-
tal dynamics section.Audio precision compress level (dbu) versus measured genampl (dbu) 21 oct 97 14:31:13
0.0 AP
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30.0
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